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Using early-release data from the JWST, Mowla et al. and Claeyssens et al. recently measured various properties for gravitationally lensed compact sources (‘sparkles’) around the ‘Sparkler’ galaxy at a redshift of 1.378 (a look-back time of 9.1 Gyr). Here, we focus on the Mowla et al. as they were able to break the age-metallicity degeneracy and derive independent ages, metallicities, and extinctions for each source. They identified five metal-rich, old Globular cluster (GC) candidates (with formation ages up to ∼13 Gyr). We examine the age–metallicity relation (AMR) for the GC candidates and other Sparkler compact sources. The Sparkler galaxy, which has a current estimated stellar mass of 109 M⊙, is compared to the Large Magellanic Cloud (LMC), the disrupted dwarf galaxy Gaia–Enceladus and the Milky Way (MW). The Sparkler galaxy appears to have undergone very rapid chemical enrichment in the first few hundred Myr after formation, with its GC candidates similar to those of the MW’s metal-rich subpopulation. We also compare the Sparkler to theoretical AMRs and formation ages from the E-MOSAICS simulation, finding the early formation age of its GCs to be in some tension with these predictions for MW-like galaxies. The metallicity of the Sparkler’s star-forming regions are more akin to a galaxy of stellar mass ≥ 1010.5 M⊙, that is, at the top end of the expected mass growth over 9.1 Gyr of cosmic time. We conclude that the Sparkler galaxy may represent a progenitor of a MW-like galaxy, even including the ongoing accretion of a satellite galaxy.

We examine ultra-diffuse galaxies (UDGs) and their relation to non-UDGs in mass-radius-luminosity space. We begin by publishing Keck/KCWI spectroscopy for the Coma cluster UDG Y358, for which we measure both a recessional velocity and velocity dispersion. Our recessional velocity confirms association with the Coma cluster and Y358's status as a UDG. From our velocity dispersion (19 $\pm$ 3 km s$^{-1}$) we calculate a dynamical mass within the half-light radius which provides evidence for a core in Y358's dark matter halo. We compare this dynamical mass, along with those for globular cluster (GC)-rich/-poor UDGs in the literature, to mass profiles for isolated, gas-rich UDGs and UDGs in the NIHAO/FIRE simulations. We find GC-poor UDGs have dynamical masses similar to isolated, gas-rich UDGs, suggesting an evolutionary pathway may exist between the two. Conversely, GC-rich UDGs have dynamical masses too massive to be easily explained as the evolution of the isolated, gas-rich UDGs. The simulated UDGs match the dynamical masses of the GC-rich UDGs. However, once compared in stellar mass -- halo mass space, the FIRE/NIHAO simulated UDGs do not match the halo masses of either the isolated, gas-rich UDGs or the GC-rich UDGs at the same stellar mass. Finally, we supplement our data for Y358 with other UDGs that have measured velocity dispersions in the literature. We compare this sample to a wide range of non-UDGs in mass-radius-luminosity space, finding UDGs have a similar locus to non-UDGs of similar luminosity with the primary difference being their larger half-light radii., Comment: 16 pages, 9 figures, accepted for publication in MNRAS

The globular cluster (GC) systems of low-mass late-type galaxies, such as NGC 2403, have been poorly studied to date. As a low mass galaxy (M$_{\ast}$ = 7 $\times$ 10$^{9}$ M$_{\odot}$), cosmological simulations predict NGC 2403 to contain few, if any, accreted GCs. It is also isolated, with a remarkably undisturbed HI disk. Based on candidates from the literature, Sloan Digital Sky Survey and Hyper Suprime-Cam imaging, we selected several GCs for follow-up spectroscopy using the Keck Cosmic Web Imager. From their radial velocities, and other properties, we identify 8 bona-fide GCs associated with either the inner halo or the disk of this bulgeless galaxy. A stellar population analysis suggests a wide range of GC ages from shortly after the Big Bang until the present day. We find all of the old GCs to be metal-poor with [Fe/H] $\le$ --1. The age--metallicity relation for the observed GCs suggests that they were formed over many Gyr from gas with a low effective yield, similar to that observed in the SMC. Outflows of enriched material may have contributed to the low yield. With a total system of $\sim$50 GCs expected, our study is the first step in fully mapping the star cluster history of NGC 2403 in both space and time., 9 pages, 7 figures, accepted for publication in MNRAS

We investigate the isolated, quiescent ultra-diffuse galaxy (UDG) DGSAT I and its globular cluster (GC) system using two orbits of Hubble Space Telescope Advanced Camera for Surveys imaging in the F606W and F814W filters. This is the first study of GCs around a UDG in a low-density environment. DGSAT I was previously found to host an irregular blue low surface brightness clump, which we confirm as very likely belonging to the galaxy rather than being a chance projection, and represents a recent episode of star formation (∼500 Myr) that challenges some UDG formation scenarios. We select GC candidates based on colours and magnitudes, and construct a self-consistent model of the GC radial surface density profile along with the background. We find a half-number radius of RGC = 2.7 ± 0.1 kpc (more compact than the diffuse starlight) and a total of 12 ± 2 GCs. The total mass fraction in GCs is relatively high, supporting an overmassive dark matter halo as also implied by the high velocity dispersion previously measured. The GCs extend to higher luminosities than expected, and have colours that are unusually similar to their host galaxy colour, with a very narrow spread, all of which suggest an early, intense burst of cluster formation. The nature and origin of this galaxy remain puzzling, but the most likely scenario is a ‘failed galaxy’ that formed relatively few stars for its halo mass, and could be related to cluster UDGs whose size and quiescence pre-date their infall. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society., SRJ acknowledges funding support from the Australian Research Council through Discovery Project grant DP200102574 during the course of this work. AJR was supported as a Research Corporation for Science Advancement Cottrell Scholar. DMD acknowledges financial support from the Talentia Senior Program (through the incentive ASE-136) from Secretaría General de Universidades, Investigación y Tecnología, de la Junta de Andalucía. DMD acknowledges funding from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and project (PDI2020-114581GB-C21/ AEI / 10.13039/501100011033). Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Support for Program number HST-GO-14846 was provided through a grant from the STScI under NASA contract NAS5-26555. This research has used the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research used ASTROPY,11 a community-developed core Python package for Astronomy (Astropy Collaboration 2013, 2018). This research used PHOTUTILS, an Astropy package for detection and photometry of astronomical sources (Bradley et al. 2021). This research has used the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has used NASA’s Astrophysics Data System Bibliographic Services.

We use spectral energy distribution (SED) fitting to place constraints on the stellar population properties of 29 quiescent ultra-diffuse galaxies (UDGs) across different environments. We use the fully Bayesian routine PROSPECTOR coupled with archival data in the optical, near, and mid-infrared from Spitzer and WISE under the assumption of an exponentially declining star formation history. We recover the stellar mass, age, metallicity, dust content, star formation time scales and photometric redshifts (photo-zs) of the UDGs studied. Using the mid-infrared data, we probe the existence of dust in UDGs. Although its presence cannot be confirmed, we find that the inclusion of small amounts of dust in the models brings the stellar populations closer to those reported with spectroscopy. Additionally, we fit the redshifts of all galaxies. We find a high accuracy in recovering photo-zs compared to spectroscopy, allowing us to provide new photo-z estimates for three field UDGs with unknown distances. We find evidence of a stellar population dependence on the environment, with quiescent field UDGs being systematically younger than their cluster counterparts. Lastly, we find that all UDGs lie below the mass--metallicity relation for normal dwarf galaxies. Particularly, the globular cluster (GC)-poor UDGs are consistently more metal-rich than GC-rich ones, suggesting that GC-poor UDGs may be puffed-up dwarfs, while most GC-rich UDGs are better explained by a failed galaxy scenario. As a byproduct, we show that two galaxies in our sample, NGC 1052-DF2 and NGC 1052-DF4, share equivalent stellar population properties, with ages consistent with 8 Gyr. This finding supports formation scenarios where the galaxies were formed together., Accepted for publication in MNRAS. 20 pages, 8 figures, 4 tables, and 4 appendices

We study the star formation history (SFH) of the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) based on the simultaneous fit to near-ultraviolet to near-infrared photometry and high signal-to-noise optical spectroscopy. In fitting the observations we adopt an advanced physical model with a flexible SFH, and we discuss the results in the context of the degeneracies between stellar population parameters. Through reconstructing the mass-assembly history with a prior for extended star formation (akin to methods in the literature) we find that DF44 formed 90 per cent of its stellar mass by $z\sim 0.9$ ($\sim 7.2$ Gyr ago). In comparison, using a prior that prefers concentrated star formation (as informed by previous studies of DF44's stellar populations) suggests that DF44 formed as early as $z\sim 8$ ($\sim 12.9$ Gyr ago). Regardless of whether DF44 is old or very old, the SFHs imply early star formation and rapid quenching. This result, together with DF44's large size and evidence that it is on its first infall into the Coma cluster, challenges UDG formation scenarios from simulations that treat all UDGs as contiguous with the canonical dwarf population. While our results cannot confirm any particular formation scenario, we can conclude from this that DF44 experienced a rare quenching event., 25 pages, 15 figures. Accepted for publication in MNRAS

It was recently proposed that the dark matter-deficient ultra-diffuse galaxies DF2 and DF4 in the NGC1052 group could be the products of a "bullet dwarf" collision between two gas-rich progenitor galaxies. In this model DF2 and DF4 formed at the same time in the immediate aftermath of the collision, and a strong prediction is that their globular clusters should have nearly identical stellar populations. Here we test this prediction by measuring accurate F606W-F814W colors from deep HST/ACS imaging. We find that the clusters are extremely homogeneous. The mean color difference between the globular clusters in DF2 and DF4 is $-0.003\pm 0.005$ mag and the observed scatter for the combined sample of 18 clusters with $M_V, Published in ApJ Letters. It is difficult to visualize the uniformity of these strange clusters. Our best attempt is Fig. 2, where we compare them to globular clusters in Virgo galaxies

A detailed study of the morphology of lenticular galaxies is an important way to understand how this type of galaxy formed and evolves over time. Decomposing a galaxy into its components (disc, bulge, bar, ...) allows recovering the colour gradients present in each system, its star formation history, and its assembly history. We use GALFITM to perform a multi-wavelength structural decomposition of the closest lenticular galaxy, NGC 3115, resulting in the description of its stellar light into several main components: a bulge, a thin disc, a thick disc and also evidence of a bar. We report the finding of central bluer stellar populations in the bulge, as compared to the colour of the galaxy outskirts, indicating either the presence of an Active Galactic Nucleus (AGN) and/or recent star formation activity. From the spectral energy distribution results, we show that the galaxy has a low luminosity AGN component, but even excluding the effect of the nuclear activity, the bulge is still bluer than the outer-regions of the galaxy, revealing a recent episode of star formation. Based on all of the derived properties, we propose a scenario for the formation of NGC 3115 consisting of an initial gas-rich merger, followed by accretions and feedback that quench the galaxy, until a recent encounter with the companion KK084 that reignited the star formation in the bulge, provoked a core displacement in NGC 3115 and generated spiral-like features. This result is consistent with the two-phase formation scenario, proposed in previous studies of this galaxy., Comment: Accepted for publication in MNRAS: 22 pages, 15 figures, 7 tables, 1 appendix

We introduce a new method for detecting ultra-diffuse galaxies by searching for over-densities in intergalactic globular cluster populations. Our approach is based on an application of the log-Gaussian Cox process, which is a commonly used model in the spatial statistics literature but rarely used in astronomy. This method is applied to the globular cluster data obtained from the PIPER survey, a \textit{Hubble Space Telescope} imaging program targeting the Perseus cluster. We successfully detect all confirmed ultra-diffuse galaxies with known globular cluster populations in the survey. We also identify a potential galaxy that has no detected diffuse stellar content. Preliminary analysis shows that it is unlikely to be merely an accidental clump of globular clusters or other objects. If confirmed, this system would be the first of its kind. Simulations are used to assess how the physical parameters of the globular cluster systems within ultra-diffuse galaxies affect their detectability using our method. We quantify the correlation of the detection probability with the total number of globular clusters in the galaxy and the anti-correlation with increasing half-number radius of the globular cluster system. The S\'{e}rsic index of the globular cluster distribution has little impact on detectability., Comment: 35 pages, 17 figures, accepted to ApJ

Globular clusters (GCs) are dense, gravitationally bound systems of thousands to millions of stars. They are preferentially associated with the oldest components of galaxies, and measurements of their composition can therefore provide insight into the build-up of the chemical elements in galaxies in the early Universe. We report a massive GC in the Andromeda Galaxy (M31) that is extremely depleted in heavy elements. Its iron abundance is about 800 times lower than that of the Sun, and about three times lower than in the most iron-poor GCs previously known. It is also strongly depleted in magnesium. These measurements challenge the notion of a metallicity floor for GCs and theoretical expectations that massive GCs could not have formed at such low metallicities., Comment: Accepted for publication in Science. For supplementary materials, see under ancillary files

We use the Keck Cosmic Web Imager integral field unit spectrograph to (1) measure the global stellar population parameters for the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) to much higher precision than previously possible for any UDG and (2) for the first time measure spatially resolved stellar population parameters of a UDG. We find that DF44 falls below the mass–metallicity relation established by canonical dwarf galaxies both in and beyond the Local Group. We measure a flat radial age gradient ( m logage = + 0.01 − 0.08 + 0.08 log Gyr kpc−1) and a flat to positive metallicity gradient ( m [ Fe / H ] = + 0.09 − 0.12 + 0.11 dex kpc−1), which are inconsistent with the gradients measured in similarly pressure-supported dwarf galaxies. We also measure a negative [Mg/Fe] gradient ( m [ Mg / Fe ] = − 0.20 − 0.18 + 0.18 ) dex kpc−1 such that the central 1.5 kpc of DF44 has stellar population parameters comparable to metal-poor globular clusters. Overall, DF44 does not have internal properties similar to other dwarf galaxies and is inconsistent with it having been puffed up through a prolonged, bursty star formation history, as suggested by some simulations. Rather, the evidence indicates that DF44 experienced an intense epoch of “inside-out” star formation and then quenched early and catastrophically, such that star formation was cut off more quickly than in canonical dwarf galaxies.

We study the present-day rotational velocity ($V_{rot}$) and velocity dispersion (${\sigma}$) profiles of the globular cluster (GC) systems in a sample of 50 lenticular (S0) galaxies from the E-MOSAICS galaxy formation simulations. We find that 82% of the galaxies have GCs that are rotating along the photometric major axis of the galaxy ($aligned$), while the remaining 18% of the galaxies do not ($misaligned$). This is generally consistent with the observations from the SLUGGS survey. For the $aligned$ galaxies, classified as $peaked$ $and$ $outwardly$ $decreasing$ (49%), $flat$ (24%) and $increasing$ (27%) based on the $V_{rot}/{\sigma}$ profiles out to large radii, we do not find any clear correlation between these present-day $V_{rot}/{\sigma}$ profiles of the GCs and the past merger histories of the S0 galaxies, unlike in previous simulations of galaxy stars. For just over half of the $misaligned$ galaxies, we find that the GC misalignment is the result of a major merger within the last 10 Gyr so that the $ex$-$situ$ GCs are misaligned by an angle between 0{\deg} (co-rotation) to 180{\deg} (counter-rotation) with respect to the $in$-$situ$ GCs, depending on the orbital configuration of the merging galaxies. For the remaining $misaligned$ galaxies, we suggest that the $in$-$situ$ metal-poor GCs, formed at early times, have undergone more frequent kinematic perturbations than the $in$-$situ$ metal-rich GCs. We also find that the GCs accreted early and the $in$-$situ$ GCs are predominantly located within 0.2 virial radii ($R_{200}$) from the centre of galaxies in 3D phase-space diagrams., Comment: 19 pages, 13 figures

We present detailed abundance measurements for 45 globular clusters (GCs) in galaxies in (and, in one case, beyond) the Local Group. The measurements are based on new high-resolution integrated-light spectra of GCs in NGC 185, NGC 205, M31, M33, and NGC 2403, combined with reanalysis of previous observations of GCs in the Fornax dSph, WLM, NGC 147, NGC 6822, and the Milky Way. The GCs cover the range -2.8 < [Fe/H] < -0.1 and we determined abundances for Fe, Na, Mg, Si, Ca, Sc, Ti, Cr, Mn, Ni, Cu, Zn, Zr, Ba, and Eu. Corrections for non local thermodynamic equilibrium effects are included for Na, Mg, Ca, Ti, Mn, Fe, Ni, and Ba. For several of the galaxies, our measurements provide the first quantitative constraints on the detailed composition of their metal-poor stellar populations. Overall, the GCs in different galaxies exhibit remarkably uniform abundance patterns of the alpha-, iron-peak, and neutron-capture elements, with a dispersion of less than 0.1 dex in [alpha/Fe] for the full sample. There is a hint that GCs in dwarf galaxies are slightly less alpha-enhanced (by about 0.04 dex on average) than those in larger galaxies. One GC in M33 (HM33-B) resembles the most metal-rich GCs in the Fornax dSph (Fornax 4) and NGC 6822 (SC7) by having alpha-element abundances closer to scaled-solar values, possibly hinting at an accretion origin. We find that the alpha-element abundances strongly correlate with those of Na, Sc, Ni, and Zn. Several GCs with [Fe/H], Comment: 34 pages + 6 appendices. Accepted for publication in Astronomy & Astrophysics

It is clear that within the class of ultra-diffuse galaxies (UDGs) there is an extreme range in the richness of their associated globular cluster (GC) systems. Here, we report the structural properties of five UDGs in the Perseus cluster based on deep Subaru / Hyper Suprime-Cam imaging. Three appear GC-poor and two appear GC-rich. One of our sample, PUDG\_R24, appears to be undergoing quenching and is expected to fade into the UDG regime within the next $\sim0.5$ Gyr. We target this sample with Keck Cosmic Web Imager (KCWI) spectroscopy to investigate differences in their dark matter halos, as expected from their differing GC content. Our spectroscopy measures both recessional velocities, confirming Perseus cluster membership, and stellar velocity dispersions, to measure dynamical masses within their half-light radius. We supplement our data with that from the literature to examine trends in galaxy parameters with GC system richness. We do not find the correlation between GC numbers and UDG phase space positioning expected if GC-rich UDGs environmentally quench at high redshift. We do find GC-rich UDGs to have higher velocity dispersions than GC-poor UDGs on average, resulting in greater dynamical mass within the half-light radius. This agrees with the first order expectation that GC-rich UDGs have higher halo masses than GC-poor UDGs., 14 pages, 9 figures. Accepted for publication in MNRAS

We study the globular clusters (GCs) in the spiral galaxy NGC~5907 well-known for its spectacular stellar stream -- to better understand its origin. Using wide-field Subaru/Suprime-Cam $gri$ images and deep Keck/DEIMOS multi-object spectroscopy, we identify and obtain the kinematics of several GCs superimposed on the stellar stream and the galaxy disk. We estimate the total number of globular clusters in NGC 5907 to be $154\pm44$, with a specific frequency of $0.73\pm0.21$. Our analysis also reveals a significant, new population of young star cluster candidates found mostly along the outskirts of the stellar disk. Using the properties of the stream GCs, we estimate that the disrupted galaxy has a stellar mass similar to the Sagittarius dwarf galaxy accreted by the Milky Way, i.e. $\sim10^8~M_\odot$., Comment: 9 pages, 9 figures, catalog of GCs is available in online version

We investigate the kinematic properties of nine nearby early-type galaxies with evidence of a disc-like component. Three of these galaxies are located in the field, five in the group, and only one in the cluster environment. By combining the kinematics of the stars with those of the globular clusters (GCs) and planetary nebulae (PNe), we probe the outer regions of our galaxies out to $\sim 4\!-\!6\, R_{\mathrm{e}}$. Six galaxies have PNe and red GCs that show good kinematic alignment with the stars, whose rotation occurs along the photometric major-axis of the galaxies, suggesting that both the PNe and red GCs are good tracers of the underlying stellar population beyond that traced by the stars. Additionally, the blue GCs also show rotation that is overall consistent with that of the red GCs in these six galaxies. The remaining three galaxies show kinematic twists and misalignment of the PNe and GCs with respect to the underlying stars, suggesting recent galaxy interactions. From the comparison with simulations, we propose that all six aligned galaxies that show similar dispersion-dominated kinematics at large radii ($\gt 2\!-\!3\, R_{\mathrm{e}}$) had similar late (z ≲ 1) assembly histories characterized by mini mergers (mass-ratio < 1:10). The different Vrot/σ profiles are then the result of an early (z > 1) minor merger (1:10 < mass-ratio < 1:4) for the four galaxies with peaked and decreasing Vrot/σ profiles and of a late minor merger for the two galaxies with flat Vrot/σ profiles. The three misaligned galaxies likely formed through multiple late minor mergers that enhanced their velocity dispersion at all radii, or a late major merger that spun-up both the GC subpopulations at large radii. Therefore, lenticular galaxies can have complex merger histories that shape their characteristic kinematic profile shapes., DF, WC, KB, and AD acknowledge support from the Australian Research Council under Discovery Project 170102344. AFM has received financial support through the Postdoctoral Junior Leader Fellowship Programme from “la Caixa” Foundation(LCF/BQ/LI18/11630007). AJR was supported by National Science Foundation grant AST-1616710 and by a Research Corporation for Science Advancement Cottrell Scholar.

The large and diffuse galaxies NGC1052-DF2 and NGC1052-DF4 have been found to have very low dark matter content and a population of luminous globular clusters. Accurate distance measurements are key to interpreting these observations. Recently, the distance to NGC1052-DF4 was found to be $20.0\pm 1.6$ Mpc by identifying the tip of the red giant branch (TRGB) in 12 orbits of Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. Here we present 40 orbits of HST ACS data for NGC1052-DF2 and use these data to measure its TRGB. The TRGB is readily apparent in the color-magnitude diagram. Using a forward model that incorporates photometric uncertainties, we find a TRGB magnitude of $m_{\rm F814W, TRGB} = 27.67 \pm 0.10$ mag. The inferred distance is $D_{\rm TRGB} = 22.1 \pm 1.2$ Mpc, consistent with the previous surface brightness fluctuation distances to the bright elliptical galaxy NGC1052. The new HST distance rules out the idea that some of NGC1052-DF2's unusual properties can be explained if it were at $\sim 13$ Mpc; instead, it implies that the galaxy's globular clusters are even more luminous than had been derived using the previous distance of 20 Mpc. The distance from NGC1052-DF2 to NGC1052-DF4 is well-determined at $2.1\pm 0.5$ Mpc, significantly larger than the virial diameter of NGC1052. We discuss the implications for formation scenarios of the galaxies and for the external field effect, which has been invoked to explain the intrinsic dynamics of these objects in the context of modified Newtonian dynamics., Comment: Accepted for publication in ApJ Letters

We recently found the globular cluster (GC) EXT8 in M31 to have an extremely low metallicity of [Fe/H]=-2.91+/-0.04 using high-resolution spectroscopy. Here we present a colour-magnitude diagram (CMD) for EXT8, obtained with the Wide Field Camera 3 on board the Hubble Space Telescope. Compared with the CMDs of metal-poor Galactic GCs, we find that the upper red giant branch (RGB) of EXT8 is about 0.03 mag bluer in F606W-F814W and slightly steeper, as expected from the low spectroscopic metallicity. The observed colour spread on the upper RGB is consistent with being caused entirely by the measurement uncertainties, and we place an upper limit of sigma(F606W-F814W)=0.015 mag on any intrinsic colour spread. The corresponding metallicity spread can be up to sigma([Fe/H])=0.2 dex or >0.7 dex, depending on the isochrone library adopted. The horizontal branch (HB) is located mostly on the blue side of the instability strip and has a tail extending to at least M(F606W)=+3, as in the Galactic GC M15. We identify two candidate RR Lyrae variables and several UV-luminous post-HB/post AGB star candidates, including one very bright (M(F300X)=-3.2) source near the centre of EXT8. The surface brightness of EXT8 out to a radius of 25 arcsec is well fitted by a Wilson-type profile with an ellipticity of epsilon=0.20, a semi-major axis core radius of 0.25", and a central surface brightness of 15.2 mag per square arcsec in the F606W band, with no evidence of extra-tidal structure. Overall, EXT8 has properties consistent with it being a "normal", but very metal-poor GC, and its combination of relatively high mass and very low metallicity thus remains challenging to explain in the context of GC formation theories operating within the hierarchical galaxy assembly paradigm., Accepted for publication in Astronomy & Astrophysics

We present spatially-resolved two-dimensional maps and radial trends of the stellar populations and kinematics for a sample of six compact elliptical galaxies (cE) using spectroscopy from the Keck Cosmic Web Imager (KCWI). We recover their star formation histories, finding that all except one of our cEs are old and metal rich, with both age and metallicity decreasing toward their outer radii. We also use the integrated values within one effective radius to study different scaling relations. Comparing our cEs with others from the literature and from simulations we reveal the formation channel that these galaxies might have followed. All our cEs are fast rotators, with relatively high rotation values given their low ellipticites. In general, the properties of our cEs are very similar to those seen in the cores of more massive galaxies, and in particular, to massive compact galaxies. Five out of our six cEs are the result of stripping a more massive (compact or extended) galaxy, and only one cE is compatible with having been formed intrinsically as the low-mass, compact object that we see today. These results further confirm that cEs are a mixed-bag of galaxies that can be formed following different formation channels, reporting for the first time an evolutionary link within the realm of compact galaxies (at all stellar masses)., Comment: Aceepted for publication in MNRAS. 19 pages, 13 figures

The ultra diffuse galaxy in the NGC 5846 group (NGC 5846UDG1) was shown to have a large number of globular cluster (GC) candidates from deep imaging as part of the VEGAS survey. Recently, Müller et al. published a velocity dispersion, based on a dozen of its GCs. Within their quoted uncertainties, the resulting dynamical mass allowed for either a dark matter free or a dark-matter-dominated galaxy. Here, we present spectra from KCWI that reconfirms membership of the NGC 5846 group and reveals a stellar velocity dispersion for UDG1 of σGC = 17 ± 2 km s-1. Our dynamical mass, with a reduced uncertainty, indicates a very high contribution of dark matter within the effective radius. We also derive an enclosed mass from the locations and motions of the GCs using the tracer mass estimator, finding a similar mass inferred from our stellar velocity dispersion. We find no evidence that the galaxy is rotating and is thus likely pressure supported. The number of confirmed GCs, and the total number inferred for the system (∼45), suggests a total halo mass of ∼2 × 1011 M. A cored mass profile is favoured when compared to our dynamical mass. Given its stellar mass of 1.1 × 108 M, NGC 5846UDG1 appears to be an ultra diffuse galaxy with a dwarf-like stellar mass and an overly massive halo., DAF thanks the ARC for financial support via DP160101608. AFM has received financial support through the Postdoctoral Junior Leader Fellowship Programme from La Caixa Banking Foundation (LCF/BQ/LI18/11630007). AJR was supported by National Science Foundation grant AST-1616710 and as a Research Corporation for Science Advancement Cottrell Scholar.

We present new spectra obtained using Keck/KCWI and perform kinematics and stellar population analyses of the shell galaxy NGC 474, from both the galaxy centre and a region from the outer shell. We show that both regions have similarly extended star formation histories although with different stellar population properties. The central region of NGC 474 is dominated by intermediate-aged stars (8.3 \pm 0.3 Gyr) with subsolar metallicity ([Z/H]= -0.24 \pm 0.07 dex) while the observed shell region, which hosts a substantial population of younger stars, has a mean luminosity-weighted age of 4.0 \pm 0.5 Gyr with solar metallicities ([Z/H]=-0.03 \pm 0.09 dex). Our results are consistent with a scenario in which NGC 474 experienced a major to intermediate merger with a log\((M_*/M_\odot)\sim10 \) mass satellite galaxy at least \sim 2 Gyr ago which produced its shell system. This work shows that the direct spectroscopic study of low-surface brightness stellar features, such as shells, is now feasible and opens up a new window to understanding galaxy formation and evolution., Accepted for publication in MNRAS, 6 pages, 5 figures

We present a catalogue of low surface brightness (LSB) galaxies in the Coma cluster obtained from deep Subaru/Suprime-Cam V and R-band imaging data within a region of $\sim$4 deg$^2$. We increase the number of LSB galaxies presented in Yagi et al. (2016) by a factor of $\sim$3 and report the discovery of 29 new ultra-diffuse galaxies (UDGs). We compile the largest sample of ultra-diffuse galaxies with colours and structural parameters in the Coma cluster. While most UDGs lie along the red-sequence relation of the colour-magnitude diagram, $\sim$5per cent are outside (bluer or redder) the red-sequence region of Coma cluster galaxies. Our analyses show that there is no special distinction in the basic photometric parameters between UDGs and other LSB galaxies. We investigate the clustercentric colour distribution and find a remarkable transition at a projected radius of $\sim$0.6 Mpc. Within this cluster core region, LSB galaxies are, on average, redder than co-spatial higher surface brightness galaxies, highlighting how vulnerable LSB galaxies are to the physical processes at play in the dense central region of the cluster. The position of the transition radius agrees with expectations from recent cosmological simulation of massive galaxy clusters within which ancient infalls are predicted to dominate the LSB galaxy population., Accepted for publication in MNRAS. The complete version of Table 2 is available online as supplementary material

Ultra diffuse galaxies (UDGs) reveal extreme properties. Here we compile the largest study to date of 85 globular cluster (GC) systems around UDGs in the Coma cluster, using new deep ground-based imaging of the known UDGs and existing imaging from the Hubble Space Telescope of their GC systems. We find that the richness of GC systems in UDGs generally exceeds that found in normal dwarf galaxies of the same stellar mass. These GC-rich UDGs imply halos more massive than expected from the standard stellar mass-halo mass relation. The presence of such overly massive halos presents a significant challenge to the latest simulations of UDGs in cluster environments. In some exceptional cases, the mass in the GC system is a significant fraction of the stellar content of the host galaxy. We find that rich GC systems tend to be hosted in UDGs of lower luminosity, smaller size and fainter surface brightness. Similar trends are seen for normal dwarf galaxies in the Coma cluster. A toy model is presented in which the GC-rich UDGs are assumed to be `failed' galaxies within massive halos that have largely old, metal-poor, alpha-element enhanced stellar populations. On the other hand, GC-poor UDGs are more akin to normal, low surface brightness dwarfs that occupy less massive dark matter halos. Additional data on the stellar populations of UDGs with GC systems will help to further refine and test this simplistic model., 11 pages, 11 figures, accepted for publication in MNRAS

We describe the goals and first results of a Program for Imaging of the PERseus cluster of galaxies (PIPER). The first phase of the program builds on imaging of fields obtained with the Hubble Space Telescope (HST) ACS/WFC and WFC3/UVIS cameras. Our PIPER target fields with HST include major early-type galaxies including the active central giant NGC 1275; known Ultra-Diffuse Galaxies; and the Intracluster Medium. The resulting two-color photometry in F475W and F814W reaches deep enough to resolve and measure the globular cluster (GC) populations in the Perseus member galaxies. Here we present initial results for eight pairs of outer fields that confirm the presence of Intergalactic GCs (IGCs) in fields as distant as 740 kpc from the Perseus center (40\% of the virial radius of the cluster). Roughly 90% of these IGCs are identifiably blue (metal-poor) but there is a clear trace of a red (metal-rich) component as well, even at these very remote distances., In press for The Astrophysical Journal

We recently discovered the X-ray/optical outbursting source 3XMM J215022.4-055108. It was best explained as the tidal disruption of a star by an intermediate-mass black hole of mass of a few tens of thousand solar masses in a massive star cluster at the outskirts of a large barred lenticular galaxy at D_L=247 Mpc. However, we could not completely rule out a Galactic cooling neutron star as an alternative explanation for the source. In order to further pin down the nature of the source, we have obtained new multiwavelength observations by XMM-Newton and Hubble Space Telescope (HST). The optical counterpart to the source in the new HST image is marginally resolved, which rules out the Galactic cooling neutron star explanation for the source and suggests a star cluster of half-light radius ~27 pc. The new XMM-Newton observation indicates that the luminosity was decaying as expected for a tidal disruption event and that the disk was still in the thermal state with a super-soft X-ray spectrum. Therefore, the new observations confirm the source as one of the best intermediate-mass black hole candidates., Accepted for publication in ApJL, 5 pages, 4 figures

We present an extensive study of spectroscopically-derived chemical abundances for M87 and its globular cluster (GC) system. Using observations from the Mitchell spectrograph at McDonald, LRIS at Keck, and Hectospec on the MMT, we derive new metallicity gradients from $\sim 2$ to $140$ kpc. We use a novel hierarchical statistical framework to simultaneously separate the GC system into subpopulations while measuring the metallicity gradients of those subpopulations. We create physically-motivated spectral stacks of the GC subpopulations by leveraging the output of this statistical framework to perform the first application of abundance tagging in a massive ETG to better constrain the origins of the GC subpopulations and, thus, the assembly history of M87. We find a metal-poor, $\alpha$-enhanced population of GCs in both in the inner and outer halo unanticipated by current cosmological simulations of galaxy evolution. We use the remarkably flat metallicity gradients we find for both the metal-rich and metal-poor GC subpopulations in the inner halo as tentative evidence that some amount of the metal-poor GCs formed directly in the halo of M87 at high redshift., Comment: Accepted to ApJ

Here, we present a kinematical analysis of the Virgo cluster ultradiffuse galaxy (UDG) VCC 1287 based on data taken with the Keck Cosmic Web Imager (KCWI). We confirm VCC 1287's association both with the Virgo cluster and its globular cluster (GC) system, measuring a recessional velocity of 1116 ± 2 km s-1. We measure a stellar velocity dispersion (19 ± 6 km s-1) and infer both a dynamical mass ($1.11^{+0.81}_{-0.81} \times 10^{9} \ \mathrm{M_{\odot }}$) and mass-To-light ratio (M/L) ($13^{+11}_{-11}$) within the half-light radius (4.4 kpc). This places VCC 1287 slightly above the well-established relation for normal galaxies, with a higher M/L for its dynamical mass than normal galaxies. We use our dynamical mass, and an estimate of GC system richness, to place VCC 1287 on the GC number-dynamical mass relation, finding good agreement with a sample of normal galaxies. Based on a total halo mass derived from GC counts, we then infer that VCC 1287 likely resides in a cored or low-concentration dark matter halo. Based on the comparison of our measurements to predictions from simulations, we find that strong stellar feedback and/or tidal effects are plausibly the dominant mechanisms in the formation of VCC 1287. Finally, we compare our measurement of the dynamical mass with those for other UDGs. These dynamical mass estimates suggest relatively massive haloes and a failed galaxy origin for at least some UDGs., The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We thank the staff of the W. M. Keck Observatory for their assistance in using the telescope and wish to express particular gratitude to Luca Rizzi in helping us fix issues that presented themselves. We have also made heavy use of a number of open source packages in the creation of this work. We therefore wish to thank contributors to ASTROPY (Price-Whelan et al. 2018), SCIPY (Virtanen et al. 2020), SCIKIT–LEARN (Pedregosa et al. 2011), NUMPY (Oliphant 2019), SEABORN (Waskom et al. 2017), CORNER (Foreman-Mackey 2016), and EMCEE (Foreman-Mackey et al. 2013) for making their software publicly available. JSG acknowledges financial support received through a Swinburne University Postgraduate Research Award throughout the creation of this work. AFM has received financial support through the Post-doctoral Junior Leader Fellowship Programme from ‘La Caixa’ Banking Foundation (LCF/BQ/LI18/11630007). AJR was supported by National Science Foundation grant AST-1616710, and as a Research Corporation for Science Advancement Cottrell Scholar. JPB gratefully acknowledges support from National Science foundation grants AST-1518294 and AST-1616598.

Using the Keck Cosmic Web Imager, we obtain spectra of several globular clusters (GCs), ultracompact dwarfs (UCDs), and the inner halo starlight of M87, at a similar projected galactocentric radius of ∼5 kpc. This enables us, for the first time, to apply the same stellar population analysis to the GCs, UCDs, and starlight consistently to derive ages, metallicities, and alpha-element abundances in M87. We find evidence for a dual stellar population in the M87 halo light, i.e. an ∼80 per cent component by mass that is old and metal-rich and a ∼20 per cent component that is old but metal-poor. Two red GCs share similar stellar populations to the halo light suggesting they may have formed contemporaneously with the dominant halo component. Three UCDs, and one blue GC, have similar stellar populations, with younger mean ages, lower metallicities, and near solar alpha-element abundances. Combined with literature data, our findings are consistent with the scenario that UCDs are the remnant nucleus of a stripped galaxy. We further investigate the discrepancy in the literature for M87's kinematics at large radii, favouring a declining velocity dispersion profile. This work has highlighted the need for more self-consistent studies of galaxy haloes., DAF thanks the ARC for financial support via DP160101608. AFM has received financial support through the Postdoctoral Junior Leader Fellowship Programme from “la Caixa” Banking Foundation (LCF/BQ/LI18/11630007). AJR was supported by National Science Foundation grant AST-1616710 and as a Research Corporation for Science Advancement Cottrell Scholar.

Using new and archival data, we study the kinematic properties of the nearest field S0 galaxy, NGC 3115, out to $\sim6.5$ half-light radii ($R_\mathrm{e}$) from its stars (integrated starlight), globular clusters (GCs) and planetary nebulae (PNe). We find evidence of three kinematic regions with an inner transition at $\sim0.2\ R_\mathrm{e}$ from a dispersion-dominated bulge ($V_\mathrm{rot}/\sigma 1$), and then an additional transition from the disk to a slowly rotating spheroid at $\sim2-2.5\, R_\mathrm{e}$, as traced by the red GCs and PNe (and possibly by the blue GCs beyond $\sim5\, R_\mathrm{e}$). From comparison with simulations, we propose an assembly history in which the original progenitor spiral galaxy undergoes a gas-rich minor merger that results in the embedded kinematically cold disk that we see today in NGC 3115. At a later stage, dwarf galaxies, in mini mergers (mass-ratio $, Comment: 22 pages (including 3 pages of Appendix material), 14 figures, published in MNRAS

Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5x10^{10} solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5 kilometers per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8 solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales., Comment: To appear in the 29 March issue of Nature

We use Keck/DEIMOS spectroscopy to confirm the cluster membership of 16 ultra-diffuse galaxies (UDGs) in the Coma cluster, bringing the total number of spectroscopically con- firmed UDGs to 24. We also identify a new cluster background UDG. In this pilot study of Coma UDGs in velocity phase-space, we find evidence that most present-day Coma UDGs have a recent infall epoch while a few may be ancient infalls. These recent infall UDGs have higher absolute relative line-of-sight velocities, bluer optical colors, and are smaller in size, unlike the ancient infalls. The kinematics of the spectroscopically confirmed Coma UDG sample is similar to that of the cluster late-type galaxy population. Our velocity phase-space analysis suggests that present-day cluster UDGs have a predominantly accretion origin from the field, acquire velocities corresponding to the mass of the cluster at accretion as they are accelerated towards the cluster center, and become redder and bigger as they experience the various physical processes at work within the cluster., Comment: 9 Figures, 1 Table, submitted to MNRAS

In order to investigate the formation mechanisms of the rare compact elliptical galaxies (cE) we have compiled a sample of 25 cEs with good SDSS spectra, covering a range of stellar masses, sizes and environments. They have been visually classified according to the interaction with their host, representing different evolutionary stages. We have included clearly disrupted galaxies, galaxies that despite not showing signs of interaction are located close to a massive neighbor (thus are good candidates for a stripping process), and cEs with no host nearby. For the latter, tidal stripping is less likely to have happened and instead they could simply represent the very low-mass, faint end of the ellipticals. We study a set of properties (structural parameters, stellar populations, star formation histories and mass ratios) that can be used to discriminate between an intrinsic or stripped origin. We find that one diagnostic tool alone is inconclusive for the majority of objects. However, if we combine all the tools a clear picture emerges. The most plausible origin, as well as the evolutionary stage and progenitor type, can be then determined. Our results favor the stripping mechanism for those galaxies in groups and clusters that have a plausible host nearby, but favors an intrinsic origin for those rare cEs without a plausible host and that are located in looser environments., Accepeted for publication in MNRAS. 24 pages, 21 figures, 5 tables

We use globular cluster kinematics data, primarily from the SLUGGS survey, to measure the dark matter fraction ($f_{\rm DM}$) and the average dark matter density ($\left< ��_{\rm DM} \right>$) within the inner 5 effective radii ($R_{\rm e}$) for 32 nearby early--type galaxies (ETGs) with stellar mass log $(M_*/\rm M_\odot)$ ranging from $10.1$ to $11.8$. We compare our results with a simple galaxy model based on scaling relations as well as with cosmological hydrodynamical simulations where the dark matter profile has been modified through various physical processes. We find a high $f_{\rm DM}$ ($\geq0.6$) within 5~$R_{\rm e}$ in most of our sample, which we interpret as a signature of a late mass assembly history that is largely devoid of gas-rich major mergers. However, around log $(M_*/M_\odot) \sim 11$, there is a wide range of $f_{\rm DM}$ which may be challenging to explain with any single cosmological model. We find tentative evidence that lenticulars (S0s), unlike ellipticals, have mass distributions that are similar to spiral galaxies, with decreasing $f_{\rm DM}$ within 5~$R_{\rm e}$ as galaxy luminosity increases. However, we do not find any difference between the $\left< ��_{\rm DM} \right>$ of S0s and ellipticals in our sample, despite the differences in their stellar populations. We have also used $\left< ��_{\rm DM} \right>$ to infer the epoch of halo assembly ($z{\sim}2-4$). By comparing the age of their central stars with the inferred epoch of halo formation, we are able to gain more insight into their mass assembly histories. Our results suggest a fundamental difference in the dominant late-phase mass assembly channel between lenticulars and elliptical galaxies., Accepted for publication in MNRAS, 10 figures. Figures 2 and 3 have been slightly modified from the previous version, with the introduction of a new simple galaxy model

Given the absence of directly detected dark matter (DM) as weakly interacting massive particles, there is strong interest in the possibility that DM is an ultra-light scalar field, here denoted as fuzzy DM. Ultra-diffuse galaxies, with the sizes of giant galaxies and the luminosities of dwarf galaxies, have a wide range of DM halo masses, thus providing new opportunities for exploring the connections between galaxies and their DM halos. Following up on new integral field unit spectroscopic observations and dynamics modeling of the DM-dominated ultra-diffuse galaxy Dragonfly 44 in the outskirts of the Coma Cluster, we present models of fuzzy DM constrained by the stellar dynamics of this galaxy. We infer a scalar field mass of $\sim 3 \times 10^{-22}$ eV, consistent with other constraints from galaxy dynamics but in tension with constraints from Ly$\alpha$ forest power spectrum modeling. While we are unable to statistically distinguish between fuzzy DM and normal cold DM models, we find that the inferred properties of the fuzzy DM halo satisfy a number of predictions for halos in a fuzzy DM cosmology. In particular, we find good agreement with the predicted core size--halo mass relation and the predicted transition radius between the quantum pressure-dominated inner region and the outer halo region., Comment: 15 pages, 11 figures. Submitted to ApJ, comments are welcome!

Globular cluster ages provide both an important test of models of globular cluster formation and a powerful method to constrain the assembly history of galaxies. Unfortunately, measuring the ages of unresolved old stellar populations has proven challenging. Here, we present a novel technique that combines optical photometry with metallicity constraints from near-infrared spectroscopy in order to measure ages. After testing the method on globular clusters in the Milky Way and its satellite galaxies, we apply our technique to three massive early-type galaxies using data from the SLUGGS Survey. The three SLUGGS galaxies and the Milky Way show dramatically different globular cluster age and metallicity distributions, with NGC 1407 and the Milky Way showing mostly old globular clusters while NGC 3115 and NGC 3377 show a range of globular ages. This diversity implies different galaxy formation histories and that the globular cluster optical colour-metallicity relation is not universal as is commonly assumed in globular cluster studies. We find a correlation between the median age of the metal rich globular cluster populations and the age of the field star populations, in line with models where globular cluster formation is a natural outcome of high intensity star formation., 12 pages, 6 figures. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

The NGC 1052 group, and in particular the discovery of two ultra diffuse galaxies with very low internal velocity dispersions, has been the subject of much attention recently. Here we present radial velocities for a sample of 77 globular clusters associated with NGC 1052 obtained on the Keck telescope. Their mean velocity and velocity dispersion are consistent with that of the host galaxy. Using a simple tracer mass estimator, we infer the enclosed dynamical mass and dark matter fraction of NGC 1052. Extrapolating our measurements with an NFW mass profile we infer a total halo mass of 6.2 ($\pm$0.2) $\times$ 10$^{12}$ M$_{\odot}$. This mass is fully consistent with that expected from the stellar mass--halo mass relation, suggesting that NGC 1052 has a normal dark matter halo mass (i.e. it is not deficient in dark matter in contrast to two ultra diffuse galaxies in the group). We present a phase space diagram showing the galaxies that lie within the projected virial radius (390 kpc) of NGC 1052. Finally, we briefly discuss the two dark matter deficient galaxies (NGC 1052--DF and DF4) and consider whether MOND can account for their low observed internal velocity dispersions., 5 pages, 2 figures, 1 abridged Table, MNRAS accepted

We report the discovery of a $>1^\circ$ ($\sim50$ kpc) long stellar tidal stream emanating from the dwarf galaxy DDO 44, a likely satellite of Local Volume galaxy NGC 2403 located $\sim70$ kpc in projection from its companion. NGC 2403 is a roughly Large Magellanic Cloud stellar-mass galaxy 3 Mpc away, residing at the outer limits of the M 81 group. We are mapping a large region around NGC 2403 as part of our MADCASH (Magellanic Analogs' Dwarf Companions and Stellar Halos) survey, reaching point source depths (90% completeness) of ($g, i$) = (26.5, 26.2). Density maps of old, metal-poor RGB stars reveal tidal streams extending on two sides of DDO 44, with the streams directed toward NGC 2403. We estimate total luminosities of the original DDO 44 system (dwarf and streams combined) to be $M_{i, \rm{tot}} = -13.4$ and $M_{g, \rm{tot}} = -12.6$, with $\sim25-30\%$ of the luminosity in the streams. Analogs of $\sim$LMC-mass hosts with massive tidally disrupting satellites are rare in the Illustris simulations, especially at large separations such as that of DDO 44. The few analogs that are present in the models suggest that even low-mass hosts can efficiently quench their massive satellites., Accepted for publication in ApJ. Updated with minor changes in response to referee

The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that it has little or no dark matter. Here we report the discovery of a second galaxy in this class, residing in the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of its size, surface brightness, and morphology; has a similar distance of $D=19.9\pm 2.8$ Mpc; and also has a population of luminous globular clusters extending out to 7 kpc from the center of the galaxy. Accurate radial velocities of the diffuse galaxy light and seven of the globular clusters were obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. The velocity of the diffuse light is identical to the median velocity of the clusters, $v_{\rm sys}=\langle v_{\rm gc} \rangle=1445$ km/s, and close to the central velocity of the NGC1052 group. The rms spread of the observed velocities is very small at $\sigma_{\rm obs}=5.8$ km/s. Taking observational uncertainties into account we determine an intrinsic velocity dispersion of $\sigma_{\rm intr}=4.2^{+4.4}_{-2.2}$ km/s, consistent with the expected value from the stars alone ($\sigma_{\rm stars}\approx 7$ km/s) and lower than expected from a standard NFW halo ($\sigma_{\rm halo}\sim 30$ km/s). We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects exists. The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood., Comment: ApJ Letters, in press. The revised version has a new figure and an expanded table with the sizes, axis ratios, and luminosities of the globular clusters. Thanks to an insightful comment of the referee it also includes a velocity measurement of the diffuse stellar light of the galaxy. In v3 the references were updated and the coordinates of one of the clusters (GC-943) were corrected

The observed characteristics of globular cluster (GC) systems, such as metallicity distributions, are commonly used to place constraints on galaxy formation models. However, obtaining reliable metallicity values is particularly difficult because of our limited means to obtain high quality spectroscopy of extragalactic GCs. Often, `color--metallicity relations' are invoked to convert easier-to-obtain photometric measurements into metallicities, but there is no consensus on what form these relations should take. In this paper we make use of multiple photometric datasets and iron metallicity values derived from applying full-spectrum stellar population synthesis models to deep Keck/LRIS spectra of 177 GCs centrally located around M87 to obtain a new color--metallicity relation. Our new relation differs substantially from previous relations in the blue, and we present evidence that the M87 relation differs from that of the Milky Way GCs, suggesting environmental dependence of GC properties. We use our color--metallicity relation to derive a new GC metallicity-host galaxy luminosity relation for red and blue GCs and find a shallower relation for the blue GCs than what previous work has found and that the metal-poor GCs are more enriched than what was previously found. This could indicate that the progenitor satellite galaxies that now make up the stellar halos of early-type galaxies are more massive and formed later than previously thought, or that the properties of metal-poor GCs are less dependent on their present-day host, indicating a common origin., Comment: Accepted to ApJ. Comments welcome!

Stellar halos offer fossil evidence for hierarchical structure formation. Since halo assembly is predicted to be scale-free, stellar halos around low-mass galaxies constrain properties such as star formation in the accreted subhalos and the formation of dwarf galaxies. However, few observational searches for stellar halos in dwarfs exist. Here we present gi photometry of resolved stars in isolated Local Group dwarf irregular galaxy IC 1613 ($M_{\star} \sim 10^8 M_{\odot})$. These Subaru/Hyper Suprime-Cam observations are the widest and deepest of IC 1613 to date. We measure surface density profiles of young main-sequence, intermediate to old red giant branch, and ancient horizontal branch stars outside of 12' ($\sim 2.6$ kpc; 2.5 half-light radii) from the IC 1613 center. All of the populations extend to ~24' ($\sim 5.2$ kpc; 5 half-light radii), with the older populations best fit by a broken exponential in these outer regions. Comparison with earlier studies sensitive to IC 1613's inner regions shows that the density of old stellar populations steepens substantially with distance from the center; we trace the $g$-band effective surface brightness to an extremely faint limit of $\sim 33.7$ mag arcsec$^{-2}$. Conversely, the distribution of younger stars follows a single, shallow exponential profile in the outer regions, demonstrating different formation channels for the younger and older components of IC 1613. The outermost, intermediate-age and old stars have properties consistent with those expected for accreted stellar halos, though future observational and theoretical work is needed to definitively distinguish this scenario from other possibilities., Comment: 13 pages, 12 figures, Published in ApJ