The claimed detection of a diffuse galaxy lacking dark matter represents a possible challenge to our understanding of the properties of these galaxies and galaxy formation in general. The galaxy, already identified in photographic plates taken in the summer of 1976 at the UK 48-in Schmidt telescope, presents normal distance-independent properties (e.g. colour, velocity dispersion of its globular clusters). However, distance-dependent quantities are at odds with those of other similar galaxies, namely the luminosity function and sizes of its globular clusters, mass-to-light ratio and dark matter content. Here we carry out a careful analysis of all extant data and show that they consistently indicate a much shorter distance (13 Mpc) than previously indicated (20 Mpc). With this revised distance, the galaxy appears to be a rather ordinary low surface brightness galaxy (R_e=1.4+-0.1 kpc; M*=6.0+-3.6x10^7 Msun) with plenty of room for dark matter (the fraction of dark matter inside the half mass radius is >75% and M_halo/M*>20) corresponding to a minimum halo mass >10^9 Msun. At 13 Mpc, the luminosity and structural properties of the globular clusters around the object are the same as those found in other galaxies.

A distance of 13 Mpc resolves the claimed anomalies of the galaxy lacking dark matter

Author(s): Chan, TK; Keres, D; Wetzel, A; Hopkins, PF; Faucher-Giguere, C-A; El-Badry, K; Garrison-Kimmel, S; Boylan-Kolchin, M | Abstract: We test if the cosmological zoom-in simulations of isolated galaxies from the FIRE project reproduce the properties of ultra diffuse galaxies (UDGs). We show that outflows that dynamically heat galactic stars, together with a passively aging stellar population after imposed quenching, naturally reproduce the observed population of red UDGs, without the need for high spin haloes, or dynamical influence from their host cluster. We reproduce the range of surface brightness, radius, and absolute magnitude of the observed red UDGs by quenching simulated galaxies at a range of different times. They represent a mostly uniform population of dark matter-dominated dwarf galaxies with M * ~ 108 M☉, low metallicity, and a broad range of ages; the more massive the UDGs, the older they are. The most massive red UDG in our sample(M * ~ 3 × 108 M☉) requires quenching at z ~ 3 when its halo reached M h ~ 1011M☉. Our simulated UDGs form with normal stellar-to-halo ratios and match the central enclosed masses and the velocity dispersions of the observed UDGs. Enclosed masses remain largely fixed across a broad range of quenching times because the central regions of their dark matter haloes complete their growth early. If our simulated dwarfs are not quenched, they evolve into bluer low surface brightness galaxies with M/L similar to observed field dwarfs. While our simulation sample covers a limited range of formation histories and halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy type around M * ~ 108 M☉, both in the field and in clusters.

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The origin of ultra diffuse galaxies: stellar feedback and quenching.

Author(s): Carleton, T; Errani, R; Cooper, M; Kaplinghat, M; Penarrubia, J; Guo, Y | Abstract: We propose that the ultra-diffuse galaxy (UDG) population represents a set of satellite galaxies born in ∼1010-1011 M⊙ haloes, similar to field dwarfs, which suffer a dramatic reduction in surface brightness due to tidal stripping and heating. This scenario is observationally motivated by the radial alignment of UDGs in Coma as well as the significant dependence of UDG abundance on cluster mass. As a test of this formation scenario, we apply a semi-analytic model describing the change in stellar mass and half-light radius of dwarf satellites, occupying either cored or cuspy haloes, to cluster subhaloes in the Illustris-dark simulation. Key to this model is results from simulations that indicate that galaxies in cored dark matter haloes expand significantly in response to tidal stripping and heating, whereas galaxies in cuspy haloes experience limited size evolution. Our analysis indicates that a population of tidally stripped dwarf galaxies, residing in cored haloes (such as those hosting low surface brightness field dwarfs), is able to reproduce the observed sizes and stellar masses of UDGs in clusters remarkably well.

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The formation of ultra-diffuse galaxies in cored dark matter haloes through tidal stripping and heating

We use data from the HST Coma Cluster Treasury program to assess the richness of the Globular Cluster Systems (GCSs) of 54 Coma ultra-diffuse galaxies (UDGs), 18 of which have a half-light radius exceeding 1.5 kpc. We use a hierarchical Bayesian method tested on a large number of mock datasets to account consistently for the high and spatially varying background counts in Coma. These include both background galaxies and intra-cluster GCs (ICGCs), which are disentangled from the population of member GCs in a probabilistic fashion. We find no candidate for a GCS as rich as that of the Milky Way, our sample has GCSs typical of dwarf galaxies. For the standard relation between GCS richness and halo mass, 33 galaxies have a virial mass $M_{vir}\leq10^{11}M_\odot$ at 90% probability. Only three have $M_{vir}>10^{11}M_\odot$ with the same confidence. The mean colour and spread in colour of the UDG GCs are indistinguishable from those of the abundant population of ICGCs. The majority of UDGs in our sample are consistent with the relation between stellar mass and GC richness of 'normal' dwarf galaxies. Nine systems, however, display GCSs that are richer by a factor of 3 or more (at 90% probability). Six of these have sizes $\lesssim1.4$ kpc. Our results imply that the physical mechanisms responsible for the extended size of the UDGs and for the enhanced GC richness of some cluster dwarfs are at most weakly correlated.

The globular cluster systems of 54 Coma ultra-diffuse galaxies: statistical constraints from HST data

The study of dwarf galaxies and their environments provides crucial testbeds for predictions of cosmological models and insights on the structure formation on small cosmological scales. In recent years, many problems on the scale of groups of galaxies challenged the current standard model of cosmology. We aim to increase the sample of known galaxies in the Leo-I group, containing the M\,96 subgroup and the Leo Triplet. This galaxy aggregate is located at the edge of the Local Volume at a mean distance of 10.7 Mpc. We employ image enhancing techniques to search for low-surface brightness objects in publicly available gr images taken by the Sloan Digital Sky Survey within 500 square degrees around the Leo-I group. Once detected, we perform surface photometry and compare their structural parameters to other known dwarf galaxies in the nearby universe. We found 36 new dwarf galaxy candidates within the search area. Their morphology and structural parameters resemble known dwarfs in other groups. Among the candidates 5 to 6 galaxies are considered as ultra diffuse galaxies candidates. If confirmed, they would be some of the closest examples of this galaxy type. We assessed the luminosity function of the Leo-I group and find it to be considerably rich in dwarf galaxies, with twice the number of galaxies as the Local Group at a limiting magnitude of M_V=-10 and a steeper faint-end slope.

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The Leo-I group: new dwarf galaxy and UDG candidates

Studies of low surface brightness (LSB) galaxies in nearby clusters have revealed a sub-population of extremely diffuse galaxies with central surface brightness $\mu_{0,g'}$ > 24 mag arcsec$^{-2}$ and effective radius between 1.5 kpc 23 mag arcsec$^{-2}$. We classified the objects based on their appearance and performed 2D Sersic model fitting with GALFIT. We analyzed their distribution and orientations in the cluster, and studied their colors and compared the LSB galaxies in Fornax with those in other environments. Our sample consists of 205 galaxies of which 196 are LSB dwarfs (with R$_e$ 1.5 kpc). We show that the UDGs have g'-r' colors similar to those of LSB dwarfs. The largest UDGs in our sample appear different from the other LSB galaxies, in that they are significantly more elongated and extended, whereas the smaller UDGs differ from the LSB dwarfs only by their effective radii. We do not find clear differences between the structural parameters of the UDGs in our sample and those of UDGs in other galaxy environments. We find that the dwarf LSB galaxies in our sample are less concentrated in the cluster center than the galaxies with higher surface brightness, and that their number density drops in the core of the cluster. Our findings are consistent with the small UDGs forming the tail of a continuous distribution of less extended LSB galaxies. However, the elongated and distorted shapes of the large UDGs could imply that they are tidally disturbed galaxies.

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The Fornax Deep Survey (FDS) with the VST: III. Low Surface Brightness (LSB) dwarfs and Ultra Diffuse Galaxies (UDGs) in the center of the Fornax cluster

C. Wittman

Papers

The Fornax Deep Survey (FDS) with the VST: III. Low Surface Brightness (LSB) dwarfs and Ultra Diffuse Galaxies (UDGs) in the center of the Fornax cluster

The Fornax Deep Survey (FDS) with the VST: III. Low Surface Brightness (LSB) dwarfs and Ultra Diffuse Galaxies (UDGs) in the center of the Fornax cluster