There is a growing consensus that the vast majority of ultra-diffuse galaxies (UDGs) are dwarf galaxies. However, there remain a few UDGs that seem to be special in terms of their globular cluster (GC) systems. In particular, according to some authors, certain UDGs exhibit large GC populations when compared to expectations from their stellar (or total) mass. Among these special UDGs, DF44 in the Coma cluster is one of the better-known examples. DF44 has been claimed to have a relatively high number of GCs, $N_{GC}=74^{+18}_{-18}$, for a stellar mass of only 3 × 108M⊙ which would indicate a much larger dark halo mass than dwarfs of similar stellar mass. In this paper we revisit this number and, contrary to previous results, find $N_{GC}=21^{+7}_{-9}$ assuming that the distribution of the GCs follows the same geometry as the galaxy. If we assume that the GCs around DF44 are distributed in a (projected) circularly symmetric way and, if we use a less strict criterion for the selection of the GCs, we find $N_{GC}=18^{+23}_{-12}$. Making use of the MGC − Mhalo relation, this number of GCs suggests a dark matter halo mass of $M_{halo}=1.1^{+0.4}_{-0.5} \times 10^{11} M_{\odot }$, a value which is consistent with the expected total mass for DF44 based on its velocity dispersion, $\sigma =33^{+3}_{-3}$ km s−1. We conclude that the number of GCs around DF44 is as expected for regular dwarf galaxies of similar stellar mass and DF44 is not extraordinary in this respect.

We present an analysis of Hubble Space Telescope observations of globular clusters (GCs) in six ultra-diffuse galaxies (UDGs) in the Coma cluster, a sample that represents UDGs with large effective radii (Re), and use the results to evaluate competing formation models. We eliminate two significant sources of systematic uncertainty in the determination of the number of GCs, NGC by using sufficiently deep observations that (i) reach the turnover of the GC luminosity function and (ii) provide a sufficient number of GCs with which to measure the GC number radial distribution. We find that NGC for these galaxies is on average ∼ 20, which implies an average total mass, Mtotal, ∼ 1011 M⊙ when applying the relation between NGC and Mtotal. This value of NGC lies at the upper end of the range observed for dwarf galaxies of the same stellar mass and is roughly a factor of two larger than the mean. The GC luminosity function, radial profile and average colour are more consistent with those observed for dwarf galaxies than with those observed for the more massive (L*) galaxies, while both the radial and azimuthal GC distributions closely follow those of the stars in the host galaxy. Finally, we discuss why our observations, specifically the GC number and GC distribution around these six UDGs, pose challenges for several of the currently favoured UDG formation models.

Ultra-compact dwarf galaxies (UCDs) were serendipitously discovered by spectroscopic surveys in the Fornax cluster 20 yr ago. Nowadays, it is commonly accepted that many bright UCDs are the nuclei of galaxies that have been stripped. However, this conclusion might be driven by biased samples of UCDs in high-density environments, on which most searches are based. With the deep optical images of the Fornax Deep Survey, combined with public near-infrared data, we revisit the UCD population of the Fornax cluster and search for UCD candidates, for the first time, systematically out to the virial radius of the galaxy cluster. Our search is complete down to magnitude mg = 21 mag or M$_g\, \sim$ −10.5 mag at the distance of the Fornax cluster. The UCD candidates are identified and separated from foreground stars and background galaxies by their optical and near-infrared colours. This primarily utilizes the u−i/i−Ks diagram and a machine learning technique is employed to incorporate other colour combinations to reduce the number of contaminants. The newly identified candidates (44) in addition to the spectroscopically confirmed UCDs (61), increase the number of known Fornax UCD considerably (105). Almost all of the new UCD candidates are located outside the Fornax cluster core (360 kpc), where all of the known UCDs were found. The distribution of UCDs within the Fornax cluster shows that a population of UCDs may form in low-density environments. This most likely challenges the current models of UCD formation.

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Context. Substructures in stellar haloes are a strong prediction of galaxy formation models in ΛCDM. Cold streams such as those from small satellite galaxies are extremely difficult to detect and kinematically characterize. The COld STream finder Algorithm (COSTA) is a novel algorithm able to find streams in the phase space of planetary nebulae (PNe) and globular cluster (GC) populations. COSTA isolates groups of (N) particles with small velocity dispersion (between 10 km s−1 and ∼120 km s−1) using an iterative (n) sigma-clipping over a defined number of (k) neighbor particles. Aims. We applied COSTA to a catalog of PNe and GCs from the Fornax Cluster VLT Spectroscopic Survey (FVSS) within ∼200 kpc from the cluster core in order to detect cold substructures and characterize their kinematics (mean velocity and velocity dispersion). Methods. We selected more than 2000 PNe and GCs from the FVSS catalogs and adopted a series of optimized setups of the COSTA parameters based on Montecarlo simulations of the PN and GC populations to search for realistic stream candidates. We find 13 cold substructures with velocity dispersion ranging from ∼20 to ∼100 km s−1, which are likely associated either to large galaxies or to ultra-compact dwarf (UCD) galaxies in the Fornax core. Results. The luminosities of these streams show a clear correlation with internal velocity dispersion, and their surface brightness correlates with their size and distance from the cluster center, which is compatible with the dissipative processes producing them. However, we cannot exclude that some of these substructures formed by violent relaxation of massive satellites that finally merged into the central galaxy. Among these substructures we have: (1) a stream connecting NGC 1387 to the central galaxy, NGC 1399, previously reported in the literature; (2) a new giant stream produced by the interaction of NGC 1382 with NGC 1380 and (possibly) NGC 1381; (3) a series of streams kinematically connected to nearby UCDs; and (4) clumps of tracers with no clear kinematical association to close cluster members. Conclusions. We show evidence for a variety of cold substructures predicted in simulations. Most of the streams are kinematically connected to UCDs, supporting the scenario that they can be remnants of disrupted dwarf systems. However, we also show the presence of long coherent substructures connecting cluster members and isolated clumps of tracers possibly left behind by their parent systems before these merged into the central galaxy. Unfortunately, the estimated low-surface brightness of these streams does not allow us to find their signatures in the current imaging data and deeper observations are needed to confirm them.