Article

Deep imaging of Eridanus II and its lone star cluster

Authors:
Denija Crnojevic at Texas Tech University
Denija Crnojevic
D. J. Sand
D. J. Sand
D. J. Sand
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Dennis Zaritsky at University of Arizona
Dennis Zaritsky
K. Spekkens
K. Spekkens
K. Spekkens
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Abstract

We present deep imaging of the most distant dwarf discovered by the Dark Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry reaches ∼\sim3 mag deeper than previous work, and allows us to confirm the presence of a stellar cluster whose position is consistent with Eri II's center. This makes Eri II, at MV=−7.1M_V=-7.1, the least luminous galaxy known to host a (possibly central) cluster. The cluster is partially resolved, and at MV=−3.5M_V=-3.5 it accounts for ∼\sim4%4\% of Eri II's luminosity. We derive updated structural parameters for Eri II, which has a half-light radius of ∼\sim280 pc and is elongated (ϵ\epsilon∼\sim0.48), at a measured distance of D∼\sim370 kpc. The color-magnitude diagram displays a blue, extended horizontal branch, as well as a less populated red horizontal branch. The presence of the latter, together with a central concentration of stars brighter than the old main sequence turnoff, hints at a possible intermediate-age (∼\sim3 Gyr) population. Alternatively, these sources could be blue straggler stars. A deep Green Bank Telescope observation of Eri II reveals no associated atomic gas.

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... Ultrafaint dwarf galaxies (UFDs) are ideal astrophysical targets to detect signals of DM interactions [26][27][28][29][30][31][32][33][34][35][36][37]. Eridanus II (Eri II) is presently the least luminous UFD observed to host a globular cluster [38][39][40][41][42][43]. Located near the center of Eri II, the star cluster offers unique op-portunities to probe the density profile of DM [42][43][44]. ...
... III. Probing dark matter in Eridanus II As mentioned above, the star cluster in Eri II has been recognized as a powerful DM tracer. At a distance of ∼ 366 kpc from the Milky Way, Eri II is a UFD with absolute magnitude M V = −7.1 ± 0.3, a mass-to-light ratio of 420 +210 −140 M ⊙ /L ⊙ , and a compact star cluster located in its central region [38][39][40][41][42][43]46]. Observations of Eri II favor a cored DM profile [42]. ...
... The measured properties of Eri II and the star cluster reported by Refs. [40,46] are summarized in Table I The time evolution of this star cluster's half-light radius r h (t) is described by Eq. (5). With the parameters taken from Table I, we solve the differential equation for t ≤ 0 to infer its r h in the past. ...
A new probe of dark matter-baryon interactions in compact stellar systems
Preprint
  • Aug 2024
We investigate the astrophysical consequences of an attractive long-range interaction between dark matter and baryonic matter. Our study highlights the role of this interaction in inducing dynamical friction between dark matter and stars, which can significantly influence the evolution of compact stellar systems. Using the star cluster in Eridanus II as a case study, we derive a new stringent upper bound on the interaction strength α~≤333.7\tilde{\alpha}\leq 333.7 for the interaction range λ=1\lambda = 1 pc. This constraint is independent of the dark matter mass and can improve the existing model-independent limits on α~\tilde{\alpha} by a few orders of magnitude. Furthermore, we observe that the constraint is insensitive to the mass of the stellar system and the dark matter density in the stellar system as long as the system is dark matter dominated. This new approach can be applied to many other stellar systems, and we obtain comparable constraints from compact stellar halos observed in ultrafaint dwarf galaxies.
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... In addition, the mass estimates compare, with the MWNSC having a mass of ∼2 × 10 7 M e (Launhardt et al. 2002;Schödel et al. 2014;Feldmeier-Krause et al. 2017b). Other data sets from the Local Volume (a field environment with a distance 11 Mpc; green diamonds; Seth et al. 2006;Georgiev et al. 2009a;Graham & Spitler 2009;Baldassare et al. 2014;Schödel et al. 2014;Calzetti et al. 2015;Carson et al. 2015;Crnojević et al. 2016;Nguyen et al. 2017;Baumgardt & Hilker 2018;Nguyen et al. 2018;Bellazzini et al. 2020;Pechetti et al. 2020), the Virgo galaxy cluster (orange squares; Sánchez-Janssen et al. 2019a), dwarfs around massive field galaxies (green triangles; Carlsten et al. 2022), and massive field galaxies (gray circles; GB14) show the general distribution of NSCs. The Milky Way NSC is highlighted with a blue plus sign. ...
... (a field environment with distance 11 Mpc;Seth et al. 2006;Georgiev et al. 2009a;Graham & Spitler 2009;Baldassare et al. 2014;Schödel et al. 2014;Calzetti et al. 2015;Carson et al. 2015;Crnojević et al. 2016;Nguyen et al. 2017;Baumgardt & Hilker 2018;Nguyen et al. 2018;Bellazzini et al. 2020;Pechetti et al. 2020) are added for comparison. Dwarfs around massive field galaxies and Virgo cluster members are taken fromCarlsten et al. (2022) andSánchez-Janssen et al. (2019a), respectively. ...
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Article
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  • Feb 2023
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... The dynamical effects from MACHOs will disrupt wide binary stars in the galactic halo, putting an upper limit on the fraction of MACHOs as DM in the halo Yoo et al. 2004;Quinn et al. 2009;Monroy-Rodríguez & Allen 2014). Similarly, dynamical heating of dwarf galaxies will be evident if MACHOs constitute a large fraction of DM, but this dynamical heating is not observed (Crnojević et al. 2016;Brandt 2016;Koushiappas & Loeb 2017). However, the existence of an intermediate mass black hole at the centre of the dwarf galaxy would weaken any MACHO constraints (Li et al. 2017). ...
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Preprint
Full-text available
  • Mar 2018
Recent interest in primordial black holes as a possible dark matter candidate has motivated the reanalysis of previous methods for constraining massive astrophysical compact objects in the Milky Way halo and beyond. In order to derive these constraints, a model for the dark matter distribution around the Milky Way must be used. Previous microlensing searches have assumed a semi-isothermal density sphere for this task. We show this model is no longer consistent with data from the Milky Way rotation curve, and test two replacement models, namely NFW and power-law. The power-law model is the most flexible as it can break spherical symmetry, and best fits the data. Thus, we recommend the power-law model as a replacement, although it still lacks the flexibility to fully encapsulate all possible shapes of the Milky Way halo. We then use the power-law model to rederive some previous microlensing constraints in the literature, while propagating the primary halo-shape uncertainties through to our final constraints. Our analysis reveals that the microlensing constraints towards the Large Magellanic Cloud weaken somewhat for MACHO masses around 10 M⊙10\, M_\odot when this uncertainty is taken into account, but the constraints tighten at lower masses. Exploring some of the simplifying assumptions of previous constraints we also study the effect of wide mass distributions of compact halo objects, as well as the effect of spatial clustering on microlensing constraints. We find that both effects induce a shift in the constraints towards smaller masses, and can effectively remove the microlensing constraints from M∼1−10M⊙M \sim 1-10 M_\odot for certain MACHO populations.
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... Nonetheless, it is tantalising that Leo T lies right on the edge of the SDSS survey footprint (Koposov et al. 2009). Any closer to the Milky Way and Leo T would have been stripped of its gas, similarly to the recently discovered Eridanus II galaxy (Crnojević et al. 2016). Any further away, and it would have been too faint to be seen. ...
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Preprint
  • Jul 2016
We fit the rotation curves of isolated dwarf galaxies to directly measure the stellar mass-halo mass relation (M∗−M200M_*-M_{200}) over the mass range 5×105<M∗/M⊙<1085 \times 10^5 < M_{*}/{\rm M}_\odot < 10^{8}. By accounting for cusp-core transformations due to stellar feedback, we find a monotonic relation with little scatter. Such monotonicity implies that abundance matching should yield a similar M∗−M200M_*-M_{200} if the cosmological model is correct. Using the 'field galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the halo mass function from the Λ\Lambda Cold Dark Matter Bolshoi simulation, we find remarkable agreement between the two. This holds down to M200∼5×109M_{200} \sim 5 \times 10^9M⊙_\odot, and to M200∼5×108M_{200} \sim 5 \times 10^8M⊙_\odot if we assume a power law extrapolation of the SDSS stellar mass function below M∗∼107M_* \sim 10^7M⊙_\odot. However, if instead of SDSS we use the stellar mass function of nearby galaxy groups, then the agreement is poor. This occurs because the group stellar mass function is shallower than that of the field below M∗∼109M_* \sim 10^9M⊙_\odot, recovering the familiar 'missing satellites' and 'too big to fail' problems. Our result demonstrates that both problems are confined to group environments and must, therefore, owe to 'galaxy formation physics' rather than exotic cosmology. Finally, we repeat our analysis for a Λ\Lambda Warm Dark Matter cosmology, finding that it fails at 68% confidence for a thermal relic mass of mWDM<1.25m_{\rm WDM} < 1.25keV, and mWDM<2m_{\rm WDM} < 2keV if we use the power law extrapolation of SDSS. We conclude by making a number of predictions for future surveys based on these results.
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... In fainter hosts, GCs can act as tracers of galaxies that would be beyond the detection limits of present or future surveys. There are four local group dwarfs fainter than Fornax that host a star cluster: Crnojević et al. 2016). With the possible exception of PegDIG, the progenitors of these galaxies will not be observable at redshifts relevant to reionization, even with JWST (Boylan- . ...
Globular Clusters in High-Redshift Dwarf Galaxies: A Case Study from the Local Group
Preprint
  • Feb 2018
We present the reconstructed evolution of rest-frame ultra-violet (UV) luminosities of the most massive Milky Way dwarf spheroidal satellite galaxy, Fornax, and its five globular clusters (GCs) across redshift, based on analysis of the stellar fossil record and stellar population synthesis modeling. We find that (1) Fornax's (proto-)GCs can generate 10−10010-100 times more UV flux than the field population, despite comprising <∼5%<\sim 5\% of the stellar mass at the relevant redshifts; (2) due to their respective surface brightnesses, it is more likely that faint, compact sources in the Hubble Frontier Fields (HFFs) are GCs hosted by faint galaxies, than faint galaxies themselves. This may significantly complicate the construction of a galaxy UV luminosity function at z>3z>3. (3) GC formation can introduce order-of-magnitude errors in abundance matching. We also find that some compact HFF objects are consistent with the reconstructed properties of Fornax's GCs at the same redshifts (e.g., surface brightness, star formation rate), suggesting we may already have detected proto-GCs in the early Universe. Finally, we discuss the prospects for improving the connections between local GCs and proto-GCs detected in the early Universe.
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... This probability drops precipitously at lower masses: 1 in 10 halos with M vir (z = 0) = 1.01 × 10 10 M will host at least one blue GC, while fewer than 1 in 100 halos with M vir = 7×10 9 M will host a blue GC. These numbers are useful in the context of considering possible dwarf galaxy hosts of GCs near the Milky Way ) and the halo masses of dwarf galaxies known to host blue GCs, e.g., Eridanus II (Koposov et al. 2015;Crnojević et al. 2016) and Pegasus (Cole et al. 2017). More sophisticated (and perhaps more physically plausible) models of the occupation number of GCs as a function of z = 0 halo mass might include the effects of environment on P(N GCs |M halo ); for example, patchy reionization might contribute to scatter in formation redshifts for globular clusters (Spitler et al. 2012). ...
The Globular Cluster - Dark Matter Halo Connection
Preprint
  • May 2017
I present a simple phenomenological model for the observed linear scaling of the stellar mass in old globular clusters (GCs) with z=0 halo mass in which the stellar mass in GCs scales linearly with progenitor halo mass at z=6 above a minimum halo mass for GC formation. This model reproduces the observed MGCs−MhaloM_{\rm GCs}-M_{\rm halo} relation at z=0 and results in a prediction for the minimum halo mass at z=6 required for hosting one GC: Mmin(z=6)=1.07×109 M⊙M_{\rm min}(z=6)=1.07 \times 10^9\,M_{\odot}. Translated to z=0, the mean threshold mass is Mhalo(z=0)≈2×1010 M⊙M_{\rm halo}(z=0) \approx 2\times 10^{10}\,M_{\odot}. I explore the observability of GCs in the reionization era and their contribution to cosmic reionization, both of which depend sensitively on the (unknown) ratio of GC birth mass to present-day stellar mass, ξ\xi. Based on current detections of z≳6z \gtrsim 6 objects with M1500<−17M_{1500} < -17, values of ξ>10\xi > 10 are strongly disfavored; this, in turn, has potentially important implications for GC formation scenarios. Even for low values of ξ\xi, some observed high-z galaxies may actually be GCs, complicating estimates of reionization-era galaxy ultraviolet luminosity functions and constraints on dark matter models. GCs are likely important reionization sources if 5≲ξ≲105 \lesssim \xi \lesssim 10. I also explore predictions for the fraction of accreted versus in situ GCs in the local Universe and for descendants of systems at the halo mass threshold of GC formation (dwarf galaxies). An appealing feature of the model presented here is the ability to make predictions for GC properties based solely on dark matter halo merger trees.
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... Since neither object is surrounded by a visible galaxy today, the hosts must have been tidally disrupted, leaving behind just their clusters and perhaps a faint (not yet observed) stream. The prototypes for clusters within ultra-faint dwarfs are the central star cluster in Eridanus II (Koposov et al. 2015;Crnojević et al. 2016;Simon et al. 2021;Alzate et al. 2021;Martínez-Vázquez et al. 2021;Weisz et al. 2023) and the cluster in Ursa Major II (Zucker et al. 2006;Eadie et al. 2022). The UMa II cluster is extremely poorly studied, with neither any published spectroscopy nor an analysis of its stellar population, so nothing can currently be said about its chemical properties. ...
Eridanus III and DELVE 1: Carbon-rich Primordial Star Clusters or the Smallest Dwarf Galaxies?
Preprint
Full-text available
  • Oct 2024
We present spectroscopy of the ultra-faint Milky Way satellites Eridanus III (Eri III) and DELVE 1. We identify eight member stars in each satellite and place non-constraining upper limits on their velocity and metallicity dispersions. The brightest star in each object is very metal-poor, at [Fe/H] = -3.1 for Eri III and [Fe/H] = -2.8 for DELVE 1. Both of these stars exhibit large overabundances of carbon and very low abundances of the neutron-capture elements Ba and Sr, and we classify them as CEMP-no stars. Because their metallicities are well below those of the Milky Way globular cluster population, and because no CEMP-no stars have been identified in globular clusters, these chemical abundances could suggest that Eri III and DELVE 1 are dwarf galaxies. On the other hand, the two systems have half-light radii of 8 pc and 6 pc, respectively, which is more compact than any known ultra-faint dwarfs. We conclude that Eri III and DELVE 1 are either the smallest dwarf galaxies yet discovered, or they are representatives of a new class of star clusters that underwent chemical evolution distinct from that of ordinary globular clusters. In the latter scenario, such objects are likely the most primordial star clusters surviving today. These possibilities can be distinguished by future measurements of carbon and/or iron abundances for larger samples of stars or improved stellar kinematics for the two systems.
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... The EDGE NSC data is displayed in large star symbols -the V-band magnitude for these has been calculated from the stars within 4 × NSC half-light radii. Observational data for both globular clusters (Harris 2010;Cerny et al. 2023;Crnojević et al. 2016;Laevens et al. 2015) and dwarf galaxies (Simon 2019;Sand et al. 2022;McQuinn et al. 2023;Conn et al. 2018;Collins et al. 2023) are shown in grey circles and teal diamonds, respectively. Globular clusters identified as Type II from Milone et al. 2017 are circled in red and listed on the right of the plot. ...
EDGE: A new model for Nuclear Star Cluster formation in dwarf galaxies
Preprint
  • May 2024
Nuclear Star Clusters (NSCs) are amongst the densest stellar systems in the Universe and are found at the centres of many bright spiral and elliptical galaxies, and up to ∼{\sim}40% of dwarf galaxies. However, their formation mechanisms, and possible links to globular clusters (GCs), remain debated. This paper uses the EDGE simulations - a collection of zoom-in, cosmological simulations of isolated dwarf galaxies -- to present a new formation mechanism for NSCs. We find that, at a gas spatial and mass resolution of ∼3 {\sim}3\,pc and ∼161{\sim}161 M⊙_\odot, respectively, NSCs naturally emerge in a subset of our EDGE dwarfs with redshift-zero halo masses of Mr200c∼5×109\rm{M}_{\rm{r}200\rm{c}} \sim 5 \times 10^9 M⊙_\odot. These dwarfs are quenched by reionisation, but retain a significant reservoir of gas that is unable to cool and form stars. Sometime after reionisation, the dwarfs then undergo a major (∼{\sim}1:1) merger that excites rapid gas cooling, leading to a significant starburst. An NSC forms in this starburst that then quenches star formation thereafter. The result is a nucleated dwarf that has two stellar populations with distinct age: one pre-reionisation and one post-reionisation. Our mechanism is unique for two key reasons. Firstly, the low mass of the host dwarf means that NSCs, formed in this way, can accrete onto galaxies of almost all masses, potentially seeding the formation of NSCs everywhere. Secondly, our model predicts that NSCs should have at least two stellar populations with a large (≳\gtrsim1 billion year) age separation. This yields a predicted colour magnitude diagram for our nucleated dwarfs that has two distinct main sequence turnoffs. Several GCs orbiting the Milky Way, including Omega Centauri and M54, show exactly this behaviour, suggesting that they may, in fact, be accreted NSCs.
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... On the outskirts of the Local Group the WLM dIrr shows a single old GC (Hodge et al. 1999). In addition, the Eridanus II dSph hosts a single low mass star cluster (Koposov et al. 2015;Crnojević et al. 2016) and the dIrr IC 1613 hosts a small number of low mass star clusters (Wyder et al. 2000). ...
Rubin Observatory LSST Stars Milky Way and Local Volume Star Clusters Roadmap
Article
Full-text available
  • Jul 2023
The Vera C. Rubin Observatory will undertake the Legacy Survey of Space and Time, providing an unprecedented, volume-limited catalog of star clusters in the Southern Sky, including Galactic and extragalactic star clusters. The Star Clusters subgroup of the Stars, Milky Way and Local Volume Working Group has identified key areas where Rubin Observatory will enable significant progress in star cluster research. This roadmap represents our science cases and preparation for studies of all kinds of star clusters from the Milky Way out to distances of tens of megaparsecs.
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... On the outskirts of the Local Group the WLM dIrr shows a single old GC (Hodge et al. 1999). In addition, the Eridanus II dSph hosts a single low mass star cluster (Koposov et al. 2015;Crnojević et al. 2016) and the dIrr IC 1613 hosts a small number of low mass star clusters (Wyder et al. 2000). ...
Rubin Observatory LSST Stars Milky Way and Local Volume Star Clusters Roadmap
Preprint
  • Jun 2023
The Vera C. Rubin Observatory will undertake the Legacy Survey of Space and Time, providing an unprecedented, volume-limited catalog of star clusters in the Southern Sky, including Galactic and extragalactic star clusters. The Star Clusters subgroup of the Stars, Milky Way and Local Volume Working Group has identified key areas where Rubin Observatory will enable significant progress in star cluster research. This roadmap represents our science cases and preparation for studies of all kinds of star clusters from the Milky Way out to distances of tens of megaparsecs.
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... The Eridanus II cluster is thought to be at least 3Gyr old, however it has been stated that it can be up to 12Gyr old (Crnojević et al. 2016) whereby tighter constraints would be made (Brandt 2016). ...
Primordial black holes as possible candidates for dark matter
Preprint
Full-text available
  • Mar 2017
The capture of dark matter by pre-stellar cores is considered, subsequently the dark matter will be trapped inside the compact remnant that the star becomes. If the dark matter is made up of primordial black holes (PBHs) then these will rapidly destroy the compact remnant and so constraints on the abundance of PBHs are implied by observations of compact remnants. Observational constraints based on black hole evaporation and gravitational lensing, as well as various dynamical constraints, are all considered and applied to the three allowed PBH mass ranges in which they could comprise the dark matter
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... CMD simulations indicate that unseen lower-mass stars contribute 15% more light than we measure. In total, we find M V,GC ∼ −3.4 within 2r h , in good agreement with Crnojević et al. (2016), and which yields a reasonable M/L V . ...
On the Reionization-era Globular Cluster in the Low-mass Galaxy Eridanus II
Article
Full-text available
  • May 2023
Using color–magnitude diagrams from deep archival Hubble Space Telescope imaging, we self-consistently measure the star formation history of Eridanus II (Eri II ), the lowest-mass galaxy ( M ⋆ ( z = 0) ∼ 10 ⁵ M ⊙ ) known to host a globular cluster (GC), and the age, mass, and metallicity of its GC. The GC (∼13.2 ± 0.4 Gyr, 〈[Fe/H]〉 = −2.75 ± 0.2 dex) and field (mean age ∼13.5 ± 0.3 Gyr, 〈[Fe/H]〉 = −2.6 ± 0.15 dex) have similar ages and metallicities. Both are reionization-era relics that formed before the peak of cosmic star and GC formation ( z ∼ 2–4). The ancient star formation properties of Eri II are not extreme and appear similar to z = 0 dwarf galaxies. We find that the GC was ≲4 times more massive at birth than today and was ∼10% of the galaxy's stellar mass at birth. At formation, we estimate that the progenitor of Eri II and its GC had M UV ∼ −7 to −12, making it one of the most common type of galaxy in the early universe, though it is fainter than direct detection limits, absent gravitational lensing. Archaeological studies of GCs in nearby low-mass galaxies may be the only way to constrain GC formation in such low-mass systems. We discuss the strengths and limitations in comparing archaeological and high-redshift studies of cluster formation, including challenges stemming from the Hubble Tension, which introduces uncertainties into the mapping between age and redshift.
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... CMD simulations indicate that unseen lower mass stars contribute 15% more light than we measure. In total, we find M V,GC ∼ −3.4 within 2r h , in good agreement with Crnojević et al. (2016), and yields a reasonable M/L V . ...
On the Reionization-Era Globular Cluster in Low-Mass Galaxy Eridanus II
Preprint
Full-text available
  • Mar 2023
Using color-magnitude diagrams from deep archival Hubble Space Telescope imaging, we self-consistently measure the star formation history of Eridanus II (Eri II), the lowest-mass galaxy (M⋆(z=0)∼105M⊙M_{\star}(z=0) \sim 10^5 M_{\odot}) known to host a globular cluster (GC), and the age, mass, and metallicity of its GC. The GC (∼13.2±0.4\sim13.2\pm0.4 Gyr, ⟨\langle[Fe/H]⟩=−2.75±0.2\rangle = -2.75\pm0.2 dex) and field (mean age ∼13.5±0.3\sim13.5\pm0.3 Gyr, ⟨\langle[Fe/H]⟩=−2.6±0.15\rangle = -2.6\pm0.15 dex) have similar ages and metallicities. Both are reionization-era relics that formed before the peak of cosmic star and GC formation (z∼2−4z\sim2-4). The ancient star formation properties of Eri II are not extreme and appear similar to z=0 dwarf galaxies. We find that the GC was ≲4\lesssim4 times more massive at birth than today and was ∼\sim10% of the galaxy's stellar mass at birth. At formation, we estimate that the progenitor of Eri II and its GC had MUV∼−7M_{\rm UV} \sim -7 to −12-12, making it one of the most common type of galaxy in the early Universe, though it is fainter than direct detection limits, absent gravitational lensing. Archaeological studies of GCs in nearby low-mass galaxies may be the only way to constrain GC formation in such low-mass systems. We discuss the strengths and limitations in comparing archaeological and high redshift studies of cluster formation, including challenges stemming from the Hubble Tension, which introduces uncertainties into the mapping between age and redshift.
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... For the smaller dwarfs where the numbers of clusters were too low to permit a bimodal-Gaussian fit, the list of GCs was simply divided at [Fe/H] = −1 with lower-metallicity ones counted as "blue" and higher-metallicity ones as "red." Finally, to extend the galaxy mass range down to the lowest possible levels, the Local Group dwarfs containing GCs were added, with spectroscopic and photometric data from a range of individual sources (Da Costa & Mould 1988;Georgiev et al. 2010;Law & Majewski 2010;Colucci et al. 2011;Veljanoski et al. 2013;Larsen et al. 2014;Veljanoski et al. 2015;Crnojević et al. 2016;Cusano et al. 2016;Dalessandro et al. 2016;de Boer & Fraser 2016;Caldwell et al. 2017;Cole et al. 2017;Forbes et al. 2018a;Piatti et al. 2018;Kruijssen et al. 2019b;Beasley et al. 2019;Massari et al. 2019;Wang et al. 2019;Forbes 2020;Pace et al. 2021;Eadie et al. 2022). For these dwarfs, the numbers of clusters are too small for GMM solutions, so again the samples were simply divided at [Fe/ H] = −1 as above. ...
A Photometric Survey of Globular Cluster Systems in Brightest Cluster Galaxies
Article
Full-text available
  • Feb 2023
Hubble Space Telescope imaging for 26 giant early-type galaxies, all drawn from the MAST archive, is used to carry out photometry of their surrounding globular cluster (GC) systems. Most of these targets are Brightest Cluster Galaxies and their distances range from 24–210 Mpc. The catalogs of photometry, completed with DOLPHOT, are publicly available. The GC color indices are converted to [Fe/H] through a combination of 12 Gyr single stellar population models and direct spectroscopic calibration of the fiducial color index (F475W–F850LP). All of the resulting metallicity distribution functions (MDFs) can be accurately matched by bimodal-Gaussian functions. The GC systems in all of the galaxies also exhibit shallow metallicity gradients with projected galactocentric distance that average Z ∼ R gc − 0.3 . Several parameters of the MDFs including the means, dispersions, and blue/red fractions are summarized. Perhaps the most interesting new result is the trend of blue/red GC fraction with galaxy mass, which connects with predictions from recent simulations of GC formation within hierarchical assembly of large galaxies. The observed trend reveals two major transition stages: for low-mass galaxies, the metal-rich (red) GC fraction f (red) increases steadily with galaxy mass, until halo mass M h ≃ 3 × 10 ¹² M ⊙ . Above this point, more than half the metal-poor (blue) GCs come from accreted satellites and f (red) starts declining. But above a still higher transition point near M h ≃ 10 ¹⁴ M ⊙ , the data hint that f (red) may start to increase again because the metal-rich GCs also become dominated by accreted systems.
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... Over the past several decades, our understanding of the sizes of the globular cluster systems of nearby galaxies has expanded considerably (e.g., Harris & Racine 1979;Harris 1991;Brodie & Strader 2006;Peng et al. 2008;Georgiev et al. 2010;Harris et al. 2013), thanks in large part to recent surveys enabled by the Hubble Space Telescope (e.g., Larsen et al. 2001;Côté et al. 2004;Lotz et al. 2004;Harris et al. 2006;Jordán et al. 2007;Georgiev et al. 2008;Harris 2009). It is now wellestablished that old globular clusters are common features of essentially all galaxy types, from ultra-faint dwarfs (e.g., Crnojević et al. 2016;van Dokkum et al. 2017;Danieli et al. 2022), to spirals (e.g., Harris 1996; Galleti et al. 2004), and to massive ellipticals (e.g., Jordán et al. 2009). This boon in observations has been complemented by a boon in computational methods-current state-of-the-art simulations are able to perform star-by-star realizations of clusters with up to a million stars over their full lifetimes incorporating not only N-body dynamical processes but also stellar evolution (e.g., Aarseth 2003;Giersz et al. 2013;Wang et al. 2016;Rodriguez et al. 2022). ...
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Full-text available
  • Feb 2023
The recent detection of a repeating fast radio burst (FRB) in an old globular cluster in M81 challenges traditional FRB formation mechanisms based on the magnetic activity of young neutron stars formed in core-collapse supernovae. Furthermore, the detection of this repeater in such a nearby galaxy implies a high local universe rate of similar events in globular clusters. Building off the properties inferred from the M81 FRB, we predict the number of FRB sources in nearby ( d ≲ 20 Mpc) galaxies with large globular cluster systems known. Incorporating the uncertain burst energy distribution, we estimate the rate of bursts detectable in these galaxies by radio instruments such as FAST and MeerKat. Of all local galaxies, we find M87 is the best candidate for FRB detections. We predict that M87's globular cluster system contains  ( 10 ) FRB sources at present and that a dedicated radio survey (by either FAST or MeerKat) of  ( 10 ) hr has a 90% probability of detecting a globular cluster FRB in M87. The detection of even a handful of additional globular cluster FRBs would provide invaluable constraints on FRB mechanisms and population properties. Previous studies have demonstrated young neutron stars formed following the collapse of dynamically formed massive white dwarf binary mergers may provide the most natural mechanism for these bursts. We explore the white dwarf merger scenario using a suite of N -body cluster models, focusing in particular on such mergers in M87's clusters. We describe a number of outstanding features of this scenario that in principle may be testable with an ensemble of observed FRBs in nearby globular clusters.
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Discovery and Spectroscopic Confirmation of Aquarius III: A Low-mass Milky Way Satellite Galaxy
Article
Full-text available
  • Jan 2025
  • ASTROPHYS J
We present the discovery of Aquarius III, an ultra-faint Milky Way satellite galaxy identified in the second data release of the DECam Local Volume Exploration survey. Based on deeper follow-up imaging with DECam, we find that Aquarius III is a low-luminosity ( M V = − 2.5 − 0.5 + 0.3 ; L V = 850 − 260 + 380 L ⊙ ), extended ( r 1 / 2 = 41 − 8 + 9 pc) stellar system located in the outer halo ( D ⊙ = 85 ± 4 kpc). From medium-resolution Keck/DEIMOS spectroscopy, we identify 11 member stars and measure a mean heliocentric radial velocity of v sys = − 13.1 − 0.9 + 1.0 km s − 1 for the system and place an upper limit of σ v < 3.5 km s ⁻¹ ( σ v < 1.6 km s ⁻¹ ) on its velocity dispersion at the 95% (68%) credible level. Based on calcium-triplet metallicities of the six brightest red giant members, we find that Aquarius III is very metal-poor ([Fe/H]= − 2.61 ± 0.21) with a statistically significant metallicity spread ( σ [ Fe / H ] = 0.46 − 0.14 + 0.26 dex). We interpret this metallicity spread as strong evidence that the system is a dwarf galaxy as opposed to a star cluster. Combining our velocity measurement with Gaia proper motions, we find that Aquarius III is currently situated near its orbital pericenter in the outer halo ( r peri = 78 ± 7 kpc) and that it is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, notably unlike other satellites with comparably low velocity dispersion limits in the literature. Thus, if further velocity measurements confirm that its velocity dispersion is truly below σ v ≲ 2 km s ⁻¹ , Aquarius III may serve as a useful laboratory for probing galaxy formation physics in low-mass halos.
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Deep Photometric Observations of Ultra-Faint Milky Way Satellites Centaurus I and Eridanus IV
Preprint
  • Jan 2025
We present deep Magellan+Megacam imaging of Centaurus I (Cen I) and Eridanus IV (Eri IV), two recently discovered Milky Way ultra-faint satellites. Our data reach ∼2−3\sim2-3 magnitudes deeper than the discovery data from the DECam Local Volume Exploration (DELVE) Survey. We use these data to constrain their distances, structural properties (e.g., half-light radii, ellipticity, and position angle), and luminosities. We investigate whether these systems show signs of tidal disturbance, and identify new potential member stars using Gaia EDR3. Our deep color-magnitude diagrams show that Cen I and Eri IV are consistent with an old (τ∼13.0\tau\sim 13.0 Gyr) and metal-poor ([Fe/H]≤−2.2\text{[Fe/H]}\le-2.2) stellar population. We find Cen I to have a half-light radius of rh=2.60±0.30′r_{h}=2.60\pm0.30' (90.6±1190.6\pm11 pc), an ellipticity of ϵ=0.36±0.05\epsilon=0.36\pm0.05, a distance of D=119.8±4.1D=119.8\pm4.1 kpc (m−M=20.39±0.08m-M=20.39\pm0.08 mag), and an absolute magnitude of MV=−5.39±0.19M_{V}=-5.39\pm0.19. Similarly, Eri IV has rh=3.24±0.48′r_{h}=3.24\pm0.48' (65.9±1065.9\pm10 pc), ϵ=0.26±0.09\epsilon=0.26\pm0.09, D=69.9±3.6D=69.9\pm3.6 kpc (m−M=19.22±0.11m-M=19.22\pm0.11 mag), and MV=−3.55±0.24M_{V}=-3.55\pm0.24. These systems occupy a space on the size-luminosity plane consistent with other known Milky Way dwarf galaxies which supports the findings from our previous spectroscopic follow-up. Cen I has a well-defined morphology which lacks any clear evidence of tidal disruption, whereas Eri IV hosts a significant extended feature with multiple possible interpretations.
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Globular Cluster Formation and Evolution in the Context of Cosmological Galaxy Assembly: Open Questions
Preprint
  • Jan 2018
We discuss some of the key open questions regarding the formation and evolution of globular clusters (GCs) during galaxy formation and assembly within a cosmological framework. The current state-of-the-art for both observations and simulations is described, and we briefly mention directions for future research. The oldest GCs have ages ≥\ge 12.5 Gyr and formed around the time of reionisation. Resolved colour-magnitude diagrams of Milky Way GCs and direct imaging of lensed proto-GCs at z ∼\sim 6 with JWST promise further insight. Globular clusters are known to host multiple populations of stars with variations in their chemical abundances. Recently, such multiple populations have been detected in ∼\sim2 Gyr old compact, massive star clusters. This suggests a common, single pathway for the formation of GCs at high and low redshift. The shape of the initial mass function for GCs remains unknown, however for massive galaxies a power-law mass function is favoured. Significant progress has been made recently modelling GC formation in the context of galaxy formation, with success in reproducing many of the observed GC-galaxy scaling relations.
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Tidal stripping and the structure of dwarf galaxies in the Local Group
Preprint
  • Jul 2017
The shallow faint-end slope of the galaxy mass function is usually reproduced in Λ\LambdaCDM galaxy formation models by assuming that the fraction of baryons that turns into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities Vmax<20V_{\rm max}<20-30 km/s. Dark matter-dominated dwarfs should therefore have characteristic velocities of about that value, unless they are small enough to probe only the rising part of the halo circular velocity curve (i.e., half-mass radii, r1/2≪1r_{1/2}\ll 1 kpc). Many dwarfs have properties in disagreement with this prediction: they are large enough to probe their halo VmaxV_{\rm max} but their characteristic velocities are well below 20 km/s. These `cold faint giants' (an extreme example is the recently discovered Crater 2 Milky Way satellite) can only be reconciled with our Λ\LambdaCDM models if they are the remnants of once massive objects heavily affected by tidal stripping. We examine this possibility using the APOSTLE cosmological hydrodynamical simulations of the Local Group. Assuming that low velocity dispersion satellites have been affected by stripping, we infer their progenitor masses, radii, and velocity dispersions, and find them in remarkable agreement with those of isolated dwarfs. Tidal stripping also explains the large scatter in the mass discrepancy-acceleration relation in the dwarf galaxy regime: tides remove preferentially dark matter from satellite galaxies, lowering their accelerations below the amin∼10−11m/s2a_{\rm min}\sim 10^{-11} m/s^2 minimum expected for isolated dwarfs. In many cases, the resulting velocity dispersions are inconsistent with the predictions from Modified Newtonian Dynamics, a result that poses a possibly insurmountable challenge to that scenario.
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Scaling Relations for Dark Matter Annihilation and Decay Profiles in Dwarf Spheroidal Galaxies
Preprint
  • Feb 2018
Measuring the dark matter distribution in dwarf spheroidal galaxies (dSphs) from stellar kinematics is crucial for indirect dark matter searches, as these distributions set the fluxes for both dark matter annihilation (J-Factor) and decay (D-Factor). Here we produce a compilation of J and D-Factors for dSphs, including new calculations for several newly-discovered Milky Way (MW) satellites, for dSphs outside of the MW virial radius, and for M31 satellites. From this compilation we test for scaling relations between the J and D-factors and physical properties of the dSphs such as the velocity dispersion (σlos\sigma_{\mathrm{los}}), the distance (d), and the stellar half-light radius (r1/2r_{1/2}). We find that the following scaling relation minimizes the residuals as compared to different functional dependencies on the observed dSphs properties J(0.5deg)=1017.72(σlos/5 km s−1)4(d/100 kpc)−2(r1/2/100 pc)−1J(0.5 {\rm deg}) = 10^{17.72} \left(\sigma_{\mathrm{los}}/5\,{\rm km \, s^{-1}}\right)^4 \left(d / 100\,{\rm kpc}\right)^{-2}\left( r_{1/2}/100 \,{\rm pc} \right)^{-1}. We find this relation has considerably smaller scatter as compared to the simpler relations that scale only as 1/d21/d^2. We further explore scalings with luminosity (LVL_V), and find that the data do not strongly prefer a scaling including LVL_V as compared to a pure 1/d21/d^2 scaling. The scaling relations we derive can be used to estimate the J-Factor without the full dynamical analysis, and will be useful for estimating limits on particle dark matter properties from new systems that do not have high-quality stellar kinematics.
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Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback
Article
Full-text available
  • Dec 2024
We report the discovery of three faint and ultrafaint dwarf galaxies—Sculptor A, Sculptor B, and Sculptor C—in the direction of NGC 300 ( D = 2.0 Mpc), a Large Magellanic Cloud–mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or H i gas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C ( D = 2.04 − 0.13 + 0.10 Mpc; M V = −9.1 ± 0.1 mag or L V = (3.7 − 0.3 + 0.4 ) × 10 ⁵ L ⊙ ) is consistent with being a satellite of NGC 300. Sculptor A ( D = 1.35 − 0.08 + 0.22 Mpc; M V = −6.9 ± 0.3 mag or L V = (5 − 1 + 1 ) × 10 ⁴ L ⊙ ) is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultrafaint Tucana B in terms of its physical properties and environment. Sculptor B ( D = 2.48 − 0.24 + 0.21 Mpc; M V = −8.1 ± 0.3 mag or L V = (1.5 − 0.4 + 0.5 ) × 10 ⁵ L ⊙ ) is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are ≳2–4 r vir from NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram-pressure stripping, reionization, and internal feedback in influencing the star formation history of the faintest stellar systems.
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Euclid preparation. LVIII. Detecting extragalactic globular clusters in the Euclid survey
Article
Full-text available
  • Nov 2024
  • ASTRON ASTROPHYS
Extragalactic globular clusters (EGCs) are an abundant and powerful tracer of galaxy dynamics and formation, and their own formation and evolution is also a matter of extensive debate. The compact nature of globular clusters means that they are hard to spatially resolve and thus study outside the Local Group. In this work we have examined how well EGCs will be detectable in images from the telescope, using both simulated pre-launch images and the first early-release observations of the Fornax galaxy cluster. The Euclid Wide Survey will provide high-spatial resolution VIS imaging in the broad band as well as near-infrared photometry ( and We estimate that the 24 719 known galaxies within 100 Mpc in the footprint of the survey host around 830 000 EGCs of which about 350 000 are within the survey's detection limits. For about half of these EGCs, three infrared colours will be available as well. For any galaxy within 50 Mpc the brighter half of its GC luminosity function will be detectable by the Euclid Wide Survey. The detectability of EGCs is mainly driven by the residual surface brightness of their host galaxy. We find that an automated machine-learning EGC-classification method based on real data of the Fornax galaxy cluster provides an efficient method to generate high purity and high completeness GC candidate catalogues. We confirm that EGCs are spatially resolved compared to pure point sources in VIS images of Fornax. Our analysis of both simulated and first on-sky data show that Euclid will increase the number of GCs accessible with high-resolution imaging substantially compared to previous surveys, and will permit the study of GCs in the outskirts of their hosts. is unique in enabling systematic studies of EGCs in a spatially unbiased and homogeneous manner and is primed to improve our understanding of many understudied aspects of GC astrophysics.
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The Milky Way satellite galaxy Leo T: A perturbed cored dwarf
Article
Full-text available
  • Nov 2024
The impact of the dynamical state of gas-rich satellite galaxies at the early moments of their infall into their host systems and the relation to their quenching process are not completely understood at the low-mass regime. Two such nearby systems are the infalling Milky Way (MW) dwarfs Leo T and Phoenix located near the MW virial radius at 414bothofwhichpresentintriguingoffsetsbetweentheirgaseousandstellardistributions.HerewepresenthydrodynamicsimulationswithramsestoreproducetheobserveddynamicsofLeoT:its414 both of which present intriguing offsets between their gaseous and stellar distributions. Here we present hydrodynamic simulations with ramses to reproduce the observed dynamics of Leo T: its 80 stellar- offset and the 35 offset between its older ( ) and younger (stellarpopulation.Weconsideredinternalandenvironmentalpropertiessuchasstellarwinds,twocomponents,coredandcuspydarkmatterprofiles,anddifferentsatelliteorbitsconsideringtheMWcircumgalacticmedium.Wefindthatthemodelsthatbestmatchtheobservedmorphologyofthegasandstarsincludemildstellarwindsthatinteractwiththegeneratingtheobservedoffset,anddarkmatterprofileswithextendedcores.Thelatterallowlongoscillationsoftheoff−centredyoungerstellarcomponent,duetolongmixingtimescales( stellar population. We considered internal and environmental properties such as stellar winds, two components, cored and cuspy dark matter profiles, and different satellite orbits considering the MW circumgalactic medium. We find that the models that best match the observed morphology of the gas and stars include mild stellar winds that interact with the generating the observed offset, and dark matter profiles with extended cores. The latter allow long oscillations of the off-centred younger stellar component, due to long mixing timescales ( and the slow precession of near-closed orbits in the cored potentials; instead, cuspy and compact cored dark matter models result in the rapid mixing of the material ($ These models predict that non-equilibrium substructures, such as spatial and kinematic offsets, are likely to persist in cored low-mass dwarfs and to remain detectable on long timescales in systems with recent star formation.
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Hedgehog: An Isolated Quiescent Dwarf Galaxy at 2.4 Mpc
Article
Full-text available
  • Oct 2024
It is well known that almost all isolated dwarf galaxies are actively forming stars. We report the discovery of dw1322m2053 (nicknamed Hedgehog), an isolated quiescent dwarf galaxy at a distance of 2.40 ± 0.15 Mpc with a stellar mass of M ⋆ ≈ 10 5.8 M ⊙ . The distance is measured using surface brightness fluctuations with both Legacy Surveys and deep Magellan/IMACS imaging data. Hedgehog is 1.7 Mpc from the nearest galaxy group, Centaurus A, and has no neighboring galaxies within 1 Mpc, making it one of the most isolated quiescent dwarf galaxies at this stellar mass. It has a red optical color and early-type morphology and shows no UV emission. This indicates that Hedgehog has an old stellar population and no ongoing star formation. Compared with other quiescent dwarfs in the Local Group and Local Volume, Hedgehog appears smaller in size for its luminosity but is consistent with the mass–size relations. Hedgehog might be a backsplash galaxy from the Centaurus A group, but it could also have been quenched in the field by ram pressure stripping in the cosmic web, reionization, or internal processes such as supernova and stellar feedback. Future observations are needed to fully unveil its formation, history, and quenching mechanisms.
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Stellar Metallicities and Gradients in the Faint M31 Satellites Andromeda XVI and Andromeda XXVIII
Article
Full-text available
  • Oct 2024
We present ∼300 stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI ( M V = −7.5) and Andromeda XXVIII ( M V = –8.8), derived using metallicity-sensitive calcium H and K narrowband Hubble Space Telescope imaging. These are the first individual stellar metallicities in And XVI (95 stars). Our And XXVIII sample (191 stars) is a factor of ∼15 increase over literature metallicities. For And XVI, we measure 〈 [Fe/H] 〉 = − 2.17 − 0.05 + 0.05 , σ [Fe/H] = 0.33 − 0.07 + 0.07 , and ∇ [Fe/H] = −0.23 ± 0.15 dex R e − 1 . We find that And XVI is more metal-rich than Milky Way ultrafaint dwarf galaxies of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure 〈 [Fe/H] 〉 = − 1.95 − 0.04 + 0.04 , σ [Fe/H] = 0.34 − 0.05 + 0.05 , and ∇ [Fe/H] = −0.46 ± 0.10 dex R e − 1 , placing it on the dwarf galaxy mass–metallicity relation. Neither galaxy has a metallicity distribution function (MDF) with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint ( L ≲ 10 ⁶ L ⊙ ) galaxies outside the Milky Way halo. Both galaxies’ gradients are consistent with predictions from the FIRE simulations, where an age–gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal-poor ([Fe/H] < –3.0) star candidates, which make up 7% of And XVI’s MDF and 6% of And XXVIII’s.
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Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback
Preprint
  • Sep 2024
We report the discovery of three faint and ultra-faint dwarf galaxies -- Sculptor A, Sculptor B and Sculptor C -- in the direction of NGC 300 (D=2.0 Mpc), a Large Magellanic Cloud-mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or HI gas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C (D=2.04−0.13+0.10^{+0.10}_{-0.13} Mpc; MVM_V=−-9.1±\pm0.1 mag or LVL_V=(3.7−0.3+0.4^{+0.4}_{-0.3})×\times105^5 L⊙L_{\odot}) is consistent with being a satellite of NGC 300. Sculptor A (D=1.35−0.08+0.22^{+0.22}_{-0.08} Mpc; MVM_V=−-6.9±\pm0.3 mag or LVL_V=(5−1+1^{+1}_{-1})×\times104^4 L⊙L_{\odot}) is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultra-faint Tucana B in terms of its physical properties and environment. Sculptor B (D=2.48−0.24+0.21^{+0.21}_{-0.24} Mpc; MVM_V=−-8.1±\pm0.3 mag or LVL_V=(1.5−0.4+0.5^{+0.5}_{-0.4})×\times105^5 L⊙L_{\odot}) is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are ≳\gtrsim2-4 rvirr_{vir} from NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram pressure stripping, reionization and internal feedback in influencing the star formation history of the faintest stellar systems.
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Capture of field stars by dark substructures
Article
  • Aug 2024
  • MON NOT R ASTRON SOC
We use analytical and N-body methods to study the capture of field stars by gravitating substructures moving across a galactic environment. The majority of stars captured by a substructure move on temporarily-bound orbits that are lost to galactic tides after a few orbital revolutions. In numerical experiments where a substructure model is immersed into a sea of field particles on a circular orbit, we find a population of particles that remain bound to the substructure potential for indefinitely-long times. This population is absent from substructure models initially placed outside the galaxy on an eccentric orbit. We show that gravitational capture is most efficient in dwarf spheroidal galaxies (dSphs) on account of their low velocity dispersions and high stellar phase-space densities. In these galaxies ‘dark’ sub-subhaloes which do not experience in-situ star formation may capture field stars and become visible as stellar overdensities with unusual properties: (i) they would have a large size for their luminosity, (ii) contain stellar populations indistinguishable from the host galaxy, and (iii) exhibit dark matter (DM)-dominated mass-to-light ratios. We discuss the nature of several ‘anomalous’ stellar systems reported as star clusters in the Fornax and Eridanus II dSphs which exhibit some of these characteristics. DM sub-subhaloes with a mass function dN/dM∙∼M∙−α{\rm d}N/{\rm d}M_\bullet \sim M_\bullet ^{-\alpha } are expected to generate stellar systems with a luminosity function, dN/dM⋆∼M⋆−β{\rm d}N/{\rm d}M_\star \sim M_\star ^{-\beta }, where β = (2α + 1)/3 = 1.6 for α = 1.9. Detecting and characterizing these objects in dSphs would provide unprecedented constraints on the particle mass and cross section of a large range of DM particle candidates.
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Stellar Metallicities and Gradients in the Faint M31 Satellites Andromeda XVI and Andromeda XXVIII
Preprint
Full-text available
  • Jul 2024
We present ∼300\sim300 stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI (MV=−7.5M_V = -7.5) and Andromeda XXVIII (MV=−8.8M_V = -8.8) derived using metallicity-sensitive Calcium H & K narrow-band Hubble Space Telescope imaging. These are the first individual stellar metallicities in And~XVI (95 stars). Our And~XXVIII sample (191 stars) is a factor of ∼15\sim15 increase over literature metallicities. For And~XVI, we measure \langle \mbox{[Fe/H]}\rangle = -2.17^{+0.05}_{-0.05}, \sigma_{\mbox{[Fe/H]}}=0.33^{+0.07}_{-0.07}, and \nabla_{\mbox{[Fe/H]}} = -0.23\pm0.15 dex Re−1R_e^{-1}. We find that And XVI is more metal-rich than MW UFDs of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure \langle \mbox{[Fe/H]}\rangle = -1.95^{+0.04}_{-0.04}, \sigma_{\mbox{[Fe/H]}}=0.34^{+0.07}_{-0.07}, and \nabla_{\mbox{[Fe/H]}} = -0.46 \pm 0.10~dex~Re−1R_e^{-1}, placing it on the dwarf galaxy mass-metallicity relation. Neither galaxy has a metallicity distribution function with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint (L≲106 L⊙L \lesssim 10^6~L_{\odot}) galaxies outside the Milky Way halo. Both galaxies' gradients are consistent with predictions from the FIRE simulations, where an age-gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal poor (\mbox{[Fe/H]} < -3.0) star candidates, which make up 7% of And~XVI's MDF and 6% of And~XXVIII's.
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Properties of Nuclear Star Clusters in Low Surface Brightness Galaxies
Article
Full-text available
  • Jul 2024
Using the Systematically Measuring Ultra-Diffuse Galaxies and Sloan Digital Sky Survey catalogs and our own reprocessing of the Legacy Survey imaging, we investigate the properties of nuclear star clusters (NSCs) in galaxies having central surface brightnesses as low as 27 mag arcsec ⁻² . We identify 273 (123 with known redshift) and 32 NSC-bearing galaxies in the two samples, respectively, where we require candidate NSCs to have a separation of less than 0.10 r e from the galaxy center. We find that galaxies with low central surface brightness ( μ 0, g > 24 mag arcsec ⁻² ) are more likely to contain an NSC if they (1) have a higher stellar mass, (2) have a higher stellar-to-total mass ratio, (3) have a brighter central surface brightness, (4) have a larger axis ratio, or (5) lie in a denser environment. Because of the correlations among these various quantities, it is likely that only one or two are true physical drivers. We also find scaling relations for the NSC mass with stellar mass ( M NSC / M ⊙ = 10 6.02 ± 0.03 ( M * , gal / 10 8 M ⊙ ) 0.77 ± 0.04 ) and halo mass ( M NSC / M ⊙ = 10 6.11 ± 0.05 ( M h , gal / 10 10 M ⊙ ) 0.92 ± 0.05 ), although it is the scaling with halo mass that is consistent with a direct proportionality. In galaxies with an NSC, M NSC ≈ 10 ⁻⁴ M h ,gal . This proportionality echoes the finding of a direct proportionality between the mass (or number) of globular clusters (GCs) in galaxies and the galaxy’s total mass. These findings favor a related origin for GCs and NSCs.
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Identifying the possible ex-situ origin of the globular clusters of the Milky Way: a kinematic study
Article
  • Oct 2023
  • MON NOT R ASTRON SOC
This is the second paper in a series, which studies the likelihood that some globular clusters (GCs) of the Milky Way (MW) could have originated from a dwarf satellite galaxy (DSG). Using a large suite of three-body simulations we determine the present-day orbital properties of 154 GCs that could have escaped from 41 MW DSGs over the past 8 Gyr8\ \mathrm{Gyr}. For the MW we considered two sets of static and dynamic models which account for the sustained growth of the MW since its birth. We focus on the Magellanic clouds and Sagittarius. We compare the apogalactic distance, eccentricity, and orbital inclination of the MW GCs with those of runaway GCs from DSGs, to constrain their possible ex-situ origin. We observe a positive correlation between a DSG mass and the dispersion of its runaway GCs in the orbital parameter space of (Rap, e). We provide tables of the identified MW GCs and their likely associated progenitors. In total, we find 29 (19 per cent) MW GCs which could be kinematically associated with MW DSGs. We report, for the first time, 6 and 10 new associations with the Large Magellanic Cloud and the Sagittarius, respectively. For the Sagittarius we predict a concentration of runaway GCs at large apogalactic distances of Rap≈275−375 kpcR_\mathrm{ap}\approx 275-375 \ \mathrm{kpc}, e ≈ 0.8, and a relative inclination of Δθ ≈ 20°. So far, there has not been any observed GCs with such orbital elements. Complemented with photometric and spectroscopic observations, and cosmological simulations, the findings from the present study could conclusively settle the debate over the in-situ versus ex-situ origin of the MW GCs.
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Properties of Nuclear Star Clusters in Low Surface Brightness Galaxies
Preprint
Full-text available
  • May 2024
Using the SMUDGes and SDSS catalogs, and our own reprocessing of the Legacy Surveys imaging, we investigate the properties of nuclear star clusters (NSCs) in galaxies having central surface brightnesses as low as 27 mag arcsec−2^{-2}. We identify 273 (123 with known redshift) and 32 NSC-bearing galaxies in the two samples, respectively, where we require candidate NSCs to have a separation of less than 0.10rer_e from the galaxy center. We find that galaxies with low central surface brightness (μ0,g>24\mu_{0,g} > 24 mag arcsec−2^{-2}) are more likely to contain an NSC if 1) they have a higher stellar mass, 2) a higher stellar to total mass ratio, 3) a brighter central surface brightness, 4) a larger axis ratio, or 5) lie in a denser environment. Because of the correlations among these various quantities, it is likely that only one or two are true physical drivers. We also find scaling relations for the NSC mass with stellar mass (MNSC/M_{NSC}/\Msol=106.02±0.03(M∗,gal/108 = 10^{6.02\pm0.03}(M_{*,gal}/10^{8} \Msol)0.77±0.04)^{0.77\pm0.04}) and halo mass (MNSC/M_{NSC}/\Msol=106.11±0.05(Mh,gal/1010 = 10^{6.11\pm0.05}(M_{h,gal}/10^{10} \Msol)0.92±0.05)^{0.92\pm0.05}), although it is the scaling with halo mass that is consistent with a direct proportionality. In galaxies with an NSC, MNSC≈10−4Mh,galM_{NSC} \approx 10^{-4}M_{h,gal}. This proportionality echoes the finding of a direct proportionality between the mass (or number) of globular clusters (GCs) in galaxies and the galaxy's total mass. These findings favor a related origin for GCs and NSCs.
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Deep Hubble Space Telescope Photometry of Large Magellanic Cloud and Milky Way Ultrafaint Dwarfs: A Careful Look into the Magnitude–Size Relation
Article
Full-text available
  • May 2024
We present deep Hubble Space Telescope photometry of 10 targets from Treasury Program GO-14734, including six confirmed ultrafaint dwarf (UFD) galaxies, three UFD candidates, and one likely globular cluster. Six of these targets are satellites of, or have interacted with, the Large Magellanic Cloud (LMC). We determine their structural parameters using a maximum-likelihood technique. Using our newly derived half-light radius ( r h ) and V -band magnitude ( M V ) values in addition to literature values for other UFDs, we find that UFDs associated with the LMC do not show any systematic differences from Milky Way UFDs in the magnitude–size plane. Additionally, we convert simulated UFD properties from the literature into the M V – r h observational space to examine the abilities of current dark matter (DM) and baryonic simulations to reproduce observed UFDs. Some of these simulations adopt alternative DM models, thus allowing us to also explore whether the M V – r h plane could be used to constrain the nature of DM. We find no differences in the magnitude–size plane between UFDs simulated with cold, warm, and self-interacting DM, but note that the sample of UFDs simulated with alternative DM models is quite limited at present. As more deep, wide-field survey data become available, we will have further opportunities to discover and characterize these ultrafaint stellar systems and the greater low surface-brightness universe.
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Strong outflows and inefficient star formation in the reionization-era ultrafaint dwarf galaxy Eridanus ii
Article
  • Apr 2024
  • MON NOT R ASTRON SOC
We present novel constraints on the underlying galaxy formation physics (e.g. mass-loading factor, star formation history, and metal retention) at z ≳ 7 for the low-mass (M* ∼ 105 M⊙) Local Group ultrafaint dwarf galaxy (UFD) Eridanus ii (Eri ii). Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri ii’s CaHK-based photometric metallicity distribution function (MDF; [Fe/H]) and find that the evolution of Eri ii is well characterized by a short, exponentially declining star formation history (τSFH=0.39±0.130.18\tau _\text{SFH}=0.39\pm _{0.13}^{0.18} Gyr), a low star formation efficiency (τSFE=27.56±12.9225.14\tau _\text{SFE}=27.56\pm _{12.92}^{25.14} Gyr), and a large mass-loading factor (η=194.53±42.6733.37\eta =194.53\pm _{42.67}^{33.37}). Our results are consistent with Eri ii forming the majority of its stars before the end of reionization. The large mass-loading factor implies strong outflows in the early history of Eri ii and is in good agreement with theoretical predictions for the mass scaling of galactic winds. It also results in the ejection of >90 per cent of the metals produced in Eri ii. We make predictions for the distribution of [Mg/Fe]–[Fe/H] in Eri ii as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri ii ([Fe/H] > −2) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the reionization era.
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A comprehensive model for the formation and evolution of the faintest Milky Way dwarf satellites
Article
  • Mar 2024
  • MON NOT R ASTRON SOC
In this study, we modify the semi-analytic model Galacticus in order to accurately reproduce the observed properties of dwarf galaxies in the Milky Way. We find that reproducing observational determinations of the halo occupation fraction and mass-metallicity relation for dwarf galaxies requires us to include H2 cooling, an updated UV background radiation model, and to introduce a model for the metal content of the intergalactic medium. By fine-tuning various model parameters and incorporating empirical constraints, we have tailored the model to match the statistical properties of Milky Way dwarf galaxies, such as their luminosity function and size–mass relation. We have validated our modified semi-analytic framework by undertaking a comparative analysis of the resulting galaxy-halo connection. We predict a total of 300−99+75300 ^{+75} _{-99} satellites with an absolute V-band magnitude (MV) less than 0 within 300 kpc from our Milky Way-analogs. The fraction of subhalos that host a galaxy at least this bright drops to 50% by a halo peak mass of ∼8.9 × 107 M⊙, consistent with the occupation fraction inferred from the latest observations of Milky Way satellite population.
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Discovery of Globular Cluster Candidates in the Dwarf Irregular Galaxy IC 2574 Using HST /ACS Imaging
Article
  • Mar 2024
  • MON NOT R ASTRON SOC
We report the discovery of 23 globular cluster (GC) candidates around the relatively isolated dwarf galaxy IC 2574 within the Messier 81 (M81) group, at a distance of 3.86 Mpc. We use observations from the HST Advanced Camera for Surveys (ACS) to analyse the imaging in the F814W and F555W broadband filters. Our GC candidates have luminosities ranging from −5.9 ≥ MV ≥ −10.4 and half-light radii of 1.4 ≤ rh ≤ 11.5 pc. We find the total number of GCs (NGC) = 27 ± 5 after applying completeness corrections, which implies a specific frequency of SN = 4.0 ± 0.8, consistent with expectations based on its luminosity. The GC system appears to have a bimodal colour distribution, with 30 per cent of the GC candidates having redder colours. We also find 5 objects with extremely blue colours that could be young star clusters linked to an intense star formation episode that occurred in IC 2574 ∼1 Gyr ago. We make an independent measurement of the halo mass of IC 2574 from its kinematic data, which is rare for low mass galaxies, and find log M200 = 10.93 ± 0.08. We place the galaxy on the well-known GC system mass-halo mass relation and find that it agrees well with the observed near-linear relation. IC 2574 has a rich GC population for a dwarf galaxy, which includes an unusually bright ω Cen-like GC, making it an exciting nearby laboratory for probing the peculiar efficiency of forming massive GCs in dwarf galaxies.
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Massive black holes in nuclear star clusters: Investigation with SRG/eROSITA X-ray data
Article
Full-text available
  • Jan 2024
Context . Massive black holes (MBHs) are typically hosted in the centres of massive galaxies but they appear to become rarer in lower mass galaxies, where nuclear star clusters (NSCs) frequently appear instead. The transition region, where both an MBH and NSC can co-exist, has been poorly studied to date and only a few dozen galaxies are known to host them. One avenue for detecting new galaxies with both an MBH and NSC is to look for accretion signatures of MBHs. Aims . Here, we use new SRG/eROSITA all-sky survey eRASS:4 data to search for X-ray signatures of accreting MBHs in NSCs, while also investigating their combined occupation fraction. Methods . We collected more than 200 galaxies containing an NSC, spanning multiple orders in terms of galaxy stellar mass and morphological type, within the footprint of the German eROSITA Consortium survey. We determined the expected X-ray contamination from binary stellar systems using the galaxy stellar mass and star formation rate as estimated from far-ultraviolet and mid-infrared emission. Results . We find significant detections for 18 galaxies (~8.3%), including one ultra-luminous X-ray source; however, only three galaxies (NGC 2903, 4212, and 4639) have X-ray luminosities that are higher than the expected value from X-ray binaries, indicative of the presence of an MBH. In addition, the X-ray luminosity of six galaxies (NGC 2903, 3384, 4321, 4365, 4639, and 4701) differs from previous studies and could indicate the presence of a variable active galactic nucleus. For NGC 4701 specifically, we find a variation of X-ray flux within the eRASS:4 data set. Stacking X-ray non-detected galaxies in the dwarf regime M * gal ≤ 10 ⁹ M ⊙ ) results in luminosity upper limits of a few times 10 ³⁸ erg s ⁻¹ . The combined occupation fraction of accreting MBHs and NSCs becomes non-zero for galaxy masses above ~ 10 7.5 M ⊙ and this result is slightly elevated as compared to the literature data. Conclusions . Our data extend, for the first time, towards the dwarf elliptical galaxy regime and identify promising MBH candidates for higher resolution follow-up observations. At most galaxy masses (and with the exception of three cases), the X-ray constraints are consistent with the expected emission from binary systems or an Eddington fraction of at most 0.01%, assuming a black holes mass of 10 6.5 M ⊙ . This work confirms the known complexities in similar-type of studies, while providing the appealing alternative of using X-ray survey data of in-depth observations of individual targets with higher resolution instruments.
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A Search for Faint Resolved Galaxies Beyond the Milky Way in DES Year 6: A New Faint, Diffuse Dwarf Satellite of NGC 55
Article
Full-text available
  • Jan 2024
We report results from a systematic wide-area search for faint dwarf galaxies at heliocentric distances from 0.3 to 2 Mpc using the full 6 yr of data from the Dark Energy Survey (DES). Unlike previous searches over the DES data, this search specifically targeted a field population of faint galaxies located beyond the Milky Way virial radius. We derive our detection efficiency for faint, resolved dwarf galaxies in the Local Volume with a set of synthetic galaxies and expect our search to be complete to M V ∼ (−7, −10) mag for galaxies at D = (0.3, 2.0) Mpc. We find no new field dwarfs in the DES footprint, but we report the discovery of one high-significance candidate dwarf galaxy at a distance of 2.2 − 0.12 + 0.05 Mpc , a potential satellite of the Local Volume galaxy NGC 55, separated by 47′ (physical separation as small as 30 kpc). We estimate this dwarf galaxy to have an absolute V -band magnitude of − 8.0 − 0.3 + 0.5 mag and an azimuthally averaged physical half-light radius of 2.2 − 0.4 + 0.5 kpc , making this one of the lowest surface brightness galaxies ever found with μ = 32.3 mag arcsec − 2 . This is the largest, most diffuse galaxy known at this luminosity, suggesting possible tidal interactions with its host.
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Metallicity Distribution Functions of 13 Ultra-faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging
Article
Full-text available
  • Nov 2023
We present uniformly measured stellar metallicities of 463 stars in 13 Milky Way (MW) ultra-faint dwarf galaxies (UFDs; M V = −7.1 to −0.8) using narrowband CaHK (F395N) imaging taken with the Hubble Space Telescope. This represents the largest homogeneous set of stellar metallicities in UFDs, increasing the number of metallicities in these 13 galaxies by a factor of 5 and doubling the number of metallicities in all known MW UFDs. We provide the first well-populated MDFs for all galaxies in this sample, with 〈[Fe/H]〉 ranging from −3.0 to −2.0 dex, and σ [Fe/H] ranging from 0.3–0.7 dex. We find a nearly constant [Fe/H]∼ −2.6 over 3 decades in luminosity (∼10 ² –10 ⁵ L ⊙ ), suggesting that the mass–metallicity relationship does not hold for such faint systems. We find a larger fraction (24%) of extremely metal-poor ([Fe/H]< −3) stars across our sample compared to the literature (14%), but note that uncertainties in our most metal-poor measurements make this an upper limit. We find 19% of stars in our UFD sample to be metal-rich ([Fe/H] > −2), consistent with the sum of literature spectroscopic studies. MW UFDs are known to be predominantly >13 Gyr old, meaning that all stars in our sample are truly ancient, unlike metal-poor stars in the MW, which have a range of possible ages. Our UFD metallicities are not well matched to known streams in the MW, providing further evidence that known MW substructures are not related to UFDs. We include a catalog of our stars to encourage community follow-up studies, including priority targets for ELT-era observations.
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EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies
Article
Full-text available
  • Nov 2023
  • MON NOT R ASTRON SOC
Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the ‘Engineering Dwarfs at Galaxy Formation’s Edge’ (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities (0.03 < ϵ < 0.85), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of an UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with a sample of 19 Local Group dwarf galaxies and a sample of 11 isolated dwarf galaxies. Given ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models.
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A comprehensive model for the formation and evolution of the faintest Milky Way dwarf satellites
Preprint
Full-text available
  • Aug 2023
In this study, we modify the semi-analytic model Galacticus in order to accurately reproduce the observed properties of dwarf galaxies in the Milky Way. We find that reproducing observational determinations of the halo occupation fraction and mass-metallicity relation for dwarf galaxies requires us to include H2_2 cooling, an updated UV background radiation model, and to introduce a model for the metal content of the intergalactic medium. By fine-tuning various model parameters and incorporating empirical constraints, we have tailored the model to match the statistical properties of Milky Way dwarf galaxies, such as their luminosity function and size−-mass relation. We have validated our modified semi-analytic framework by undertaking a comparative analysis of the resulting galaxy-halo connection. We predict a total of 300−99+75300 ^{+75} _{-99} satellites with an absolute V-band magnitude (MV_{V}) less than 0 within 300 kpc from our Milky Way-analogs. The fraction of subhalos that host a galaxy at least this bright drops to 50%50\% by a halo peak mass of ∼8.9×107\sim 8.9 \times 10^{7} M⊙_{\odot}, consistent with the occupation fraction inferred from the latest observations of Milky Way satellite population.
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DELVE 6: An Ancient, Ultra-faint Star Cluster on the Outskirts of the Magellanic Clouds
Article
Full-text available
  • Aug 2023
We present the discovery of DELVE 6, an ultra-faint stellar system identified in the second data release of the DECam Local Volume Exploration (DELVE) survey. Based on a maximum-likelihood fit to its structure and stellar population, we find that DELVE 6 is an old ( τ > 9.8 Gyr at 95% confidence) and metal-poor ([Fe/H] < −1.17 dex at 95% confidence) stellar system with an absolute magnitude of M V = − 1.5 − 0.6 + 0.4 mag and an azimuthally averaged half-light radius of r 1 / 2 = 10 − 3 + 4 pc. These properties are consistent with the population of ultra-faint star clusters uncovered by recent surveys. Interestingly, DELVE 6 is located at an angular separation of ∼10° from the center of the Small Magellanic Cloud (SMC), corresponding to a 3D physical separation of ∼20 kpc given the system’s observed distance ( D ⊙ = 80 kpc). This also places the system ∼35 kpc from the center of the Large Magellanic Cloud (LMC), lying within recent constraints on the size of the LMC’s dark matter halo. We tentatively measure the proper motion of DELVE 6 using data from Gaia, which we find supports a potential association between the system and the LMC/SMC. Although future kinematic measurements will be necessary to determine its origins, we highlight that DELVE 6 may represent only the second or third ancient ( τ > 9 Gyr) star cluster associated with the SMC, or one of fewer than two dozen ancient clusters associated with the LMC. Nonetheless, we cannot currently rule out the possibility that the system is a distant Milky Way halo star cluster.
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EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies
Preprint
Full-text available
  • Jul 2023
Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities (0.03<ε<0.850.03 < \varepsilon < 0.85), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of an UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with that of 21 Local Group dwarf galaxies. Given the ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a similar non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models.
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Spectroscopic Analysis of Milky Way Outer Halo Satellites: Aquarius II and Boötes II
Article
Full-text available
  • Jun 2023
In this paper, we present a chemical and kinematic analysis of two ultrafaint dwarf galaxies (UFDs), Aquarius II (Aqu II) and Boötes II (Boo II), using Magellan/IMACS spectroscopy. We present the largest sample of member stars for Boo II (12), and the largest sample of red giant branch members with metallicity measurements for Aqu II (eight). In both UFDs, over 80% of targets selected based on Gaia proper motions turned out to be spectroscopic members. In order to maximize the accuracy of stellar kinematic measurements, we remove the identified binary stars and RR Lyrae variables. For Aqu II, we measure a systemic velocity of −65.3 ± 1.8 km s ⁻¹ and a metallicity of [Fe/H] = − 2.57 − 0.17 + 0.17 . When compared with previous measurements, these values display a ∼6 km s ⁻¹ difference in radial velocity and a decrease of 0.27 dex in metallicity. Similarly for Boo II, we measure a systemic velocity of − 130.4 − 1.1 + 1.4 km s ⁻¹ , more than 10 km s ⁻¹ different from the literature, a metallicity almost 1 dex smaller at [Fe/H] = − 2.71 − 0.10 + 0.11 , and a velocity dispersion 3 times smaller at σ v hel = 2.9 − 1.2 + 1.6 km s ⁻¹ . Additionally, we derive systemic proper-motion parameters and model the orbits of both UFDs. Finally, we highlight the extremely dark-matter-dominated nature of Aqu II and compute the J-factor for both galaxies to aid searches of dark matter annihilation. Despite the small size and close proximity of Boo II, it is an intermediate target for the indirect detection of dark matter annihilation due to its low-velocity dispersion and corresponding low dark matter density.
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Metallicity Distribution Functions of 13 Ultra-Faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging
Preprint
Full-text available
  • Jun 2023
We present uniformly measured stellar metallicities for 463 stars in 13 Milky Way (MW) ultra-faint dwarf galaxies (UFDs; MV=−7.1M_V = -7.1 to −0.8-0.8) using narrowband CaHK (F395N) imaging taken with the Hubble Space Telescope (HST). This represents the largest homogeneous set of stellar metallicities in UFDs, increasing the number of metallicities in these 13 galaxies by a factor of 5 and doubling the number of metallicities in all known MW UFDs. We provide the first well-populated MDFs for all galaxies in this sample, with [Fe/H] ranging from -3.0 dex to -2.0 dex, and \sigma_{\mbox{[Fe/H]}} ranging from 0.3 dex to 0.7 dex. We find a nearly constant [Fe/H] ~ -2.6 over 3 decades in luminosity (~102−105L⊙10^2 - 10^5 L_{\odot}), suggesting that the mass-metallicity relationship does not hold for such faint systems. We find a larger fraction (24%) of EMP ([Fe/H]<-3.0) stars across our sample compared to the literature (14%), but note that uncertainties in our most metal-poor measurements make this an upper limit. We find 19% of stars in our UFD sample to be metal-rich ([Fe/H]>-2.0), consistent with the sum of literature spectroscopic studies. MW UFDs are known to be predominantly >13 Gyr old, meaning that all stars in our sample are truly ancient, unlike metal-poor stars in the MW, which have a range of possible ages. Our UFD metallicities are not well-matched to known streams in the MW, providing further evidence that known MW substructures are not related to UFDs. We include a catalog of our stars to encourage community follow-up studies, including priority targets for ELT-era observations.
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Multifield ultralight dark matter
Article
  • Apr 2023
Ultralight dark matter (ULDM) is usually taken to be a single scalar field. Here we explore the possibility that ULDM consists of N light scalar fields with only gravitational interactions. This configuration is more consistent with the underlying particle physics motivations for these scenarios than a single ultralight field. ULDM halos have a characteristic granular structure that increases stellar velocity dispersion and can be used as observational constraints on ULDM models. In multifield simulations, we find that inside a halo the amplitude of the total density fluctuations decreases as 1/N and that the fields do not become significantly correlated over cosmological timescales. Smoother halos heat stellar orbits less efficiently, reducing the velocity dispersion relative to the single field case and thus weakening the observational constraints on the field mass. Analytically, we show that for N equal-mass fields with mass m the ULDM contribution to the stellar velocity dispersion scales as 1/(Nm3). Lighter fields heat the most efficiently and if the smallest mass mL is significantly below the other field masses the dispersion scales as 1/(N2mL3).
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Observations of R-Process Stars in the Milky Way and Dwarf Galaxies
Chapter
  • Nov 2022
This chapter presents an overview of the recent progress on spectroscopic observations of metal-poor stars with r-process element signatures found in the Milky Way’s stellar halo and satellite dwarf galaxies. Major empirical lessons related to the origins of the r-process are discussed, including the universality of the observed r-process pattern and deviations from universality among the light r-process elements and actinides. Different astrophysical sites of the r-process based on theoretical expectations are presented, including common and rare supernovae and neutron star mergers. A major distinguishing factor between r-process sites is their delay time distribution. The best constraints on the detailed r-process pattern come from galactic halo r-process stars, but these cannot provide information on the environment of the stars’ birth gas clouds. Studying r-process enrichment within dwarf galaxies can remedy the situation despite the fact that high-resolution spectroscopic observations of individual stars in these systems are very difficult to obtain. A general overview of dwarf galaxy properties and chemical evolution expectations depending on their mass and star formation duration is provided. The r-process trends depend on the stellar mass and star formation durations of dwarf galaxies in a way that clearly shows that the r-process is rare and prolific and has both prompt and delayed sources. This work complements ongoing theoretical heavy-element nucleosynthesis explorations and experimental measurements of the properties of r-process nuclei, such as with the Facility for Rare Isotope Beams.
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Spectroscopic analysis of Milky Way outer halo satellites: Aquarius II and Bootes II
Preprint
  • Feb 2023
In this paper we present a chemical and kinematic analysis of two ultra-faint dwarf galaxies (UFDs), Aquarius II (Aqu~II) and \text{Bo\"{o}tes II} (Boo~II), using Magellan/IMACS spectroscopy. We present the largest sample of member stars for Boo~II (12), and the largest sample of red-giant-branch members with metallicity measurements for Aqu~II (8). In both UFDs, over 80\% of targets selected based on Gaia proper motions turned out to be spectroscopic members. In order to maximize the accuracy of stellar kinematic measurements, we remove the identified binary stars and RR Lyrae variables. For Aqu~II we measure a systemic velocity of −65.3±1.8-65.3 \pm 1.8 km s−1^{-1} and a metallicity of [Fe/H] = −2.57−0.17+0.17-2.57^{+0.17}_{-0.17}. When compared with previous measurements, these values display a ∼6\sim 6 km s−1^{-1} difference in radial velocity and a decrease of 0.27 dex in metallicity. Similarly for Boo~II, we measure a systemic velocity of −130.4−1.1+1.4-130.4^{+1.4}_{-1.1} km s−1^{-1}, more than 10 km s−1^{-1} different from the literature, a metallicity almost 1 dex smaller at [Fe/H] = −2.71−0.10+0.11-2.71^{+0.11}_{-0.10}, and a velocity dispersion 3 times smaller at σvhel=2.9−1.2+1.6\sigma_{v_{\rm hel}} = 2.9^{+1.6}_{-1.2} km s−1^{-1}. Additionally, we derive systemic proper motion parameters and model the orbits of both UFDs. Finally, we highlight the extremely dark matter dominated nature of Aqu~II and compute the J-factor for both galaxies to aid searches of dark matter annihilation. Despite the small size and close proximity of Boo~II, it is an intermediate target for the indirect detection of dark matter annihilation due to its low velocity dispersion and corresponding low dark matter density.
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The extended halo of Centaurus A: uncovering satellites, streams and substructures
Article
Full-text available
  • Dec 2015
  • ASTROPHYS J
We present the widest-field resolved stellar map to date of the closest (D∼3.8D\sim3.8 Mpc) massive elliptical galaxy NGC 5128 (Centaurus A; Cen A), extending out to a projected galactocentric radius of ∼150\sim150 kpc. The dataset is part of our ongoing Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) utilizing the Magellan/Megacam imager. We resolve a population of old red giant branch stars down to ∼1.5\sim1.5 mag below the tip of the red giant branch, reaching surface brightness limits as low as μV,0∼32\mu_{V,0}\sim32 mag arcsec−2^{-2}. The resulting spatial stellar density map highlights a plethora of previously unknown streams, shells, and satellites, including the first tidally disrupting dwarf around Cen A, which underline its active accretion history. We report 13 previously unknown dwarf satellite candidates, of which 9 are confirmed to be at the distance of Cen A (the remaining 4 are not resolved into stars), with magnitudes in the range MV=−7.2M_V=-7.2 to −13.0-13.0, central surface brightness values of μV,0=25.4−26.9\mu_{V,0}=25.4-26.9 mag arcsec−2^{-2}, and half-light radii of rh=0.22−2.92r_h=0.22-2.92 kpc. These values are in line with Local Group dwarfs but also lie at the faint/diffuse end of their distribution; most of the new dwarfs are fainter than the previously known Cen A satellites. The newly discovered dwarfs and halo substructures are discussed in light of their stellar populations, and they are compared to those discovered by the PAndAS survey of M31.
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Unveiling a Rich System of Faint Dwarf Galaxies in the Next Generation Fornax Survey
Article
Full-text available
  • Oct 2015
We report the discovery of 158 previously undetected dwarf galaxies in the Fornax cluster central regions using a deep coadded u,gu, g and i-band image obtained with the DECam wide-field camera mounted on the 4-meter Blanco telescope at the Cerro Tololo Interamerican Observatory as part of the {\it Next Generation Fornax Survey} (NGFS). The new dwarf galaxies have quasi-exponential light profiles, effective radii 0.1 ⁣< ⁣re ⁣< ⁣2.80.1\!<\!r_e\!<\!2.8 kpc and average effective surface brightness values 22.0 ⁣< ⁣μi ⁣< ⁣28.022.0\!<\!\mu_i\!<\!28.0 mag arcsec−2^{-2}. We confirm the existence of ultra-diffuse galaxies (UDGs) in the Fornax core regions that resemble counterparts recently discovered in the Virgo and Coma galaxy clusters.~We also find extremely low surface brightness NGFS dwarfs, which are several magnitudes fainter than the classical UDGs. The faintest dwarf candidate in our NGFS sample has an absolute magnitude of Mi ⁣= ⁣−8.0M_i\!=\!-8.0\,mag. The nucleation fraction of the NGFS dwarf galaxy sample appears to decrease as a function of their total luminosity, reaching from a nucleation fraction of > ⁣75%>\!75\% at luminosities brighter than Mi ⁣≃ ⁣−15.0M_i\!\simeq\!-15.0 mag to 0%0\% at luminosities fainter than Mi ⁣≃ ⁣−10.0M_i\!\simeq\!-10.0 mag. The two-point correlation function analysis of the NGFS dwarf sample shows an excess on length scales below ∼ ⁣100\sim\!100 kpc, pointing to the clustering of dwarf galaxies in the Fornax cluster core.
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Eight Ultra-faint Galaxy Candidates Discovered in Year Two of the Dark Energy Survey
Article
Full-text available
  • Aug 2015
  • ASTROPHYS J
We report the discovery of eight new ultra-faint dwarf galaxy candidates in the second year of optical imaging data from the Dark Energy Survey (DES). Six of these candidates are detected at high confidence, while two additional lower-confidence candidates are identified in regions of incomplete or non-uniform survey coverage. The new stellar systems are found using three independent automated search techniques, and are identified as statistically significant overdensities of individually resolved stars consistent with the isochrone and luminosity function of an old and metal-poor simple stellar population. The new systems are faint (Mv > -4.7 mag) and span a broad range of physical sizes (17 pc < r1/2r_{1/2} < 181 pc) and heliocentric distances (25 kpc < D < 214 kpc). All of the new systems have central surface brightnesses (\mu > 27.5 mag arcsec2^2) consistent with known ultra-faint dwarf galaxies. Roughly half of the DES candidates are more distant, less luminous, and/or have lower surface brightnesses than previously known Milky Way satellite galaxies, and would have had a low probability of detection if observed by the Sloan Digital Sky Survey. A large fraction of satellite candidates are found in the southern half of the DES footprint in proximity to the Magellanic Clouds. We find that the DES data alone exclude (p < 0.001) a spatially isotropic distribution of Milky Way satellites, and that this distribution can be well, although not uniquely, explained by a model in which several of the observed DES satellites are associated with the Magellanic system. Including the current sample, our model predicts that ~100 ultra-faint galaxies with physical properties comparable to the DES satellites might exist over the full sky and that 20-30% of these would be spatially associated with the Magellanic Clouds.
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Sagittarius II, Draco II and Laevens 3: three new Milky Way satellites discovered in the Pan-STARRS 1 3pi Survey
Article
Full-text available
  • Jul 2015
  • ASTROPHYS J
We present the discovery of three new Milky Way satellites from our search for compact stellar overdensities in the photometric catalog of the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS 1, or PS1) 3pi survey. The first satellite, Laevens 3, is located at a heliocentric distance of d=67+/-3 kpc. With a total magnitude of Mv=-4.4+/-0.3 and a half-light radius rh=7+/-2 pc, its properties resemble those of outer halo globular clusters. The second system, Draco II/Laevens 4 (Dra II), is a closer and fainter satellite (d~20 kpc, Mv =-2.9+/-0.8), whose uncertain size (rh = 19 +8/-6 pc) renders its classification difficult without kinematic information; it could either be a faint and extended globular cluster or a faint and compact dwarf galaxy. The third satellite, Sagittarius II/Laevens 5 (Sgr II), has an ambiguous nature as it is either the most compact dwarf galaxy or the most extended globular cluster in its luminosity range (rh = 37 +9/-8 pc and Mv=-5.2+/-0.4). At a heliocentric distance of 67+/-5 kpc, this satellite lies intriguingly close to the expected location of the trailing arm of the Sagittarius stellar stream behind the Sagittarius dwarf spheroidal galaxy (Sgr dSph). If confirmed through spectroscopic follow up, this connection would locate this part of the trailing arm of the Sagittarius stellar stream that has so far gone undetected. It would further suggest that Sgr II was brought into the Milky Way halo as a satellite of the Sgr dSph.
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On the neutral gas content of nine new Milky Way satellite galaxy candidates
Article
Full-text available
  • Jul 2015
  • MON NOT R ASTRON SOC
We use a new, improved version of the H i Parkes All-Sky Survey to search for H i emission from nine new, ultrafaint Milky Way satellite galaxy candidates recently discovered in data from the Dark Energy Survey. None of the candidates is detected in H i, implying upper limits for their H i masses of typically several hundred to a few thousand solar masses. The resulting upper limits on MHI/LVM_{\rm H\small {I}} / L_{\rm V} and MHI/M⋆M_{\rm H\small {I}} / M_{\star } suggest that at least some of the new galaxy candidates are H i deficient. This finding is consistent with the general H i deficiency of satellite galaxies located within the Milky Way's virial radius and supports the hypothesis that gas is being removed from satellites by tidal and ram-pressure forces during perigalactic passages. In addition, some of the objects may be embedded in, and interacting with, the extended neutral and ionized gas filaments of the Magellanic Stream.
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Hydra II: a faint and compact Milky Way dwarf galaxy found in the Survey of the Magellanic Stellar History
Article
Full-text available
  • Mar 2015
We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the MAgellanic Stellar History (SMASH) conducted with the Dark Energy Camera on the Blanco 4m Telescope. The new satellite is compact (r_h = 68 +/- 11 pc) and faint (M_V = -4.8 +/- 0.3), but well within the realm of dwarf galaxies. The stellar distribution of HydraII in the color-magnitude diagram is well-described by a metal-poor ([Fe/H] = -2.2) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 +/- 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds.
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A new faint milky way satellite discovered in the Pan-STARRS1 3π Survey
Article
Full-text available
  • Mar 2015
We present the discovery of a faint Milky Way satellite, Laevens 2/Triangulum II, found in the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS 1) 3 pi imaging data and confirmed with follow-up wide-field photometry from the Large Binocular Cameras. The stellar system, with an absolute magnitude of M_V=-1.8 +/-0.5, a heliocentric distance of 30 +2/-2 kpc, and a half-mass radius of 34 +9/-8 pc, shows remarkable similarity to faint, nearby, small satellites such as Willman 1, Segue 1, Segue 2, and Bo\"otes II. The discovery of Laevens 2/Triangulum II further populates the region of parameter space for which the boundary between dwarf galaxies and globular clusters becomes tenuous. Follow-up spectroscopy will ultimately determine the nature of this new satellite, whose spatial location hints at a possible connection with the complex Triangulum-Andromeda stellar structures.
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Beasts of the Southern Wild: Discovery of nine Ultra Faint satellites in the vicinity of the Magellanic Clouds
Article
Full-text available
  • Mar 2015
  • ASTROPHYS J
We have used the publicly released Dark Energy Survey data to hunt for new satellites of the Milky Way in the Southern hemisphere. Our search yielded a large number of promising candidates. In this paper, we announce the discovery of 9 new unambiguous ultra-faint objects, whose authenticity can be established with the DES data alone. Based on the morphological properties, three of the new satellites are dwarf galaxies, one of which is located at the very outskirts of the Milky Way, at a distance of 380 kpc. The remaining 6 objects have sizes and luminosities comparable to the Segue 1 satellite and can not be classified straightforwardly without follow-up spectroscopic observations. The satellites we have discovered cluster around the LMC and the SMC. We show that such spatial distribution is unlikely under the assumption of isotropy, and, therefore, conclude that at least some of the new satellites must have been associated with the Magellanic Clouds in the past.
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Eight New Milky Way Companions Discovered in First-Year Dark Energy Survey Data
Article
Full-text available
  • Mar 2015
  • ASTROPHYS J
We report the discovery of eight new Milky Way companions in ~1,800 deg^2 of optical imaging data collected during the first year of the Dark Energy Survey (DES). Each system is identified as a statistically significant over-density of individual stars consistent with the expected isochrone and luminosity function of an old and metal-poor stellar population. The objects span a wide range of absolute magnitudes (M_V from -2.2 mag to -7.4 mag), physical sizes (10 pc to 170 pc), and heliocentric distances (30 kpc to 330 kpc). Based on the low surface brightnesses, large physical sizes, and/or large Galactocentric distances of these objects, several are likely to be new ultra-faint satellite galaxies of the Milky Way and/or Magellanic Clouds. We introduce a likelihood-based algorithm to search for and characterize stellar over-densities, as well as identify stars with high satellite membership probabilities. We also present completeness estimates for detecting ultra-faint galaxies of varying luminosities, sizes, and heliocentric distances in the first-year DES data.
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The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
Article
Full-text available
  • Oct 2014
  • ASTROPHYS J
We present new constraints on the star formation histories of six ultra-faint dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I. Our analysis employs a combination of high-precision photometry obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, medium-resolution spectroscopy obtained with the DEep Imaging Multi-Object Spectrograph on the W.M. Keck Observatory, and updated Victoria-Regina isochrones tailored to the abundance patterns appropriate for these galaxies. The data for five of these Milky Way satellites are best fit by a star formation history where at least 75% of the stars formed by z~10 (13.3 Gyr ago). All of the galaxies are consistent with 80% of the stars forming by z~6 (12.8 Gyr ago) and 100% of the stars forming by z~3 (11.6 Gyr ago). The similarly ancient populations of these galaxies support the hypothesis that star formation in the smallest dark matter sub-halos was suppressed by a global outside influence, such as the reionization of the universe.
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The Dearth of Neutral Hydrogen in Galactic Dwarf Spheroidal Galaxies
Article
Full-text available
  • Sep 2014
We present new upper limits on the neutral hydrogen (HI) content within the stellar half-light ellipses of 15 Galactic dwarf spheroidal galaxies (dSphs), derived from pointed observations with the Green Bank Telescope (GBT) as well as Arecibo L-band Fast ALFA (ALFALFA) survey and Galactic All-Sky Survey (GASS) data. All of the limits Mlim are more stringent than previously reported values, and those from the GBT improve upon contraints in the literature by a median factor of 23. Normalizing by V-band luminosity Lv and dynamical mass Mdyn, we find Mlim/Lv ~ 10^{-3} Mo/Lo and Mlim/Mdyn ~ 5 x 10^{-5}, irrespective of location in the Galactic halo. Comparing these relative HI contents to those of the Local Group and nearby neighbor dwarfs compiled by McConnachie, we find that the Galactic dSphs are extremely gas-poor. Our HI upper limits therefore provide the clearest picture yet of the environmental dependence of the HI content in Local Volume dwarfs. If ram pressure stripping explains the dearth of HI in these systems, then orbits in a relatively massive Milky Way are favored for the outer halo dSph Leo I, while Leo II and Canes Venatici I have had a pericentric passage in the past. For Draco and Ursa Minor, the interstellar medium mass that should accumulate through stellar mass loss in between pericentric passages exceeds Mlim by a factor of ~30. In Ursa Minor, this implies that either this material is not in the atomic phase, or that another mechanism clears the recycled gas on shorter timescales.
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A PAndAS view of M31 dwarf elliptical satellites: NGC147 and NGC185
Article
Full-text available
  • Sep 2014
  • MON NOT R ASTRON SOC
We exploit data from the Pan-Andromeda Archaeological Survey (PAndAS) to study the extended structures of M31's dwarf elliptical companions, NGC 147 and NGC 185. Our wide-field, homogeneous photometry allows us to construct deep colour–magnitude diagrams which reach down to ∼3 mag below the red giant branch (RGB) tip. We trace the stellar components of the galaxies to surface brightness of μg ∼ 32 mag arcsec−2 and show that they have much larger extents (∼5 kpc radii) than previously recognized. While NGC 185 retains a regular shape in its peripheral regions, NGC 147 exhibits pronounced isophotal twisting due to the emergence of symmetric tidal tails. We fit single Sérsic models to composite surface brightness profiles constructed from diffuse light and star counts and find that NGC 147 has an effective radius almost three times that of NGC 185. In both cases, the effective radii that we calculate are larger by a factor of ∼2 compared to most literature values. We also calculate revised total magnitudes of Mg = −15.36 ± 0.04 for NGC 185 and Mg = −16.36 ± 0.04 for NGC 147. Using photometric metallicities computed for RGB stars, we find NGC 185 to exhibit a metallicity gradient of [Fe/H] ∼ −0.15 dex kpc−1 over the radial range 0.125–0.5 deg. On the other hand, NGC 147 exhibits almost no metallicity gradient, ∼−0.02 dex kpc−1 from 0.2 to 0.6 deg. The differences in the structure and stellar populations in the outskirts of these systems suggest that tidal influences have played an important role in governing the evolution of NGC 147.
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The HST/ACS Coma Cluster Survey - X. Nuclear star clusters in low-mass early-type galaxies: Scaling relations
Article
Full-text available
  • Sep 2014
  • MON NOT R ASTRON SOC
We present scaling relations between structural properties of nuclear star clusters and their host galaxies for a sample of early-type dwarf galaxies observed as part of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Coma Cluster Survey. We have analysed the light profiles of 200 early-type dwarf galaxies in the magnitude range 16.0<mF814W<22.616.0 < m_{F814W} < 22.6 mag, corresponding to −19.0<MF814W<−12.4-19.0 < M_{F814W} < -12.4 mag. Nuclear star clusters are detected in 80% of the galaxies, thus doubling the sample of HST-observed early-type dwarf galaxies with nuclear star clusters. \changed{We confirm that the} nuclear star cluster detection fraction decreases strongly toward faint magnitudes. The luminosities of nuclear star clusters do not scale linearly with host galaxy luminosity. A linear fit yields Lnuc∼_{nuc} \sim Lgal0.57±0.05_{gal}^{0.57\pm0.05}. The nuclear star cluster-host galaxy luminosity scaling relation for low-mass early-type dwarf galaxies is consistent with formation by globular cluster accretion. We find that at similar luminosities, galaxies with higher S\'ersic indices have slightly more luminous nuclear star clusters. Rounder galaxies have on average more luminous clusters. Some of the nuclear star clusters are resolved, despite the distance of Coma. We argue that the relation between nuclear star cluster mass and size is consistent with both formation by globular cluster accretion and in situ formation. Our data are consistent with GC inspiraling being the dominant mechanism at low masses, although the observed trend with S\'ersic index suggests that in situ star formation is an important second order effect.
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Galactic Globular and Open Cluster Fiducial Sequences in the Pan-STARRS1 Photometric System
Article
Full-text available
  • May 2014
  • MON NOT R ASTRON SOC
We present the fiducial sequences of a sample of Galactic star clusters in the five bands of the Pan-STARRS1 (PS1) photometric system (gP1, rP1, iP1, zP1, and yP1). These empirical sequences – which include the red giant and sub-giant branches, the main sequence, and the horizontal branch – were defined from deep colour–magnitude diagrams reaching below the oldest main-sequence turn-offs of 13 globular and 3 old open clusters covering a wide range of metallicities ( − 2.4 ≲ [Fe/H] ≲ +0.4). We find excellent agreement for the nine clusters in common with previous studies in similar photometric systems when transformed to the PS1 system. Because the photometric and spectroscopic properties of these stellar populations are accurately known, the fiducials provide a solid basis for the interpretation of observations in the PS1 system, as well as valuable constraints to improve the empirical colour–Teff relations.
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ATLAS lifts the Cup: Discovery of a New Milky Way satellite in Crater
Article
Full-text available
  • Mar 2014
  • MON NOT R ASTRON SOC
We announce the discovery of a new Galactic companion found in data from the ESO VST ATLAS survey, and followed up with deep imaging on the 4-m William Herschel Telescope. The satellite is located in the constellation of Crater (the Cup) at a distance of ∼170 kpc. Its half-light radius is rh = 30 pc and its luminosity is MV = −5.5. The bulk of its stellar population is old and metal poor. We would probably have classified the newly discovered satellite as an extended globular cluster were it not for the presence of a handful of blue loop stars and a sparsely populated red clump. The existence of the core helium burning population implies that star formation occurred in Crater perhaps as recently as 400 Myr ago. No globular cluster has ever accomplished the feat of prolonging its star formation by several Gyr. Therefore, if our hypothesis that the blue bright stars in Crater are blue loop giants is correct, the new satellite should be classified as a dwarf galaxy with unusual properties. Note that only 10°to the north of Crater, two ultrafaint galaxies Leo IV and Leo V orbit the Galaxy at approximately the same distance. This hints that all three satellites may once have been closely associated before falling together into the Milky Way halo.
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H I in Local Group Dwarf Galaxies and Stripping by the Galactic Halo
Article
Full-text available
  • Jan 2009
We examine the H I content and environment of all of the Local Group dwarf galaxies (Mtot < 1010 M⊙), including the numerous newly discovered satellites of the Milky Way and M31. All of the new dwarfs, with the exception of Leo T, have no detected H I. The majority of dwarf galaxies within ∼270 kpc of the Milky Way or Andromeda are undetected in H I (< 104 M⊙ for Milky Way dwarfs), while those further than ∼270 kpc are predominantly detected with masses ∼105 to 108 M⊙. Analytical ram-pressure arguments combined with velocities obtained via proper motion studies allow for an estimate of the halo density of the Milky Way at several distances. This halo density is constrained to be greater than 2× 10 -4-3 × 10-4 cm-3 out to distances of at least 70 kpc. This is broadly consistent with theoretical models of the diffuse gas in a Milky Way-like halo and is consistent with this component hosting a large fraction of a galaxy's baryons. Accounting for completeness in the dwarf galaxy count, gasless dwarf galaxies could have provided at most 2.1 × 108 M⊙ of H I gas to the Milky Way, which suggests that most of our Galaxy's star formation fuel does not come from accreted small satellites in the current era.
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Evidence for an Accretion Origin for the Outer Halo Globular Cluster System of M31
Article
Full-text available
  • Jun 2010
We use a sample of newly discovered globular clusters from the Pan-Andromeda Archaeological Survey (PAndAS) in combination with previously cataloged objects to map the spatial distribution of globular clusters in the M31 halo. At projected radii beyond 30 kpc, where large coherent stellar streams are readily distinguished in the field, there is a striking correlation between these features and the positions of the globular clusters. Adopting a simple Monte Carlo approach, we test the significance of this association by computing the probability that it could be due to the chance alignment of globular clusters smoothly distributed in the M31 halo. We find that the likelihood of this possibility is low, below 1%, and conclude that the observed spatial coincidence between globular clusters and multiple tidal debris streams in the outer halo of M31 reflects a genuine physical association. Our results imply that the majority of the remote globular cluster system of M31 has been assembled as a consequence of the accretion of cluster-bearing satellite galaxies. This constitutes the most direct evidence to date that the outer halo globular cluster populations in some galaxies are largely accreted.
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A Deeper Look at Leo IV: Star Formation History and Extended Structure
Article
Full-text available
  • Jul 2010
We present MMT/Megacam imaging of the Leo IV dwarf galaxy in order to investigate its structure and star formation history, and to search for signs of association with the recently discovered Leo V satellite. Based on parameterized fits, we find that Leo IV is round, with < 0.23 (at the 68% confidence limit) and a half-light radius of rh 130 pc. Additionally, we perform a thorough search for extended structures in the plane of the sky and along the line of sight. We derive our surface brightness detection limit by implanting fake structures into our catalog with stellar populations identical to that of Leo IV. We show that we are sensitive to stream-like structures with surface brightness μ r 29.6 mag arcsec–2, and at this limit we find no stellar bridge between Leo IV (out to a radius of ~0.5 kpc) and the recently discovered, nearby satellite Leo V. Using the color-magnitude fitting package StarFISH, we determine that Leo IV is consistent with a single age (~14 Gyr), single metallicity ([Fe/H] ~ –2.3) stellar population, although we cannot rule out a significant spread in these values. We derive a luminosity of MV = –5.5 ± 0.3. Studying both the spatial distribution and frequency of Leo IV's "blue plume" stars reveals evidence for a young (~2 Gyr) stellar population which makes up ~2% of its stellar mass. This sprinkling of star formation, only detectable in this deep study, highlights the need for further imaging of the new Milky Way satellites along with theoretical work on the expected, detailed properties of these possible "reionization fossils."
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The Structural Properties and Star Formation History of Leo T from Deep LBT Photometry
Article
Full-text available
  • Dec 2008
We present deep, wide-field g and r photometry of the transition-type dwarf galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The data confirm the presence of both very young (<1 Gyr) and much older (>5 Gyr) stars. We study the structural properties of the old and young stellar populations by preferentially selecting either population on the basis of their color and magnitude. The young population is significantly more concentrated than the old population, with half-light radii of 104 ± 8 and 148 ± 16 pc, respectively, and their centers are slightly offset. Approximately 10% of the total stellar mass is estimated to be represented by the young stellar population. Comparison of the color-magnitude diagram (CMD) with theoretical isochrones, as well as numerical CMD fitting, suggests that star formation began over 10 Gyr ago and continued in recent times until at least a few hundred Myr ago. The CMD-fitting results are indicative of two distinct star formation bursts, with a quiescent period around 3 Gyr ago, albeit at low significance. The results are consistent with no metallicity evolution and a value of [ Fe/H ] ~ − 1.5 over the entire age of the system. Finally, the data show little, if any, sign of tidal distortion of Leo T.
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The pan-starrs1 photometric system
Article
Full-text available
  • Feb 2012
The Pan-STARRS1 survey is collecting multi-epoch, multi-color observations of the sky north of declination –30° to unprecedented depths. These data are being photometrically and astrometrically calibrated and will serve as a reference for many other purposes. In this paper, we present our determination of the Pan-STARRS1 photometric system: g P1, r P1, i P1, z P1, y P1, and w P1. The Pan-STARRS1 photometric system is fundamentally based on the Hubble Space Telescope Calspec spectrophotometric observations, which in turn are fundamentally based on models of white dwarf atmospheres. We define the Pan-STARRS1 magnitude system and describe in detail our measurement of the system passbands, including both the instrumental sensitivity and atmospheric transmission functions. By-products, including transformations to other photometric systems, Galactic extinction, and stellar locus, are also provided. We close with a discussion of remaining systematic errors.
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The Star Formation History of Leo T from Hubble Space Telescope Imaging
Article
Full-text available
  • Jan 2012
We present the star formation history (SFH) of the faintest known star-forming galaxy, Leo T, based on imaging taken with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2). The HST/WFPC2 color-magnitude diagram (CMD) of Leo T is exquisitely deep, extending ~ 2 magnitudes below the oldest main sequence turnoff, permitting excellent constraints on star formation at all ages. We use a maximum likelihood CMD fitting technique to measure the SFH of Leo T assuming three different sets of stellar evolution models: Padova (solar-scaled metallicity) and BaSTI (both solar-scaled and alpha-enhanced metallicities). The resulting SFHs are remarkably consistent at all ages, indicating that our derived SFH is robust to the choice of stellar evolution model. From the lifetime SFH of Leo T, we find that 50% of the total stellar mass formed prior to z ~ 1 (7.6 Gyr ago). Subsequent to this epoch, the SFH of Leo T is roughly constant until the most recent ~ 25 Myr, where the SFH shows an abrupt drop. This decrease could be due to a cessation of star formation or stellar initial mass function sampling effects, but we are unable to distinguish between the two scenarios. Overall, our measured SFH is consistent with previously derived SFHs of Leo T. However, the HST-based solution provides improved age resolution and reduced uncertainties at all epochs. The SFH, baryonic gas fraction, and location of Leo T are unlike any of the other recently discovered faint dwarf galaxies in the Local Group, and instead bear strong resemblance to gas-rich dwarf galaxies (irregular or transition), suggesting that gas-rich dwarf galaxies may share common modes of star formation over a large range of stellar mass (~ 10^5-10^9 Msun).
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Tidal Signatures in the Faintest Milky Way Satellites: The Detailed Properties of Leo V, Pisces II and Canes Venatici II
Article
Full-text available
  • Nov 2011
We present deep wide-field photometry of three recently discovered faint Milky Way satellites: Leo V, Pisces II, and Canes Venatici II. Our main goals are to study the structure and star formation history of these dwarfs; we also search for signs of tidal disturbance. The three satellites have similar half-light radii (∼60−90\sim 60-90 pc) but a wide range of ellipticities. Both Leo V and CVn II show hints of stream-like overdensities at large radii. An analysis of the satellite color-magnitude diagrams shows that all three objects are old (>> 10 Gyr) and metal-poor ([Fe/H] ∼−2\sim -2), though neither the models nor the data have sufficient precision to assess when the satellites formed with respect to cosmic reionization. The lack of an observed younger stellar population (\la 10 Gyr) possibly sets them apart from the other satellites at Galactocentric distances \ga 150 kpc. We present a new compilation of structural data for all Milky Way satellite galaxies and use it to compare the properties of classical dwarfs to the ultra-faints. The ellipticity distribution of the two groups is consistent at the ∼\sim2-σ\sigma level. However, the faintest satellites tend to be more aligned toward the Galactic center, and those satellites with the highest ellipticity (\ga 0.4) have orientations (ΔθGC\Delta \theta_{GC}) in the range 20∘≲ΔθGC≲40∘20^{\circ} \lesssim \Delta \theta_{GC} \lesssim 40^{\circ}. This latter observation is in rough agreement with predictions from simulations of dwarf galaxies that have lost a significant fraction of their dark matter halos and are being tidally stripped.
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On the Absolute Age of the Globular Cluster M921
Article
Full-text available
  • Jun 2010
We present precise and deep optical photometry of the globular M92. Data were collected in three different photometric systems: Sloan Digital Sky Survey (g',r',i',z'; MegaCam@CFHT), Johnson-Kron-Cousins (B, V, I; various ground-based telescopes) and Advanced Camera for Surveys (ACS) Vegamag (F475W, F555W, F814W; Hubble Space Telescope). Special attention was given to the photometric calibration, and the precision of the ground-based data is generally better than 0.01 mag. We computed a new set of {\alpha}-enhanced evolutionary models accounting for the gravitational settling of heavy elements at fixed chemical composition ([{\alpha}/Fe]=+0.3, [Fe/H]=-2.32 dex, Y=0.248). The isochrones -- assuming the same true distance modulus ({\mu}=14.74 mag), the same reddening (E(B-V)=0.025+-0.010 mag), and the same reddening law -- account for the stellar distribution along the main sequence and the red giant branch in different Color-Magnitude Diagrams (i',g'-i' ; i',g'-r' ; i',g'-z' ; I,B-I ; F814W,F475W-F814W). The same outcome applies to the comparison between the predicted Zero-Age-Horizontal-Branch (ZAHB) and the HB stars. We also found a cluster age of 11 +/- 1.5 Gyr, in good agreement with previous estimates. The error budget accounts for uncertainties in the input physics and the photometry. To test the possible occurrence of CNO-enhanced stars, we also computed two sets of {\alpha}- and CNO-enhanced (by a factor of three) models both at fixed total metallicity ([M/H]=-2.10 dex) and at fixed iron abundance. We found that the isochrones based on the former set give the same cluster age (11 +/- 1.5 Gyr) as the canonical {\alpha}-enhanced isochrones. The isochrones based on the latter set also give a similar cluster age (10 +/- 1.5 Gyr). These indings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures.
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Scl-dE1 GC1: An Extended Globular Cluster in a Low-Luminosity Dwarf Elliptical Galaxy
Article
Full-text available
  • Mar 2009
  • ASTRON J
We report the discovery from Hubble Space Telescope ACS images of an extended globular cluster, denoted by Scl-dE1 GC1, in the Sculptor Group dwarf Elliptical galaxy Scl-dE1 (Sc22). The distance of the dE is determined as 4.3 +/- 0.25 Mpc from the I magnitude of the tip of the red giant branch in the color-magnitude diagram. At this distance the half-light radius of Scl-dE1 GC1 is ~22 pc, placing it among the largest clusters known, particularly for globular clusters associated with dwarf galaxies. The absolute magnitude of Scl-dE1 GC1 is Mv = -6.7 and, to within the photometric uncertainties of the data, the cluster stellar population appears indistinguishable from that of the dE. We suggest that there may be two modes of globular cluster formation in dwarf galaxies, a "normal" mode with half-light radii of typically 3 pc, and an "extended" mode with half-light radii of ~10 pc or more. Comment: 18 pages, 5 figures, 1 Table. Accepted for publication in the Astronomical Journal
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A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites
Article
Full-text available
  • May 2008
We derive the structural parameters of the recently discovered very low luminosity Milky Way satellites through a Maximum Likelihood algorithm applied to SDSS data. For each satellite, even when only a few tens of stars are available down to the SDSS flux limit, the algorithm yields robust estimates and errors for the centroid, position angle, ellipticity, exponential half-light radius and number of member stars. This latter parameter is then used in conjunction with stellar population models of the satellites to derive their absolute magnitudes and stellar masses, accounting for `CMD shot-noise'. We find that faint systems are somewhat more elliptical than initially found and ascribe that to the previous use of smoothed maps which can be dominated by the smoothing kernel. As a result, the faintest half of the Milky Way dwarf galaxies (M_V>-7.5) is significantly (4-sigma) flatter (e=0.47+/-0.03) than its brightest half (M_V<-7.5, e=0.32+/-0.02). From our best models, we also investigate whether the seemingly distorted shape of the satellites, often taken to be a sign of tidal distortion, can be quantified. We find that, except for tentative evidence of distortion in CVnI and UMaII, these can be completely accounted for by Poisson scatter in the sparsely sampled systems. We consider three scenarios that could explain the rather elongated shape of faint satellites: rotation supported systems, stars following the shape of more triaxial dark matter subhalos, or elongation due to tidal interaction with the Milky Way. Although none of these is entirely satisfactory, the last one appears the least problematic, but warrants much deeper observations to track evidence of such tidal interaction.
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The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater
Article
  • Apr 2016
  • MON NOT R ASTRON SOC
We announce the discovery of the Crater 2 dwarf galaxy, identified in imaging data of the VLT Survey Telescope ATLAS survey. Given its half-light radius of ∼1100 pc, Crater 2 is the fourth largest satellite of the Milky Way, surpassed only by the Large Magellanic Cloud, Small Magellanic Cloud and the Sgr dwarf. With a total luminosity of MV ≈ −8, this galaxy is also one of the lowest surface brightness dwarfs. Falling under the nominal detection boundary of 30 mag arcsec−2, it compares in nebulosity to the recently discovered Tuc 2 and Tuc IV and UMa II. Crater 2 is located ∼120 kpc from the Sun and appears to be aligned in 3D with the enigmatic globular cluster Crater, the pair of ultrafaint dwarfs Leo IV and Leo V and the classical dwarf Leo II. We argue that such arrangement is probably not accidental and, in fact, can be viewed as the evidence for the accretion of the Crater-Leo group.
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A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular Cluster Crater
Article
  • Oct 2015
  • ASTROPHYS J
We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging taken with the Hubble Space Telescope. The HST/ACS-based color-magnitude diagram (CMD) of Crater extends ∼\sim4 magnitudes below the oldest main sequence turnoff, providing excellent leverage on Crater's physical properties. Structurally, Crater has a half-light radius of ∼\sim20 pc and shows no evidence for tidal distortions. Crater is well-described by a simple stellar population with an age of ∼\sim7.5 Gyr, [M/H]∼−1.65\sim-1.65, a M⋆∼104_{\star}\sim10^4 M⊙_{\odot}, MV∼−5.3_{\rm V}\sim -5.3, located at a distance of (d⊙_{\odot}, dGC_{\rm GC}) ∼\sim (145, 110) kpc, with modest uncertainties in these properties due to differences in the underlying stellar evolution models. The sparse sampling of stars above the turnoff and sub-giant branch are likely to be 1.0-1.4 M⊙_{\odot} binary star systems (blue stragglers) and their evolved descendants, as opposed to intermediate age main sequence stars. Confusion of these populations highlights a substantial challenge in accurately characterizing sparsely populated stellar systems. Our analysis shows that Crater is not a dwarf galaxy, but instead is an unusually young cluster given its location in the Milky Way's very outer stellar halo. Crater is similar to SMC cluster Lindsay 38, and its position and velocity are in good agreement with observations and models of the Magellanic stream debris, suggesting it may have accreted from the Magellanic Clouds. However, its age and metallicity are also in agreement with the age-metallicity relationships of lower mass dwarf galaxies such as Leo I or Carina. Despite uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently than z∼1z\sim1 (8 Gyr ago), providing an important new constraint on the accretion history of the Milky Way. [abridged]
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Horologium II: a Second Ultra-faint Milky Way Satellite in the Horologium Constellation
Article
  • May 2015
We report the discovery of a new ultra-faint Milky Way satellite candidate, Horologium II, detected in the Dark Energy Survey Y1A1 public data. Horologium II features a half light radius of rh=47±10r_{h}=47\pm10 pc and a total luminosity of MV=−2.6−0.3+0.2M_{V}=-2.6^{+0.2}_{-0.3} that place it in the realm of ultra-faint dwarf galaxies on the size-luminosity plane. The stellar population of the new satellite is consistent with an old (∼13.5\sim13.5 Gyr) and metal-poor ([Fe/H]∼−2.1\sim-2.1) isochrone at a distance modulus of (m−M)=19.46(m-M)=19.46, or a heliocentric distance of 78 kpc, in the color-magnitude diagram. Horologium II has a distance similar to the Sculptor dwarf spheroidal galaxy (79 kpc) and the recently reported ultra-faint satellites Eridanus III (87 kpc) and Horologium I (79 kpc). All four satellites are well aligned on the sky, which suggests a possible common origin. As Sculptor is moving on a retrograde orbit within the Vast Polar Structure when compared to the other classical MW satellite galaxies including the Magellanic Clouds, this hypothesis can be tested once proper motion measurements become available.
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Megacam: A wide-field CCD imager for the MMT and magellan
Article
  • Apr 2015
Megacam is a large-format optical camera that can be operated at the f/5 Cassegrain foci of the MMT on Mount Hopkins, Arizona, and the Magellan Clay telescope at Las Campanas Observatory, Chile. Megacam’s focal plane is composed of 36 closely packed e2v CCD42-90 CCDs, each with 2048 × 4608 pixels, assembled in an 18,432 × 18,432 array. Two additional CCD42-90s are provided for autoguiding and focus control. The CCDs have 13.5 μm square pixels that subtend at the f/5 foci, yielding a 25′ × 25′ field-of-view. The camera system includes a focal plane shutter, two filter wheels, two liquid nitrogen reservoirs, a central chamber that holds the CCD mosaic array, and two electronics boxes. Megacam is equipped with a variety of broadband and narrowband filters. Software features include automatic calculation of twilight flat exposure times.
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