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Candidate Dark Galaxy-2 (CDG-2) is a potential dark galaxy consisting of four globular clusters (GCs) in the Perseus cluster, first identified in D. Li et al. through a sophisticated statistical method. The method searched for overdensities of GCs from a Hubble Space Telescope (HST) survey targeting Perseus. Using the same HST images and new imaging data from the Euclid survey, we report the detection of extremely faint but significant diffuse emission around the four GCs of CDG-2. We thus have exceptionally strong evidence that CDG-2 is a galaxy. This is the first galaxy detected purely through its GC population. Under the conservative assumption that the four GCs make up the entire GC population, preliminary analysis shows that CDG-2 has a total luminosity of L_V,gal = 6.2 ± 3.0 × 10^6 L_⊙ and a minimum GC luminosity of L_V,GC = 1.03 ± 0.2 × 10^6 L_⊙. Our results indicate that CDG-2 is one of the faintest galaxies having associated GCs, while at least ∼16.6% of its light is contained in its GC population. This ratio is likely to be much higher (∼33%) if CDG-2 has a canonical GC luminosity function (GCLF). In addition, if the previously observed GC-to-halo mass relations apply to CDG-2, it would have a minimum dark matter halo mass fraction of 99.94% to 99.98%. If it has a canonical GCLF, then the dark matter halo mass fraction is ≳99.99%. Therefore, CDG-2 may be the most GC dominated galaxy and potentially one of the most dark matter dominated galaxies ever discovered. 

Two low surface brightness (LSB) dwarf galaxies were identified recently as having little or no dark matter (DM), provoking widespread interest in their formation histories. These galaxies also host populous systems of star clusters that are on average larger and more luminous than typical globular clusters (GCs). We report an initial attempt to identify new candidate DM-deficient dwarfs via their unusual GC systems. Using a large catalog of LSB galaxies from the Dark Energy Survey, we inspect their Dark Energy Camera Legacy Survey (DECaLS) imaging and identify FCC 224 as a candidate found on the outskirts of the Fornax cluster. We analyze the GC system using DECaLS and archival Hubble Space Telescope WFPC2 imaging, and find an apparent population of overluminous GCs. More detailed follow-up of FCC 224 is in progress. 

We present MArk-dependently THinned POint Process (Mathpop), a novel method to infer the globular cluster (GC) counts in ultra-diffuse galaxies (UDGs) and low-surface brightness galaxies (LSBGs). Many known UDGs have a surprisingly high ratio of GC number to surface brightness. However, standard methods to infer GC counts in UDGs face various challenges, such as photometric measurement uncertainties, GC membership uncertainties, and assumptions about the GC luminosity functions (GCLFs). Mathpop tackles these challenges using the mark-dependent thinned point process, enabling joint inference of the spatial and magnitude distributions of GCs. In doing so, Mathpop allows us to infer and quantify the uncertainties in both GC counts and GCLFs with minimal assumptions. As a precursor to Mathpop, we also address the data uncertainties coming from the selection process of GC candidates: we obtain probabilistic GC candidates instead of the traditional binary classification based on the color–magnitude diagram. We apply Mathpop to 40 LSBGs in the Perseus cluster using GC catalogs from a Hubble Space Telescope imaging program. We then compare our results to those from an independent study using the standard method. We further calibrate and validate our approach through extensive simulations. Our approach reveals two LSBGs having GCLF turnover points much brighter than the canonical value with Bayes’ factor being ∼4.5 and ∼2.5, respectively. An additional crude maximum-likelihood estimation and simulation study show that their GCLF TO points are approximately 0.9 mag and 1.1 mag brighter than the canonical value, with p-values of ∼10^−8 and ∼10^−5, respectively. 

Recent studies of ultra-diffuse galaxies (UDGs) have shown their globular cluster (GC) systems to be central in unveiling their remarkable properties and halo masses. Deep Hubble Space Telescope imaging revealed 54 GC candidates around the UDG NGC5846_UDG1 (UDG1), with a remarkable 13 per cent of the stellar light contained in the GC system. We present a kinematic analysis of UDG1’s GC system from observations with the integral field spectrograph Keck Cosmic Web Imager on the Keck II telescope. We measure recessional velocities for 19 GCs, confirming them as members of UDG1, giving a total of 20 confirmed GCs when combined with literature. Approximately, 9 per cent of the stellar light are contained just in the confirmed GCs. We determine the GC system’s velocity dispersion to be sigma_GC = 29.8^+6.4_-4.9 km s^-1. We find that sigma _GC increases with increasing magnitude, consistent with predictions for a GC system that evolved under the influence of dynamical friction. The GC system velocity dispersion is constant out to ~1 R_eff. Using sigma _GC, we calculate M_dyn = 2.09^+1.00_-0.64 x 10^9 M_\odot as the dynamical mass enclosed within ~2.5 kpc. The dark matter halo mass suggested by the GC number–halo mass relationship agrees with our dynamical mass estimate, implying a halo more massive than suggested by common stellar mass–halo mass relationships. UDG1, being GC-rich with a massive halo, fits the picture of a failed galaxy. 

Some ultra diffuse galaxies (UDGs) reveal many more globular clusters (GCs) than classical dwarf galaxies of the same stellar mass. These UDGs, with a mass in their GC system (M_GC) approaching 10 per cent of their host galaxy stellar mass (M_*), are also inferred to have high halo mass to stellar mass ratios (M_halo/M_*). They have been dubbed Failed Galaxies. It is unknown what role high GC formation efficiencies and/or low destruction rates play in determining the high M_GC/M_* ratios of some UDGs. Here we present a simple model, which is informed by recent JWST observations of lensed galaxies and by a simulation in the literature of GC mass loss and tidal disruption in dwarf galaxies. With this simple model, we aim to constrain the effects of GC efficiency/destruction on the observed GC richness of UDGs and their variation with the integrated stellar populations of UDGs. We assume no ongoing star formation (i.e. quenching at early times) and that the disrupted GCs contribute their stars to those of the host galaxy. We find that UDGs, with high M_GC/M_* ratios today, are most likely the result of very high GC formation efficiencies combined with modest rates of GC destruction. The current data loosely follow the model that ranges from the mean stellar population of classical dwarfs to that of metal-poor GCs as M_GC/M_* increases. As more data becomes available for UDGs, our simple model can be refined and tested further. 

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. 

NGC 1052-DF2 and -DF4 are two ultradiffuse galaxies that have been reported as deficient in dark matter and associated with the same galaxy group. Recent findings suggest that DF2 and DF4 are part of a large linear substructure of dwarf galaxies that could have been formed from a high-velocity head-on encounter of two gas-rich galaxies, known as a “bullet dwarf” collision. Based on new observations from the Hubble Space Telescope, combined with existing imaging from theuband to mid-infrared, we test the bullet dwarf scenario by studying the morphologies and stellar populations of the trail dwarfs. We find no significant morphological differences between the trail dwarfs and other dwarfs in the group, while for both populations, their photometric major axes unexpectedly align parallel with the trail. We find that the trail dwarfs have significantly older ages and higher metallicities than the comparison sample, supporting the distinctiveness of the trail. These observations provide key constraints for any formation model, and we argue that they are currently best explained by the bullet dwarf collision scenario, with additional strong tests anticipated with future observations. 

High-resolution imaging and strong gravitational lensing of high-redshift galaxies have enabled the detection of compact sources with properties similar to nearby massive star clusters. Often found to be very young, these sources may be globular clusters detected in their earliest stages. In this work, we compare predictions of high-redshift (z ~ 1–10) star cluster properties from the E-MOSAICS simulation of galaxy and star cluster formation with those of the star cluster candidates in strongly lensed galaxies from JWST and Hubble Space Telescope (HST) imaging. We select galaxies in the simulation that match the luminosities of the majority of lensed galaxies with star cluster candidates observed with JWST. We find that the luminosities, ages, and masses of the brightest star cluster candidates in the high-redshift galaxies are consistent with the E-MOSAICS model. In particular, the brightest cluster ages are in excellent agreement. The results suggest that star clusters in both low- and high-redshift galaxies may form via common mechanisms. However, the brightest clusters in the lensed galaxies tend to be ~= 1–1.5 mag brighter and ~= 0.5 dex more massive than the median E-MOSAICS predictions. We discuss the large number of effects that could explain the discrepancy, including simulation and observational limitations, stellar population models, cluster detection biases, and nuclear star clusters. Understanding these limitations would enable stronger tests of globular cluster formation models. 

This study compiles stellar populations and internal properties of ultra-diffuse galaxies (UDGs) to highlight correlations with their local environment, globular cluster (GC) richness, and star formation histories. Complementing our sample of 88 UDGs, we include 36 low surface brightness dwarf galaxies with UDG-like properties, referred to as NUDGes (nearly UDGs). All galaxies were studied using the same spectral energy distribution fitting methodology to explore what sets UDGs apart from other galaxies. We show that NUDGes are similar to UDGs in all properties except for being, by definition, smaller and having higher surface brightness. We find that UDGs and NUDGes show similar behaviours in their GC populations, with the most metal-poor galaxies hosting consistently more GCs on average. This suggests that GC content may provide an effective way to distinguish extreme galaxies within the low surface brightness regime alongside traditional parameters like size and surface brightness. We confirm previous results using clustering algorithms that UDGs split into two main classes, which might be associated with the formation pathways of a puffy dwarf and a failed galaxy. The clustering applied to the UDGs + NUDGes data set yields an equivalent result. The difference in mass contained in the GC system suggests that galaxies in different environments have not simply evolved from one another but may have formed through distinct processes.