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Abstract Using the Systematically Measuring Ultra-Diffuse Galaxies and Sloan Digital Sky Survey catalogs and our own reprocessing of the Legacy Survey imaging, we investigate the properties of nuclear star clusters (NSCs) in galaxies having central surface brightnesses as low as 27 mag arcsec⁻². We identify 273 (123 with known redshift) and 32 NSC-bearing galaxies in the two samples, respectively, where we require candidate NSCs to have a separation of less than 0.10r_efrom the galaxy center. We find that galaxies with low central surface brightness (μ_0,g> 24 mag arcsec⁻²) are more likely to contain an NSC if they (1) have a higher stellar mass, (2) have a higher stellar-to-total mass ratio, (3) have a brighter central surface brightness, (4) have a larger axis ratio, or (5) lie in a denser environment. Because of the correlations among these various quantities, it is likely that only one or two are true physical drivers. We also find scaling relations for the NSC mass with stellar mass (M_NSC/ $M_{\odot }={10}^{6.02\pm 0.03}{(M_{*,\mathrm{gal}}/{10}^8{M}_{\odot })}^{0.77\pm 0.04}$) and halo mass (M_NSC/ $M_{\odot }={10}^{6.11\pm 0.05}{(M_{h,\mathrm{gal}}/{10}^{10}{M}_{\odot })}^{0.92\pm 0.05}$), although it is the scaling with halo mass that is consistent with a direct proportionality. In galaxies with an NSC,M_NSC≈ 10⁻⁴M_h,gal. This proportionality echoes the finding of a direct proportionality between the mass (or number) of globular clusters (GCs) in galaxies and the galaxy’s total mass. These findings favor a related origin for GCs and NSCs. 

Abstract We present our photometric search for potential nuclear star clusters (NSCs) in ultra-diffuse galaxies (UDGs) as an extension of the SMUDGes catalog. We identify 325 SMUDGes galaxies with NSCs and, from the 144 with existing distance estimates, identify 33 NSC hosts as UDGs (μ_0,g≥ 24 mag arcsec⁻²,r_e≥ 1.5 kpc). The SMUDGes with NSCs lie on the galaxy red sequence, satisfy the relationship between NSC and host galaxy stellar masses, have a mean NSC stellar mass fraction of 0.02 but reach as high as 0.1, have NSCs that are displaced from the host center with a standard deviation of 0.10r_e, and weakly favor higher-density environments. All of these properties are consistent with previous results from higher surface brightness galaxy samples, allowing for at most a relatively weak dependence of NSC behavior on host galaxy surface brightness. 

Abstract We report the discovery of Corvus A, a low-mass, gas-rich galaxy at a distance of approximately 3.5 Mpc, identified in DR10 of the Dark Energy Camera Legacy Imaging Survey during the initial phase of our ongoing SEmi-Automated Machine LEarning Search for Semi-resolved galaxies (SEAMLESS). Jansky Very Large Array observations of Corvus A detect Hiline emission at a radial velocity of 523 ± 2 km s⁻¹. Magellan/Megacam imaging reveals an irregular and complex stellar population with both young and old stars. We detect UV emission in Neil Gehrels Swift observations, indicative of recent star formation. However, there are no signs of Hiiregions in Hαimaging from Steward Observatory’s Kuiper telescope. Based on the Megacam color–magnitude diagram we measure the distance to Corvus A via the tip of the red giant branch standard candle as 3.48 ± 0.24 Mpc. This makes Corvus A remarkably isolated, with no known galaxy within ∼1 Mpc. Based on this distance, we estimate the Hiand stellar mass of Corvus A to be $\log M_{\mathrm{H}\mathrm{I}}/M_{\odot }=6.59$and $\log M_{*}/M_{\odot }=6.0$, respectively. Although there are some signs of rotation, the Hidistribution of Corvus A appears to be close to face on, analogous to that of Leo T, and we therefore do not attempt to infer a dynamical mass from its Hiline width. Higher-resolution synthesis imaging is required to confirm this morphology and to draw robust conclusions from its gas kinematics. 

Abstract To better understand the formation of large, low-surface-brightness galaxies, we measure the correlation function between ultradiffuse galaxy (UDG) candidates and Milky Way analogs (MWAs). We find that: (1) the projected radial distribution of UDG satellites (projected surface density ∝r^{−0.84±0.06}) is consistent with that of normal satellite galaxies; (2) the number of UDG satellites per MWA (S_UDG) is ∼0.5 ± 0.1 over projected radii from 20 to 250 kpc and −17 <M_r< −13.5; (3)S_UDGis consistent with a linear extrapolation of the relationship between the number of UDGs per halo versus halo mass obtained over galaxy group and cluster scales; (4) red UDG satellites dominate the population of UDG satellites (∼80%); (5) over the range of satellite magnitudes studied, UDG satellites comprise ∼10% of the satellite galaxy population of MWAs; and (6) a significant fraction of these (∼13%) have estimated total masses >10^10.9M_⊙or, equivalently, at least half the halo mass of the LMC, and populate a large fraction (∼18%) of the expected subhalos down to these masses. All of these results suggest a close association between the overall low-mass galaxy population and UDGs, which we interpret as favoring models where UDG formation principally occurs within the general context of low-mass galaxy formation over models invoking more exotic physical processes specifically invoked to form UDGs. 

ABSTRACT We present a serendipitously detected system consisting of an S0/a galaxy, which we refer to as the ‘Kite,’ and a highly collimated tail of gas and stars that extends over 380 kpc and contains pockets of star formation. In its length, narrowness, and linearity the Kite’s tail is an extreme example relative to known tails. The Kite (PGC 1000273) has a companion galaxy, Mrk 0926 (PGC 070409), which together comprise a binary galaxy system in which both galaxies host active galactic nuclei. Despite this systems being previously searched for signs of tidal interactions, the tail had not been discovered prior to our identification as part of the validation process of the SMUDGes survey for low surface brightness galaxies. We confirm the kinematic association between various H α knots along the tail, a small galaxy, and the Kite galaxy using optical spectroscopy obtained with the Magellan telescope and measure a velocity gradient along the tail. The Kite shares characteristics common to those formed via ram pressure stripping (‘jellyfish’ galaxies) and formed via tidal interactions. However, both scenarios face significant challenges that we discuss, leaving open the question of how such an extreme tail formed. We propose that the tail resulted from a three-body interaction from which the lowest mass galaxy was ejected at high velocity. 

Abstract We report the discovery of Pavo, a faint (M_V= −10.0), star-forming, irregular, and extremely isolated dwarf galaxy atD≈ 2 Mpc. Pavo was identified in Dark Energy Camera Legacy Survey imaging via a novel approach that combines low surface brightness galaxy search algorithms and machine-learning candidate classifications. Follow-up imaging with the Inamori-Magellan Areal Camera and Spectrograph on the 6.5 m Magellan Baade telescope revealed a color–magnitude diagram (CMD) with an old stellar population, in addition to the young population that dominates the integrated light, and a tip of the red giant branch distance estimate of ${1.99}_{-0.22}^{+0.20}$Mpc. The blue population of stars in the CMD is consistent with the youngest stars having formed no later than 150 Myr ago. We also detected no Hαemission with SOAR telescope imaging, suggesting that we may be witnessing a temporary low in Pavo’s star formation. We estimate the total stellar mass of Pavo to be $\log M_{*}/M_{\odot }=5.6\pm 0.2$and measure an upper limit on its Higas mass of 1.0 × 10⁶M_⊙based on the HIPASS survey. Given these properties, Pavo’s closest analog is Leo P (D= 1.6 Mpc), previously the only known isolated, star-forming, Local Volume dwarf galaxy in this mass range. However, Pavo appears to be even more isolated, with no other known galaxy residing within over 600 kpc. As surveys and search techniques continue to improve, we anticipate an entire population of analogous objects being detected just outside the Local Group. 

ABSTRACT We present an analysis of Hubble Space Telescope observations of globular clusters (GCs) in six ultradiffuse 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 globular cluster luminosity function (GCLF) 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 GCLF, 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. 

Abstract We present 226 large ultra-diffuse galaxy (UDG) candidates ( r e > 5.″3, μ 0, g > 24 mag arcsec −2 ) in the SDSS Stripe 82 region recovered using our improved procedure developed in anticipation of processing the entire Legacy Surveys footprint. The advancements include less constrained structural parameter fitting, expanded wavelet filtering criteria, consideration of Galactic dust, estimates of parameter uncertainties and completeness based on simulated sources, and refinements of our automated candidate classification. We have a sensitivity ∼1 mag fainter in μ 0, g than the largest published catalog of this region. Using our completeness-corrected sample, we find that (1) there is no significant decline in the number of UDG candidates as a function of μ 0, g to the limit of our survey (∼26.5 mag arcsec −2 ); (2) bluer candidates have smaller Sérsic n ; (3) most blue ( g – r < 0.45 mag) candidates have μ 0, g ≲ 25 mag arcsec −2 and will fade to populate the UDG red sequence we observe to ∼26.5 mag arcsec −2 ; (4) any red UDGs that exist significantly below our μ 0, g sensitivity limit are not descendent from blue UDGs in our sample; and (5) candidates with lower μ 0, g tend to smaller n . We anticipate that the final SMUDGes sample will contain ∼30 × as many candidates. 

Abstract We present a catalog of 5598 ultra-diffuse galaxy (UDG) candidates with effective radiusr_e> 5.″3 distributed throughout the southern portion of the DESI Legacy Imaging Survey covering ∼15,000 deg². The catalog is most complete for physically large (r_e> 2.5 kpc) UDGs lying in the redshift range 1800 ≲cz/km s⁻¹≲ 7000, where the lower bound is defined by where incompleteness becomes significant for large objects on the sky and the upper bound by our minimum angular size selection criterion. Because physical size is integral to the definition of a UDG, we develop a method of distance estimation using existing redshift surveys. With three different galaxy samples, two of which contain UDGs with spectroscopic redshifts, we estimate that the method has a redshift accuracy of ∼75% when the method converges, although larger, more representative spectroscopic UDG samples are needed in order to fully understand the behavior of the method. We are able to estimate distances for 1079 of our UDG candidates (19%). Finally, to illustrate some uses of the catalog, we present both distance-independent and distance-dependent results. In the latter category, we establish that the red sequence of UDGs lies on the extrapolation of the red sequence relation for bright ellipticals and that the environment–color relation is at least qualitatively similar to that of high surface brightness galaxies. Both of these results challenge some of the models proposed for UDG evolution.