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The volumetric rate of tidal disruption events (TDEs) encodes information on the still-unknown demographics of central massive black holes (MBHs) in low-mass galaxies ($\lesssim 10^9$~M$_\odot$). Theoretical TDE rates from model galaxy samples can extract this information, but this requires accurately defining the nuclear stellar density structures. This region is typically dominated by nuclear star clusters (NSCs), which have been shown to increase TDE rates by orders of magnitude. Thus, we assemble the largest available sample of pc-scale 3-D density profiles that include NSC components. We deproject the PSF-deconvolved surface brightness profiles of 91 nearby galaxies of varying morphology and combine these with nuclear mass-to-light ratios estimated from measured colors or spectral synthesis to create 3-D mass density profiles. We fit the inner 3-D density profile to find the best-fit power-law density profile in each galaxy. We compile this information as a function of galaxy stellar mass to fit new empirical density scaling relations. These fits reveal positive correlations between galaxy stellar mass and central stellar density in both early- and late-type galaxies. We find that early-type galaxies have somewhat higher densities and shallower profiles relative to late-type galaxies at the same mass. We also use the density profiles to estimate the influence radius of each galaxy's MBH and find that the sphere of influence was likely resolved in most cases. These new relations will be used in future works to build mock galaxy samples for dynamical TDE rate calculations, with the aim of constraining MBH demographics in low-mass galaxies., Comment: 18 pages, 6 figures, 1 table. Accepted by AAS Journals

We measure the high-mass stellar initial mass function (IMF) from resolved stars in M33 young stellar clusters. Leveraging \textit{Hubble Space Telescope's} high resolving power, we fully model the IMF probabilistically. We first model the optical CMD of each cluster to constrain its power-law slope $\Gamma$, marginalized over other cluster parameters in the fit (e.g., cluster age, mass, and radius). We then probabilistically model the distribution of MF slopes for a highly strict cluster sample of 9 clusters more massive than log(Mass/M$_{\odot}$)=3.6; above this mass, all clusters have well-populated main sequences of massive stars and should have accurate recovery of their MF slopes, based on extensive tests with artificial clusters. We find the ensemble IMF is best described by a mean high-mass slope of $\overline{\Gamma} = 1.49\pm0.18$, with an intrinsic scatter of $\sigma^{2}_{\Gamma} = 0.02^{+0.16}_{0.00}$, consistent with a universal IMF. We find no dependence of the IMF on environmental impacts such as the local star formation rate or galactocentric radius within M33, which serves as a proxy for metallicity. This $\overline{\Gamma}$ measurement is consistent with similar measurements in M31, despite M33 having a much higher star formation rate intensity. While this measurement is formally consistent with the canonical Kroupa ($\Gamma = 1.30$) IMF, as well as the Salpeter ($\Gamma = 1.35)$) value, it is the second Local Group cluster sample to show evidence for a somewhat steeper high-mass IMF slope. We explore the impacts a steeper IMF slope has on a number of astronomical sub-fields., Comment: Accepted for publication in ApJ. 9 Figures, 1 Table

Omega Centauri ($\omega$ Cen) is the most massive globular cluster of the Milky Way. It is thought to be the nucleus of an accreted dwarf galaxy because of its high mass and its complex stellar populations. To decipher its formation history and study its dynamics, we created the most comprehensive kinematic catalog for its inner region, by analyzing both archival and new Hubble Space Telescope (HST) data. Our catalog contains 1 395 781 proper-motion measurements out to the half-light radius of the cluster ($\sim$5.0') and down to $m_{F625W}\approx$25. The typical baseline for our proper-motion measurements is 20 years, leading to a median 1D proper motion precision of $\sim$11 $\mu$as yr$^{-1}$ for stars with $m_{F625W}\approx$18 mag, with even better precision ($\sim$6.6 $\mu$as yr$^{-1}$) achieved in the extensively observed centermost (r$<$1.5') region. In addition to our astrometric measurements, we also obtained precise HST photometry in seven filters spanning from the ultraviolet to the near-infrared. This allows detailed color-magnitude-diagram studies and to separate the multiple stellar populations of the cluster. In this work, we describe the data reduction used to obtain both the photometric and the proper-motion measurements. We also illustrate the creation and the content of our catalog, which is made publicly available. Finally, we present measurements of the plane-of-sky rotation of $\omega$ Cen in the previously unprobed inner few arcminutes and a precise measurement of the inclination $i = (43.9\pm1.3)^\circ$., Comment: 43 pages, 25 figures, 9 tables. Published by ApJ. The full catalog is publicly available under: https://doi.org/10.5281/zenodo.11104046

We have conducted a systematic search around the Milky Way (MW) analog NGC 253 (D=3.5 Mpc), as a part of the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) - a Magellan+Megacam survey to identify dwarfs and other substructures in resolved stellar light around MW-mass galaxies outside of the Local Group. In total, NGC 253 has five satellites identified by PISCeS within 100 kpc with an absolute V-band magnitude $M_V<-7$. We have additionally obtained deep Hubble Space Telescope imaging of four reported candidates beyond the survey footprint: Do III, Do IV, and dw0036m2828 are confirmed to be satellites of NGC 253, while SculptorSR is found to be a background galaxy. We find no convincing evidence for the presence of a plane of satellites surrounding NGC 253. We construct its satellite luminosity function, which is complete down to $M_V$$\lesssim$$-8$ out to 100 kpc and $M_V$$\lesssim$$-9$ out to 300 kpc, and compare it to those calculated for other Local Volume galaxies. Exploring trends in satellite counts and star-forming fractions among satellite systems, we find relationships with host stellar mass, environment, and morphology, pointing to a complex picture of satellite formation, and a successful model has to reproduce all of these trends., Comment: Submitted to AAS Journal. Comments are welcome

As the nearest accessible massive early-type galaxy, NGC 5128 presents an exceptional opportunity to measure dark matter halo parameters for a representative elliptical galaxy. Here we take advantage of rich new observational datasets of large-radius tracers to perform dynamical modeling of NGC 5128, using a discrete axisymmetric anisotropic Jeans approach with a total tracer population of nearly 1800 planetary nebulae, globular clusters, and dwarf satellite galaxies extending to a projected distance of $\sim250$ kpc from the galaxy center. We find that a standard NFW halo provides an excellent fit to nearly all the data, excepting a subset of the planetary nebulae that appear to be out of virial equilibrium. The best-fit dark matter halo has a virial mass of ${\rm M}_{vir}=4.4^{+2.4}_{-1.4}\times10^{12} {\rm M}_{\odot}$, and NGC 5128 appears to sit below the mean stellar mass--halo mass and globular cluster mass--halo mass relations, which both predict a halo virial mass closer to ${\rm M}_{vir} \sim 10^{13} {\rm M}_{\odot}$. The inferred NFW virial concentration is $c_{vir}=5.6^{+2.4}_{-1.6}$, nominally lower than $c_{vir} \sim 9$ predicted from published $c_{vir}$--${\rm M}_{vir}$ relations, but within the $\sim 30\%$ scatter found in simulations. The best-fit dark matter halo constitutes only $\sim10\%$ of the total mass at 1 effective radius but $\sim50\%$ at 5 effective radii. The derived halo parameters are relatively insensitive to reasonable variations in the tracer population considered, tracer anisotropies, and system inclination. Our analysis highlights the value of comprehensive dynamical modeling of nearby galaxies, and the importance of using multiple tracers to allow cross-checks for model robustness., Comment: Submitted to A&A on 20.06.2023

We use young clusters and giant molecular clouds (GMCs) in the galaxies M33 and M31 to constrain temporal and spatial scales in the star formation process. In M33, we compare the PHATTER catalogue of 1214 clusters with ages measured via colour-magnitude diagram (CMD) fitting to 444 GMCs identified from a new 35 pc resolution ALMA $^{12}$CO(2-1) survey. In M31, we compare the PHAT catalogue of 1249 clusters to 251 GMCs measured from a CARMA $^{12}$CO(1-0) survey with 20 pc resolution. Through two-point correlation analysis, we find that young clusters have a high probability of being near other young clusters, but correlation between GMCs is suppressed by the cloud identification algorithm. By comparing the positions, we find that younger clusters are closer to GMCs than older clusters. Through cross-correlation analysis of the M33 cluster data, we find that clusters are statistically associated when they are $\leq$10 Myr old. Utilizing the high precision ages of the clusters, we find that clusters older than $\approx 18$ Myr are uncorrelated with the molecular ISM. Using the spatial coincidence of the youngest clusters and GMCs in M33, we estimate that clusters spend $\approx$4-6 Myr inside their parent GMC. Through similar analysis, we find that the GMCs in M33 have a total lifetime of $\approx 11$-15 Myr. We also develop a drift model and show that the above correlations can be explained if the clusters in M33 have a 5-10 km s$^{-1}$ velocity dispersion relative to the molecular ISM., Comment: 14 pages, 13 figures, 1 tables, accepted for publication at MNRAS

We present a new approach to measuring the thickness of a partially face-on stellar disk, using dust geometry. In a moderately-inclined disk galaxy, the fraction of reddened stars is expected to be 50% everywhere, assuming that dust lies in a thin midplane. In a thickened disk, however, a wide range of radii project onto the line of sight. Assuming stellar density declines with radius, this geometrical projection leads to differences in the numbers of stars on the near and far sides of the thin dust layer. The fraction of reddened stars will thus differ from the 50% prediction, with a deviation that becomes larger for puffier disks. We map the fraction of reddened red giant branch (RGB) stars across M31, which shows prominent dust lanes on only one side of the major axis. The fraction of reddened stars varies systematically from 20% to 80%, which requires that these stars have an exponential scale height h_z that is 0.14+/-0.015 times the exponential scale length (h_r~5.5kpc). M31's RGB stars must therefore have h_z=770+/-80pc, which is far thicker than the Milky Way's thin disk, but comparable to its thick disk. The lack of a significant thin disk in M31 is unexpected, but consistent with its interaction history and high disk velocity dispersion. We suggest that asymmetric reddening be used as a generic criteria for identifying ``thick disk'' dominated systems, and discuss prospects for future 3-dimensional tomographic mapping of the gas and stars in M31., Comment: 22 pages. Accepted to the Astrophysical Journal

We use high-resolution Hubble Space Telescope imaging data of dwarf galaxies in the Local Volume ($\lesssim 11$ Mpc) to parameterise 19 newly discovered nuclear star clusters (NSCs). Most of the clusters have stellar masses of $M_{\star}^{nsc} \lesssim 10^6$ M$_{\odot}$ and compare to Galactic globular clusters in terms of ellipticity, effective radius, stellar mass, and surface density. The clusters are modelled with a S\'ersic profile and their surface brightness evaluated at the effective radius reveals a tight positive correlation to the host galaxy stellar mass. Our data also indicate an increase in slope of the density profiles with increasing mass, perhaps indicating an increasing role for in-situ star formation in more massive hosts. We evaluate the scaling relation between the clusters and their host galaxy stellar mass to find an environmental dependence: for NSCs in field galaxies, the slope of the relation is $\alpha = 0.82^{+0.08}_{-0.08}$ whereas $\alpha = 0.55^{+0.06}_{-0.05}$ for dwarfs in the core of the Virgo cluster. Restricting the fit for the cluster to $M_{\star}^{gal} \geq 10^{6.5}$ M$_{\odot}$ yields $\alpha = 0.70^{+0.08}_{-0.07}$, in agreement with the field environment within the $1\sigma$ interval. The environmental dependence is due to the lowest-mass nucleated galaxies and we speculate that this is either due to an increased number of progenitor globular clusters merging to become an NSC, or due to the formation of more massive globular clusters in dense environments, depending on the initial globular cluster mass function. Our results clearly corroborate recent results in that there exists a tight connection between NSCs and globular clusters in dwarf galaxies., Comment: 19 pages, 12 figures, 5 tables. Resubmitted to MNRAS

We combine archival HST and new JWST imaging data, covering the ultraviolet to mid-infrared regime, to morphologically analyze the nuclear star cluster (NSC) of NGC 628, a grand-design spiral galaxy. The cluster is located in a 200 pc x 400 pc cavity, lacking both dust and gas. We find roughly constant values for the effective radius (r_eff ~ 5 pc) and ellipticity ({\epsilon} ~ 0.05), while the S\'ersic index (n) and position angle (PA) drop from n ~ 3 to ~ 2 and PA ~ 130{\deg} to 90{\deg}, respectively. In the mid-infrared, r_eff ~ 12pc, {\epsilon} ~ 0.4, and n ~ 1-1.5, with the same PA ~ 90{\deg}. The NSC has a stellar mass of log10 (M_nsc / M_Sun) = 7.06 +- 0.31, as derived through B-V, confirmed when using multi-wavelength data, and in agreement with the literature value. Fitting the spectral energy distribution, excluding the mid-infrared data, yields a main stellar population's age of (8 +- 3) Gyr with a metallicity of Z = 0.012 +- 0.006. There is no indication of any significant star formation over the last few Gyr. Whether gas and dust were dynamically kept out or evacuated from the central cavity remains unclear. The best-fit suggests an excess of flux in the mid-infrared bands, with further indications that the center of the mid-infrared structure is displaced with respect to the optical center of the NSC. We discuss five potential scenarios, none of them fully explaining both the observed photometry and structure., Comment: 26 pages, 10 figures, 6 tables. Accepted for publication by ApJL

We present comprehensive multi-component dynamical models of M54 (NGC6715), the nuclear star cluster of the Sagittarius dwarf galaxy (Sgr), which is undergoing a tidal disruption in the Milky Way halo. Previous papers in the series used a large MUSE mosaic data set to identify multiple stellar populations in the system and study their kinematic differences. Here we use Jeans-based dynamical models that fit the population properties (mean age and metallicity), spatial distributions, and kinematics simultaneously. They provide a solid physical explanation to our previous findings. The population-dynamical models deliver a comprehensive view of the whole system, and allow us to disentangle the different stellar populations. We explore their dynamical interplay and confirm our previous findings about the build-up of Sgr's nuclear cluster via contributions from globular cluster stars, Sgr inner field stars, and in-situ star formation. We explore various parameterisations of the gravitational potential and show the importance of a radially varying mass-to-light ratio for the proper treatment of the mass profile. We find a total dynamical mass within M54's tidal radius ($\sim75$ pc) of $1.60\pm0.07\times10^6$ Msun in excellent agreement with $N$-body simulations. The metal-poor globular cluster stars contribute about $65\%$ of the total mass or $1.04\pm0.05\times10^6$ Msun. The metal-rich stars can be further divided into young and intermediate age populations that contribute $0.32\pm0.02\times10^6$ Msun ($20\%$) and $0.24\pm0.02\times10^6$ Msun ($15\%$), respectively. Our population-dynamical models successfully distinguish the different stellar populations in Sgr's nucleus because of their different spatial distributions, ages, metallicities, and kinematic features., Comment: Accepted for publication in ApJ

We construct a catalog of star clusters from Hubble Space Telescope images of the inner disk of the Triangulum Galaxy (M33) using image classifications collected by the Local Group Cluster Search, a citizen science project hosted on the Zooniverse platform. We identify 1214 star clusters within the Hubble Space Telescope imaging footprint of the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) survey. Comparing this catalog to existing compilations in the literature, 68% of the clusters are newly identified. The final catalog includes multi-band aperture photometry and fits for cluster properties via integrated light SED fitting. The cluster catalog's 50% completeness limit is ~1500 solar masses at an age of 100 Myr, as derived from comprehensive synthetic cluster tests., Comment: 24 pages, 15 figures, 9 tables, Accepted to ApJ

Studies of resolved stellar populations in the Milky Way and nearby galaxies reveal an amazingly detailed and clear picture of galaxy evolution. Within the Local Group, the ability to probe the stellar populations of small and large galaxies opens up the possibility of exploring key questions such as the nature of dark matter, the detailed formation history of different galaxy components, and the role of accretion in galactic formation. Upcoming wide-field surveys promise to extend this ability to all galaxies within 10~Mpc, drastically increasing our capability to decipher galaxy evolution and enabling statistical studies of galaxies' stellar populations. To facilitate the optimum use of these upcoming capabilities we develop a simple formalism to predict the density of resolved stars for an observation of a stellar population at fixed surface brightness and population parameters. We provide an interface to calculate all quantities of interest to this formalism via a public release of the code: \texttt{walter}. This code enables calculation of (i) the expected number density of detected stars, (ii) the exposure time needed to reach certain population features, such as the horizontal branch, and (iii) an estimate of the crowding limit, among other features. These calculations will be very useful for planning surveys with NASA's upcoming Nancy Grace Roman Space Telescope (Roman, formerly WFIRST), which we use for example calculations throughout this work., Comment: 16 pages, 6 figures, submitted to AJ. Comments welcome

In the coming decade, thousands of stellar streams will be observed in the halos of external galaxies. What fundamental discoveries will we make about dark matter from these streams? As a first attempt to look at these questions, we model Magellan/Megacam imaging of the Centaurus A's (Cen A) disrupting dwarf companion Dwarf 3 (Dw3) and its associated stellar stream, to find out what can be learned about the Cen A dark-matter halo. We develop a novel external galaxy stream-fitting technique and generate model stellar streams that reproduce the stream morphology visible in the imaging. We find that there are many viable stream models that fit the data well, with reasonable parameters, provided that Cen A has a halo mass larger than M$_{200}$ $>4.70\times 10^{12}$ M$_{\odot}$. There is a second stream in Cen A's halo that is also reproduced within the context of this same dynamical model. However, stream morphology in the imaging alone does not uniquely determine the mass or mass distribution for the Cen A halo. In particular, the stream models with high likelihood show covariances between the inferred Cen A mass distribution, the inferred Dw3 progenitor mass, the Dw3 velocity, and the Dw3 line-of-sight position. We show that these degeneracies can be broken with radial-velocity measurements along the stream, and that a single radial velocity measurement puts a substantial lower limit on the halo mass. These results suggest that targeted radial-velocity measurements will be critical if we want to learn about dark matter from extragalactic stellar streams., Comment: 11 figures, 23 pages, Accepted to ApJ

We measure the star cluster mass function for the Local Group galaxy M33. We use the catalog of stellar clusters selected from the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) survey. We analyze 711 clusters in M33 with $\rm 7.0 < log(Age/yr) < 8.5$, and log($M/M_{\odot}$) $>$ 3.0 as determined from color-magnitude diagram fits to individual stars. The M33 cluster mass function is best described by a Schechter function with power law slope $\alpha = -2.06^{+0.14}_{-0.13}$, and truncation mass log($M_c/M_{\odot}$) $= 4.24^{+0.16}_{-0.13}$. The data show strong evidence for a high-mass truncation, thus strongly favoring a Schechter function fit over a pure power law. M33's truncation mass is consistent with the previously identified linear trend between $M_c$, and star formation rate surface density, \SigSFR. We also explore the effect that individual cluster mass uncertainties have on derived mass function parameters, and find evidence to suggest that large cluster mass uncertainties have the potential to bias the truncation mass of fitted mass functions on the one sigma level., Comment: 18 pages, 15 figures, 1 table, Accepted to ApJ (February 2, 2022)

The dense central regions of tidally disrupted galaxies can survive as ultra-compact dwarfs (UCDs) that hide among the luminous globular clusters (GCs) in the halo of massive galaxies. An exciting confirmation of this model is the detection of overmassive black holes in the centers of some UCDs, which also lead to elevated dynamical mass-to-light ratios ($M/L_{dyn}$). Here we present new high-resolution spectroscopic observations of 321 luminous GC candidates in the massive galaxy NGC 5128/Centaurus A. Using these data we confirm 27 new luminous GCs, and measure velocity dispersions for 57 luminous GCs (with $g$-band luminosities between $2.5 \times 10^5$ and $2.5 \times 10^7 L_{\odot}$), of which 48 are new measurements. Combining these data with size measurements from Gaia, we determine the $M/L_{dyn}$ for all 57 luminous GCs. We see a clear bimodality in the $M/L_{dyn}$ distribution, with a population of normal GCs with mean $M/L_{dyn}=1.51\pm0.31$, and a second population of $\sim$20 GCs with elevated mean $M/L_{dyn}=2.68\pm0.22$. We show that black holes with masses $\sim4$-18 % of the luminous GCs can explain the elevated mass-to-light ratios. Hence, it is plausible that the NGC 5128 sources with elevated $M/L_{dyn}$ are mostly stripped galaxy nuclei that contain massive central black holes, though future high spatial resolution observations are necessary to confirm this hypothesis for individual sources. We also present a detailed discussion of an extreme outlier, \textit{VHH81-01}, one of the largest and most massive GC in NGC 5128, making it an exceptionally strong candidate to be a tidally stripped nucleus., Comment: Accepted to ApJ

We investigated whether the two recently discovered nuclei in NGC7727 both host a super-massive black hole (SMBH). We used the high spatial resolution mode of the integral-field spectrograph MUSE on the VLT in adaptive optics mode to resolve the stellar kinematics within the sphere of influence of both putative black holes. We combined the kinematic data with an HST-based mass model and used Jeans models to measure their SMBH mass. We report the discovery of a dual SMBH system in NGC7727. We detect a SMBH in the photometric center of the galaxy in Nucleus 1, with a mass of $M_{\rm SMBH}=1.54^{+0.18}_{-0.15} \times10^{8}M_{\odot}$. In the second nucleus, which is 500pc offset from the main nucleus, we also find a clear signal for a SMBH with a mass of $M_{\rm BH}=6.33^{+3.32}_{-1.40}\times10^{6}M_{\odot}$. Both SMBHs are detected at high significance. The off-axis nature of Nucleus 2 makes modeling the system challenging; but a number of robustness tests suggest that a black hole is required to explain the kinematics. The SMBH in the offset Nucleus 2 makes up 3.0% of its total mass, which means its SMBH is over-massive compared to the $M_{\rm BH}-M_{\rm Bulge}$ scaling relation. This confirms it as the surviving nuclear star cluster of a galaxy that has merged with NGC7727. This discovery is the first dynamically confirmed dual SMBH system with a projected separation of less than a kiloparsec and the nearest dynamically confirmed dual SMBH at a distance of 27.4Mpc. The second Nucleus is in an advanced state of inspiral, and it will eventually result in a 1:24 mass ratio SMBH merger. Optical emission lines suggest Nucleus 2 is a Seyfert galaxy, making it a low-luminosity Active Galactic Nuclei (AGN). There are likely many more quiescent SMBHs as well as dual SMBH pairs in the local Universe that have been missed by surveys that focus on bright accretion signatures., Comment: Accepted to A&A, 16 pages, 15 figures

The formation of nuclear star clusters (NSCs) remains an open question. In this work, we use spatially resolved HST/STIS spectroscopic observations of three nearby NSCs (hosted by NGC 5102, NGC 5206, and NGC 205) to constrain their formation histories by exploring radial variations of the stellar populations within each cluster. Utilizing full-spectrum fitting, we find substantial age and metallicity gradients within the central 0."9 (16~pc) of the NSC in NGC 5102 where populations near the center are young/metal-rich (age ~400~Myr and [M/H] ~-0.4) and become older/metal-poor at larger radii (mean age ~1~Gyr and mean [M/H] ~-1.6 in the radial range [0."3, 0."9]). This behavior suggests that the young/metal-rich population at the center was formed from a period of in situ formation, while the older/metal-poor populations were likely formed by inspiraled globular clusters. The two broad populations observed in the NGC 5102 NSC (young/metal-rich and old/metal-poor) appear to be linked to the transition between the two morphological components of the NSC derived from the surface-brightness profile in Nguyen et al. (2018)(arXiv:1711.04314). The radial ranges explored in NGC 5206 and NGC 205 were much smaller due to poor data quality; in NGC 5206 we find a similar metallicity gradient to NGC 5102 (but with much lower significance), while the data for NGC 205 is too poor to reach any conclusions. Overall, this data highlights the links between the morphological and stellar population complexity of NSCs and their formation mechanisms., Comment: 18 pages, 12 figures, to be published in The Astronomical Journal

We present deep Hubble Space Telescope imaging of five faint dwarf galaxies associated with the nearby spiral NGC 253 (D$\approx$3.5 Mpc). Three of these are newly discovered ultra-faint dwarf galaxies, while all five were found in the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), a Magellan$+$Megacam survey to identify faint dwarfs and other substructures in resolved stellar light around massive galaxies outside of the Local Group. Our HST data reach $\gtrsim$3 magnitudes below the tip of the red giant branch for each dwarf, allowing us to derive their distances, structural parameters, and luminosities. All five systems contain predominantly old, metal-poor stellar populations (age$\sim$12 Gyr, [M/H]$\lesssim$$-$1.5) and have sizes ($r_{h}$$\sim$110-3000 pc) and luminosities ($M_V$$\sim$$-7$ to $-12$ mag) largely consistent with Local Group dwarfs. The three new NGC 253 satellites are among the faintest systems discovered beyond the Local Group. We also use archival HI data to place limits on the gas content of our discoveries. Deep imaging surveys such as our program around NGC 253 promise to elucidate the faint end of the satellite luminosity function and its scatter across a range of galaxy masses, morphologies, and environments in the decade to come., Comment: 11 pages, 4 figures, 1 table; submitted to ApJL; comments welcome

Nuclear star clusters (NSCs) are a common phenomenon in galaxy centres and are found in a vast majority of galaxies of intermediate stellar mass $10^{9}$ M$_{\odot}$. Recent investigations suggest that they are rarely found in the least and most massive galaxies and that the nucleation fraction increases in dense environments. It is unclear whether this trend holds true for field galaxies due to the limited data currently available. Here we present our results on the nucleation fraction for 601 galaxies in the Local Volume ($\lesssim 12$ Mpc). Covering more than eight orders of magnitude in stellar mass, this is the largest sample of galaxies analysed in a low-density environment. Within the Local Volume sample we find a strong dependence of the nucleation fraction on galaxy stellar mass, in agreement with previous work. We also find that for galaxies with $M_{\star} < 10^{9}$ M$_{\odot}$, early-type galaxies have a higher nucleation fraction than late-types. The nucleation fraction in the Local Volume correlates independently with stellar mass, Hubble type, and local environmental density. We compare our data to those in galaxy cluster environments (Coma, Fornax, and Virgo) by compiling previous results and calculating stellar masses in a homogeneous way. We find significantly lower nucleation fractions (up to 40$\%$) in galaxies with $M_{\star} \lesssim 10^{9.5}$ M$_{\odot}$, in agreement with previous work. Our results reinforce the connection between globular clusters and NSCs, but it remains unclear if it can explain the observed trends with Hubble type and local environment. We speculate that correlation between the nucleation fraction and cluster environment weakens for the densest clusters like Coma and Virgo., Comment: 22 pages, 21 figures, 6 tables, resubmitted to MNRAS after referee report

We present a preview of the faint dwarf galaxy discoveries that will be possible with the Vera C. Rubin Observatory and Subaru Hyper Suprime-Cam in the next decade. In this work, we combine deep ground-based images from the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) and extensive image simulations to investigate the recovery of faint, resolved dwarf galaxies in the Local Volume with a matched-filter technique. We adopt three fiducial distances - 1.5, 3.5, 5 Mpc, and quantitatively evaluate the effects on dwarf detection of varied stellar backgrounds, ellipticity, and Milky Way foreground contamination and extinction. We show that our matched-filter method is powerful for identifying both compact and extended systems, and near-future surveys will be able to probe at least ~4.5 mag below the tip of the red giant branch (TRGB) for a distance of up to 1.5 Mpc, and ~2 mag below the TRGB at 5 Mpc. This will push the discovery frontier for resolved dwarf galaxies to fainter magnitudes, lower surface brightnesses, and larger distances. Our simulations show the secure census of dwarf galaxies down to $M_{V}$$\approx$-5, -7, -8, will be soon within reach, out to 1.5 Mpc, 3.5 Mpc, and 5 Mpc, respectively, allowing us to quantify the statistical fluctuations in satellite abundances around hosts, and parse environmental effects as a function of host properties., Comment: Submitted to AAS journals. Comments are welcome

We present the results from an observing campaign to confirm the peculiar motion of the supermassive black hole (SMBH) in J0437+2456 first reported in Pesce et al. (2018). Deep observations with the Arecibo Observatory have yielded a detection of neutral hydrogen (HI) emission, from which we measure a recession velocity of 4910 km s$^{-1}$ for the galaxy as a whole. We have also obtained near-infrared integral field spectroscopic observations of the galactic nucleus with the Gemini North telescope, yielding spatially resolved stellar and gas kinematics with a central velocity at the innermost radii ($0.1^{\prime \prime} \approx 34$ pc) of 4860 km s$^{-1}$. Both measurements differ significantly from the $\sim$4810 km s$^{-1}$ H$_2$O megamaser velocity of the SMBH, supporting the prior indications of a velocity offset between the SMBH and its host galaxy. However, the two measurements also differ significantly from one another, and the galaxy as a whole exhibits a complex velocity structure that implies the system has recently been dynamically disturbed. These results make it clear that the SMBH is not at rest with respect to the systemic velocity of the galaxy, though the specific nature of the mobile SMBH -- i.e., whether it traces an ongoing galaxy merger, a binary black hole system, or a gravitational wave recoil event -- remains unclear., Comment: 17 pages, 8 figures, accepted for publication in ApJ

The presence of AGB stars in clusters provides key constraints for stellar models, as has been demonstrated with historical data from the Magellanic Clouds. In this work, we look for candidate AGB stars in M31 star clusters from the Panchromatic Hubble Andromeda Treasury (PHAT) survey. Our photometric criteria selects stars brighter than the tip of the red giant branch, which includes the bulk of the thermally-pulsing AGB stars as well as early-AGB stars and other luminous cool giants expected in young stellar populations (e.g. massive red supergiants, and intermediate-mass red helium-burning stars). The AGB stars can be differentiated, a posteriori, using the ages already estimated for our cluster sample. 937 candidates are found within the cluster aperture radii, half (450) of which are very likely cluster members. Cross-matching with additional databases reveals two carbon stars and ten secure variables among them. The field-corrected age distribution reveals the presence of young supergiants peaking at ages smaller than 100 Myr, followed by a long tail of AGB stars extending up to the oldest possible ages. This long tail reveals the general decrease in the numbers of AGB stars from initial values of 50e-6/Msun at 100 Myr down to 5e-6/Msun at 10 Gyr. Theoretical models of near-solar metallicity reproduce this general trend, although with localized discrepancies over some age intervals, whose origin is not yet identified. The entire catalogue is released together with finding charts to facilitate follow-up studies., Comment: To appear in ApJ. Data will be soon on MAST

The ultra-faint dwarf galaxy Hercules has an extremely elongated morphology with both photometric overdensities and kinematic members at large radii, suggesting that it may be tidally disrupting due to a previous close encounter with the Milky Way. To explain its observational peculiarities, we present a deep Hubble Space Telescope (HST) imaging study of Hercules and its surrounding regions and investigate its tidal history through a careful search for a distance gradient along its stretched body. Our off-center HST data clearly resolve a main sequence, showing that the stellar extension seen along the major-axis of Hercules is genuine, not a clump of background galaxies. Utilizing Gaia DR2 data, we clean the region around Hercules of field contamination, and find four new plausible member stars, all of which are located at the outskirts of the dwarf galaxy. We update the distance to Hercules, and find $130.6\pm1.2$ kpc ($m-M=20.58\pm0.02$) for the main body, which is consistent with earlier estimates in the literature. While we find no conclusive evidence for a distance gradient, our work demonstrates that constraining a distance gradient in such a faint system is not trivial, and the possible thickness of the dwarf along the line of sight and field contamination make it harder to make decisive conclusions even with these high-precision data. Future studies coupled with tailored theoretical models are needed to understand the true nature of Hercules and of tidal distortion observables in ultra-faint galaxies in general., Comment: 17 pages, 7 figures, submitted to ApJ

We estimate the mass of the intermediate-mass black hole at the heart of the dwarf elliptical galaxy NGC 404 using Atacama Large Millimeter/submillimeter Array (ALMA) observations of the molecular interstellar medium at an unprecedented linear resolution of ~0.5 pc, in combination with existing stellar kinematic information. These ALMA observations reveal a central disc/torus of molecular gas clearly rotating around the black hole. This disc is surrounded by a morphologically and kinematically complex flocculent distribution of molecular clouds, that we resolve in detail. Continuum emission is detected from the central parts of NGC 404, likely arising from the Rayleigh-Jeans tail of emission from dust around the nucleus, and potentially from dusty massive star-forming clumps at discrete locations in the disc. Several dynamical measurements of the black hole mass in this system have been made in the past, but they do not agree. We show here that both the observed molecular gas and stellar kinematics independently require a ~5x10$^5$ Msun black hole once we include the contribution of the molecular gas to the potential. Our best estimate comes from the high-resolution molecular gas kinematics, suggesting the black hole mass of this system is 5.5$^{+4.1}_{-3.8}\times$10$^5$ Msun (at the 99% confidence level), in good agreement with our revised stellar kinematic measurement and broadly consistent with extrapolations from the black hole mass - velocity dispersion and black hole mass - bulge mass relations. This highlights the need to accurately determine the mass and distribution of each dynamically important component around intermediate-mass black holes when attempting to estimate their masses., Comment: 18 pages, 15 figures, MNRAS accepted

The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is in an advanced stage of disruption but still hosts its nuclear star cluster (NSC), M54, at its center. In this paper, we present a detailed kinematic characterization of the three stellar populations present in M54: young metal-rich (YMR); intermediate-age metal-rich (IMR); and old metal-poor (OMP), based on the spectra of $\sim6500$ individual M54 member stars extracted from a large MUSE/VLT dataset. We find that the OMP population is slightly flattened with a low amount of rotation ($\sim0.8$ km s$^{-1}$) and with a velocity dispersion that follows a Plummer profile. The YMR population displays a high amount of rotation ($\sim5$ km s$^{-1}$) and a high degree of flattening, with a lower and flat velocity dispersion profile. The IMR population shows a high but flat velocity dispersion profile, with some degree of rotation ($\sim2$ km s$^{-1}$). We complement our MUSE data with information from \textit{Gaia DR2} and confirm that the stars from the OMP and YMR populations are comoving in 3D space, suggesting that they are dynamically bound. While dynamical evolutionary effects (e.g. energy equipartition) are able to explain the differences in velocity dispersion between the stellar populations, the strong differences in rotation indicate different formation paths for the populations, as supported by an $N$-body simulation tailored to emulate the YMR-OMP system. This study provides additional evidence for the M54 formation scenario proposed in our previous work, where this NSC formed via GC accretion (OMP) and in situ formation from gas accretion in a rotationally supported disc (YMR)., Comment: 21 Pages, 10 figures. Accepted for publication by ApJ

Tidally stripped galaxy nuclei and luminous globular clusters (GCs) are important tracers of the halos and assembly histories of nearby galaxies, but are difficult to reliably identify with typical ground-based imaging data. In this paper we present a new method to find these massive star clusters using Gaia DR2, focusing on the massive elliptical galaxy Centaurus A (Cen A). We show that stripped nuclei and globular clusters are partially resolved by Gaia at the distance of Cen A, showing characteristic astrometric and photometric signatures. We use this selection method to produce a list of 632 new candidate luminous clusters in the halo of Cen A out to a projected radius of 150 kpc. Adding in broadband photometry and visual examination improves the accuracy of our classification. In a spectroscopic pilot program we have confirmed 5 new luminous clusters, which includes the 7th and 10th most luminous GC in Cen\,A. Three of the newly discovered GCs are further away from Cen A in than all previously known GCs. Several of these are compelling candidates for stripped nuclei. We show that our novel Gaia selection method retains at least partial utility out to distances of 25 Mpc and hence is a powerful tool for finding and studying star clusters in the sparse outskirts of galaxies in the local universe., Comment: Accepted to ApJ

The center of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) hosts a nuclear star cluster, M54, which is the only galaxy nucleus that can be resolved into individual stars at optical wavelengths. It is thus a key target for understanding the formation of nuclear star clusters and their relation to globular clusters. We present a large Multi-Unit Spectroscopic Explorer (MUSE) data set that covers M54 out to $\sim$2.5 half-light radius, from which we extracted the spectra of $\sim$6600 cluster member stars. We use these data in combination with HST photometry to derive age and metallicity for each star. The stellar populations show a well defined age-metallicity relation, implying an extended formation history for the central region of Sgr dSph. We classify these populations into three groups, all with the same systemic velocity: young metal-rich (YMR; 2.2\,Gyr, \mbox{[Fe/H]$=-0.04$}); intermediate-age metal-rich (IMR; 4.3\,Gyr, \mbox{[Fe/H]$=-0.29$}); and old metal-poor (OMP; 12.2\,Gyr, \mbox{[Fe/H]$=-1.41$}). The YMR and OMP populations are more centrally concentrated than the IMR population, which are likely stars of the Sgr dSph. We suggest the OMP population is the result of accretion and merging of two or more old and metal-poor globular clusters dragged to the center by dynamical friction. The YMR is consistent with being formed by in situ star formation in the nucleus. The ages of the YMR population suggest that it may have been triggered into forming when the Sgr dSph began losing its gas during the most recent interaction with the Milky Way, $\sim$3\,Gyr ago., Comment: 24 pages, 13 figures. Accepted for publication in ApJ

Nuclear star clusters are found at the centers of most galaxies. They are the densest stellar systems in the Universe, and thus have unique and interesting stellar dynamics. We review how common nuclear star clusters are in galaxies of different masses and types, and then discuss the typical properties of NSCs. We close by discussing the formation of NSCs, and how a picture is emerging of different formation mechanisms being dominant in lower and higher mass galaxies., Comment: Invited Review from IAUS351: "Star Clusters: from the Milky Way to the Early Universe"

The ultra-faint dwarf galaxy Leo V has shown both photometric overdensities and kinematic members at large radii, along with a tentative kinematic gradient, suggesting that it may have undergone a close encounter with the Milky Way. We investigate these signs of disruption through a combination of i) high-precision photometry obtained with the Hubble Space Telescope (HST), ii) two epochs of stellar spectra obtained with the Hectochelle Spectrograph on the MMT, and iii) measurements from the Gaia mission. Using the HST data, we examine one of the reported stream-like overdensities at large radii, and conclude that it is not a true stellar stream, but instead a clump of foreground stars and background galaxies. Our spectroscopic analysis shows that one known member star is likely a binary, and challenges the membership status of three others, including two distant candidates that had formerly provided evidence for overall stellar mass loss. We also find evidence that the proposed kinematic gradient across Leo V might be due to small number statistics. We update the systemic proper motion of Leo V, finding $(\mu_\alpha \cos\delta, \mu_\delta)= (0.009\pm0.560$, $-0.777\pm0.314)$ mas yr$^{-1}$, which is consistent with its reported orbit that did not put Leo V at risk of being disturbed by the Milky Way. These findings remove most of the observational clues that suggested Leo V was disrupting, however, we also find new plausible member stars, two of which are located >5 half-light radii from the main body. These stars require further investigation. Therefore, the nature of Leo V still remains an open question., Comment: Higher resolution figures are available upon request. Submitted to the ApJ

We present the first measurement of the mass of a supermassive black hole (SMBH) in the nearby double-barred spiral galaxy NGC 3504 as part of the Measuring Black Holes Below the Milky Way ($M_{\star}$) mass galaxies (MBHBM$_{\star}$) Project. Our analysis is based on Atacama Large Millimeter/submillimeter Array (ALMA) Cycle-5 observations of the ${\rm ^{12}CO(2-1)}$ emission line. NGC 3504 has a circumnuclear gas disk (CND), which has a relatively high-velocity dispersion of 30 \kms. Our dynamical models of the CND yield a \Mbh~of $M_{\rm BH}=1.02^{+0.18}_{-0.15}\times10^7$\Msun and a mass-to-light ratio in $H$-band of \ml$_{\rm H}=0.66^{+1.44}_{-0.65}$ (\Msun/\Lsun). This black hole (BH) mass is consistent with BH--galaxy scaling relations. We also detect a central deficiency in the ${\rm ^{12}CO(2-1)}$ integrated intensity map with a diameter of 2.7 pc at the putative position of the SMBH. However, this hole is filled by a dense gas tracer ${\rm CS(5-4)}$ that peaks at the galaxy center found in one of the three low-velocity-resolution continuum spectral correlators. The ${\rm CS(5-4)}$ line has the same kinematics with the ${\rm ^{12}CO(2-1)}$ line within the CND, suggesting that it is also an alternative transition for measuring the central \Mbh~in NGC 3504 probably more accurately than the current commonly used of ${\rm ^{12}CO(2-1)}$ due to its centralization., Comment: 25 pages, 15 Figures, 6 Tables, Submitted to ApJ

We improve the dynamical black hole (BH) mass estimates in three nearby low-mass early-type galaxies--NGC 205, NGC 5102, and NGC 5206. We use new \hst/STIS spectroscopy to fit the star formation histories of the nuclei in these galaxies, and use these measurements to create local color--mass-to-light ratio (\ml) relations. We then create new mass models from \hst~imaging and combined with adaptive optics kinematics, we use Jeans dynamical models to constrain their BH masses. The masses of the central BHs in NGC 5102 and NGC 5206 are both below one million solar masses and are consistent with our previous estimates, $9.12_{-1.53}^{+1.84}\times10^5$\Msun~and $6.31_{-2.74}^{+1.06}\times10^5$\Msun~(3$\sigma$ errors), respectively. However, for NGC 205, the improved models suggest the presence of a BH for the first time, with a best-fit mass of $6.8_{-6.7}^{+95.6}\times10^3$\Msun~(3$\sigma$ errors). This is the least massive central BH mass in a galaxy detected using any method. We discuss the possible systematic errors of this measurement in detail. Using this BH mass, the existing upper limits of both X-ray, and radio emissions in the nucleus of NGC 205 suggest an accretion rate $\lesssim$$10^{-5}$ of the Eddington rate. We also discuss the color--\mleff~relations in our nuclei and find that the slopes of these vary significantly between nuclei. Nuclei with significant young stellar populations have steeper color--\mleff~relations than some previously published galaxy color--\mleff~relations., Comment: 31 pages, 19 figures, 6 tables, Accepted to ApJ

We analyze the kinematics of Andromeda's disk as a function of stellar age by using photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey and spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey. We use HI 21-cm and CO ($\rm J=1 \rightarrow 0$) data to examine the difference between the deprojected rotation velocity of the gas and that of the stars. We divide the stars into four stellar age bins, from shortest lived to longest lived: massive main sequence stars (0.03 Gyr), more luminous intermediate mass asymptotic giant branch (AGB) stars (0.4 Gyr), less luminous intermediate mass AGB stars (2 Gyr), and low mass red giant branch stars (4 Gyr). There is a clear correlation between the offset of the stellar and the gas rotation velocity, or the asymmetric drift: the longer lived populations lag farther behind the gas than short lived populations. We also examine possible causes of the substructure in the rotation curves and find that the most significant cause of scatter in the rotation curves comes from the tilted ring model being an imperfect way to account for the multiple warps in Andromeda's disk., Comment: 30 pages, 24 figures, 2 tables; accepted to ApJ November 16, 2018

The recent discovery of super-massive black holes (SMBHs) in the centers of high-mass ultra compact dwarf galaxies (UCDs) suggests that at least some UCDs are the stripped nuclear star clusters of lower mass galaxies. Tracing these former nuclei of stripped galaxies provides a unique way to track the assembly history of a galaxy or galaxy cluster. In this paper we present a new method to estimate how many UCDs host an SMBH in their center and thus are stripped galaxy nuclei. We revisit the dynamical mass measurements that suggest many UCDs have more mass than expected from stellar population estimates, which recent observations have shown is due to the presence of an SMBH. We revise the stellar population mass estimates using a new empirical relation between the mass-to-light ratio (M/L) and metallicity, and use this to predict which UCDs are most likely to host an SMBH. This enables us to calculate the fraction of UCDs that host SMBHs across their entire luminosity range for the first time. We then apply the SMBH occupation fraction to the observed luminosity function of UCDs and estimate that in the Fornax and Virgo cluster alone there should be $69^{+32}_{-25}$ stripped nuclei with SMBHs. This analysis shows that stripped nuclei with SMBHs are almost as common in clusters as present-day galaxy nuclei. We estimate the local SMBH number density in stripped nuclei to $3-8\times10^{-3}Mpc^{-3}$, which represents a significant fraction (10-40\%) of the SMBH density in the local Universe. These SMBHs hidden in stripped nuclei will increase expected event rates for tidal disruption events and SMBH-SMBH and SMBH-BH mergers. The existence of numerous stripped nuclei with SMBHs are a direct consequence of hierarchical galaxy formation, but until now their impact on the SMBH density had not been quantified., Comment: 15 pages, 8 Figures, accepted for publication in ApJ

The majority of spiral and elliptical galaxies in the Universe host very dense and compact stellar systems at their centres known as nuclear star clusters (NSCs). In this work we study the stellar populations and star formation histories (SFH) of the NSCs of six nearby galaxies with stellar masses ranging between $2$ and $8\times10^9~{\rm M_{\odot}}$ (four late-type spirals and two early-types) with high resolution spectroscopy. Our observations are taken with the X-Shooter spectrograph at the VLT. We make use of an empirical simple stellar population (SSP) model grid to fit composite stellar populations to the data and recover the SFHs of the nuclei. We find that the nuclei of all late-type galaxies experienced a prolonged SFH, while the NSCs of the two early-types are consistent with SSPs. The NSCs in the late-type galaxies sample appear to have formed a significant fraction of their stellar mass already more than $10$ Gyr ago, while the NSCs in the two early-type galaxies are surprisingly younger. Stars younger than $100$ Myr are present in at least two nuclei: NGC 247 and NGC 7793, with some evidence for young star formation in NGC 300's NSC. The NSCs of the spirals NGC 247 and NGC 300 are consistent with prolonged \in situ star formation with a gradual metallicity enrichment from $\sim-1.5$ dex more than $10$ Gyr ago, reaching super-Solar values few hundred Myr ago. NGC 3621 appears to be very metal rich already in the early Universe and NGC 7793 presents us with a very complex SFH, likely dominated by merging of various massive star clusters coming from different environments., Comment: Accepted for publication in MNRAS

We present the results of an ultra-deep, comprehensive radio continuum survey for the accretion signatures of intermediate-mass black holes in globular clusters. The sample, imaged with the Karl G.~Jansky Very Large Array and the Australia Telescope Compact Array, comprises 50 Galactic globular clusters. No compelling evidence for an intermediate-mass black hole is found in any cluster in our sample. In order to achieve the highest sensitivity to low-level emission, we also present the results of an overall stack of our sample, as well as various subsamples, also finding non-detections. These results strengthen the idea that intermediate-mass black holes with masses $\gtrsim 1000 M_{\odot}$ are rare or absent in globular clusters., Comment: 27 pages, 7 figures, 2 tables. Accepted for publication in ApJ

We examine the internal properties of the most massive ultracompact dwarf galaxy (UCD), M59-UCD3, by combining adaptive optics assisted near-IR integral field spectroscopy from Gemini/NIFS, and Hubble Space Telescope (HST) imaging. We use the multi-band HST imaging to create a mass model that suggests and accounts for the presence of multiple stellar populations and structural components. We combine these mass models with kinematics measurements from Gemini/NIFS to find a best-fit stellar mass-to-light ratio ($M/L$) and black hole (BH) mass using Jeans Anisotropic Models (JAM), axisymmetric Schwarzschild models, and triaxial Schwarzschild models. The best fit parameters in the JAM and axisymmetric Schwarzschild models have black holes between 2.5 and 5.9 million solar masses. The triaxial Schwarzschild models point toward a similar BH mass, but show a minimum $\chi^2$ at a BH mass of $\sim 0$. Models with a BH in all three techniques provide better fits to the central $V_{rms}$ profiles, and thus we estimate the BH mass to be $4.2^{+2.1}_{-1.7} \times 10^{6}$ M$_\odot$ (estimated 1$\sigma$ uncertainties). We also present deep radio imaging of M59-UCD3 and two other UCDs in Virgo with dynamical BH mass measurements, and compare these to X-ray measurements to check for consistency with the fundamental plane of BH accretion. We detect faint radio emission in M59cO, but find only upper limits for M60-UCD1 and M59-UCD3 despite X-ray detections in both these sources. The BH mass and nuclear light profile of M59-UCD3 suggests it is the tidally stripped remnant of a $\sim$10$^{9-10}$ M$_\odot$ galaxy., Comment: 17 pages, 14 figures, 5 tables

The recent discovery of massive black holes (BHs) in the centers of high-mass ultra compact dwarf galaxies (UCDs) suggests that at least some are the stripped nuclear star clusters of dwarf galaxies. We present the first study that investigates whether such massive BHs, and therefore stripped nuclei, also exist in low-mass ($M<10^{7}M_{\odot}$) UCDs. We constrain the BH masses of two UCDs located in Centaurus A (UCD320 and UCD330) using Jeans modeling of the resolved stellar kinematics from adaptive optics VLT/SINFONI data. No massive BHs are found in either UCD. We find a $3\,\sigma$ upper limit on the central BH mass in UCD\,330 of $M_{\bullet}<1.0\times10^{5}M_{\odot}$, which corresponds to 1.7\% of the total mass. This excludes a high mass fraction BH and would only allow a low-mass BHs similar to those claimed to be detected in Local Group GCs. For UCD320, poorer data quality results in a less constraining $3\,\sigma$ upper limit of $M_{\bullet}<1\times10^{6}M_{\odot}$, which is equal to 37.7\% of the total mass. The dynamical $M/L$ of UCD320 and UCD330 are not inflated compared to predictions from stellar population models. The non-detection of BHs in these low-mass UCDs is consistent with the idea that elevated dynamical $M/L$s do indicate the presence of a substantial BH. Despite not detecting massive BHs, these systems could still be stripped nuclei. The strong rotation ($v/\sigma$ of 0.3 to 0.4) in both UCDs and the two-component light profile in UCD330 support the idea that these UCDs may be stripped nuclei of low-mass galaxies where the BH occupation fraction is not yet known., Comment: Accepted for publication in ApJ, 21 pages, 10 Figures

We present a detailed study of the nuclear star clusters (NSCs) and massive black holes (BHs) of four of the nearest low-mass early-type galaxies: M32, NGC205, NGC5012, and NGC5206. We measure dynamical masses of both the BHs and NSCs in these galaxies using Gemini/NIFS or VLT/SINFONI stellar kinematics, Hubble Space Telescope (HST) imaging, and Jeans Anisotropic Models. We detect massive BHs in M32, NGC5102, and NGC5206, while in NGC205, we find only an upper limit. These BH mass estimates are consistent with previous measurements in M32 and NGC205, while those in NGC5102 and NGC5206 are estimated for the first time, and both found to be $<$$10^6~M_{\odot}$. This adds to just a handful of galaxies with dynamically measured sub-million $M_{\odot}$ central BHs. Combining these BH detections with our recent work on NGC404's BH, we find that 80\% (4/5) of nearby, low-mass ($10^9-10^{10}~M_{\odot}$; $\sigma_{\star}\sim20-70$ km/s) early-type galaxies host BHs. Such a high occupation fraction suggests the BH seeds formed in the early epoch of cosmic assembly likely resulted in abundant seeds, favoring a low-mass seed mechanism of the remnants, most likely from the first generation of massive stars. We find dynamical masses of the NSCs ranging from $2-80\times10^6~M_{\odot}$ and compare these masses to scaling relations for NSCs based primarily on photometric mass estimates. Color gradients suggest younger stellar populations lie at the centers of the NSCs in three of the four galaxies (NGC205, NGC5102, and NGC5206), while the morphology of two are complex and are best-fit with multiple morphological components (NGC5102 and NGC5206). The NSC kinematics show they are rotating, especially in M32 and NGC5102 ($V/\sigma_{\star}\sim0.7$)., Comment: 30 pages, 13 figures, 12 tables. Accepted to ApJ

Observations of globular clusters in dwarf galaxies can be used to study a variety of topics, including the structure of dark matter halos and the history of vigorous star formation in low-mass galaxies. We report on the properties of the faint globular cluster (M_V ~ -3.4) in the M31 dwarf galaxy Andromeda I. This object adds to the growing population of low-luminosity Local Group galaxies that host single globular clusters., Comment: PASA in press

Using HST, we identify circumnuclear ($100$-$500$ pc scale) structures in nine new H$_2$O megamaser host galaxies to understand the flow of matter from kpc-scale galactic structures down to the supermassive black holes (SMBHs) at galactic centers. We double the sample analyzed in a similar way by Greene et al. (2013) and consider the properties of the combined sample of 18 sources. We find that disk-like structure is virtually ubiquitous when we can resolve $<200$ pc scales, in support of the notion that non-axisymmetries on these scales are a necessary condition for SMBH fueling. We perform an analysis of the orientation of our identified nuclear regions and compare it with the orientation of megamaser disks and the kpc-scale disks of the hosts. We find marginal evidence that the disk-like nuclear structures show increasing misalignment from the kpc-scale host galaxy disk as the scale of the structure decreases. In turn, we find that the orientation of both the $\sim100$ pc scale nuclear structures and their host galaxy large-scale disks is consistent with random with respect to the orientation of their respective megamaser disks., Comment: 24 pages, 16 figures, 4 tables; Resubmitted to ApJ after referee's comments

We measure the mass function for a sample of 840 young star clusters with ages between 10-300 Myr observed by the Panchromatic Hubble Andromeda Treasury (PHAT) survey in M31. The data show clear evidence of a high-mass truncation: only 15 clusters more massive than $10^4$ $M_{\odot}$ are observed, compared to $\sim$100 expected for a canonical $M^{-2}$ pure power-law mass function with the same total number of clusters above the catalog completeness limit. Adopting a Schechter function parameterization, we fit a characteristic truncation mass of $M_c = 8.5^{+2.8}_{-1.8} \times 10^3$ $M_{\odot}$. While previous studies have measured cluster mass function truncations, the characteristic truncation mass we measure is the lowest ever reported. Combining this M31 measurement with previous results, we find that the cluster mass function truncation correlates strongly with the characteristic star formation rate surface density of the host galaxy, where $M_c \propto$ $\langle \Sigma_{\mathrm{SFR}} \rangle^{\sim1.1}$. We also find evidence that suggests the observed $M_c$-$\Sigma_{\mathrm{SFR}}$ relation also applies to globular clusters, linking the two populations via a common formation pathway. If so, globular cluster mass functions could be useful tools for constraining the star formation properties of their progenitor host galaxies in the early Universe., Comment: 15 pages, 7 figures, Accepted by ApJ

We present the detection of supermassive black holes (BHs) in two Virgo ultracompact dwarf galaxies (UCDs), VUCD3 and M59cO. We use adaptive optics assisted data from the Gemini/NIFS instrument to derive radial velocity dispersion profiles for both objects. Mass models for the two UCDs are created using multi-band Hubble Space Telescope (HST) imaging, including the modeling of mild color gradients seen in both objects. We then find a best-fit stellar mass-to-light ratio ($M/L$) and BH mass by combining the kinematic data and the deprojected stellar mass profile using Jeans Anisotropic Models (JAM). Assuming axisymmetric isotropic Jeans models, we detect BHs in both objects with masses of $4.4^{+2.5}_{-3.0} \times 10^6$ $M_{\odot}$ in VUCD3 and $5.8^{+2.5}_{-2.8} \times 10^6$ $M_{\odot}$ in M59cO (3$\sigma$ uncertainties). The BH mass is degenerate with the anisotropy parameter, $\beta_z$; for the data to be consistent with no BH requires $\beta_z = 0.4$ and $\beta_z = 0.6$ for VUCD3 and M59cO, respectively. Comparing these values with nuclear star clusters shows that while it is possible that these UCDs are highly radially anisotropic, it seems unlikely. These detections constitute the second and third UCDs known to host supermassive BHs. They both have a high fraction of their total mass in their BH; $\sim$13% for VUCD3 and $\sim$18% for M59cO. They also have low best-fit stellar $M/L$s, supporting the proposed scenario that most massive UCDs host high mass fraction BHs. The properties of the BHs and UCDs are consistent with both objects being the tidally stripped remnants of $\sim$10$^9$ M$_\odot$ galaxies., Comment: 17 pages, 9 Figures, 3 Tables, accepted for publication in The Astrophysical Journal

We present a sample of 224 stars that emit H$\alpha$ (H$\alpha$ stars) in the Andromeda galaxy (M31). The stars were selected from $\sim$ 5000 spectra, collected as part of the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey using Keck II/DEIMOS. We used six-filter Hubble Space Telescope photometry from the Panchromatic Hubble Andromeda Treasury survey to classify and investigate the properties of the H$\alpha$ stars. We identified five distinct categories of H$\alpha$ star: B-type main sequence (MS) stars, `transitioning'-MS (T-MS) stars, red core He burning (RHeB) stars, non-C-rich asymptotic giant branch (AGB) stars, and C-rich AGB stars. We found $\sim$ 12 per cent of B-type stars exhibit H$\alpha$ emission (Be stars). The frequency of Be to all B stars is known to vary with the metallicity of their environment. Comparing this proportion of Be stars with other environments around the Local Group, the result could indicate that M31 is more metal rich than the Milky Way. We predict that the 17 T-MS H$\alpha$ stars are Be stars evolving off the MS with fading H$\alpha$ emission. We separated RHeB from AGB H$\alpha$ stars. We conclude that the 61 RHeB and AGB stars are likely to be Long Period Variables. We found that $\sim$ 14 per cent of C-rich AGB stars (C stars) emit H$\alpha$, which is an upper limit for the ratio of C-rich Miras to C stars. This catalogue of H$\alpha$ stars will be useful to constrain stellar evolutionary models, calibrate distance indicators for intermediate age populations, and investigate the properties of M31., Comment: Accepted for publication in MNRAS. 25 pages, 14 figures

We explore the nucleus of the nearby 10$^9M_{\odot}$~early-type galaxy (ETGs), NGC~404, using Hubble Space Telescope (HST)/STIS spectroscopy and WFC3 imaging. We first present evidence for nuclear variability in UV, optical, and infrared filters over a time period of 15~years. This variability adds to the already substantial evidence for an accreting black hole at the center of NGC~404. We then redetermine the dynamical black hole mass in NGC~404 including modeling of the nuclear stellar populations. We combine HST/STIS spectroscopy with WFC3 images to create a local color-$M/L$~relation derived from stellar population modeling of the STIS data. We then use this to create a mass model for the nuclear region. We use Jeans modeling to fit this mass model to adaptive optics (AO) stellar kinematic observations from Gemini/NIFS. From our stellar dynamical modeling, we find a 3$\sigma$ upper limit on the black hole mass of $1.5\times10^5M_{\odot}$. Given the accretion evidence for a black hole, this upper limit makes NGC~404 the lowest mass central black hole with dynamical mass constraints. We find that the kinematics of H$_2$ emission line gas show evidence for non-gravitational motions preventing the use of gas dynamical modeling to constrain the black hole mass. Our stellar population modeling also reveals that the central, counter-rotating region of the nuclear cluster is dominated by $\sim$1~Gyr old populations., Comment: 23 papes, 16 figures, 6 tables, Resubmitted after accounting for referee comments

We use the Panchromatic Hubble Andromeda Treasury (PHAT) survey dataset to perform spatially resolved measurements of star cluster formation efficiency ($\Gamma$), the fraction of stellar mass formed in long-lived star clusters. We use robust star formation history and cluster parameter constraints, obtained through color-magnitude diagram analysis of resolved stellar populations, to study Andromeda's cluster and field populations over the last $\sim$300 Myr. We measure $\Gamma$ of 4-8% for young, 10-100 Myr old populations in M31. We find that cluster formation efficiency varies systematically across the M31 disk, consistent with variations in mid-plane pressure. These $\Gamma$ measurements expand the range of well-studied galactic environments, providing precise constraints in an HI-dominated, low intensity star formation environment. Spatially resolved results from M31 are broadly consistent with previous trends observed on galaxy-integrated scales, where $\Gamma$ increases with increasing star formation rate surface density ($\Sigma_{\mathrm{SFR}}$). However, we can explain observed scatter in the relation and attain better agreement between observations and theoretical models if we account for environmental variations in gas depletion time ($\tau_{\mathrm{dep}}$) when modeling $\Gamma$, accounting for the qualitative shift in star formation behavior when transitioning from a H$_2$-dominated to a HI-dominated interstellar medium. We also demonstrate that $\Gamma$ measurements in high $\Sigma_{\mathrm{SFR}}$ starburst systems are well-explained by $\tau_{\mathrm{dep}}$-dependent fiducial $\Gamma$ models., Comment: 26 pages, 13 figures, Accepted by ApJ

We use new precision measurements of black hole masses from water megamaser disks to investigate scaling relations between macroscopic galaxy properties and supermassive black hole (BH) mass. The megamaser-derived BH masses span 10^6-10^8 M_sun, while all the galaxy properties that we examine (including stellar mass, central mass density, central velocity dispersion) lie within a narrow range. Thus, no galaxy property correlates tightly with M_BH in ~L* spiral galaxies. Of them all, stellar velocity dispersion provides the tightest relation, but at fixed sigma* the mean megamaser M_BH are offset by -0.6+/-0.1 dex relative to early-type galaxies. Spiral galaxies with non-maser dynamical BH masses do not show this offset. At low mass, we do not yet know the full distribution of BH mass at fixed galaxy property; the non-maser dynamical measurements may miss the low-mass end of the BH distribution due to inability to resolve the spheres of influence and/or megamasers may preferentially occur in lower-mass BHs., Comment: 6 pages, 4 figures, replaced to fix error: NGC 4594 is not a maser galaxy

Globular clusters which exhibit chemical and dynamical complexity have been suggested to be the stripped nuclei of dwarf galaxies (e.g., M54, $\omega$ Cen). We use $N$-body simulations of nuclear star clusters forming via the mergers of star clusters to explore the persistence of substructure in the phase space. We find that the observed level of differentiation is difficult to reconcile with the observed if nuclear clusters form wholly out of the mergers of star clusters. Only the star clusters that merged most recently retain sufficiently distinct kinematics to be distinguishable from the rest of the nuclear cluster though the critical factor is the number of merger events not the elapsed time. In situ star formation must therefore be included to explain the observed properties of nuclear star clusters, in good agreement with previous results., Comment: 16 pages, 17 figures, 3 tables