We present a homogeneous catalog of 275 large (effective radius 5.3 ) ultra-diffuse galaxy (UDG) candidates lying within an ≈ 290 square degree region surrounding the Coma cluster. The catalog results from our automated postprocessing of data from the Legacy Surveys, a three-band imaging survey covering 14,000 square degrees of the extragalactic sky. We describe a pipeline that identifies UDGs and provides their basic parameters. The survey is as complete in these large UDGs as previously published UDG surveys of the central region of the Coma cluster. We conclude that the majority of our detections are at roughly the distance of the Coma cluster, implying effective radii ≥ 2.5 kpc, and that our sample contains a significant number of analogs of DF 44, where the effective radius exceeds 4 kpc, both within the cluster and in the surrounding field. The g − z color of our UDGs spans a large range, suggesting that even large UDGs may reflect a range of formation histories. A majority of the UDGs are consistent with being lower stellar mass analogs of red sequence galaxies, but we find both red and blue UDG candidates in the vicinity of the Coma cluster and a relative overabundance of blue UDG candidates in the lower density environments and the field. Our eventual processing of the full Legacy Surveys data will produce the largest, most homogeneous sample of large UDGs.

We have conducted a search of a 9 deg 2 region of the CFHTLS around the Milky Way analog M101 (D∼7 Mpc), in order to look for previously unknown low surface brightness galaxies. This search has uncovered 38 new low surface brightness dwarf candidates, and confirmed 11 previously reported galaxies, all with central surface brightness µ(g,0)>23mag/arcsec 2 , potentially extending the satellite luminosity function for the M101 group by ∼1.2 magnitudes. The search was conducted using an algorithm that nearly automates the detection of diffuse dwarf galaxies. The candidates small size and low surface brightness means that the faintest of these objects would likely be missed by traditional visual or computer detection techniques. The dwarf galaxy candidates span a range of −7.1 ≥ M g ≥ −10.2 and half light radii of 118-540 pc at the distance of M101, and they are well fit by simple Sérsic surface brightness profiles. These properties are consistent with dwarfs in the Local Group, and to match the Local Group luminosity function ∼10-20 of these candidates should be satellites of M101. Association with a massive host is supported by the lack of detected star formation and the over density of candidates around M101 compared to the field. The spatial distribution of the dwarf candidates is highly asymmetric, and concentrated to the northeast of M101 and therefore distance measurements will be required to determine if these are genuine members of the M101 group.

We have obtained deep Hubble Space Telescope (HST) imaging of 19 dwarf galaxy candidates in the vicinity of M101. Advanced Camera for Surveys (ACS) HST photometry for 2 of these objects showed resolved stellar populations and Tip of the Red Giant Branch (TRGB) derived distances (D∼7 Mpc) consistent with M101 group membership. The remaining 17 were found to have no resolved stellar populations, meaning they are either part of the background NGC 5485 group or are distant low surface brightness (LSB) galaxies. It is noteworthy that many LSB objects which had previously been assumed to be M101 group members based on projection have been shown to be background objects, indicating the need for future diffuse dwarf surveys to be very careful in drawing conclusions about group membership without robust distance estimates. In this work we update the satellite luminosity function (LF) of M101 based on the presence of these new objects down to M V =−8.2. M101 is a sparsely populated system with only 9 satellites down to M V ≈−8, as compared to 26 for M31 and 24.5±7.7 for the median Milky Way (MW)-mass host in the Local Volume. This makes M101 by far the sparsest group probed to this depth, though M94 is even sparser to the depth it has been examined (M V =−9.1). M101 and M94 share several properties that mark them as unusual compared to the other local MW-mass galaxies examined: they have a very sparse satellite population but also have high star forming fractions among these satellites; such properties are also found in the galaxies examined as part of the SAGA survey. We suggest that these properties appear to be tied to the wider galactic environment, with more isolated galaxies showing sparse satellite populations which are more likely to have had recent star formation, while those in dense environments have more satellites which tend to have no ongoing star formation. Overall our results show a level of halo-to-halo scatter between galaxies of similar mass that is larger than is predicted in the ΛCDM model.

We report the discovery of the faintest known dwarf galaxy satellite of an LMC stellar-mass host beyond the Local Group, based on deep imaging with Subaru/Hyper Suprime-Cam. MAD-CASH J074238+652501-dw lies ∼35 kpc in projection from NGC 2403, a dwarf spiral galaxy at D≈3.2 Mpc. This new dwarf has M g = −7.4 ± 0.4 and a half-light radius of 168 ± 70 pc, at the calculated distance of 3.39 ± 0.41 Mpc. The color-magnitude diagram reveals no evidence of young stellar populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low luminosity dwarfs in the Local Group. The lack of either detected HI gas (M HI /L V < 0.69 M ⊙ /L ⊙ , based on Green Bank Telescope observations) or GALEX NUV/FUV flux enhancement is consistent with a lack of young stars. This is the first result from the MADCASH (Magellanic Analog Dwarf Companions And Stellar Halos) survey, which is conducting a census of the stellar substructure and faint satellites in the halos of Local Volume LMC analogs via resolved stellar populations. Models predict a total of ∼4-10 satellites at least as massive as MADCASH J074238+652501-dw around a host with the mass of NGC 2403, with 2-3 within our field of view, slightly more than the one such satellite observed in our footprint.