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The discovery of quiescent, dark matter (DM)-deficient ultra-diffuse galaxies (UDGs) with overluminous globular clusters (GCs) has challenged galaxy formation models within the Lambda cold dark matter (ΛCDM) cosmological paradigm. Previously, such galaxies were only identified in the NGC 1052 group, raising the possibility that they are the result of unique, group-specific processes, and limiting their broader significance. The recent identification of FCC 224, a putative DM-deficient UDG on the outskirts of the Fornax Cluster, suggests that such galaxies are not confined to the NGC 1052 group but rather represent a broader phenomenon. We aim to investigate the DM content of FCC 224 and to explore its similarities to the DM-free dwarfs in the NGC 1052 group, DF2 and DF4, to determine whether or not it belongs to the same class of DM-deficient UDGs. We use high-resolution Keck Cosmic Web Imager (KCWI) spectroscopy to study the kinematics, stellar populations, and GC system of FCC 224, enabling direct comparisons with DF2 and DF4. We find that FCC 224 is also DM-deficient and exhibits a distinct set of traits shared with DF2 and DF4, including slow and prolate rotation, quiescence in low-density environments, coeval formation of stars and GCs, flat stellar population gradients, a top-heavy GC luminosity function, and monochromatic GCs. These shared characteristics signal the existence of a previously unrecognised class of DM-deficient dwarf galaxies. This diagnostic framework provides a means of identifying additional examples and raises new questions for galaxy formation models within ΛCDM cosmology. 

We study the quiescent ultradiffuse galaxy FCC 224 in the Fornax cluster using Hubble Space Telescope (HST) imaging, motivated by peculiar properties of its globular cluster (GC) system revealed in shallower imaging. The surface brightness fluctuation distance of FCC 224 measured from HST is 18.6 ± 2.7 Mpc, consistent with the Fornax cluster distance. We use Prospector to infer the stellar population from a combination of multiwavelength photometry (HST, ground-based, Wide-field Infrared Survey Explorer) and Keck Cosmic Web Imager spectroscopy. The galaxy has a mass-weighted age of ∼10 Gyr, metallicity [M/H] of ∼ −1.25 dex, and a very short formation e-folding time of τ ∼ 0.3 Gyr. Its 12 candidate GCs exhibit highly homogeneous g_475−I_814 colors, merely 0.04 mag bluer than the diffuse starlight, which supports a single-burst formation scenario for this galaxy. We confirm a top-heavy GC luminosity function, similar to the two dark matter deficient galaxies NGC 1052-DF2 and DF4. However, FCC 224 differs from those galaxies with relatively small GC sizes of ∼3 pc (∼35% smaller than typical for other dwarfs), and with radial mass segregation in its GC system. We are not yet able to identify a formation scenario to explain all of the GC properties in FCC 224. Follow-up measurements of the dark matter content in FCC 224 will be crucial because of the mix of similarities and differences among FCC 224, DF2, and DF4. 

Context.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. Aims.Here we take advantage of rich new observational datasets of large-radius tracers to perform dynamical modeling of NGC 5128 Methods.We used 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 ∼250 kpc from the galaxy center to model the dynamics of NGC 5128. Results.We find that a standard Navarro-Frenk-White (NFW) halo provides an excellent fit to nearly all the data, except for a subset of the planetary nebulae that appear to be out of virial equilibrium. The best-fit dark matter halo has a virial mass ofM_vir = 4.4_−1.4^+2.4 × 10¹² M_⊙, 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 toM_vir ∼ 10¹³ M_⊙. The inferred NFW virial concentration isc_vir = 5.6_−1.6^+2.4, which is nominally lower thanc_vir ∼ 9 predicted from publishedc_vir–M_virrelations, but within the ∼30% scatter found in simulations. The best-fit dark matter halo constitutes only ∼10% of the total mass at one effective radius but ∼50% at five effective radii. The derived halo parameters are consistent within the uncertainties for models with differing tracer populations, anisotropies, and inclinations. Conclusions.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. 

We present spatially resolved Keck Cosmic Web Imager stellar spectroscopy of the Virgo cluster dwarf galaxies VCC 9 and VCC 1448. These galaxies have similar stellar masses and large half-light radii but very different globular cluster (GC) system richness (∼25 versus ∼99 GCs). Using the KCWI data, we spectroscopically confirm 10 GCs associated with VCC 1448 and one GC associated with VCC 9. We make two measurements of dynamical mass for VCC 1448 based on the stellar and GC velocities, respectively. VCC 1448’s mass measurements suggest that it resides in a halo in better agreement with the expectation of the stellar mass–halo mass relationship than the expectation from its large GC counts. For VCC 9, the dynamical mass we measure agrees with the expected halo mass from both relationships. We compare VCC 1448 and VCC 9 to the GC-rich galaxy Dragonfly 44 (∼74 GCs), which is similar in size but has ∼1 dex less stellar mass than either Virgo galaxy. In dynamical mass – GC number space, Dragonfly 44 and VCC 1448 exhibit richer GC systems given their dynamical mass than that of VCC 9 and other ‘normal’ galaxies. We also place the galaxies in kinematics–ellipticity space finding evidence of an anticorrelation between rotational support and the fraction of a galaxy’s stellar mass in its GC system, that is, VCC 9 is more rotationally supported than VCC 1448, which is more rotationally supported than Dragonfly 44. This trend may be expected if a galaxy’s GC content depends on its natal gas properties at formation. 

ABSTRACT We derive the stellar population parameters of 11 quiescent ultra-diffuse galaxies (UDGs) from Keck/KCWI data. We supplement these with 14 literature UDGs, creating the largest spectroscopic sample of UDGs to date (25). We find a strong relationship between their α-enhancement and their star formation histories: UDGs that formed on very short time-scales have elevated [Mg/Fe] abundance ratios, whereas those forming over extended periods present lower values. Those forming earlier and faster are overall found in high-density environments, being mostly early infalls into the cluster. No other strong trends are found with infall times. We analyse the stellar mass–metallicity, age–metallicity, and [Mg/Fe]–metallicity relations of the UDGs, comparing them to other types of low mass galaxies. Overall, UDGs scatter around the established stellar mass–metallicity relations of classical dwarfs. We find that GC-rich UDGs have intermediate-to-old ages, but previously reported trends of galaxy metallicity and GC richness are not reproduced with this spectroscopic sample due to the existence of GC-rich UDGs with elevated metallicities. In addition, we also find that a small fraction of UDGs could be ‘failed-galaxies’, supported by their GC richness, high alpha-abundance, fast formation time-scales and that they follow the mass–metallicity relation of z ∼2 galaxies. Finally, we also compare our observations to simulated UDGs. We caution that there is not a single simulation that can produce the diverse UDG properties simultaneously, in particular the low metallicity failed galaxy like UDGs. 

ABSTRACT Observational surveys have found that the dynamical masses of ultradiffuse galaxies (UDGs) correlate with the richness of their globular cluster (GC) system. This could be explained if GC-rich galaxies formed in more massive dark matter haloes. We use simulations of galaxies and their GC systems from the E-MOSAICS project to test whether the simulations reproduce such a trend. We find that GC-rich simulated galaxies in galaxy groups have enclosed masses that are consistent with the dynamical masses of observed GC-rich UDGs. However, simulated GC-poor galaxies in galaxy groups have higher enclosed masses than those observed. We argue that GC-poor UDGs with low stellar velocity dispersions are discs observed nearly face on, such that their true mass is underestimated by observations. Using the simulations, we show that galactic star formation conditions resulting in dispersion-supported stellar systems also leads to efficient GC formation. Conversely, conditions leading to rotationally supported discs lead to inefficient GC formation. This result may explain why early-type galaxies typically have richer GC systems than late-type galaxies. This is also supported by comparisons of stellar axis ratios and GC-specific frequencies in observed dwarf galaxy samples, which show GC-rich systems are consistent with being spheroidal, while GC-poor systems are consistent with being discs. Therefore, particularly for GC-poor galaxies, rotation should be included in dynamical mass measurements from stellar dynamics. 

ABSTRACT We use spectral energy distribution fitting to place constraints on the stellar populations of 59 ultra-diffuse galaxies (UDGs) in the low-to-moderate density fields of the MATLAS survey. We use the routine prospector, coupled with archival data in the optical from the Dark Energy Camera Legacy Survey, and near- and mid-infrared imaging from the Wide-field Infrared Survey Explorer, to recover the stellar masses, ages, metallicities, and star formation time-scales of the UDGs. We find that a subsample of the UDGs lies within the scatter of the mass–metallicity relation (MZR) for local classical dwarfs. However, another subsample is more metal-poor, being consistent with the evolving MZR at high redshift. We investigate UDG positioning trends in the mass–metallicity plane as a function of surface brightness, effective radius, axis ratio, local volume density, mass-weighted age, star formation time-scale, globular cluster (GC) counts, and GC specific frequency. We find that our sample of UDGs can be separated into two main classes: Class A: comprised of UDGs with lower stellar masses, prolonged star formation histories (SFHs), more elongated, inhabiting less dense environments, hosting fewer GCs, younger, consistent with the classical dwarf MZR, and fainter. Class B: UDGs with higher stellar masses, rapid SFHs, rounder, inhabiting the densest of our probed environments, hosting on average the most numerous GC systems, older, consistent with the high-redshift MZR (i.e. consistent with early-quenching), and brighter. The combination of these properties suggests that UDGs of Class A are consistent with a ‘puffed-up dwarf’ formation scenario, while UDGs of Class B seem to be better explained by ‘failed galaxy’ scenarios. 

ABSTRACT 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*  = 7 × 109 M⊙), cosmological simulations predict NGC 2403 to contain few, if any, accreted GCs. It is also isolated, with a remarkably undisturbed HI disc. 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 eight bona-fide GCs associated with either the inner halo or the disc 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] ≤ −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 ∼50 GCs expected, our study is the first step in fully mapping the star cluster history of NGC 2403 in both space and time. 

Abstract The dense central regions of tidally disrupted galaxies can survive as ultracompact 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 × 10 5 and 2.5 × 10 7 L ⊙ ), 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 ± 0.31, and a second population of ∼20 GCs with elevated mean M / L dyn = 2.68 ± 0.22. We show that black holes with masses ∼4%–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, 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. 

Abstract We present new radial velocity measurements from the Magellan and the Anglo-Australian Telescopes for 175 previously known and 121 newly confirmed globular clusters (GCs) around NGC 5128, the nearest accessible massive early-type galaxy atD= 3.8 Mpc. Remarkably, 28 of these newly confirmed GCs are at projected radii $>50\prime$(≳54 kpc), extending to ∼130 kpc, in the outer halo where few GCs had been confirmed in previous work. We identify several subsets of GCs that spatially trace halo substructures that are visible in red giant branch star maps of the galaxy. In some cases, these subsets of GCs are kinematically cold, and may be directly associated with and originate from these specific stellar substructures. From a combined kinematic sample of 645 GCs, we see evidence for coherent rotation at all radii, with a higher rotation amplitude for the metal-rich GC subpopulation. Using the tracer mass estimator, we measure a total enclosed mass of 2.5 ± 0.3 × 10¹²M_⊙within ∼120 kpc, an estimate that will be sharpened with forthcoming dynamical modeling. The combined power of stellar mapping and GC kinematics makes NGC 5128 an ongoing keystone for understanding galaxy assembly at mass scales inaccessible in the Local Group.