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A dark matter disc in three cosmological simulations of Milky Way mass galaxies
Abstract
Making robust predictions for the phase space distribution of dark matter at the solar neighbourhood is vital for dark matter direct detection experiments. To date, almost all such predictions have been based on simulations that model the dark matter alone. Here, we use three cosmological hydrodynamics simulations of bright, disc dominated galaxies to include the effects of baryonic matter self-consistently for the first time. We find that the addition of baryonic physics drastically alters the dark matter profile in the vicinity of the Solar neighbourhood. A stellar/gas disc, already in place at high redshift, causes merging satellites to be dragged preferentially towards the disc plane where they are torn apart by tides. This results in an accreted dark matter disc that contributes ~0.25 - 1.5 times the non-rotating halo density at the solar position. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts WIMP capture in the Earth and Sun, boosts the annual modulation signal, and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined. Comment: Final version accepted for publication in MNRAS; only minor changes from previous version
... Studies using hydrodynamical simulations have found that a dark disc has a negligible contribution (< 25%) to the local DM density and hence are not expected to contribute much to the DM direct detection signal [12,15,29,30]. However, [31] found a wider range for the contribution of dark discs, of (0.25-1.5) × the non-rotating DM halo density near the Sun. A significant dark disc can increase the WIMP detection rates, e.g., by a factor of ≈ 3 at recoil energies of 5-20 keV [32] and thus improve the constraints on the interaction cross-section. ...
... The accreted dark disc is likely to be formed from tidal debris from satellites incoming on low-inclination orbits [24]. A stellar disc also drags the incoming satellites towards the disc plane where they are more easily torn apart by tides [31]. Alternatively (or in addition to), adiabatic contraction due to the stellar disc may also lead to the formation of a dark disc. ...
... Alternatively (or in addition to), adiabatic contraction due to the stellar disc may also lead to the formation of a dark disc. The presence of a dark disc may have JCAP11(2020)016 implications for direct detection of DM, as a low-velocity dark disc with respect to the Earth can increase the rate of detection at lower recoil energies [31,32]. ...
Dark matter (DM) direct detection experiments aim to place constraints on the DM-nucleon scattering cross-section and the DM particle mass. These constraints depend sensitively on the assumed local DM density and velocity distribution function. While astrophysical observations can inform the former (in a model-dependent way), the latter is not directly accessible with observations. Here we use the high-resolution ARTEMIS cosmological hydrodynamical simulation suite of 42 Milky Way-mass halos to explore the spatial and kinematical distributions of the DM in the solar neighbourhood, and we examine how these quantities are influenced by substructures, baryons, the presence of dark discs, as well as general halo-to-halo scatter (cosmic variance). We also explore the accuracy of the standard Maxwellian approach for modelling the velocity distribution function. We find significant halo-to-halo scatter in the density and velocity functions which, if propagated through the standard halo model for predicting the DM detection limits, implies a significant scatter about the typically quoted limit. We also show that, in general, the Maxwellian approximation works relatively well for simulations that include the important gravitational effects of baryons, but is less accurate for collisionless (DM-only) simulations. Given the significant halo-to-halo scatter in quantities relevant for DM direct detection, we advocate propagating this source of uncertainty through in order to derive conservative DM detection limits.
... Such "disk dragging" is most pronounced for massive satellites (∼ 1 : 10 mergers) 1 moving on prograde orbits. Observations of the Milky Way's disk stars do allow for such an encounter [9][10][11]. The fact that accreted stellar and dark disks form from such mergers was first demonstrated using N -body simulations [6,7,[9][10][11], and has since been reproduced in cosmological simulations with baryons [12][13][14]. ...
... Observations of the Milky Way's disk stars do allow for such an encounter [9][10][11]. The fact that accreted stellar and dark disks form from such mergers was first demonstrated using N -body simulations [6,7,[9][10][11], and has since been reproduced in cosmological simulations with baryons [12][13][14]. ...
... Taking this evidence together, we argue that Nyx is a remnant of a dwarf galaxy. Its characteristics are largely consistent with previous studies of accreted stellar disks [9][10][11]14]. Namely, it is corotating with a lag speed that falls in the range motivated by simulations. Additionally, its mean metallicity of [Fe/H] ∼ −0.5 is consistent with a galaxy of stellar mass in the range [4.5 × 10 9 , 2.7 × 10 10 ] M , as estimated using the massmetallicity relation 3 of [47]. ...
Massive dwarf galaxies that merge with the Milky Way on prograde orbits can be dragged into the disk plane before being completely disrupted. Such mergers can contribute to an accreted stellar disk and a dark matter disk. Here we present Nyx, a vast stellar stream in the vicinity of the Sun, which provides the first indication that such an event occurred in the Milky Way. We identify about 200 stars that have coherent radial and prograde motion in this stream using a catalogue of accreted stars built by applying deep learning methods to the Gaia data. Taken together with chemical abundance and orbital information, these results strongly favour the interpretation that Nyx is the remnant of a disrupted dwarf galaxy. Further justified by FIRE hydrodynamic simulations, we demonstrate that prograde streams like Nyx can be found in the disk plane of galaxies and identified using our methods. Analysis of a catalogue of accreted stars by their radial and prograde motions has identified a 200-plus-member coherent stellar stream (called Nyx) that is likely to be the remnant of a dwarf galaxy that merged with the Milky Way.
... Studies using hydrodynamical simulations have found that a dark disc has a negligible contribution (< 25%) to the local DM density and hence are not expected to contribute much to the DM direct detection signal [12,15,29,30]. However, [31] found a wider range for the contribution of dark discs, of (0.25 − 1.5)× the non-rotating DM halo density near the Sun. A significant dark disc can increase the WIMP detection rates, e.g., by a factor of ≈ 3 at recoil energies of 5 − 20 keV [32] and thus improve the constraints on the interaction cross-section. ...
... The accreted dark disc is likely to be formed from tidal debris from satellites incoming on low-inclination orbits [24]. A stellar disc also drags the incoming satellites towards the disc plane where they are more easily torn apart by tides [31]. Alternatively (or in addition to), adiabatic contraction due to the stellar disc may also lead to the formation of a dark disc. ...
... Alternatively (or in addition to), adiabatic contraction due to the stellar disc may also lead to the formation of a dark disc. The presence of a dark disc may have implications for direct detection of DM, as a low-velocity dark disc with respect to the Earth can increase the rate of detection at lower recoil energies [31,32]. ...
Dark matter (DM) direct detection experiments aim to place constraints on the DM--nucleon scattering cross-section and the DM particle mass. These constraints depend sensitively on the assumed local DM density and velocity distribution function. While astrophysical observations can inform the former (in a model-dependent way), the latter is not directly accessible with observations. Here we use the high-resolution ARTEMIS cosmological hydrodynamical simulation suite of 42 Milky Way-mass halos to explore the spatial and kinematical distributions of the DM in the solar neighbourhood, and we examine how these quantities are influenced by substructures, baryons, the presence of dark discs, as well as general halo-to-halo scatter (cosmic variance). We also explore the accuracy of the standard Maxwellian approach for modelling the velocity distribution function. We find significant halo-to-halo scatter in the density and velocity functions which, if propagated through the standard halo model for predicting the DM detection limits, implies a significant scatter about the typically quoted limit. We also show that, in general, the Maxwellian approximation works relatively well for simulations that include the important gravitational effects of baryons, but is less accurate for collisionless (DM-only) simulations. Given the significant halo-to-halo scatter in quantities relevant for DM direct detection, we advocate propagating this source of uncertainty through in order to derive conservative DM detection limits.
... Their efforts were focused on the identification of an accreted or ex-situ disc component. Such an ex-situ disc is expected to arise during massive mergers at low inclination angles with respect to the plane of the main disc (Abadi et al. 2003;Read et al. 2008Read et al. , 2009Purcell et al. 2009;Pillepich et al. 2014Pillepich et al. , 2015Schaller et al. 2016). A DM disc may also form at the same time, which could have important consequences for direct DM detection experiments (see Schaller et al. 2016, and references therein). ...
... An ex-situ disc would not only be relevant for probing the merger history of the Milky Way, but would also hint at the presence of an underlying DM disc (see e.g. Read et al. 2008;Purcell et al. 2009;Read et al. 2009). The quantification and characterization of the DM discs in the Auriga simulations will be presented in another paper (Schaller et Figure 9. Two dimensional histograms of the ratio of ex-situ to in-situ disc stellar mass, ν = M exsitu /M insitu , in age and [Fe/H] space, obtained from four representative galaxies with significant ex-situ discs. ...
... However, it is clear that a single Gaussian centred at 0 km s −1 cannot describe the azimuthal velocity distributions, Vrot. Following Read et al. (2009), we used a double Gaussian to describe these distributions. As in Schaller et al. (2016) one of the Gaussian is centred at Vrot = 0 km s −1 . ...
We characterize the contribution from accreted material to the galactic discs of the Auriga Project, a set of high resolution magnetohydrodynamic cosmological simulations of late-type galaxies performed with the moving-mesh code AREPO. Our goal is to explore whether a significant accreted (or ex-situ) stellar component in the Milky Way disc could be hidden within the near-circular orbit population, which is strongly dominated by stars born in-situ. One third of our models shows a significant ex-situ disc but this fraction would be larger if constraints on orbital circularity were relaxed. Most of the ex-situ material ($\gtrsim 50\%$) comes from single massive satellites ($> 6 \times 10^{10}~M_{\odot}$). These satellites are accreted with a wide range of infall times and inclination angles (up to $85^{\circ}$). Ex-situ discs are thicker, older and more metal-poor than their in-situ counterparts. They show a flat median age profile, which differs from the negative gradient observed in the in-situ component. As a result, the likelihood of identifying an ex-situ disc in samples of old stars on near-circular orbits increases towards the outskirts of the disc. We show three examples that, in addition to ex-situ discs, have a strongly rotating dark matter component. Interestingly, two of these ex-situ stellar discs show an orbital circularity distribution that is consistent with that of the in-situ disc. Thus, they would not be detected in typical kinematic studies.
... Another possible source of DM substructure that has low velocity dispersion is a dark disk. Corotating thick dark disks are found to form in certain N -body simulations with baryons [208][209][210][211] due to the disruption of merging satellites galaxies that are pulled into the disk. In the simulations, the dark disks are found to be co-rotating with lag speeds and velocity dispersions both ∼50 km/s. ...
... In the simulations, the dark disks are found to be co-rotating with lag speeds and velocity dispersions both ∼50 km/s. They may even dominate the local DM density [208,210]; however, as we will see, even if the dark disk is only a small fraction of the local DM density, it can still leave a significant signature in the direct detection data due to the small velocity dispersion and lag speed. ...
The majority of the matter in the Universe is non-luminous and unaccounted for by any known particle, making the unknown nature of dark matter one of the most urgent problems in fundamental physics. Amidst a broad landscape of particles proposed to explain the dark matter, axions have emerged as a particularly well-motivated candidate as they naturally arise in extensions of the Standard Model and can simultaneously reproduce the observed dark matter abundance while solving other outstanding mysteries in particle physics. Despite this, axions have remained largely unprobed, and new insights and innovative approaches are required to carefully test the axion dark matter hypothesis. This thesis aims to advance prospects for axion detection by identifying how axion signals may appear, developing optimized searches for these signals, and implementing robust analysis strategies. I will begin by showing how simulations of axion production in the early universe can direct search efforts toward the best-motivated mass range for axions that solve the Strong textit{CP} Problem related to the absence of a neutron electric dipole moment in quantum chromodynamics. I will then discuss the development of rigorous analysis frameworks for axion direct detection and their application to the search for axion dark matter with the ABRACADABRA detector. Lastly, I will show how astrophysical observations with textit{X}-ray and radio telescopes can be used in novel searches for axion dark matter. This thesis contributes to an increasingly comprehensive search program that will either discover or exclude axion dark matter in the coming years.
... Another possible source of DM substructure that has low velocity dispersion is a dark disk. Co-rotating thick dark disks are found to form in certain N -body simulations with baryons [99][100][101][102] due to the disruption of merging satellites galaxies that are pulled into the disk. In the simulations, the dark disks are found to be corotating with lag speeds and velocity dispersions both ∼50 km/s. ...
... In the simulations, the dark disks are found to be corotating with lag speeds and velocity dispersions both ∼50 km/s. They may even dominate the local DM density [99,101]; however, as we will see, even if the dark disk is only a small fraction of the local DM density, it can still leave a significant signature in the direct detection data due to the small velocity dispersion and lag speed. ...
The next generation of axion direct detection experiments may rule out or confirm axions as the dominant source of dark matter. We develop a general likelihood-based framework for studying the time-series data at such experiments, with a focus on the role of dark-matter astrophysics, to search for signatures of the QCD axion or axion like particles. We illustrate how in the event of a detection the likelihood framework may be used to extract measures of the local dark matter phase-space distribution, accounting for effects such as annual modulation and gravitational focusing, which is the perturbation to the dark matter phase-space distribution by the gravitational field of the Sun. Moreover, we show how potential dark matter substructure, such as cold dark matter streams or a thick dark disk, could impact the signal. For example, we find that when the bulk dark matter halo is detected at 5$\sigma$ global significance, the unique time-dependent features imprinted by the dark matter component of the Sagittarius stream, even if only a few percent of the local dark matter density, may be detectable at $\sim$2$\sigma$ significance. A co-rotating dark disk, with lag speed $\sim$50 km$/$s, that is $\sim$20$\%$ of the local DM density could dominate the signal, while colder but as-of-yet unknown substructure may be even more important. Our likelihood formalism, and the results derived with it, are generally applicable to any time-series based approach to axion direct detection.
... 6.3), had a same mass merger at z ∼ 1. This is the same halo studied in Ref. 63 at a lower resolution compared to Sloane et al. 22 ...
... 69 The existence of a dark disk can modify the signals in direct detection experiments. 63,70,71 In particular if a large fraction of DM particles in the Solar neighborhood are in the disk, the direct detection event rates could be enhanced, especially in the low recoil energy range. Depending on the rotation speed of the DM disk compared to the stellar disk, the phase of the annual modulation signal could also be shifted. ...
In recent years, realistic hydrodynamical simulations of galaxies like the Milky Way have become available, enabling a reliable estimate of the dark matter density and velocity distribution in the Solar neighborhood. We review here the status of hydrodynamical simulations and their implications for the interpretation of direct dark matter searches. We focus in particular on: the criteria to identify Milky Way-like galaxies; the impact of baryonic physics on the dark matter velocity distribution; the possible presence of substructures like clumps, streams, or dark disks; and on the implications for the direct detection of dark matter with standard and nonstandard interactions.
... The product of the DM density and the MSP density is still sufficient to allow for some r-process nucleosynthesis in both the UFDs and the globular clusters, but we estimate that CMZ accounts for 10% to 50% of the total Galactic production. We include contributions from the GC (CMZ) and the rest of the halo (which may be comparable, within uncertainties) [42,43]. ...
... For UFD we vary the DM density in the range 0.3 GeV/cm 3 < ρ UFD DM < 15 GeV/cm 3 , which corresponds to the Navarro-Frenk-White [81] (NFW) profile evaluated in the 1 − 50 pc range from the galactic center, respectively. We consider MW DM velocity dispersion values in the range 50 km/s < v MW < 200 km/s, where the lower limit corresponds to possible DM disk within the halo [42,43] and the upper limit corresponds to NFW DM density profile without adiabatic contraction [80] at 0.1 kpc from the GC. Additionally, we take values of the pulsar velocity dispersion in the MW to be in the range of [47] 48 km/s to [84] 80 km/s. ...
We show that some or all of the inventory of $r$-process nucleosynthesis can be produced in interactions of primordial black holes (PBHs) with neutron stars (NSs) if PBHs with masses ${10}^{-14}\,{\rm M}_\odot < {\rm M}_{\rm PBH} < {10}^{-8}\,{\rm M}_\odot$ make up a few percent or more of the dark matter. A PBH captured by a neutron star (NS) sinks to the center of the NS and consumes it from the inside. When this occurs in a rotating millisecond-period NS, the resulting spin-up ejects $\sim 0.1-0.5\,{\rm M}_{\odot}$ of relatively cold neutron-rich material. This ejection process and the accompanying decompression and decay of nuclear matter can produce electromagnetic transients, such as a kilonova-type afterglow and fast radio bursts. These transients are not accompanied by significant gravitational radiation or neutrinos, allowing such events to be differentiated from compact object mergers occurring within the distance sensitivity limits of gravitational wave observatories. The PBH-NS destruction scenario is consistent with pulsar and NS statistics, the dark matter content and spatial distributions in the Galaxy and Ultra Faint Dwarfs (UFD), as well as with the $r$-process content and evolution histories in these sites. Ejected matter is heated by beta decay, which leads to emission of positrons in an amount consistent with the observed 511-keV line from the Galactic Center.
... There are two classes of explanations for the origin of the Nyx stream with disparate effects on the terrestrial directdetection experiments of dark matter. These two classes revolve around Nyx having an accompanying dark matter component, which reduces to whether Nyx is the remnant of one or more dwarf galaxy accretion events (Abadi et al. 2003;Sales et al. 2009;Read et al. 2009Read et al. , 2008Purcell et al. 2009;Ling et al. 2010;Pillepich et al. 2014;Rodriguez-Gomez et al. 2017). If Nyx is indeed a remnant of a merger, it would potentially contribute to a dark matter substructure, which could affect the local dark matter phase-space distribution and impact constraints from direct-detection experiments Bruch et al. 2009). ...
Nyx is a nearby, prograde, and high-eccentricity stellar stream physically contained in the thick disk, but its origin is unknown. Nyx could be the remnant of a disrupted dwarf galaxy, in which case the associated dark matter substructure could affect terrestrial dark matter direct-detection experiments. Alternatively, Nyx could be a signature of the Milky Way’s disk formation and evolution. To determine the origin of Nyx, we obtained high-resolution spectroscopy of 34 Nyx stars using Keck/HIRES and Magellan/MIKE. A differential chemical abundance analysis shows that most Nyx stars reside in a metal-rich ([Fe/H] > −1) high- α component that is chemically indistinguishable from the thick disk. This rules out the originally suggested scenario that Nyx is the remnant of a single massive dwarf galaxy merger. However, we also identify 5 substantially more metal-poor stars ([Fe/H] ∼ −2.0) whose chemical abundances are similar to those of the metal-weak thick disk. It remains unclear how stars that are chemically identical to the thick disk can be on such prograde, high-eccentricity orbits. We suggest two most likely scenarios: that Nyx is the result of an early minor dwarf galaxy merger, or that it is a record of the early spin-up of the Milky Way disk—although neither perfectly reproduces the chemodynamic observations. The most likely formation scenarios suggest that future spectroscopic surveys should find Nyx-like structures outside of the solar neighborhood.
... This indicates that the host halo influences the orientation of the gas disc. Furthermore, it may suggest that the highly oblate halo shape encoura g es the formation of a gas disc at later times due to the torquing of accreted gas (similar to the effect reported in Read et al. 2009 ). This will be investigated fully in future work (Rey et al. in prep.). ...
Collisionless Dark Matter Only (DMO) structure formation simulations predict that Dark Matter(DM) haloes are prolate in their centres and triaxial towards their outskirts. The addition of gas condensation transforms the central Dark Matter(DM) shape to be rounder and more oblate. It is not clear, however, whether such shape transformations occur in ‘ultra-faint’ dwarfs, which have extremely low baryon fractions. We present the first study of the shape and velocity anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of fgas(r < Rhalf) < 0.06. These dwarfs are drawn from the Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high resolution simulations that allow us to resolve Dark Matter(DM) halo shapes within the half light radius (∼100 pc). We show that gas-poor ultra-faints (M200c ≤ 1.5 × 109 M⊙; fgas < 10−5) retain their pristine prolate Dark Matter(DM) halo shape even when gas, star formation and feedback are included. This could provide a new and robust test of Dark Matter(DM) models. By contrast, gas-rich ultra-faints (M200c > 3 × 109 M⊙; fgas > 10−4) become rounder and more oblate within ∼10 half light radii. Finally, we find that most of our simulated dwarfs have significant radial velocity anisotropy that rises to $\tilde{\beta } > 0.5$ at R ≳ 3Rhalf. The one exception is a dwarf that forms a rotating gas/stellar disc because of a planar, major merger. Such strong anisotropy should be taken into account when building mass models of gas-poor ultra-faints.
... Furthermore, the dynamical model recovers a range of axis ratios for the DM haloes: Au21 = 0.93, Au23 = 0.60, and Au24 = 0.72. The higher DM halo flattening of haloes 23 and 24 could be caused by what appears to be an ex-situ disk, which would lead to a more significant baryonic effect in the more central regions (see also Read et al. (2009)). ...
In this work, we present an action-based dynamical equilibrium model to constrain the phase-space distribution of stars in the stellar halo, present-day dark matter distribution, and the total mass distribution in M31-like galaxies. The model comprises a three-component gravitational potential (stellar bulge, stellar disk, and a dark matter halo), and a double-power law distribution function (DF), $f(\mathbf{J})$, which is a function of actions. A Bayesian model-fitting algorithm was implemented that enabled both parameters of the potential and DF to be explored. After testing the model-fitting algorithm on mock data drawn from the model itself, it was applied to a set of three M31-like haloes from the Auriga simulations (Auriga 21, Auriga 23, Auriga 24). Furthermore, we tested the equilibrium assumption and the ability of a double-power law distribution function to represent the stellar halo stars. The model incurs an error in the total enclosed mass of around 10 percent out to 100 kpc, thus justifying the equilibrium assumption. Furthermore, the double-power law DF used proves to be an appropriate description of the investigated M31-like halos. The anisotropy profiles of the halos were also investigated and discussed from a merger history point of view.
... Some simulations have reported the presence of an increase in DM density inside the galactic disc [90,91], the so-called dark disc. This feature tends to be absent in more recent works [92,93] with more realistic discs. ...
The role of baryonic physics, star formation and stellar feedback, in shaping the galaxies and their host halos is an evolving topic. The dark matter aspects are illustrated in this work by showing distribution features in a Milky Way sized halo. We focus on the halo morphology, geometry, and profile as well as the phase space distribution using one dark matter only and five hydrodynamical cosmological high-resolution simulations of the same halo with different subgrid prescriptions for the baryonic physics (Kennicut versus multi-freefall star formation and delayed cooling versus mechanical supernovae feedback). If some general properties like the relative halo-galaxy orientation are similar, the modifications of the gravitational potential due to the presence of baryons are found to induce different dark matter distributions (rounder and more concentrated halo). The mass density profile as well as the velocity distribution are modified distinctively according to the specific resulting baryonic distribution highlighting the variability of those properties (e.g inner power index from 1.3 to 1.8, broader speed distribution). The uncertainties on those features are of paramount importance for dark matter phenomenology, particularly when dealing with dark matter dynamics or direct and indirect detection searches. As a consequence, dark matter properties and prospects using cosmological simulations require improvement on baryonic physics description. Modeling such processes is a key issue not only for galaxy formation but also for dark matter investigations.
... This indicates that the host halo influences the orientation of the gas disc. Furthermore, it may suggest that the highly oblate halo shape encourages the formation of a gas disc at later times due to the torquing of accreted gas (similar to the effect reported in Read et al. 2009). This will be investigated fully in future work (Rey et al. in prep.). ...
Purely collisionless Dark Matter Only (DMO) structure formation simulations predict that Dark Matter (DM) haloes are typically prolate in their centres and spheroidal towards their outskirts. The addition of gas cooling transforms the central DM shape to be rounder and more oblate. It is not clear, however, whether such shape transformations occur in `ultra-faint' dwarfs, which have extremely low baryon fractions. We present the first study of the shape and velocity anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of $f_{\rm gas}(r<R_{\rm half}) < 0.06$. These dwarfs are drawn from the Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high resolution simulations (spatial and mass resolution of 3 pc and $120$ M$_\odot$, respectively) that allow us to resolve DM halo shapes within the half light radius ($\sim 100$ pc). We show that gas-poor ultra-faints ($M_{\rm 200c} \leqslant 1.5\times10^9$ M$_\odot$; $f_{\rm gas} < 10^{-5}$) retain their pristine prolate DM halo shape even when gas, star formation and feedback are included. This could provide a new and robust test of DM models. By contrast, gas-rich ultra-faints ($M_{\rm 200c} > 3\times10^9$ M$_\odot$; $f_{\rm gas} > 10^{-4}$) become rounder and more oblate within $\sim 10$ half light radii. Finally, we find that most of our simulated dwarfs have significant radial velocity anisotropy that rises to $\tilde{\beta} > 0.5$ at $R \gtrsim 3 R_{\rm half}$. The one exception is a dwarf that forms a rotating gas/stellar disc because of a planar, major merger. Such strong anisotropy should be taken into account when building mass models of gas-poor ultra-faints.
... There are two classes of explanations for the origin of the Nyx stream with disparate effects on the terrestrial direct detection experiments of dark matter. These two classes revolve around Nyx having an accompanying dark matter component, which reduces to whether or not Nyx is the remnant of one or more dwarf galaxy accretion events (Abadi et al. 2003;Sales et al. 2009;Read et al. 2009Read et al. , 2008Purcell et al. 2009;Ling et al. 2010;Pillepich et al. 2014;Rodriguez-Gomez et al. 2017). If Nyx is indeed a remnant of a merger, it would potentially contribute to a dark matter substructure which could affect the local dark matter phase-space distribution and impact constraints from direct detection experiments Bruch et al. 2009). ...
Nyx is a nearby, prograde, and high-eccentricity stellar stream physically contained in the thick disk but with an unknown origin. Nyx could be the remnant of a disrupted dwarf galaxy, in which case the associated dark matter substructure could affect terrestrial dark matter direct detection experiments. Alternatively, Nyx could be a signature of the Milky Way's disk formation and evolution. To determine the origin of Nyx, we obtained high-resolution spectroscopy of 34 Nyx stars using Keck/HIRES and Magellan/MIKE. A differential chemical abundance analysis shows that most Nyx stars reside in a metal-rich ($\mbox{[Fe/H]} > -1$) high-$\alpha$ component that is chemically indistinguishable from the thick disk. This rules out an originally suggested scenario that Nyx is the remnant of a single massive dwarf galaxy merger. However, we also identify five substantially more metal-poor stars ($\mbox{[Fe/H]} \sim -2.0$) that have chemical abundances similar to the metal-weak thick disk. It remains unclear how stars chemically identical to the thick disk can be on such prograde, high-eccentricity orbits. We suggest two most likely scenarios: that Nyx is the result of an early minor dwarf galaxy merger or that it is a record of the early spin-up of the Milky Way disk -- although neither perfectly reproduces the chemodynamic observations. The most likely formation scenarios suggest that future spectroscopic surveys should find Nyx-like structures outside of the Solar Neighborhood.
... However, no currently known astronomical tracer can directly probe the DM contribution to the MW gravitational potential with subparsec resolution [38,39]. Progress in understanding the particle properties of DM [40][41][42], the shape, composition, and merger history of the MW [43,44] and, more broadly, the formation of galaxies is hampered by the lack of such ultralocal subparsec information about the DM density. In particular, by combining global and ultralocal measurements of the DM density, we can constrain the shape of the MW halo [39]. ...
Despite strong evidence for the existence of large amounts of dark matter (DM) in our Universe, there is no direct indication of its presence in our own solar system. All estimates of the local DM density rely on extrapolating results on much larger scales. We demonstrate for the first time the possibility of simultaneously measuring the local DM density and interaction cross section with a direct detection experiment. It relies on the assumption that incoming DM particles frequently scatter on terrestrial nuclei prior to detection, inducing an additional time-dependence of the signal. We show that for sub-GeV DM, with a large spin-independent DM-proton cross section, future direct detection experiments should be able to reconstruct the local DM density with smaller than 50% uncertainty.
... Simulations indicate that such events may lead to similar substructures in the DM phase space distribution [32,61]. Proposed structures range from cold components like co-rotating DM disks [62] to hot components like in-falling extragalactic DM, near v esc [25]. ...
... Simulations indicate that such events may lead to similar substructures in the DM phase space distribution [32,61]. Proposed structures range from cold components like co-rotating DM disks [62] to hot components like in-falling extragalactic DM, near v esc [25]. ...
DEAP-3600 is a single-phase liquid argon detector aiming to directly detect Weakly Interacting Massive Particles (WIMPs), located at SNOLAB (Sudbury, Canada). After analyzing data taken during the first year of operation, a null result was used to place an upper bound on the WIMP-nucleon spin-independent, isoscalar cross section. This study reinterprets this result within a Non-Relativistic Effective Field Theory framework, and further examines how various possible substructures in the local dark matter halo may affect these constraints. Such substructures are hinted at by kinematic structures in the local stellar distribution observed by the Gaia satellite and other recent astronomical surveys. These include the Gaia Sausage (or Enceladus), as well as a number of distinct streams identified in recent studies. Limits are presented for the coupling strength of the effective contact interaction operators $\mathcal{O}_1$, $\mathcal{O}_3$, $\mathcal{O}_5$, $\mathcal{O}_8$, and $\mathcal{O}_{11}$, considering isoscalar, isovector, and xenonphobic scenarios, as well as the specific operators corresponding to millicharge, magnetic dipole, electric dipole, and anapole interactions. The effects of halo substructures on each of these operators are explored as well, showing that the $\mathcal{O}_5$ and $\mathcal{O}_8$ operators are particularly sensitive to the velocity distribution, even at dark matter masses above 100 GeV/$c^2$.
... However, besides the isothermal sphere, other dark matter profiles like the NFW profile [28] are suggested. Other potential deviations from the SHM include that the halo might instead be shaped in a more oblate or prolate way [29,30,31,32]. Furthermore, the concentration of the dark matter halo, and hence overall local density, can depend on cosmology [33]. ...
The measurement of an annual modulation in the event rate of direct dark matter detection experiments is a powerful tool for dark matter discovery. Indeed, several experiments have already claimed such a discovery in the past decade. While most of them have later revoked their conclusions, and others have found potentially contradictory results, one still stands today. This paper explains the potential as well as the challenges of annual modulation measurements, and gives an overview on past, present and future direct detection experiments.
... The final result is that the rotating gas sphere collapses into a disk-like shape. Also, a disk may be formed by accretion Of dark electrons into the baryonic potential [95][96][97]. In the presence of mergers, though, the disk may be disrupted [62,80], and the final halo distribution may be spheroidal. ...
We present the complete history of structure formation in a simple dissipative dark-sector model. The model has only two particles: a dark electron, which is a subdominant component of dark matter, and a dark photon. Dark-electron perturbations grow from primordial overdensities, become non-linear, and form dense dark galaxies. Bremsstrahlung cooling leads to fragmentation of the dark-electron halos into clumps that vary in size from a few to millions of solar masses, depending on the particle model parameters. In particular, we show that asymmetric dark stars and black holes form within the Milky Way from the collapse of dark electrons. These exotic compact objects may be detected and their properties measured at new high-precision astronomical observatories, giving insight into the particle nature of the dark sector without the requirement of non-gravitational interactions with the visible sector. Our analysis demonstrates that a full study of the structure-formation history is needed to assess correctly the viability of exotic compact objects in dissipative dark-sector models.
... 2018; Necib et al., 2018b). Such accreted stars also trace the presence of a 'dark disk' formed from the late accretion of massive and more metal rich satellites (Lake, 1989;Read et al., 2008Read et al., , 2009Ruchti et al., 2014Ruchti et al., , 2015, and structures that are not yet fully phase mixed like 'debris flows' (e.g., Belokurov et al., 2018;Helmi et al., 2018;Necib et al., 2018a) and tidal streams (e.g., Freese et al., 2005;O'Hare et al., 2018). All of these structures imprint features on f (v) that alter the expected flux at a given recoil energy in dark matter detection experiments, and the expected annual modulation signal (e.g., Freese et al., 2005;Bruch et al., 2009;Evans et al., 2018). ...
Astrophysical observations currently provide the only robust, empirical measurements of dark matter. In the coming decade, astrophysical observations will guide other experimental efforts, while simultaneously probing unique regions of dark matter parameter space. This white paper summarizes astrophysical observations that can constrain the fundamental physics of dark matter in the era of LSST. We describe how astrophysical observations will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational interactions with the Standard Model, and compact object abundances. Additionally, we highlight theoretical work and experimental/observational facilities that will complement LSST to strengthen our understanding of the fundamental characteristics of dark matter.
... 2018; Necib et al., 2018b). Such accreted stars also trace the presence of a 'dark disk' formed from the late accretion of massive and more metal rich satellites (Lake, 1989;Read et al., 2008Read et al., , 2009Ruchti et al., 2014Ruchti et al., , 2015, and structures that are not yet fully phase mixed like 'debris flows' (e.g., Belokurov et al., 2018;Helmi et al., 2018;Necib et al., 2018a) and tidal streams (e.g., Freese et al., 2005;O'Hare et al., 2018). All of these structures imprint features on f (v) that alter the expected flux at a given recoil energy in dark matter detection experiments, and the expected annual modulation signal (e.g., Freese et al., 2005;Bruch et al., 2009;Evans et al., 2018). ...
Astrophysical and cosmological observations currently provide the only robust, empirical measurements of dark matter. Future observations with Large Synoptic Survey Telescope (LSST) will provide necessary guidance for the experimental dark matter program. This white paper represents a community effort to summarize the science case for studying the fundamental physics of dark matter with LSST. We discuss how LSST will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational couplings to the Standard Model, and compact object abundances. Additionally, we discuss the ways that LSST will complement other experiments to strengthen our understanding of the fundamental characteristics of dark matter. More information on the LSST dark matter effort can be found at https://lsstdarkmatter.github.io/ .
... The final result is that the rotating gas sphere collapses into a disk-like shape. Also, a disk may be formed by accretion of dark electrons into the baryonic potential [93][94][95]. In the presence of mergers, though, the disk may be disrupted [60,78], and the final halo distribution may be spheroidal. ...
We present the complete history of structure formation in a simple dissipative dark-sector model. The model has only two particles: a dark electron, which is a subdominant component of dark matter, and a dark photon. Dark-electron perturbations grow from primordial overdensities, become non-linear, and form dense dark galaxies. Bremsstrahlung cooling leads to fragmentation of the dark-electron halos into clumps that vary in size from a few to millions of solar masses, depending on the particle model parameters. In particular, we show that asymmetric dark stars and black holes form within the Milky Way from the collapse of dark electrons. These exotic compact objects may be detected and their properties measured at new high-precision astronomical observatories, giving insight into the particle nature of the dark sector without the requirement of non-gravitational interactions with the visible sector.
... Deviations from this simple model of the DM halo are expected in the Milky Way [35][36][37][38] and while the parameters associated with the SHM (such as the Sun's velocity [39], the local circular velocity [40] and the Galactic escape velocity [41]) can be estimated observationally, they carry with them an associated uncertainty [10,42,43]. In addition, numerical simulations have suggested the possibility of non-Maxwellian structure in the DM velocity distribution [44][45][46][47][48][49][50][51]. While some state-of-the-art hydrodynamical simulations find DM distributions consistent with the Maxwell-Boltzmann form [52,53], it is still possible that ultra-local substructures such as streams may also contribute [54,55]. ...
The theoretical interpretation of dark matter (DM) experiments is hindered by uncertainties on the dark matter density and velocity distribution inside the Solar System. In order to quantify those uncertainties, we present a parameter that characterizes the deviation of the true velocity distribution from the standard Maxwell-Boltzmann form, and we then determine for different values of this parameter the most aggressive and most conservative limits on the dark matter scattering cross section with nuclei; uncertainties in the local dark matter density can be accounted for trivially. This allows us to bracket, in a model independent way, the impact of astrophysical uncertainties on limits from direct detection experiments and/or neutrino telescopes. We find that current limits assuming the Standard Halo Model are at most a factor of ∼ 2 weaker than the most aggressive possible constraints. In addition, combining neutrino telescope and direct detection constraints (in a statistically meaningful way), we show that limits on DM in the mass range ∼ 10 - 1000 GeV cannot be weakened by more than around a factor of 10, for all possible velocity distributions. We finally demonstrate that our approach can also be employed in the event of a DM discovery, allowing us to avoid bias in the reconstruction of the DM properties.
... We (like others before us [85,119,125]) will focus on substructure in the form of streams, since the case for their presence nearby is the most compelling. However other creatures have been suggested variously in the literature such as debris flows [126][127][128][129], shadow bars [130,131] and dark disks [132][133][134][135][136]. The latter of these would lead to an enhancement of f (v) at low speeds. ...
We develop a formalism to describe extensions of existing axion haloscope designs to those that possess directional sensitivity to incoming dark matter axion velocities. The effects are measurable if experiments are designed to have dimensions that approach the typical coherence length for the local axion field. With directional sensitivity, axion detection experiments would have a greatly enhanced potential to probe the local dark matter velocity distribution. We develop our formalism generally, but apply it to specific experimental designs, namely resonant cavities and dielectric disk haloscopes. We demonstrate that these experiments are capable of measuring the daily modulation of the dark matter signal and using it to reconstruct the three-dimensional velocity distribution. This allows one to measure the Solar peculiar velocity, probe the anisotropy of the dark matter velocity ellipsoid and identify cold substructures such as the recently discovered streams near to Earth. Directional experiments can also identify features over much shorter timescales, potentially facilitating the mapping of debris from axion miniclusters.
... Deviations from this simple model of the DM halo are expected in the Milky Way [35][36][37][38] and while the parameters associated with the SHM (such as the Sun's velocity [39], the local circular velocity [40] and the Galactic escape velocity [41]) can be estimated observationally, they carry with them an associated uncertainty [10,42]. In addition, numerical simulations have suggested the possibility of non-Maxwellian structure in the DM velocity distribution [43][44][45][46][47][48][49][50]. While some state-of-the-art hydrodynamical simulations find DM distributions consistent with the Maxwell-Boltzmann form [51,52], it is still possible that ultra-local substructures such as streams may also contribute [53,54]. ...
The theoretical interpretation of dark matter (DM) experiments is hindered by uncertainties on the dark matter density and velocity distribution inside the Solar System. In order to quantify those uncertainties, we present a parameter that characterizes the deviation of the true velocity distribution from the standard Maxwell-Boltzmann form, and we then determine for different values of this parameter the most aggressive and most conservative limits on the dark matter scattering cross section with nuclei. This allows us to bracket, in a model independent way, the impact of astrophysical uncertainties on limits from direct detection experiments and/or neutrino telescopes. We find that current limits assuming the Standard Halo Model are at most a factor of $\sim 2$ weaker than the most aggressive possible constraints. In addition, combining neutrino telescope and direct detection constraints (in a statistically meaningful way), we show that limits on DM in the mass range $\sim 10 - 1000$ GeV cannot be weakened by more than around a factor of 10, for all possible velocity distributions. We finally demonstrate that our approach can also be employed in the event of a DM discovery, allowing us to avoid bias in the reconstruction of the DM properties.
... We (like others before us [84,118,123]) will focus on substructure in the form of streams, since the case for their presence nearby is the most compelling. However other creatures have been suggested variously in the literature such as debris flows [124][125][126][127], shadow bars [128,129] and dark disks [130][131][132][133][134]. The latter of these would lead to an enhancement of f (v) at low speeds. ...
We develop a formalism to describe extensions of existing axion haloscope designs to those that possess directional sensitivity to incoming dark matter axion velocities. The effects are measurable if experiments are designed to have dimensions that approach the typical coherence length for the local axion field. With directional sensitivity, axion detection experiments would have a greatly enhanced potential to probe the local dark matter velocity distribution. We develop our formalism generally, but apply it to specific experimental designs, namely resonant cavities and dielectric disk haloscopes. We demonstrate that these experiments are capable of measuring the daily modulation of the dark matter signal and using it to reconstruct the three-dimensional velocity distribution. This allows one to measure the Solar peculiar velocity, probe the anisotropy of the dark matter velocity ellipsoid and identify cold substructures such as the recently discovered streams near to Earth. Directional experiments can also identify features over much shorter timescales, potentially facilitating the mapping of debris from axion miniclusters.
... In most of direct searches, the velocity distribution of dark matter is supposed to be isotropic Maxwellian velocity distribution. However, non-Maxwellian distribution had been indicated by some simulations and observations [2][3][4][5][6][7]. In this study, an anisotropic velocity distribution derived in [4] is adopted; ...
Direct detection of dark matter with directional sensitivity has the potential to discriminate the dark matter velocity distribution. Especially, it will be suitable to discriminate isotropic distribution from anisotropic one. Analyzing data produced with Monte-Carlo simulation, required conditions for the discrimination is estimated. If energy threshold of detector is optimized, $O(10^3-10^4)$ event number is required to discriminate the anisotropy.
... The dark disc can be generated through several mechanisms, such as accretion of DM subhalos [13,14,15], or the flattening of a spherical DM halo in the response to the formation of the baryon disc [16,17,18]. The dark disc element is turned on and off for various runs. ...
... The DM velocity dispersion values are considered to lie in the 50 km/s < v < 200 km/s range. The lower v limit corresponds to a possible DM disk within the halo [106,107], while the upper limit corresponds to the Navarro-Frenk-White DM density profile without adiabatic contraction [108]. We further have included the effects of natal pulsar kicks, modifying the capture rate F 0 according to the method outlined in [27]. ...
We present several scenarios how capture of primordial black holes (PBHs) with masses $10^{-16} M_{\odot} \lesssim M_{\rm PBH} \lesssim 10^{-7} M_{\odot}$ by compact objects (white dwarfs or neutron stars) can source short gamma-ray bursts (sGRBs) as well as microquasars (MQs). A small PBH captured by a compact star will eventually consume the host, turning it into a stellar-mass BH. We argue that formation of an accretion disk around the resulting BH, which is an important prerequisite for standard sGRB production mechanisms, can be generic. The PBH-induced relativistic sGRB flares as well as continuous MQ jets will accelerate positrons to high energies. We find that if PBHs constitute a few percent or more of the dark matter, the generated positrons can address the excess observed in the positron flux by the Pamela, the AMS-02 and the Fermi-LAT experiments. The allowed resulting parameter space for primordial black holes to constitute dark matter is in a favorable region to permit for resolution of several other astronomical puzzles.
... On the other hand, N-body simulations indicate that a Maxwellian distribution might not provide a good description of the smooth halo component [10][11][12][13][14][15]. Furthermore, it has been argued that the dark matter halo of our Galaxy might contain tidal streams or a dark disk component [16][17][18][19] which may induce significant deviations in the velocity distribution from the Maxwell-Boltzmann form and in turn affect the interpretation of dark matter search experiments [20][21][22][23][24] (however, the existence of a dark disk in the Milky Way has been questioned in [25][26][27] and seems to be disfavored by observations [28]). More recently, hydrodynamical simulations have suggested that the average velocity distribution at the position of the Solar System may be well described by a Maxwell-Boltzmann form [26,27,29]. ...
We present a method to calculate, without making assumptions about the local dark matter velocity distribution, the maximal and minimal number of signal events in a direct detection experiment given a set of constraints from other direct detection experiments and/or neutrino telescopes. The method also allows to determine the velocity distribution that optimizes the signal rates. We illustrate our method with three concrete applications: i) to derive a halo-independent upper limit on the cross section from a set of null results, ii) to confront in a halo-independent way a detection claim to a set of null results and iii) to assess, in a halo-independent manner, the prospects for detection in a future experiment given a set of current null results.
... A more recent measurement using the rotation curve of the galaxy and assuming spherical symmetry of the dark halo [48] finds ρ DM = 0.420 +0.019 −0.021 (2σ) ± 0.026 GeV/cm 3 where the second error is the standard deviation of the best-fit values over all visible matter models used in the paper (and there is a small variation of this range depending on the dark halo profile assumed). The local DM density and velocity distribution could also be affected by local substructure in the dark halo, e.g. if Earth is within a DM clump, which is unlikely [49], or in a DM stream [50], or if there is a dark disk in our galaxy [51]. The DM of the leading arm of the Sagittarius Stream, tidally stripped from the Sagittarius Dwarf Galaxy, could be passing through the Solar system, perpendicularly to the galactic disk [50] and tidal disruption of earlier substructure have created "debris flows", which are spatially homogeneous structures in velocity [52]. ...
Light WIMPs are dark matter particle candidates with weak scale interaction with the known particles and mass in the GeV to 10's of GeV range. Hints of light WIMPs have appeared in several dark matter searches in the last decade.Here we review the evidence for and against light WIMPs as dark matter candidates.
... The matter has not yet been conclusively settled and, critically for direct detection experiments, this means that the local velocity distribution at the Earth's Galactic radius may contain significant departures from a Maxwellian form [56][57][58]. The distribution may also contain additional features and substructures such as debris flows [59,60], tidal streams [61,62], a corotating dark disk [63][64][65] or a 'Shadow Bar' [66,67]. We consider three astrophysical benchmarks in this work which are motivated by results from N-body simulations, but also importantly have very different velocity structures so that the different approaches for reconstructing the velocity distribution can be compared under a range of scenarios. ...
Directionally sensitive dark matter (DM) direct detection experiments present the only way to observe the full three-dimensional velocity distribution of the Milky Way halo local to Earth. In this work we compare methods for extracting information about the local DM velocity distribution from a set of recoil directions and energies in a range of hypothetical directional and non-directional experiments. We compare a model independent empirical parameterisation of the velocity distribution based on an angular discretisation with a model dependent approach which assumes knowledge of the functional form of the distribution. The methods are tested under three distinct halo models which cover a range of possible phase space structures for the local velocity distribution: a smooth Maxwellian halo, a tidal stream and a debris flow. In each case we use simulated directional data to attempt to reconstruct the shape and parameters describing each model as well as the DM particle properties. We find that the empirical parametrisation is able to make accurate unbiased reconstructions of the DM mass and cross section as well as capture features in the underlying velocity distribution in certain directions without any assumptions about its true functional form. We also find that by extracting directionally averaged velocity parameters with this method one can discriminate between halo models with different classes of substructure.
A recent Letter studied cratering during collisions between rocky bodies and primordial black holes. Hydrodynamic simulations in that work showed that ejecta blankets from these collisions are steeper because the black holes completely penetrate the target, potentially making these craters distinguishable from traditional point-like impactors. This may allow us to use lunar craters to constrain primordial black holes in the asteroid-mass window, about 1017 to 1019 g. In this work, we calculate the lunar dark matter flux from the Galactic halo and several models for a dark disk. We consider several effects that may enhance the dark matter flux, such as gravitational focusing on the solar system and historical modulations due to the solar system’s galactic orbit. We find that nondetection of novel craters on the moon can constrain relativistic compact MACHO dark matter up to 1017 g at 95 % confidence, motivating a detailed search through lunar surface scans. In addition, we show that fluxes near Earth from dark disks may be significantly enhanced by gravitational focusing and that the relative velocity between the disk and the sun can result in annual modulations out of phase with the annual modulations from the halo.
The role of baryonic physics, star formation, and stellar feedback, in shaping the galaxies and their host halos is an evolving topic. The dark matter aspects are illustrated in this work by showing distribution features in a Milky-Way-sized halo. We focus on the halo morphology, geometry, and profile as well as the phase space distribution using one dark matter only and five hydrodynamical cosmological high-resolution simulations of the same halo with different subgrid prescriptions for the baryonic physics (Kennicut versus multi-freefall star formation and delayed cooling versus mechanical supernovae feedback). If some general properties like the relative halo-galaxy orientation are similar, the modifications of the gravitational potential due to the presence of baryons are found to induce different dark matter distributions (rounder and more concentrated halo). The mass density profile as well as the velocity distribution are modified distinctively according to the specific resulting baryonic distribution highlighting the variability of those properties (e.g inner power index from 1.3 to 1.8, broader speed distribution). The uncertainties on those features are of paramount importance for dark matter phenomenology, particularly when dealing with dark matter dynamics or direct and indirect detection searches. As a consequence, dark matter properties and prospects using cosmological simulations require improvement on baryonic physics description. Modeling such processes is a key issue not only for galaxy formation but also for dark matter investigations.
This review focuses on novel astrophysical probes of dark matter at galactic and sub-galactic scales. After reviewing classical tests of cold dark matter (CDM) in galaxy formation, we discuss them in light of recent results from increasingly detailed simulations and observations, and then shift our attention to more recent, less explored tests in the context of the most popular and most studied dark matter scenarios alternative to CDM. Among them, there are warm dark matter (WDM) scenarios, arising, for example, from sterile neutrinos, as well as self-interacting dark matter (SIDM) scenarios , mixed models that combine WDM and SIDM, and Bose–Einstein condensate/fuzzy dark matter scenarios (FDM/BECDM) originating from ultra-light bosons such as axions. The hypothesis that primordial black holes constitute all or most of the dark matter is also revisited in light of the LIGO/Virgo discovery of massive black holes together with very recent constraints from the internal structure of nearby ultra-faint dwarf galaxies. The important role of baryonic physics in the interpretation of various probes of dark matter, especially how it affects the ability to infer dark matter properties from observational diagnostics, is emphasized and reviewed. The effect of baryons blurs, in many cases, the underlying differences in the properties of dark matter halos arising in various dark matter models. Nevertheless, baryons can potentially be a useful tracer of such differences, for instance during the earliest phases of star formation in the lowest mass galaxies. New promising probes which will be delivered by future gravitational wave experiments are discussed, such as the occurrence rate of gravitational wave signals from merging intermediate mass black holes in dwarf galaxies tracing the inner structure of dark halos. Recent observational discoveries and analysis methods, such as the tentative detection of dark subhalos through the analysis of stellar tidal streams in the Milky Way halo, and the prospects of gravitational lensing analysis to directly detect dark substructure down to the relevant small scales, are also illustrated.
DEAP-3600 is a single-phase liquid argon detector aiming to directly detect weakly interacting massive particles (WIMPs), located at SNOLAB (Sudbury, Canada). After analyzing data taken during the first year of operation, a null result was used to place an upper bound on the WIMP-nucleon, spin-independent, isoscalar cross section. This study reinterprets this result within a nonrelativistic effective field theory framework and further examines how various possible substructures in the local dark matter halo may affect these constraints. Such substructures are hinted at by kinematic structures in the local stellar distribution observed by the Gaia satellite and other recent astronomical surveys. These include the Gaia Sausage (or Enceladus), as well as a number of distinct streams identified in recent studies. Limits are presented for the coupling strength of the effective contact interaction operators O1, O3, O5, O8, and O11, considering isoscalar, isovector, and xenonphobic scenarios, as well as the specific operators corresponding to millicharge, magnetic dipole, electric dipole, and anapole interactions. The effects of halo substructures on each of these operators are explored as well, showing that the O5 and O8 operators are particularly sensitive to the velocity distribution, even at dark matter masses above 100 GeV/c2.
We apply the vertical Jeans equation to the kinematics of Milky Way stars in the solar neighbourhood to measure the local dark matter density. More than 90 000 G- and K-type dwarf stars are selected from the cross-matched sample of LAMOST (Large Sky Area Multi-Object Fibre Spectroscopic Telescope) fifth data release and Gaia second data release for our analyses. The mass models applied consist of a single exponential stellar disc, a razor thin gas disc, and a constant dark matter density. We first consider the simplified vertical Jeans equation that ignores the tilt term and assumes a flat rotation curve. Under a Gaussian prior on the total stellar surface density, the local dark matter density inferred from Markov chain Monte Carlo simulations is $0.0133_{-0.0022}^{+0.0024}\ {\rm M}_{\odot }\, {\rm pc}^{-3}$. The local dark matter densities for subsamples in an azimuthal angle range of −10° < ϕ < 5° are consistent within their 1σ errors. However, the northern and southern subsamples show a large discrepancy due to plateaux in the northern and southern vertical velocity dispersion profiles. These plateaux may be the cause of the different estimates of the dark matter density between the north and south. Taking the tilt term into account has little effect on the parameter estimations and does not explain the north and south asymmetry. Taking half of the difference of σz profiles as unknown systematic errors, we then obtain consistent measurements for the northern and southern subsamples. We discuss the influence of the vertical data range, the scale height of the tracer population, the vertical distribution of stars, and the sample size on the uncertainty of the determination of the local dark matter density.
In Ostdiek et al. (2019), we developed a deep neural network classifier that only relies on phase-space information to obtain a catalog of accreted stars based on the second data release of Gaia (DR2). In this paper, we apply two clustering algorithms to identify velocity substructure within this catalog. We focus on the subset of stars with line-of-sight velocity measurements that fall in the range of Galactocentric radii r∈[6.5,9.5] kpc and vertical distances |z|<3 kpc. Known structures such as Gaia Enceladus and the Helmi stream are identified. The largest previously-unknown structure, Nyx, first introduced in Necib et al. (2019a), is a vast stream consisting of at least 500 stars in the region of interest. This study displays the power of the machine learning approach by not only successfully identifying known features, but also discovering new kinematic structures that may shed light on the merger history of the Milky Way.
Context . Very metal-poor halo stars are the best candidates for being among the oldest objects in our Galaxy. Samples of halo stars with age determination and detailed chemical composition measurements provide key information for constraining the nature of the first stellar generations and the nucleosynthesis in the metal-poor regime.
Aims . Age estimates are very uncertain and are available for only a small number of metal-poor stars. We present the first results of a pilot programme aimed at deriving precise masses, ages, and chemical abundances for metal-poor halo giants using asteroseismology and high-resolution spectroscopy.
Methods . We obtained high-resolution UVES spectra for four metal-poor RAVE stars observed by the K2 satellite. Seismic data obtained from K2 light curves helped improve spectroscopic temperatures, metallicities, and individual chemical abundances. Mass and ages were derived using the code PARAM, investigating the effects of different assumptions (e.g. mass loss and [ α /Fe]-enhancement). Orbits were computed using Gaia DR2 data.
Results . The stars are found to be normal metal-poor halo stars (i.e. non C-enhanced), and an abundance pattern typical of old stars (i.e. α and Eu-enhanced), and have masses in the 0.80−1.0 M⊙ range. The inferred model-dependent stellar ages are found to range from 7.4 Gyr to 13.0 Gyr with uncertainties of ∼30%−35%. We also provide revised masses and ages for metal-poor stars with Kepler seismic data from the APOGEE survey and a set of M4 stars.
Conclusions . The present work shows that the combination of asteroseismology and high-resolution spectroscopy provides precise ages in the metal-poor regime. Most of the stars analysed in the present work (covering the metallicity range of [Fe/H] ∼ −0.8 to −2 dex) are very old >9 Gyr (14 out of 19 stars), and all of the stars are older than >5 Gyr (within the 68 percentile confidence level).
We present and discuss the stellar kinematics and populations of the S0 galaxy FCC 170 (NGC 1381) in the Fornax cluster, using deep MUSE data from the Fornax 3D survey. We show the maps of the first four moments of the stellar line-of-sight velocity distribution and of the mass-weighted mean stellar age, metallicity, and [Mg/Fe] abundance ratio. The high-quality MUSE stellar kinematic measurements unveil the structure of this massive galaxy: a nuclear disk, a bar seen as a boxy bulge with a clear higher-velocity-dispersion X shape, a fast-rotating and flaring thin disk and a slower rotating thick disk. Whereas their overall old age makes it difficult to discuss differences in the formation epoch between these components, we find a clear-cut distinction between metal-rich and less [Mg/Fe]-enhanced populations in the thin-disk, boxy-bulge and nuclear disk, and more metal-poor and [Mg/Fe]-enhanced stars in the thick disk. Located in the densest region of the Fornax cluster, where signs of tidal stripping have been recently found, the evolution of FCC 170 might have been seriously affected by its environment. We discuss the possibility of its "preprocessing" in a subgroup before falling into the present-day cluster, which would have shaped this galaxy a long time ago. The thick disk displays a composite star formation history, as a significant fraction of younger stars co-exist with the main older thick-disk population. The former subpopulation is characterized by even lower-metallicity and higher-[Mg/Fe] values, suggesting that these stars formed later and faster in a less chemically evolved satellite, which was subsequently accreted. Finally, we discuss evidence that metal-rich and less [Mg/Fe]-enhanced stars were brought in the outer parts of the thick disk by the flaring of the thin disk.
A thick dark matter disk is predicted in cold dark matter simulations as the outcome of the interaction between accreted satellites and the stellar disk in Milky Way sized halos. We study the effects of a self-interacting thick dark disk on the energetic neutrino flux from the Sun. We find that for particle masses between 100 GeV and 1 TeV and dark matter annihilation to heavy leptons either the self-interaction may not be strong enough to solve the small scale structure motivation or a dark disk cannot be present in the Milky Way.
This is a review article on the primordial black holes (PBHs), with particular focus on the massive ones ($\gtrsim 10^{15}{\rm g}$) which have not evaporated by the present epoch by the Hawking radiation. By the detections of gravitational waves by LIGO, we have gained a completely novel tool to observationally search for PBHs complementary to the electromagnetic waves. Based on the perspective that gravitational-wave astronomy will make a significant progress in the next decades, a purpose of this article is to give a comprehensive review covering a wide range of topics on PBHs. After discussing PBH formation as well as several inflation models leading to PBH production, we summarize various existing and future observational constraints. We then present topics on formation of PBH binaries, gravitational waves from PBH binaries, various observational tests of PBHs by using gravitational waves.
Although the velocity distribution of dark matter is assumed to be generally isotropic, some studies have found that $\sim\hspace{-0.1cm}25$\% of the distribution can have anisotropic components. As the directional detection of dark matter is sensitive to both the recoil energy and direction of nuclear recoil, directional information can prove useful in measuring the distribution of dark matter. Using a Monte Carlo simulation based on the modeled directional detection of dark matter, we analyze the differences between isotropic and anisotropic distributions and show that the isotropic case can be rejected at a 90\% confidence level if $O(10^4)$ events can be obtained.
Primordial black holes interacting with stars in binaries lead to a new class of gravity wave signatures that we explore. A small $10^{-16} - 10^{-7} M_{\odot}$ primordial black hole captured by a neutron star or a white dwarf will eventually consume the host. The resulting black hole will have a mass of only $\sim0.5-2 M_{\odot}$, not expected from astrophysics. For a double neutron star binary system this leads to a transmutation into a black hole--neutron star binary, with a gravity wave signal detectable by LIGO. For a neutron star--white dwarf system this leads to a black hole--white dwarf binary, with a gravity wave signal detectable by LISA. Other systems, such as cataclysmic variable binaries, can also undergo transmutations. We describe gravity wave signals of the transmuted systems, stressing the differences and similarities with the original binaries. Correlating astrophysical phenomena, such as a double kilonova, can further help to distinguish these events. A lack of signal in future searches can constrain primordial black holes as dark matter.
Directional detection of dark matter has sensitivity for both recoil energy and direction of nuclear recoil. It opens the way to measure local velocity distribution of dark matter. In this paper, we study possibility to discriminate isotropic distribution and anisotropic one suggested by a N-body simulation with directional detector. Numerical simulation is performed for two cases according to the detectors, one corresponds to angular histogram and the other is energy-angular distribution of the signals. We reveal that the anisotropy of velocity distribution can be discriminated at 90% C.L. with chi-squared test if O($10^4$) signals are obtained.
The differential event rate in Weakly Interacting Massive Particle (WIMP) direct detection experiments depends on the local dark matter density and velocity distribution. Accurate modelling of the local dark matter distribution is therefore required to obtain reliable constraints on the WIMP particle physics properties. Data analyses typically use a simple Standard Halo Model which might not be a good approximation to the real Milky Way (MW) halo. We review observational determinations of the local dark matter density, circular speed and escape speed and also studies of the local dark matter distribution in simulated MW-like galaxies. We discuss the effects of the uncertainties in these quantities on the energy spectrum and its time and direction dependence. Finally we conclude with an overview of various methods for handling these astrophysical uncertainties.
We investigate whether the inclusion of baryonic physics influences the formation of thin, coherently rotating planes of satellites such as those seen around the Milky Way and Andromeda. For four Milky Way-mass simulations, each run both as dark matter-only and with baryons included, we are able to identify a planar configuration that significantly maximizes the number of plane satellite members. The maximum plane member satellites are consistently different between the dark matter-only and baryonic versions of the same run due to the fact that satellites are both more likely to be destroyed and to infall later in the baryonic runs. Hence, studying satellite planes in dark matter-only simulations is misleading, because they will be composed of different satellite members than those that would exist if baryons were included. Additionally, the destruction of satellites in the baryonic runs leads to less radially concentrated satellite distributions, a result that is critical to making planes that are statistically significant compared to a random distribution. Since all planes pass through the centre of the galaxy, it is much harder to create a plane from a random distribution if the satellites have a low radial concentration. We identify Andromeda's low radial satellite concentration as a key reason why the plane in Andromeda is highly significant. Despite this, when co-rotation is considered, none of the satellite planes identified for the simulated galaxies are as statistically significant as the observed planes around the Milky Way and Andromeda, even in the baryonic runs.
Over a handful of rotation periods, dynamical processes in barred galaxies induce non-axisymmetric structure in dark matter halos. Using n-body simulations of a Milky Way-like barred galaxy, we identify both a trapped dark-matter component, a shadow bar, and a strong response wake in the dark-matter distribution that affects the predicted dark-matter detection rates for current experiments. The presence of a baryonic disk together with well-known dynamical processes (e.g. spiral structure and bar instabilities) increase the dark matter density in the disk plane. We find that the magnitude of the combined stellar and shadow bar evolution, when isolated from the effect of the axisymmetric gravitational potential of the disk, accounts for >30% of this overall increase in disk-plane density. This is significantly larger that of previously claimed deviations from the standard halo model. The dark-matter density and kinematic wakes driven by the Milky Way bar increase the detectability of dark matter overall, especially for the experiments with higher $v_{min}$. These astrophysical features increase the detection rate by more than a factor of two when compared to the standard halo model and by a factor of ten for experiments with high minimum recoil energy thresholds. These same features increase (decrease) the annual modulation for low (high) minimum recoil energy experiments. We present physical arguments for why these dynamics are generic for barred galaxies such as the Milky Way rather than contingent on a specific galaxy model.
We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard cold dark matter (LambdaCDM) cosmology. The resulting models are confronted with the broad range of observational data available for the Milky Way and M31 galaxies. We find that there is a narrow range of parameters that can satisfy the observational constraints, but within this range, the models score remarkably well. Our favored models have virial masses of 1012 and 1.6×1012 Msolar for the Galaxy and for M31, respectively, average spin parameters lambda~0.03-0.05, and concentrations Cvir=10-17, typical for halos of this mass in the standard LambdaCDM cosmology. The models require neither dark matter modifications nor flat cores to fit the observational data. We explore two types of models, with and without the exchange of angular momentum between the dark matter and the baryons. The models without exchange give reasonable rotation curves, fulfill constraints in the solar neighborhood, and satisfy constraints at larger radii, but they may be problematic for fast rotating central bars. We explore models in which the baryons experience additional contraction due to loss of angular momentum to the surrounding dark matter. These models produce similar global properties, but the dark matter is only a 25% of the total mass in the central 3 kpc region, allowing a fast rotating bar to persist. According to preliminary calculations, our model galaxies probably have sufficient baryonic mass in the central ~3.5 kpc to reproduce recent observational values of the optical depth to microlensing events toward the Galactic center. Our dynamical models unequivocally require that about 50% of all the gas inside the virial radius must not be in the disk or in the bulge, a result that is obtained naturally in standard semianalytic models. Assuming that the Milky Way is ``typical,'' we investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function. We find that if the Milky Way has a luminosity MK=-24.0, then these constraints are satisfied, but if it is more luminous (as expected if it lies on the Tully-Fisher relation), then the predicted space density is larger than the observed space density of galaxies of the corresponding luminosity by a factor of 1.5-2. We conclude that observed rotation curves and dynamical properties of ``normal'' spiral galaxies appear to be consistent with standard LambdaCDM.
We derive the mass density profiles of dark matter halos that are implied by high spatial resolution rotation curves of low surface brightness galaxies. We find that, at small radii, the mass density distribution is dominated by a nearly constant density core with a core radius of a few kiloparsecs. For , the distribution of inner a r(r) ∼ r slopes a is strongly peaked around . This is significantly shallower than the cuspy halos found a p 0.2 a ≤ 1 in cold dark matter simulations. While the observed distribution of a does have a tail toward such extreme values, the derived value of a is found to depend on the spatial resolution of the rotation curves: is found only a ≈ 1 for the least well resolved galaxies. Even for these galaxies, our data are also consistent with constant-density cores ( ) of modest (∼1 kpc) core radius, which can give the illusion of steep cusps when insufficiently a p 0 resolved. Consequently, there is no clear evidence for a cuspy halo in any of the low surface brightness galaxies observed. Subject headings: dark matter — galaxies: fundamental parameters — galaxies: kinematics and dynamics
We analyze the effect of dissipation on the shapes of dark matter (DM) halos using high-resolution cosmological gasdynamics simulations of clusters and galaxies in the ΛCDM cosmology. We find that halos formed in simulations with gas cooling are significantly more spherical than corresponding halos formed in adiabatic simulations. Gas cooling results in an average increase of the principle axis ratios of halos by ~0.2-0.4 in the inner regions. The systematic difference decreases slowly with radius but persists almost to the virial radius. We argue that the differences in simulations with and without cooling arise both during periods of quiescent evolution, when gas cools and condenses toward the center, and during major mergers. We perform a series of high-resolution N-body simulations to study the shapes of remnants in major mergers of DM halos and halos with embedded stellar disks. In the DM halo-only mergers, the shape of the remnants depends only on the orbital angular momentum of the encounter and not on the internal structure of the halos. However, significant shape changes in the DM distribution may result if stellar disks are included. In this case the shape of the DM halos is correlated with the morphology of the stellar remnants.
This paper focuses on cosmological constraints derived from analysis of WMAP data alone. A simple ΛCDM cosmological model fits the five-year WMAP temperature and polarization data. The basic parameters of the model are consistent with the three-year data and now better constrained: Ω b h 2 = 0.02273 ± 0.00062, Ω c h 2 = 0.1099 ± 0.0062, ΩΛ = 0.742 ± 0.030, ns = 0.963+0.014 –0.015, τ = 0.087 ± 0.017, and σ8 = 0.796 ± 0.036, with h = 0.719+0.026 –0.027. With five years of polarization data, we have measured the optical depth to reionization, τ>0, at 5σ significance. The redshift of an instantaneous reionization is constrained to be z reion = 11.0 ± 1.4 with 68% confidence. The 2σ lower limit is z reion > 8.2, and the 3σ limit is z reion > 6.7. This excludes a sudden reionization of the universe at z = 6 at more than 3.5σ significance, suggesting that reionization was an extended process. Using two methods for polarized foreground cleaning we get consistent estimates for the optical depth, indicating an error due to the foreground treatment of τ ~ 0.01. This cosmological model also fits small-scale cosmic microwave background (CMB) data, and a range of astronomical data measuring the expansion rate and clustering of matter in the universe. We find evidence for the first time in the CMB power spectrum for a nonzero cosmic neutrino background, or a background of relativistic species, with the standard three light neutrino species preferred over the best-fit ΛCDM model with N eff = 0 at >99.5% confidence, and N eff > 2.3(95%confidence limit (CL)) when varied. The five-year WMAP data improve the upper limit on the tensor-to-scalar ratio, r < 0.43(95%CL), for power-law models, and halve the limit on r for models with a running index, r < 0.58(95%CL). With longer integration we find no evidence for a running spectral index, with dns /dln k = –0.037 ± 0.028, and find improved limits on isocurvature fluctuations. The current WMAP-only limit on the sum of the neutrino masses is ∑m ν < 1.3 eV(95%CL), which is robust, to within 10%, to a varying tensor amplitude, running spectral index, or dark energy equation of state.
M giants selected from the Two Micron All Sky Survey (2MASS) have been used to trace streams of tidal debris apparently associated with the Sagittarius dwarf spheroidal galaxy (Sgr) that entirely encircle the Galaxy. While the Sgr M giants are generally aligned with a single great circle on the sky, we measure a difference of 104 ± 26 between the mean orbital poles of the great circles that best fit debris leading and trailing Sgr, which can be attributed to the precession of Sgr's orbit over the range of phases explored by the data set. Simulations of the destruction of Sgr in potentials containing bulge, disk, and halo components best reproduce this level of precession along the same range of orbital phases if the potential contours of the halo are only slightly flattened, with the ratio of the axis length perpendicular to and in the disk in the range q = 0.90-0.95 (corresponding to isodensity contours with qρ ~ 0.83-0.92). Oblate halos are strongly preferred over prolate (qρ > 1) halos, and flattenings in the potential of q ≤ 0.85 (qρ ≤ 0.75) and q ≥ 1.05 (qρ ≥ 1.1) are ruled out at the 3 σ level. More extreme values of q ≤ 0.80 (qρ ≤ 0.6) and q ≥ 1.25 (qρ ≥ 1.6) are ruled out at the 7 and 5 σ levels, respectively. These constraints will improve as debris with larger separation in orbital phases is found.
We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxy mapped by M-giant star tracers detected in the complete Two Micron All Sky Survey (2MASS). Near-infrared photometry of Sgr's prominent M-giant population permits an unprecedentedly clear view of the center of Sgr. The main body is fitted with a King profile of limiting major-axis radius 30°-substantially larger than previously found or assumed-beyond which is a prominent break in the density profile from stars in the Sgr tidal tails; thus the Sgr radial profile resembles that of Galactic dwarf speroidal (dSph) satellites. Adopting traditional methods for analyzing dSph light profiles, we determine the brightness of the main body of Sgr to be MV=-13.27 (the brightest of the known Galactic dSph galaxies) and the total Sgr mass-to-light ratio to be 25 in solar units. However, we regard the latter result with suspicion and argue that much of the observed structure beyond the King-fit core radius (224') may be outside the actual Sgr tidal radius as the former dwarf spiral/irregular satellite undergoes catastrophic disruption during its last orbits. The M-giant distribution of Sgr exhibits a central density cusp at the same location as, but not due to, the old stars constituting the globular cluster M54. A striking trailing tidal tail is found to extend from the Sgr center and arc across the south Galactic hemisphere with approximately constant density and mean distance varying from ~20 to 40 kpc. A prominent leading debris arm extends from the Sgr center northward of the Galactic plane to an apogalacticon ~45 kpc from the Sun and then turns toward the north Galactic cap (NGC), from where it descends back toward the Galactic plane, becomes foreshortened, and, at brighter magnitudes, covers the NGC. The leading and trailing Sgr tails lie along a well-defined orbital plane about the Galactic center. The Sun lies within a kiloparsec of that plane and near the path of leading Sgr debris; thus, it is possible that former Sgr stars are near or in the solar neighborhood. We discuss the implications of this new view of the Sgr galaxy and its entrails for the character of the Sgr orbit, mass, mass-loss rate, and contribution of stars to the Milky Way halo. The minimal precession displayed by the Sgr tidal debris along its inclined orbit supports the notion of a nearly spherical Galactic potential. The number of M giants in the Sgr tails is at least 15% that contained within the King limiting radius of the main Sgr body. The fact that M giants, presumably formed within the past few gigayears in the Sgr nucleus, are nevertheless so widespread along the Sgr tidal arms not only places limits on the dynamical age of these arms but also poses a timing problem that bears on the recent binding energy of the Sgr core and that is most naturally explained by recent and catastrophic mass loss. Sgr appears to contribute more than 75% of the high-latitude, halo M giants, despite substantial reservoirs of M giants in the Magellanic Clouds. No evidence of extended M-giant tidal debris from the Magellanic Clouds is found. Generally good correspondence is found between the M-giant, all-sky map of the Sgr system and all previously published detections of potential Sgr debris, with the exception of Sgr carbon stars, which must be subluminous compared with counterparts in other Galactic satellites in order to resolve the discrepancy.
We used fully cosmological, high-resolution N-body + smooth particle hydrodynamic (SPH) simulations to follow the formation of disc galaxies with rotational velocities between 135 and 270 km s−1 in a Λ cold dark matter (CDM) universe. The simulations include gas cooling, star formation, the effects of a uniform ultraviolet (UV) background and a physically motivated description of feedback from supernovae (SNe). The host dark matter haloes have a spin and last major merger redshift typical of galaxy-sized haloes as measured in recent large-scale N-body simulations. The simulated galaxies form rotationally supported discs with realistic exponential scalelengths and fall on both the I band and baryonic Tully–Fisher relations. An extended stellar disc forms inside the Milky Way (MW)-sized halo immediately after the last major merger. The combination of UV background and SN feedback drastically reduces the number of visible satellites orbiting inside a MW-sized halo, bringing it in fair agreement with observations. Our simulations predict that the average age of a primary galaxy's stellar population decreases with mass, because feedback delays star formation in less massive galaxies. Galaxies have stellar masses and current star formation rates as a function of total mass that are in good agreement with observational data. We discuss how both high mass and force resolution and a realistic description of star formation and feedback are important ingredients to match the observed properties of galaxies.
The highly radiopure ≃ 250kg NaI(Tl) DAMA/LIBRA set-up is running at the Gran Sasso National Laboratory of the INFN. In this
paper the first result obtained by exploiting the model independent annual modulation signature for Dark Matter (DM) particles
is presented. It refers to an exposure of 0.53 ton×yr. The collected DAMA/LIBRA data satisfy all the many peculiarities of
the DM annual modulation signature. Neither systematic effects nor side reactions able to account for the observed modulation
amplitude and to contemporaneously satisfy all the several requirements of this DM signature are available. Considering the
former DAMA/NaI and the present DAMA/LIBRA data all together (total exposure 0.82 ton×yr), the presence of Dark Matter particles
in the galactic halo is supported, on the basis of the DM annual modulation signature, at 8.2σ C.L.; in particular, in the energy interval (2–6)keV, the modulation amplitude is (0.0131±0.0016)cpd/kg/keV and the phase
and the period are well compatible with June 2
nd
and one year, respectively.
There is almost universal agreement among astronomers that most of the mass in the Universe and most of the mass in the Galactic halo is dark. Many lines of reasoning suggest that the dark matter consists of some new, as yet undiscovered, weakly interacting massive particle (WIMP). There is now a vast experimental effort being surmounted to detect WIMPs in the halo. The most promising techniques involve direct detection in low-background laboratory detectors and indirect detection through observation of energetic neutrinos from annihilation of WIMPs that have accumulated in the Sun and/or the Earth. Of the many WIMP candidates, perhaps the best motivated and certainly the most theoretically developed is the neutralino, the lightest superpartner in many supersymmetric theories. We review the minimal supersymmetric extension of the standard model and discuss prospects for detection of neutralino dark matter. We review in detail how to calculate the cosmological abundance of the neutralino and the event rates for both direct- and indirect-detection schemes, and we discuss astrophysical and laboratory constraints on supersymmetric models. We isolate and clarify the uncertainties from particle physics, nuclear physics, and astrophysics that enter at each step in the calculations. We briefly review other related dark-matter candidates and detection techniques.
Dark matter is the dominant form of matter in the universe, but its nature is unknown. It is plausibly an elementary particle, perhaps the lightest supersymmetric partner of known particle species. In this case, annihilation of dark matter in the halo of the Milky Way should produce g -rays at a level which may soon be observable.
We report results from the Cryogenic Dark Matter Search at the Soudan Underground Laboratory (CDMS II) featuring the full complement of 30 detectors. A blind analysis of data taken between October 2006 and July 2007 sets an upper limit on the weakly interacting massive particle (WIMP) nucleon spin-independent cross section of 6.6x10;{-44} cm;{2} (4.6x10;{-44} cm;{2} when combined with previous CDMS II data) at the 90% confidence level for a WIMP mass of 60 GeV/c;{2}. This achieves the best sensitivity for dark matter WIMPs with masses above 44 GeV/c;{2}, and significantly restricts the parameter space for some favored supersymmetric models.
We compare assembly of DM halos with and without baryons, within the context of cosmological evolution in the LCDM WMAP3 Universe (baryons+DM, BDM model, and pure DM, PDM model). In representative PDM and BDM models we find that baryons contribute decisively to the evolution of the central region, leading to an isothermal DM cusp, and to a flat DM density core -- the result of heating by dynamical friction of the substructure during a quiescent evolution epoch. This process ablates the cold gas from an embedded disk, cutting the star formation rate by ~10, and heats up the spheroidal gas and stellar components, triggering their expansion. The substructure is more resilient in the presence of baryons. The disk which formed from inside-out as gas dominated, is transformed into an intermediate Hubble type by z ~ 2 and to an early type by z ~ 0.5, based on its gas contents and spheroidal-to-disk stellar mass ratio. Only a relatively small ~20% fraction of DM particles in PDM and BDM models are bound within the radius of maximal circular velocity in the halo -- most of the DM particles perform larger radial excursions. We also find that the fraction of baryons within the halo virial radius somewhat increases during the major mergers and decreases during the minor mergers. The net effect appears to be negligible. While the substructure is being tidally-disrupted, mixing of its debris in the halo is not efficient and becomes even less so with z. The streamers formed after z ~ 1 survive largely to the present time -- an important implication for embedded disk evolution. Comment: 19 pages, 19 figures, to be published by the Astrophysical Journal; Minor revision following the referee comments; The high-resolution figures and animation mpeg can be found at this http://www.pa.uky.edu/~shlosman/research/galdyn/galdiss1
We use high resolution cosmological hydrodynamical simulations to demonstrate
that cold flow gas accretion, particularly along filaments, modifies the
standard picture of gas accretion and cooling onto galaxy disks. In the
standard picture, all gas is initially heated to the virial temperature of the
galaxy as it enters the virial radius. Low mass galaxies are instead dominated
by accretion of gas that stays well below the virial temperature, and even when
a hot halo is able to develop in more massive galaxies there exist dense
filaments that penetrate inside of the virial radius and deliver cold gas to
the central galaxy. For galaxies up to ~L*, this cold accretion gas is
responsible for the star formation in the disk at all times to the present.
Even for galaxies at higher masses, cold flows dominate the growth of the disk
at early times. Within this modified picture, galaxies are able to accrete a
large mass of cold gas, with lower initial gas temperatures leading to shorter
cooling times to reach the disk. Although star formation in the disk is
mitigated by supernovae feedback, the short cooling times allow for the growth
of stellar disks at higher redshifts than predicted by the standard model.
(Abridged) We conduct a series of high-resolution, dissipationless N-body simulations to investigate the cumulative effect of substructure mergers onto thin disk galaxies in the context of the LCDM paradigm of structure formation. Our simulation campaign is based on a hybrid approach. Substructure properties are culled directly from cosmological simulations of galaxy-sized cold dark matter (CDM) halos. In contrast to what can be inferred from statistics of the present-day substructure populations, accretions of massive subhalos onto the central regions of host halos, where the galactic disk resides, since z~1 should be common occurrences. One host halo merger history is subsequently used to seed controlled numerical experiments of repeated satellite impacts on an initially-thin Milky Way-type disk galaxy. We show that these accretion events produce several distinctive observational signatures in the stellar disk including: a ring-like feature in the outskirts; a significant flare; a central bar; and faint filamentary structures that (spuriously) resemble tidal streams. The final distribution of disk stars exhibits a complex vertical structure that is well-described by a standard ``thin-thick'' disk decomposition. We conclude that satellite-disk encounters of the kind expected in LCDM models can induce morphological features in galactic disks that are similar to those being discovered in the Milky Way, M31, and in other disk galaxies. These results highlight the significant role of CDM substructure in setting the structure of disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys and astrometric satellites may be able to distinguish between competing cosmological models by testing whether the detailed structure of galactic disks is as excited as predicted by the CDM paradigm. Comment: Accepted version to appear in ApJ, 24 pages, 8 figures, LaTeX (uses emulateapj.cls). Comparison between the simulated ring-like features and the Monoceros ring stellar structure in the Milky Way performed; conclusions unaltered
According to the hierarchical scenario, galaxies form via merging and accretion of small objects. Using N-body simulations, we study the frequency of merging events in the history of the halos. We find that at z<~2 the merging rate of the overall halo population can be described by a simple power law (1+z)^3. The main emphasis of the paper is on the effects of environment of halos at the present epoch (z=0). We find that the halos located inside clusters have formed earlier (dz \approx 1) than isolated halos of the same mass. At low redshifts (z<1), the merger rate of cluster halos is 3 times lower than that of isolated halos and 2 times lower than merger rate of halos that end up in groups by z=0. At higher redshifts (z~1-4), progenitors of cluster and group halos have 3--5 times higher merger rates than isolated halos. We briefly discuss implications of our results for galaxy evolution in different environments. Comment: submitted to the Astrophys. Journal; 11 pages, 9 figs., LaTeX (uses emulateapj.sty)
(abridged) We present an analysis of B-R and R-K_s color maps for 47 late-type, edge-on, unwarped, bulgeless disk galaxies spanning a wide range of mass. The color maps show that the thin disks of these galaxies are embedded within a low surface brightness red envelope that is substantially thicker than the thin disk (a/b~4:1 vs a/b>8:1), extends to at least 5 vertical disk scale heights above the galaxy midplane, and has a radial scale length that appears to be uncorrelated with that of the embedded thin disk. The color of the red envelope is similar from galaxy to galaxy and is consistent with a relatively old (>6Gyr) stellar population that is not particularly metal-poor. The color difference between the thin disk and the envelope varies systematically with rotation speed, indicating a younger thin disk relative to the red envelope in lower mass galaxies. The red stellar envelopes are similar to the MW thick disk, having common surface brightnesses, spatial distributions, mean ages, and metallicities. The ubiquity of the red stellar envelopes implies that the formation of the thick disk is a nearly universal feature of disk formation and need not be associated with bulge formation. Furthermore, our data suggest that the thick disk forms early, even in cases where the majority of star formation was recent. Finally, we find that our data and the observed properties of the MW thick disk argue in favor of a merger origin for the thick disk population. If so, then the age of the thick disk marks the end of the epoch of major merging, and the age difference between the thin and thick disks can become a strong constraint on cosmological constants and models of galaxy and/or structure formation. Comment: accepted to the September 2002 Astronomical Journal, LaTeX, 31 pages including figures
The formation of disk galaxies is one of the most outstanding problems in modern astrophysics and cosmology. We review the progress made by numerical simulations carried out on large parallel supercomputers. Recent progress stems from a combination of increased resolution and improved treatment of the astrophysical processes modeled in the simulations, such as the phenomenological description of the interstellar medium and of the process of star formation. High mass and spatial resolution is a necessary condition in order to obtain large disks comparable with observed spiral galaxies avoiding spurious dissipation of angular momentum. A realistic model of the star formation history. gas-to-stars ratio and the morphology of the stellar and gaseous component is instead controlled by the phenomenological description of the non-gravitational energy budget in the galaxy. We show that simulations of gas collapse within cold dark matter halos including a phenomenological description of supernovae blast-waves allow to obtain stellar disks with nearly exponential surface density profiles as those observed in real disk galaxies, counteracting the tendency of gas collapsing in such halos to form cuspy baryonic profiles. However, the ab-initio formation of a realistic rotationally supported disk galaxy with a pure exponential disk in a fully cosmological simulation is still an open problem. We argue that the suppression of bulge formation is related to the physics of galaxy formation during the merger of the most massive protogalactic lumps at high redshift, where the reionization of the Universe likely plays a key role. A sufficiently high resolution during this early phase of galaxy formation is also crucial to avoid artificial angular momentum loss (Abridged). Comment: 41 pages, 15 figures, Invited Review accepted for publication on Advanced Science Letters. High resolution version can be found at http://www.exp-astro.phys.ethz.ch/mayer/galform.ps.gz
We study the effects of substructure in the Galactic halo on direct detection of dark matter, on searches for energetic neutrinos from WIMP annihilation in the Sun and Earth, and on the enhancement in the WIMP annihilation rate in the halo. Our central result is a probability distribution function (PDF) P(\rho) for the local dark-matter density. This distribution must be taken into account when using null dark-matter searches to constrain the properties of dark-matter candidates. We take two approaches to calculating the PDF. The first is an analytic model that capitalizes on the scale-invariant nature of the structure--formation hierarchy in order to address early stages in the hierarchy (very small scales; high densities). Our second approach uses simulation-inspired results to describe the PDF that arises from lower-density larger-scale substructures which formed in more recent stages in the merger hierarchy. The distributions are skew positive, and they peak at densities lower than the mean density. The local dark-matter density may be as small as 1/10th the canonical value of ~ 0.4 GeV/cm^3, but it is probably no less than 0.2 GeV/cm^3.
We present a new set of multi-million particle SPH simulations of the formation of disk dominated galaxies in a cosmological context. Some of these galaxies are higher resolution versions of the models already described in Governato et al (2007). To correctly compare simulations with observations we create artificial images of our simulations and from them measure photometric Bulge to Disk (B/D) ratios and disk scale lengths. We show how feedback and high force and mass resolution are necessary ingredients to form galaxies that have flatter rotation curves, larger I band disk scale lengths and smaller B/D ratios. A new simulated disk galaxy has an I-band disk scale length of 9.2 kpc and a B/D flux ratio of 0.64 (face on, dust reddened).
Cold dark matter cosmogony predicts triaxial dark matter halos, whereas observations find quite round halos. This is most likely due to the condensation of baryons leading to rounder halos. We examine the halo phase space distribution basis for such shape changes. Triaxial halos are supported by box orbits, which pass arbitrarily close to the density center. The decrease in triaxiality caused by baryons is thought to be due to the scattering of these orbits. We test this hypothesis with simulations of disks grown inside triaxial halos. After the disks are grown we check whether the phase space structure has changed by evaporating the disks and comparing the initial and final states. While the halos are substantially rounder when the disk is at full mass, their final shape after the disk is evaporated is not much different from the initial. Likewise, the halo becomes (more) radially anisotropic when the disk is grown, but the final anisotropy is consistent with the initial. Only if the baryons are unreasonably compact or massive does the halo change irreversibly. We show that the character of individual orbits is not generally changed by the growing mass. Thus the central condensation of baryons does not destroy enough box orbits to cause the shape change. Rather, box orbits merely become rounder along with the global potential. However, if angular momentum is transferred to the halo, either via satellites or via bars, a large irreversible change in the halo distribution occurs. The ability of satellites to alter the phase space distribution of the halo is of particular concern to galaxy formation simulations since halo triaxiality can profoundly influence the evolution of disks. Comment: 35 pages, 13 figures (3 in color). Accepted to ApJ. This version is expanded, with new simulations included in response to referee. Conclusions remain unchanged
Abridged: We estimate the distances to ~48 million stars detected by the
Sloan Digital Sky Survey and map their 3D number density distribution in 100 <
D < 20 kpc range over 6,500 deg^2 of sky. The data show strong evidence for a
Galaxy consisting of an oblate halo, a disk component, and a number of
localized overdensities with exponential disk parameters (bias-corrected for an
assumed 35% binary fraction) H_1 = 300 pc, L_1 = 2600 pc, H_2 = 900 pc, L_2 =
3600 pc, and local density normalization of 12%. We find the halo to be oblate,
with best-fit axis ratio c/a = 0.64, r^{-2.8} profile, and the local
halo-to-thin disk normalization of 0.5%. We estimate the errors of derived
model parameters to be no larger than ~20% (disk scales) and ~10% (thick disk
normalization). While generally consistent with the above model, the density
distribution shows a number of statistically significant localized deviations.
We detect two overdensities in the thick disk region at (R, Z) ~ (6.5, 1.5)kpc
and (R, Z) ~ (9.5, 0.8) kpc, and a remarkable density enhancement in the halo
covering >1000deg^2 of sky towards the constellation of Virgo, at distances of
~6-20 kpc. Compared to a region symmetric with respect to the l=0 line, the
Virgo overdensity is responsible for a factor of 2 number density excess and
may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging
with the Milky Way. After removal of the resolved overdensities, the remaining
data are consistent with a smooth density distribution; we detect no evidence
of further unresolved clumpy substructure at scales ranging from ~50pc in the
disk, to ~1 - 2 kpc in the halo.
The cooling of gas in the centers of dark matter halos is expected to lead to
a more concentrated dark matter distribution. The response of dark matter to
the condensation of baryons is usually calculated using the model of adiabatic
contraction, which assumes spherical symmetry and circular orbits. In contrast,
halos in the hierarchical structure formation scenarios grow via multiple
violent mergers and accretion along filaments, and particle orbits in the halos
are highly eccentric. We study the effects of the cooling of gas in the inner
regions of halos using high-resolution cosmological simulations which include
gas dynamics, radiative cooling, and star formation. We find that the
dissipation of gas indeed increases the density of dark matter and steepens its
radial profile in the inner regions of halos compared to the case without
cooling. For the first time, we test the adiabatic contraction model in
cosmological simulations and find that the standard model systematically
overpredicts the increase of dark matter density in the inner 5% of the virial
radius. We show that the model can be improved by a simple modification of the
assumed invariant from M(r)r to M(r_av)r, where r and r_av are the current and
orbit-averaged particle positions. This modification approximately accounts for
orbital eccentricities of particles and reproduces simulation profiles to
within 10-20%. We present analytical fitting functions that accurately describe
the transformation of the dark matter profile in the modified model and can be
used for interpretation of observations.
The GAIA astrometric mission has recently been approved as one of the next two `cornerstones' of ESA's science programme, with a launch date target of not later than mid-2012. GAIA will provide positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy (and into the Local Group), amounting to about 1 per cent of the Galactic stellar population. GAIA's main scientific goal is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. It will advance questions such as when the stars in our Galaxy formed, when and how it was assembled, and its distribution of dark matter. The survey aims for completeness to V=20 mag, with accuracies of about 10 microarcsec at 15 mag. Combined with astrophysical information for each star, provided by on-board multi-colour photometry and (limited) spectroscopy, these data will have the precision necessary to quantify the early formation, and subsequent dynamical, chemical and star formation evolution of our Galaxy. Additional products include detection and orbital classification of tens of thousands of extra-Solar planetary systems, and a comprehensive survey of some 10^5-10^6 minor bodies in our Solar System, through galaxies in the nearby Universe, to some 500,000 distant quasars. It will provide a number of stringent new tests of general relativity and cosmology. The complete satellite system was evaluated as part of a detailed technology study, including a detailed payload design, corresponding accuracy assesments, and results from a prototype data reduction development.
Simulations of nearby (0.015 < z < 0.025) SDSS galaxies have been used to reproduce as accurately as possible the appearance that they would have on COSMOS ACS images if they had been observed at z ~ 0.7 and z ~ 1.2. By adding the SDSS galaxies to random locations in the COSMOS images, we simulate the effects of chance superpositions of high redshift galaxies with unrelated foreground or background objects. We have used these simulated images, together with those of real COSMOS galaxies at these same redshifts, to undertake a "blind" morphological classification of galaxies to identify those that appear to be undergoing mergers and thus to estimate the change in merger fraction with redshift. We find that real mergers are harder to recognize at high redshift, and also that the chance superposition of unrelated galaxies often produces the appearance of mergers where in reality none exists. In particular, we estimate that 1.5 - 2.0% of objects randomly added to ACS images are misclassified as mergers due to projection with unrelated objects, and as a result, that 40% of the apparent mergers in COSMOS at z=0.7 are likely to be spurious. We find that the fraction of galaxies undergoing mergers increases as (1+z)^3.8+/-1.2 to z ~ 0.7 and that this trend appears to continue to z = 1.2. Merger candidates at z ~ 0.7 are bluer than the parent population, especially when the statistical effects of the chance projections are accounted for. Merger candidates are more asymmetric than the population as a whole, and are often associated with irregular morphology. Nevertheless, the majority (~60%) of the merger candidates appear to be associated with spiral galaxies although in this case we cannot correct for the effects of chance projections. Comment: 24 pages, 7 figures; accepted for publication in the ApJ
We examine the origin and evolution of the mass-metallicity relationship (MZR, M-Z) for galaxies using high resolution cosmological SPH + N-Body simulations that include a physically motivated description of supernovae feedback and subsequent metal enrichment. We discriminate between two sources that may contribute to the origin of the MZR: 1) metal and baryon loss due to gas outflow, or 2) inefficient star formation at the lowest galaxy masses. Our simulated galaxies reproduce the observed MZR in shape and normalization both at z=0 and z=2. We find that baryon loss occurs due to UV heating before star formation turns on in galaxies with M_baryon < 10^8 M_sun, but that some gas loss due to supernovae induced winds is required to subsequently reproduce the low effective chemical yield observed in low mass galaxies. Despite this, we show that low star formation efficiencies, regulated by supernovae feedback, are primarily responsible for the lower metallicities of low mass galaxies and the overall M-Z trend. We find that the shape of the MZR is relatively constant with redshift, but that its normalization increases with time. Simulations with no energy feedback from supernovae overproduce metals at low galaxy masses by rapidly transforming a large fraction of their gas into stars. Despite the fact that our low mass galaxies have lost a majority of their baryons, they are still the most gas rich objects in our simulations due to their low star formation efficiencies. Comment: 3 figures. Accepted for ApJL, in press
We present new weak lensing observations of 1E0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen. Comment: Accepted for publication in ApJL
We present an improved analytic calculation for the tidal radius of satellites and test our results against N-body simulations.
The tidal radius in general depends upon four factors: the potential of the host galaxy, the potential of the satellite, the orbit of the satellite and the orbit of the star within the satellite. We demonstrate that this last point is critical and suggest using three tidal radii to cover the range of orbits of stars within the satellite. In this way we show explicitly that prograde star orbits will be more easily stripped than radial orbits; while radial orbits are more easily stripped than retrograde ones. This result has previously been established by several authors numerically, but can now be understood analytically. For point mass, power-law (which includes the isothermal sphere), and a restricted class of split power-law potentials our solution is fully analytic. For more general potentials, we provide an equation which may be rapidly solved numerically.
Over short times (≲1–2 Gyr ∼1 satellite orbit), we find excellent agreement between our analytic and numerical models. Over longer times, star orbits within the satellite are transformed by the tidal field of the host galaxy. In a Hubble time, this causes a convergence of the three limiting tidal radii towards the prograde stripping radius. Beyond the prograde stripping radius, the velocity dispersion will be tangentially anisotropic.
We investigate the population of z= 3 damped Lyman α systems (DLAs) in a recent series of high-resolution galaxy formation simulations. The simulations are of interest because they form at z= 0 some of the most realistic disc galaxies to date. No free parameters are available in our study: the simulation parameters have been fixed by physical and z= 0 observational constraints, and thus our work provides a genuine consistency test. The precise role of DLAs in galaxy formation remains in debate, but they provide a number of strong constraints on the nature of our simulated bound systems at z= 3 because of their coupled information on neutral H i densities, kinematics, metallicity and estimates of star formation activity.
Our results, without any parameter tuning, closely match the observed incidence rate and column density distributions of DLAs. Our simulations are the first to reproduce the distribution of metallicities (with a median of ZDLA≃ Z⊙/20) without invoking observationally unsupported mechanisms such as significant dust biasing. This is especially encouraging given that these simulations have previously been shown to have a realistic 0 < z < 2 stellar mass–metallicity relation. Additionally, we see a strong positive correlation between sightline metallicity and low-ion velocity width, the normalization and slope of which come close to matching recent observational results. However, we somewhat underestimate the number of observed high-velocity width systems; the severity of this disagreement is comparable to other recent DLA-focused studies.
DLAs in our simulations are predominantly associated with dark-matter haloes with virial masses in the range 109 < Mvir/M⊙ < 1011. We are able to probe DLAs at high resolution, irrespective of their masses, by using a range of simulations of differing volumes. The fully constrained feedback prescription in use causes the majority of DLA haloes to form stars at a very low rate, accounting for the low metallicities. It is also responsible for the mass–metallicity relation which appears essential for reproducing the velocity–metallicity correlation. By z= 0, the majority of the z= 3 neutral gas forming the DLAs has been converted into stars, in agreement with rough physical expectations.
The interaction of a spiral galaxy with its system of satellites leads
to the orbital decay of the satellites on time scales comparable to
their orbital periods. Over a Hubble time, the disk of a spiral and the
distribution of satellites can be significantly altered by the action of
dynamical friction which leads to the exchange of mass and energy
between the satellites and the disk. The authors have used a restricted
N-body method to uncover the essential dynamics of friction in
disk-satellite systems and to investigate the orbital decay rates as a
function of the satellite's mass and orbital parameters.
The relationship between the dark matter in the Galactic halo and the
Galacitc disk is examined. It is suggested that these two types of dark
matter may have the same origin. It is found that, if the dark halo was
initially composed of lumps resembling present-day dwarf galaxies, some
of these lumps could behave like sinking satellites and settle into the
disk. It is shown that, if these two types of dark matter have the same
origin, the disk dark matter would dominate the signal in proposed dark
matter detection schemes.
We use a series of cosmological N-body simulations for a flat LCDM cosmology to investigate the properties of dark matter haloes in the mass range 3.0e9-3.0e13 Msun. These properties include the concentration parameter (c), the spin parameter (lambda) and the mean axis ratio (q). For the concentration-mass relation we find c~M^(-0.11) in agreement with the model proposed by Bullock et al. even if we find a lower normalization (15%). The results for lambda and q are in good agreement with previous studies, while c and lambda are anti-correlated. In an attempt to remove unrelaxed haloes, we use the offset parameter (xoff), defined as the distance between the most bound particle and the center of mass. Removing haloes with large xoff increases the c by ~10%, lowers the lambda by ~15%, and removes the most prolate haloes. In addition, it largely removes the anti-correlation between c and lambda though not entirely. We also investigate the effects of the large-scale environment. We find that more concentrated haloes live in denser environments. Note, however, that the trend is weak compared to the scatter. For the spin parameters we find no environment dependence, while there is a weak indication that the most spherical haloes reside in denser region. Finally, using a simple model for disk galaxy formation we show that haloes that host low surface brightness galaxies are expected to be hosted by a biased sub-set of haloes. Not only do these haloes have spin parameters that are larger than average, they also have c that are 15% percent lower than the average at a given halo mass. We discuss the implications of all these findings for the claimed disagreement between halo concentrations inferred from LSB rotation curves, and those expected for a LCDM cosmology. (abridged)
WE have detected a large, extended group of comoving stars in the direction of the Galactic Centre, which we interpret as belonging to a dwarf galaxy that is closer to our own Galaxy than any other yet known. Located in the constellation of Sagittarius, and on the far side of the Galactic Centre, it has not previously been seen because of the large number of foreground stars (in the Milky Way) in that direction. Following convention, we propose to call it the Sagittarius dwarf galaxy. Its properties are similar to those of the eight other dwarf spheroidal companions to the Milky Way, and it is comparable in size and luminosity to the largest of them— the Fornax system. The Sagittarius dwarf is elongated towards the plane of the Milky Way, suggesting that it is undergoing some tidal disruption before being absorbed by the Milky Way.
In the standard model of disk galaxy formation, a dark matter disk forms as massive satellites are preferentially dragged into the disk plane and dissolve. Here, we show the importance of the dark disk for direct dark matter detection. The low velocity of the dark disk with respect to the Earth enhances detection rates at low recoil energy. For weakly interacting massive particle (WIMP) masses M WIMP 50 GeV/c 2, the detection rate increases by up to a factor of 3 in the 5-20 keV recoil energy range. Comparing this with rates at higher energy is sensitive to M WIMP, providing stronger mass constraints particularly for M WIMP 100 GeV/c 2. The annual modulation signal is significantly boosted and the modulation phase is shifted by ~3 weeks relative to the dark halo. The variation of the observed phase with recoil energy determines M WIMP, once the dark disk properties are fixed by future astronomical surveys. The constraints on the WIMP interaction cross section from current experiments improve by factors of 1.4-3.5 when a typical contribution from the dark disk is included.
We investigate the evolution of angular momentum in simulations of galaxy formation in a cold dark matter universe. We analyse two model galaxies generated in the N-body/hydrodynamic simulations of Okamoto et al. Starting from identical initial conditions, but using different assumptions for the baryonic physics, one of the simulations produced a bulge-dominated galaxy and the other one a disc-dominated galaxy. The main difference is the treatment of star formation and feedback, both of which were designed to be more efficient in the disc-dominated object. We find that the specific angular momentum of the disc-dominated galaxy tracks the evolution of the angular momentum of the dark matter halo very closely: the angular momentum grows as predicted by linear theory until the epoch of maximum expansion and remains constant thereafter. By contrast, the evolution of the angular momentum of the bulge-dominated galaxy resembles that of the central, most bound halo material: it also grows at first according to linear theory, but 90 per cent of it is rapidly lost as pre-galactic fragments, into which gas had cooled efficiently, merge, transferring their orbital angular momentum to the outer halo by tidal effects. The disc-dominated galaxy avoids this fate because the strong feedback reheats the gas, which accumulates in an extended hot reservoir and only begins to cool once the merging activity has subsided. Our analysis lends strong support to the classical theory of disc formation whereby tidally torqued gas is accreted into the centre of the halo conserving its angular momentum.
We study the formation of galaxies in a Λ cold dark matter (ΛCDM) universe using high-resolution hydrodynamical simulations with a multiphase treatment of gas, cooling and feedback, focusing on the formation of discs. Our simulations follow eight isolated haloes similar in mass to the Milky Way and extracted from a large cosmological simulation without restriction on spin parameter or merger history. This allows us to investigate how the final properties of the simulated galaxies correlate with the formation histories of their haloes. We find that, at z= 0, none of our galaxies contains a disc with more than 20 per cent of its total stellar mass. Four of the eight galaxies nevertheless have well-formed disc components, three have dominant spheroids and very small discs, and one is a spheroidal galaxy with no disc at all. The z= 0 spheroids are made of old stars, while discs are younger and formed from the inside-out. Neither the existence of a disc at z= 0 nor the final disc-to-total mass ratio seems to depend on the spin parameter of the halo. Discs are formed in haloes with spin parameters as low as 0.01 and as high as 0.05; galaxies with little or no disc component span the same range in spin parameter. Except for one of the simulated galaxies, all have significant discs at z≳ 2, regardless of their z= 0 morphologies. Major mergers and instabilities which arise when accreting cold gas is misaligned with the stellar disc trigger a transfer of mass from the discs to the spheroids. In some cases, discs are destroyed, while in others, they survive or reform. This suggests that the survival probability of discs depends on the particular formation history of each galaxy. A realistic ΛCDM model will clearly require weaker star formation at high redshift and later disc assembly than occurs in our models.
We present a new and completely general technique for calculating the fine-grained phase-space structure of dark matter (DM) throughout the Galactic halo. Our goal is to understand this structure on the scales relevant for direct and indirect detection experiments. Our method is based on evaluating the geodesic deviation equation along the trajectories of individual DM particles. It requires no assumptions about the symmetry or stationarity of the halo formation process. In this paper we study general static potentials which exhibit more complex behaviour than the separable potentials studied previously. For ellipsoidal logarithmic potentials with a core, phase mixing is sensitive to the resonance structure, as indicated by the number of independent orbital frequencies. Regions of chaotic mixing can be identified by the very rapid decrease in the real-space density of the associated DM streams. We also study the evolution of stream-density in ellipsoidal NFW haloes with radially varying isopotential shape, showing that if such a model is applied to the Galactic halo, at least 105 streams are expected near the Sun. The most novel aspect of our approach is that general non-static systems can be studied through implementation in a cosmological N-body code. Such an implementation allows a robust and accurate evaluation of the enhancements in annihilation radiation due to fine-scale structure such as caustics. We embed the scheme in the current state-of-the-art code gadget-3 and present tests which demonstrate that N-body discreteness effects can be kept under control in realistic configurations.
We determine the velocity distribution and space density of a volume-complete sample of A and F stars, using parallaxes and
proper motions from the Hipparcos satellite. We use these data to solve for the gravitational potential vertically in the local Galactic disc, by comparing
the Hipparcos measured space density with predictions from various disc models. We derive an estimate of the local dynamical mass density
of 0.102±0.010 pc−3, which may be compared with an estimate of 0.095 M⊙ pc−3 in visible disc matter. Our estimate is found to be in reasonable agreement with other estimates by Crézé et al. and Pham,
also based on Hipparcos data. We conclude that there is no compelling evidence for significant amounts of dark matter in the disc.
We have carried out a comparison study of hydrodynamical codes by investigating their performance in modelling interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin–Helmholtz or Rayleigh–Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of an SPH smoothing kernel radius over which interactions are severely damped.
In a Λ cold dark matter (ΛCDM) cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc.
On average, a Milky Way-sized galaxy has three subhaloes with vmax > 80 km s−1 ; seven with vmax > 60 km s−1 and 15 with vmax > 40 km s−1 merge at redshift z≳ 1. Assuming isotropic accretion, a third of these merge at an impact angle θ < 20° and are dragged into the disc plane by dynamical friction. Their accreted stars and dark matter settle into a thick disc. The stellar thick disc qualitatively reproduces the observed thick disc at the solar neighbourhood, but is less massive by a factor ∼2 − 10. The dark matter disc contributes ρDDISC= 0.25 − 1ρHALO at the solar position. Although not likely to be dynamically interesting, the dark disc has important implications for the direct detection of dark matter because of its low velocity with respect to the Earth.
Higher inclination encounters θ > 20° are twice as likely as low-inclination ones. These lead to structures that closely resemble the recently discovered inner and outer stellar haloes. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (vmax≳ 80 km s−1) heat the thin disc enough to produce a thick disc. These heated thin-disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some ∼50–90 per cent of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z= 1.
Only one in four of our sample Milky Way haloes experiences mergers massive and late enough to fully destroy the thin disc. We conclude that thick, thin and dark discs occur naturally within a ΛCDM cosmology.
The key features of the Gasoline code for parallel hydrodynamics with self-gravity are described. Gasoline is an extension of the efficient Pkdgrav parallel N-body code using smoothed particle hydrodynamics. Accuracy measurements, performance analysis and tests of the code are presented. Recent successful Gasoline applications are summarized. These cover a diverse set of areas in astrophysics including galaxy clusters, galaxy formation and gas-giant planets. Future directions for gasdynamical simulations in astrophysics and code development strategies for tackling cutting edge problems are discussed.
The XENON experiment aims at the direct detection of dark matter in the form of Weakly Interacting Massive Particles (WIMPs) via their elastic scattering off Xe nuclei. A fiducial mass of 1000 kg, distributed in 10 independent liquid xenon time projection chambers will be used to probe the lowest interaction cross-section predicted by SUSY models. The TPCs are operated in dual (liquid/gas) phase, to allow a measurement of nuclear recoils down to 16 keV energy, via simultaneous detection of the ionization, through secondary scintillation in the gas, and primary scintillation in the liquid. The distinct ratio of primary to secondary scintillation for nuclear recoils from WIMPs (or neutrons), and for electron recoils from background, is key to the event-by-event discrimination capability of XENON. A dual phase xenon prototype has been realized and is currently being tested, along with other prototypes dedicated to other measurements relevant to the XENON program. As part of the R&D phase, we will realize and move underground a first XENON module (XENON10) with at least 10 kg fiducial mass to measure the background rejection capability and to optimize the conditions for continuous and stable detector operation underground. We present some of the results from the on-going R&D and summarize the expected performance of the 10 kg experiment, from Monte-Carlo simulations. The main design features of the 100 kg detector unit(XENON100), with which we envisage to make up the 1 ton sensitive mass of XENON1T will also be presented.
We analyse the dark matter (DM) distribution in an ≈ 1012 M⊙ halo extracted from a simulation consistent with the concordance cosmology, where the physics regulating the transformation
of gas into stars was allowed to change producing galaxies with different morphologies. Although the DM profiles get more
concentrated as baryons are collected at the centre of the haloes compared to a pure dynamical run, the total baryonic mass
alone is not enough to fully predict the reaction of the DM profile. We also note that baryons affect the DM distribution
even outside the central regions. Those systems where the transformation of gas into stars is regulated by supernova (SN)
feedback, so that significant disc structures are able to form, are found to have more concentrated DM profiles than a galaxy
which has efficiently transformed most of its baryons into stars at early times. The accretion of satellites is found to be
associated with an expansion of the DM profiles, triggered by angular momentum transfer from the incoming satellites. As the
impact of SN feedback increases, the satellites get less massive and are even strongly disrupted before getting close to the
main structure causing less angular momentum transfer. Our findings suggest that the response of the DM halo is driven by
the history of assembly of baryons into a galaxy along their merger tree.
We use the recently completed one billion particle Via Lactea IIΛ cold dark matter simulation to investigate local properties like density, mean velocity, velocity dispersion, anisotropy,
orientation and shape of the velocity dispersion ellipsoid, as well as the structure in velocity space of dark matter haloes.
We show that at the same radial distance from the halo centre, these properties can deviate by orders of magnitude from the
canonical, spherically averaged values, a variation that can only be partly explained by triaxiality and the presence of subhaloes.
The mass density appears smooth in the central relaxed regions but spans four orders of magnitude in the outskirts, both because
of the presence of subhaloes as well as of underdense regions and holes in the matter distribution. In the inner regions,
the local velocity dispersion ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in the outer parts
where the orientation becomes more isotropic. The clumpy structure in local velocity space of the outer halo cannot be well
described by a smooth multivariate normal distribution. Via Lactea II also shows the presence of cold streams made visible by their high 6D phase space density. Generally, the structure of dark
matter haloes shows a high degree of graininess in phase space that cannot be described by a smooth distribution function.
In hierarchical structure formation models of disk galaxies, a dark matter disk forms as massive satellites are preferentially dragged into the disk-plane where they dissolve. Here, we quantify the importance of this dark disk for direct and indirect dark matter detection. The low velocity of the dark disk with respect to the Earth enhances detection rates in direct detection experiments at low recoil energy. For WIMP masses M_{WIMP} >~ 50 GeV, the detection rate increases by up to a factor of 3 in the 5 - 20 keV recoil energy range. Comparing this with rates at higher energy is sensitive to M_{WIMP}, providing stronger mass constraints particularly for M_{WIMP}>~100 GeV. The annual modulation signal is significantly boosted by the dark disk and the modulation phase is shifted by ~3 weeks relative to the dark halo. The variation of the observed phase with recoil energy determines M_{WIMP}, once the dark disk properties are fixed by future astronomical surveys. The low velocity of the particles in the dark disk with respect to the solar system significantly enhances the capture rate of WIMPs in the Sun, leading to an increased flux of neutrinos from the Sun which could be detected in current and future neutrino telescopes. The dark disk contribution to the muon flux from neutrino back conversion at the Earth is increased by a factor of ~5 compared to the SHM, for rho_d/rho_h=0.5. Comment: 5 pages, 7 figures, To appear in the proceedings of Identification of Dark Matter 2008 (IDM2008), Stockholm, 18-22 August 2008; corrected one reference
The study of the Milky Way stellar discs in the context of galaxy formation is discussed. In particular we explore the properties of the Milky Way disc using a new sample of about 550 dwarf stars for which we have recently obtained elemental abundances and ages based on high resolution spectroscopy. For all the stars we also have full kinematic information as well as information about their stellar orbits. We confirm results from previous studies that the thin and the thick disc have distinct abundance patterns. But we also explore a larger range of orbital parameters than what has been possible in our previous studies. Several new results are presented. We find that stars that reaches high above the galactic plane and have eccentric orbits show remarkably tight abundance trends. This implies that these stars formed out of well mixed gas that had been homogenized over large volumes. We find some evidence that point to that the event that most likely caused the heating of this stellar population happened a few billion years ago. Through a simple, kinematic exploration of stars with super-solar [Fe/H] we show that the solar neighbourhood contains metal-rich, high velocity stars that very likely are associated with the thick disc. Additionally, the HR1614 moving group and the Hercules and Arcturus stellar streams are discussed and it is concluded that, probably, a large fraction of the so far identified groups and streams in the disc are the result of evolution and interactions within the stellar disc rather than being dissolved stellar clusters or engulfed dwarf galaxies. Comment: 20 pages, Review talk at the conference "A stellar journey", A symposium in celebration of Bengt Gustafsson's 65th birthday, held in Uppsala, June 2008, In press in Physica Scripta, eds. Paul Barklem, Andreas Korn, and Bertrand Plez
We show that the Galactic thick disk reaches at least solar metallicities, and that it experienced strong chemical enrichment during a period of ~3 Gyr, ending around 8-9 Gyr ago. This finding puts further constraints on the relation and interface between the thin and thick disks, and their formation processes. Our results are based on a detailed elemental abundance analysis of 261 kinematically selected F and G dwarf stars in the solar neighborhood: 194 likely members of the thick disk and 67 likely members of the thin disk, in the range -1.3<[Fe/H]<+0.4. Comment: Accepted for publication in ApJ Letters
We present non-parametric radial mass profiles for ten QSO strong lensing galaxies. Five of the galaxies have profiles close to $\rho(r)\propto r^{-2}$, while the rest are closer to r^{-1}, consistent with an NFW profile. The former are all relatively isolated early-types and dominated by their stellar light. The latter --though the modeling code did not know this-- are either in clusters, or have very high mass-to-light, suggesting dark-matter dominant lenses (one is a actually pair of merging galaxies). The same models give $H_0^{-1} = 15.2_{-1.7}^{+2.5}\Gyr$ ($H_0 = 64_{-9}^{+8} \legacy$), consistent with a previous determination. When tested on simulated lenses taken from a cosmological hydrodynamical simulation, our modeling pipeline recovers both H_0 and $\rho(r)$ within estimated uncertainties. Our result is contrary to some recent claims that lensing time delays imply either a low H_0 or galaxy profiles much steeper than r^{-2}. We diagnose these claims as resulting from an invalid modeling approximation: that small deviations from a power-law profile have a small effect on lensing time-delays. In fact, as we show using using both perturbation theory and numerical computation from a galaxy-formation simulation, a first-order perturbation of an isothermal lens can produce a zeroth-order change in the time delays. Comment: Replaced with final version accepted for publication in ApJ; very minor changes to text; high resolution figures may be obtained at justinread.net
A parametrized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scalelength Rd* of the stellar disc to R0. The disc and bulge dominate vc(R) at R≲R0 only for Rd,*/R0≲0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R « R0 to affect profoundly the structure of the best-fitting model at R » R0. For example, changing the disc slightly from an exponential surface-density profile significantly changes the form of vc(R) at R » R0, where the disc makes a negligible contribution to vc. Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct haloes rather than outwards-increasing mass-to-light ratios.
The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z≳R and at R > R0 are especially important.
We present the results of an AAT wide-field camera survey of the stars in the Monoceros Ring and purported Canis Major overdensity in the Galactic longitudes of {\it l} = (193 - 276)$^\circ$. Current numerical simulations suggest that both of these structures are the result of a single on-going accretion event, although an alternative solution is that the warped and flared disc of the Galaxy can explain the origin of both of these structures. Our results show that, with regards the Monoceros Ring, the warped and flared disc is unable to reproduce the locations and strengths of the detections observed around the Galaxy. This supports a non-Galactic origin for this structure. We report 8 new detections and 2 tentative detections of the Monoceros Ring in this survey. The exact nature of the Canis Major overdensity is still unresolved although this survey provides evidence that invoking the Galactic warp is not a sufficient solution when compared with observation. Several fields in this survey are highly inconsistent with the current Galactic disc models that include a warp and flare, to such an extent that explaining their origins with these structures is problematic. (Abridged)
We describe our new ‘mlapm halo finder’ (mhf), which is based on the adaptive grid structure of the N-body code mlapm. We then extend the mhf code in order to track the orbital evolution of gravitationally bound objects through any given cosmological N-body simulation – our so-called ‘mlapm halo tracker’ (mht). The mode of operation of mht is demonstrated using a series of eight high-resolution N-body simulations of galaxy clusters. Each of these haloes hosts more than one million particles within their virial radii
rvir. We use mht as well as mhf to follow the temporal evolution of hundreds of individual satellites, and show that the radial distribution of these substructure
satellites follows a ‘universal’ radial distribution irrespective of the environment and formation history of the host halo.
This in fact might pose another problem for simulations of cold dark matter structure formation, as there are recent findings
by Taylor, Silk & Babul that the Milky Way satellites are found preferentially closer to the Galactic Centre and simulations
underestimate the amount of central substructure. Further, this universal substructure profile is anti-biased with respect
to the underlying dark matter profile. The halo finder mhf will become part of the open source mlapm distribution.