Facundo A. Gómez

Michigan State University, Ист-Лансинг, Michigan, United States

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Publications (24)82.3 Total impact

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    ABSTRACT: A recent observational study of haloes of nearby Milky Way-like galaxies shows that only half of the current sample exhibits strong negative metallicity ([Fe/H]) gradients. This is at odds with predictions from hydrodynamical simulations where such gradients are ubiquitous. In this Letter, we use high resolution cosmological hydrodynamical simulations to study the [Fe/H] distribution of galactic haloes. We find that kinematically selected stellar haloes, including both in-situ and accreted particles, have an oblate [Fe/H] distribution. Spherical [Fe/H] radial profiles show strong negative gradients within 100 kpc, in agreement with previous numerical results. However, the projected median [Fe/H] profiles along the galactic disc minor axis, typically obtained in observations, are significantly flatter. The median [Fe/H] values at a given radius are larger for the spherical profiles than for the minor axis profiles by as much as 0.4 dex within the inner 50 kpc. Similar results are obtained if only the accreted stellar component is considered indicating that the differences between spherical and minor axis profiles are not purely driven by `kicked-out' disc star particles formed in situ. Our study highlights the importance of performing careful comparisons between models and observations of halo [Fe/H] distributions.
    Preview · Article · Dec 2015
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    ABSTRACT: Vertically extended, high velocity dispersion stellar distributions appear to be a ubiquitous feature of disc galaxies, and both internal and external mechanisms have been proposed to be the major driver of their formation. However, it is unclear to what extent each mechanism can generate such a distribution, which is likely to depend on the assembly history of the galaxy. To this end, we perform 16 high resolution cosmological-zoom simulations of Milky Way-sized galaxies using the state-of-the-art cosmological magneto-hydrodynamical code \textlcsc{AREPO}, and analyse the evolution of the vertical kinematics of the stellar disc in connection with various heating mechanisms. We find that the bar is the dominant heating mechanism in most cases, whereas spiral arms, radial migration, and adiabatic heating from mid-plane density growth are all sub-dominant. The strongest source, though less prevalent than bars, originates from external perturbations from satellites/sub-halos of masses log$_{10} (M/\rm M_{\odot}) \gtrsim 10$. However, in many simulations the orbits of newborn star particles become cooler with time, such that they dominate the shape of the age-velocity dispersion relation and overall vertical disc structure unless a strong external perturbation takes place.
    Preview · Article · Dec 2015
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    ABSTRACT: We study the vertical structure of a stellar disc obtained from a fully cosmological high-resolution hydrodynamical simulation of the formation of a Milky Way-like galaxy. At the present day, the disc's mean vertical height shows a well defined and strong pattern, with amplitudes as large as 3 kpc in its outer regions. This pattern is the result of a satellite– host halo–disc interaction and reproduces, qualitatively, many of the observable properties of the Monoceros Ring. In particular we find disc material at the distance of Monoceros (R ∼ 12–16 kpc, galactocentric) extending far above the mid plane (30°, 〈Z〉 ∼ 1–2 kpc) in both hemispheres, as well as well-defined arcs of disc material at heliocentric distances ≳5 kpc. The pattern was first excited ≈3 Gyr ago as an m = 1 mode that later winds up into a leading spiral pattern. Interestingly, the main driver behind this perturbation is a low-mass low-velocity fly-by encounter. The satellite has total mass, pericentre distance and pericentric velocity of ∼5 per cent of the host, ∼80 kpc and 215 km s−1, respectively. The satellite is not massive enough to directly perturb the galactic disc but we show that the density field of the host dark matter halo responds to this interaction resulting in a strong amplification of the perturbative effects. This subsequently causes the onset and development of the Monoceros-like feature.
    No preview · Article · Sep 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present the largest number of Milky Way sized dark matter halos simulated at very high mass ($\sim$$10^4$ M$_\odot$/particle) and temporal resolution ($\sim$5 Myrs/snapshot) done to date, quadrupling what is currently available in the literature. This initial suite consists of the first 24 halos of the $Caterpillar$ $Project$ (www.caterpillarproject.org) whose project goal of 60 - 70 halos will be made public when complete. We resolve $\sim$20,000 gravitationally bound subhalos within the virial radius of each host halo. Over the ranges set by our spatial resolution our convergence is excellent and improvements were made upon current state-of-the-art halo finders to better identify substructure at such high resolutions (e.g., on average we recover $\sim$4 subhalos in each host halo above 10$^8$ M$_\odot$ which would have otherwise not been found using conventional methods). For our relaxed halos, the inner profiles are reasonably fit by Einasto profiles ($\alpha$ = 0.169 $\pm$ 0.023) though this depends on the relaxed nature and assembly history of a given halo. Averaging over all halos, the substructure mass fraction is $f_{m,subs} = 0.121 \pm 0.041$, and mass function slope is d$N$/d$M\propto M^{-1.88 \pm 0.10}$ though we find scatter in the normalizations for fixed halo mass due to more concentrated hosts having less subhalos at fixed subhalo mass. There are no biases stemming from Lagrangian volume selection as all Lagrangian volume types are included in our sample. Our detailed contamination study of 264 low resolution halos has resulted in obtaining very large and unprecedented, high-resolution regions around our host halos for our target resolution (sphere of radius $\sim$$1.4 \pm 0.4$ Mpc) allowing for accurate studies of low mass dwarf galaxies at large galactocentric radii and the very first stellar systems at high redshift ($z \geq$ 10).
    Full-text · Article · Sep 2015 · The Astrophysical Journal
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    ABSTRACT: We show that diffusion due to chaotic mixing in the Neighbourhood of the Sun may not be as relevant as previously suggested in erasing phase space signatures of past Galactic accretion events. For this purpose, we analyse Solar Neighbourhood–like volumes extracted from cosmological simulations that naturally account for chaotic or-bital behaviour induced by the strongly triaxial and cuspy shape of the resulting dark matter haloes, among other factors. In the approximation of an analytical static triax-ial model, our results show that a large fraction of stellar halo particles in such local volumes have chaos onset times (i.e., the timescale at which stars commonly associated with chaotic orbits will exhibit their chaotic behaviour) significantly larger than a Hubble time. Furthermore, particles that do present a chaotic behaviour within a Hubble time do not exhibit significant diffusion in phase space.
    Full-text · Article · Sep 2015
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    ABSTRACT: We report the discovery of a substantial stellar stream in the periphery of the Large Magellanic Cloud (LMC), found using public imaging from the first year of the Dark Energy Survey. The stream appears to emanate from the edge of the outer LMC disk at a radius $\approx 13.5$ degrees due north of its centre, and stretches more than $10$ kpc towards the east. It is roughly $1.5$ kpc wide and has an integrated $V$-band luminosity of at least $M_V = -7.4$. The stellar populations in the stream are indistinguishable from those in the outer LMC disk. We attempt to quantify the geometry of the outer disk using simple planar models, and find that only a disk with mild intrinsic ellipticity can simultaneously explain the observed stellar density on the sky and the azimuthal line-of-sight distance profile. We also see possible non-planar behaviour in the outer disk that may reflect a warp and/or flare. Based on all these observations, we conclude that the stream is most likely comprised of material that has been stripped from the outskirts of the LMC disk. We conduct a simple $N$-body simulation to show that this is plausibly due to the tidal force of the Milky Way, but we cannot rule out a recent close interaction between the LMC and the SMC as the source of the stripping. Finally, we observe tentative evidence for extremely diffuse LMC populations beyond the outer edge of the stream, at radii of up to $\sim 18.5$ kpc in the disk plane. These stars could serve as useful tracers of the total mass and orbital history of the LMC.
    Preview · Article · Aug 2015
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    Jorge Peñarrubia · Facundo A. Gómez · Gurtina Besla · Denis Erkal · Yin-Zhe Ma
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    ABSTRACT: This paper explores the effect of the Large Magellanic Cloud (LMC) on the mass estimates obtained from the timing argument. We show that accounting for the presence of the LMC systematically lowers the Local Group mass (MLG) derived from the relative motion of the Milky Way–Andromeda pair. Motivated by this result, we apply a Bayesian technique devised by Peñarrubia et al. to simultaneously fit (i) distances and velocities of galaxies within 3 Mpc and (ii) the relative motion between the Milky Way and Andromeda derived from HST observations, with the LMC mass (MLMC) as a free parameter. Our analysis returns a Local Group mass $M_{\rm LG}=2.64^{+0.42}_{-0.38}\times 10^{12}\,\mathrm{M}_{\odot }$ at a 68 per cent confidence level. The masses of the Milky Way, $M_{\rm MW}=1.04_{-0.23}^{+0.26}\times 10^{12}\,\mathrm{M}_{\odot }$, and Andromeda, $M_{{\rm M}31}=1.33_{-0.33}^{+0.39}\times 10^{12}\,\mathrm{M}_{\odot }$, are consistent with previous estimates that neglect the impact of the LMC on the observed Hubble flow. We find a (total) LMC mass $M_{\rm LMC}=0.25_{-0.08}^{+0.09}\times 10^{12}\,\mathrm{M}_{\odot }$, which is indicative of an extended dark matter halo and supports the scenario where this galaxy is just past its first pericentric approach. Consequently, these results suggest that the LMC may induce significant perturbations on the Galactic potential.
    Preview · Article · Jul 2015 · Monthly Notices of the Royal Astronomical Society Letters
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    ABSTRACT: Motivated by recent studies suggesting that the Large Magellanic Cloud (LMC) could be significantly more massive than previously thought, we explore whether the approximation of an inertial Galactocentric reference frame is still valid in the presence of such a massive LMC. We find that previous estimates of the LMC's orbital period and apocentric distance derived assuming a fixed Milky Way are significantly shortened for models where the Milky Way is allowed to move freely in response to the gravitational pull of the LMC. Holding other parameters fixed, the fraction of models favoring first infall is reduced. Due to this interaction, the Milky Way center of mass within the inner 50 kpc can be significantly displaced in phase-space in a very short period of time that ranges from 0.3 to 0.5 Gyr by as much as 20 kpc and 75 km/s. Furthermore, we show that the gravitational pull of the LMC and response of the Milky Way are likely to significantly affect the orbit and phase space distribution of tidal debris from the Sagittarius dwarf galaxy (Sgr). Such effects are larger than previous estimates based on the torque of the LMC alone. As a result, Sgr deposits debris in regions of the sky that are not aligned with the present-day Sgr orbital plane. In addition, we find that properly accounting for the movement of the Milky Way around its common center of mass with the LMC significantly modifies the angular distance between apocenters and tilts its orbital pole, alleviating tensions between previous models and observations. While these models are preliminary in nature, they highlight the central importance of accounting for the mutual gravitational interaction between the MW and LMC when modeling the kinematics of objects in the Milky Way and Local Group.
    Preview · Article · Aug 2014 · The Astrophysical Journal
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    ABSTRACT: Effective visualization of high-likelihood regions of parameter space is severely hampered by the large number of parameter dimensions that many models have. We present a novel technique, Optimal Percentile Region Projection, to visualize a high-dimensional likelihood density function that enables the viewer to understand the shape of the high-likelihood region. Optimal Percentile Region Projection has three novel components: first, we select the region of high likelihood in the high-dimensional space before projecting its shadow into a lower-dimensional projected space. Second, we analyze features on the surface of the region in the projected space to select the projection direction that shows the most interesting parameter dependencies. Finally, we use a three-dimensional projection space to show features that are not salient in only two dimensions. The viewer can also choose sets of axes to project along to explore subsets of the parameter space, using either the original parameter axes or principal-component axes. The technique was evaluated by our domain-science collaborators, who found it to be superior to their existing workflow both when there were interesting dependencies between parameters and when there were not.
    No preview · Article · Jun 2014 · Computers & Graphics
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    ABSTRACT: [Abridged] We present an application of statistical tools to characterize the relationship between input parameters and observational predictions of semi-analytic models of galaxy formation coupled to cosmological $N$-body simulations. We use statistical emulators to efficiently explore the input parameter space of our model, ChemTreeN. We show how a sensitivity analysis can be performed on these model emulators to characterize and quantify the relationship between model input parameters and predicted observable properties. The result of this analysis provides the user with information about which parameters are most important and likely to affect the prediction of a given observable. It can also be used to simplify models by identifying input parameters that have no effect on the outputs of interest. Conversely, it allow us to identify what model parameters can be most efficiently constrained by the given observational data set. We have applied this technique to real observational data sets associated to the Milky Way, such as its luminosity function of satellite galaxies. A statistical comparison of model outputs and real observables is used to obtain a "best-fitting" parameter set. We consider different Milky Way-like dark matter halos to account for the dependence of the best-fitting parameters selection process on underlying the merger history of the models. For all formation histories considered, running ChemTreeN with best-fitting parameters produced luminosity functions that tightly fit their observed counterpart. However, only one models was able to reproduce the observed stellar halo mass within 40 kpc of the Galactic center. On the basis of this analysis it is possible to disregard certain models, and their corresponding merger histories, as good representations of the underlying merger history of the Milky Way.
    Full-text · Article · Nov 2013 · The Astrophysical Journal
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    Daniel D. Carpintero · Facundo A. Gómez · Andrés E. Piatti
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    ABSTRACT: We model the dynamical interaction between the Small and Large Magellanic Clouds and their corresponding stellar cluster populations. Our goal is to explore whether the lack of old clusters (≳7 Gyr) in the Small Magellanic Cloud (SMC) can be the result of the capture of clusters by the Large Magellanic Cloud (LMC) as well as their ejection due to the tidal interaction between the two galaxies. For this purpose, we perform a suite of numerical simulations probing a wide range of parameters for the orbit of the SMC about the LMC. We find that, for orbital eccentricities e ≥ 0.4, approximately 15 per cent of the SMC clusters are captured by the LMC. In addition, another 20–50 per cent of its clusters are ejected into the intergalactic medium. In general, the clusters lost by the SMC are the less tightly bound cluster population. The final LMC cluster distribution shows a spatial segregation between clusters that originally belonged to the LMC and those that were captured from the SMC. Clusters that originally belonged to the SMC are more likely to be found in the outskirts of the LMC. Within this scenario, it is possible to interpret the difference observed between the star field and cluster SMC age–metallicity relationships for ages ≳7 Gyr.
    Preview · Article · Jul 2013 · Monthly Notices of the Royal Astronomical Society Letters
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    ABSTRACT: We use the very high resolution, fully cosmological simulations from the Aquarius Project, coupled to a semi-analytical model of galaxy formation, to study the phase-space distribution of halo stars in ‘solar neighbourhood’ like volumes. We find that this distribution is very rich in substructure in the form of stellar streams for all five stellar haloes we have analysed. These streams can be easily identified in velocity space, as well as in spaces of pseudo-conserved quantities such as E versus Lz. In our best resolved local volumes, the number of identified streams ranges from ≈300 to 600, in very good agreement with previous analytical predictions, even in the presence of chaotic mixing. The fraction of particles linked to (massive) stellar streams in these volumes can be as large as 84 per cent. The number of identified streams is found to decrease as a power law with galactocentric radius. We show that the strongest limitation to the quantification of substructure in our poorest resolved local volumes is particle resolution rather than strong diffusion due to chaotic mixing.
    Full-text · Article · Jun 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We use the semi-analytic model ChemTreeN, coupled to cosmological N-body simulations, to explore how different galaxy formation histories can affect observational properties of Milky Way like galaxies' stellar halos and their satellite populations. Gaussian processes are used to generate model emulators that allow one to statistically estimate a desired set of model outputs at any location of a p-dimensional input parameter space. This enables one to explore the full input parameter space orders of magnitude faster than could be done otherwise. Using mock observational data sets generated by ChemTreeN itself, we show that it is possible to successfully recover the input parameter vectors used to generate the mock observables if the merger history of the host halo is known. However, our results indicate that for a given observational data set, the determination of 'best-fit' parameters is highly susceptible to the particular merger history of the host. Very different halo merger histories can reproduce the same observational data set, if the 'best-fit' parameters are allowed to vary from history to history. Thus, attempts to characterize the formation history of the Milky Way using these kind of techniques must be performed statistically, analyzing large samples of high-resolution N-body simulations.
    Full-text · Article · Dec 2012 · The Astrophysical Journal
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    ABSTRACT: We explore two consequences of hierarchical structure formation on galaxy evolution: the effect that a particular Milky Way-sized galaxy's merger history has on the properties of its stellar halo and dwarf galaxy population, and the signatures of minor mergers in the thick disk of the Milky Way. In the first case, we use semi-analytical models (which include phenomenological descriptions of the evolution of stellar populations coupled to N-body produced merger trees) to demonstrate that the formation history of galaxies of approximately equal mass can significantly affect bulk properties of the dwarf galaxy population, but that the galaxy's stellar halo metallicity is much more robust. In the second project, we show that a carefully-chosen sample of Solar neighborhood thick disk stars exhibit distributions of energies that are consistent with the predictions of a minor-merger event that corresponds to recent models of Sagittarius' interactions with the disk of the Milky Way.
    No preview · Article · Aug 2012
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    ABSTRACT: Recently, Widrow and collaborators announced the discovery of vertical density waves in the Milky Way disc. Here we investigate a scenario where these waves were induced by the Sagittarius dwarf galaxy as it plunged through the Galaxy. Using numerical simulations, we find that the Sagittarius impact produces north–south asymmetries and vertical wave-like behaviour that qualitatively agrees with what is observed. The extent to which vertical modes can radially penetrate into the disc, as well as their amplitudes, depends on the mass of the perturbing satellite. We show that the mean height of the disc is expected to vary more rapidly in the radial than in the azimuthal direction. If the observed vertical density asymmetry is indeed caused by vertical oscillations, we predict radial and azimuthal variations of the mean vertical velocity, correlating with the spatial structure. These variations can have amplitudes as large as 8 km s−1.
    Full-text · Article · Jul 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: It is now known that minor mergers are capable of creating structure in the phase-space distribution of their host galaxy’s disc. In order to search for such imprints in the Milky Way, we analyse the Sloan Extension for Galactic Understanding and Exploration (SEGUE) F/G dwarf and the Schuster et al. stellar samples. We find similar features in these two completely independent stellar samples, consistent with the predictions of a Milky Way minor-merger event. We next apply the same analyses to high-resolution, idealized N-body simulations of the interaction between the Sagittarius dwarf galaxy and the Milky Way. The energy distributions of stellar particle samples in small spatial regions in the host disc reveal strong variations of structure with position. We find good matches to the observations for models with a mass of Sagittarius’ dark matter halo progenitor ⪅1011 M⊙. Thus, we show that this kind of analysis could be used to provide unprecedentedly tight constraints on Sagittarius’ orbital parameters, as well as place a lower limit on its mass.
    Full-text · Article · Jan 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: By means of N-body simulations, we study the response of a galactic disc to a minor merger event. We find that non-self-gravitating, spiral-like features are induced in the thick disc. As we have shown in a previous work, this ‘ringing’ also leaves an imprint in velocity space (the u–v plane) in small spatial regions, such as the solar neighbourhood. As the disc relaxes after the event, clumps in the u–v plane get closer with time, allowing us to estimate the time of impact. In addition to confirming the possibility of this diagnostic, here we show that in a more realistic scenario, the infall trajectory of the perturber gives rise to an azimuthal dependence of the structure in phase space. We also find that the space defined by the energy and angular momentum of stars is a better choice than velocity space, as clumps remain visible even in large local volumes. This makes their observational detection much easier since one need not be restricted to a small spatial volume. We show that information about the time of impact, the mass of the perturber and its trajectory is stored in the kinematics of disc stars.
    Full-text · Article · May 2011 · Monthly Notices of the Royal Astronomical Society
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    Facundo A. Gómez · Amina Helmi · Anthony G. A. Brown · Yang-Shyang Li
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    ABSTRACT: We model the formation of the Galactic stellar halo via the accretion of satellite galaxies on to a time-dependent semicosmological galactic potential. Our goal is to characterize the substructure left by these accretion events in a close manner to what may be possible with the Gaia mission. We have created a synthetic Gaia solar neighbourhood catalogue by convolving the six-dimensional phase-space coordinates of stellar particles from our disrupted satellites with the latest estimates of the Gaia measurement errors, and included realistic background contamination due to the Galactic disc(s) and bulge. We find that, even after accounting for the expected observational errors, the resulting phase space is full of substructure. We are able to successfully isolate roughly 50 per cent of the different satellites contributing to the ‘solar neighbourhood’ by applying the mean shift clustering algorithm in energy–angular momentum space. Furthermore, a Fourier analysis of the space of orbital frequencies allows us to obtain accurate estimates of the time since accretion for approximately 30 per cent of the recovered satellites.
    Preview · Article · Apr 2010 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We study the orbital properties of stars in four (published) simulations of thick discs formed by (i) accretion from disrupted satellites, (ii) heating of a pre-existing thin disc by a minor merger, (iii) radial migration and (iv) gas-rich mergers. We find that the distribution of orbital eccentricities is predicted to be different for each model: a prominent peak at low eccentricity is expected for the heating, migration and gas-rich merging scenarios, while the eccentricity distribution is broader and shifted towards higher values for the accretion model. These differences can be traced back to whether the bulk of the stars in each case is formed in situ or is accreted, and is robust to the peculiarities of each model. A simple test based on the eccentricity distribution of nearby thick-disc stars may thus help elucidate the dominant formation mechanism of the Galactic thick disc.
    Full-text · Article · Nov 2009 · Monthly Notices of the Royal Astronomical Society Letters
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    ABSTRACT: We study the orbital properties of stars in four (published) simulations of thick disks formed by: i) accretion from disrupted satellites, ii) heating of a pre-existing thin disk by a minor merger, iii) radial migration and iv) gas rich mergers. We find that the distribution of orbital eccentricities are predicted to be different for each model: a prominent peak at low eccentricity is expected for the heating, migration and gas-rich merging scenarios, while the eccentricity distribution is broader and shifted towards higher values for the accretion model. These differences can be traced back to whether the bulk of the stars in each case is formed 'in-situ' or is 'accreted', and are robust to the peculiarities of each model. A simple test based on the eccentricity distribution of nearby thick disk stars may thus help elucidate the dominant formation mechanism of the Galactic thick disk. Comment: 5 pages, 3 figures, accepted to MNRAS Letters. Typo corrected
    Full-text · Article · Sep 2009