Systematically Measuring Ultra-diffuse Galaxies (SMUDGes). I. Survey Description and First Results in the Coma Galaxy Cluster and Environs

Abstract

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.

Observational properties of ultra-diffuse galaxies in low-density environments: field UDGs are predominantly blue and star forming

“…UDGs in clusters are typically on the red sequence (Koda et al 2015;van der Burg et al 2016;RS et al 2019) and show little evidence for tidal interaction even close to the cluster centres (Mowla et al 2017), suggesting relatively high mass-to-light ratios. There is tentative evidence that UDGs tend to be bluer towards the outskirts of galaxy groups (Román & Trujillo 2017b;Alabi et al 2018;Zaritsky et al 2019) (and more generally in lower-density environments cf. Greco et al 2018a,b), suggesting that interactions with the environment during in-fall can diminish star formation in the UDGs.…”

“…Indeed, this property has been exploited by several authors in their studies of UDGs in such environments (e.g. Koda et al 2015;Muñoz et al 2015;Yagi et al 2016;van der Burg et al 2016van der Burg et al , 2017Janssens et al 2017;Venhola et al 2017;Zaritsky et al 2019;Mancera Piña et al 2019). This bias towards studies in dense environments is exacerbated by the difficulty of obtaining spectroscopic redshifts (and therefore distances) for large samples of LSB galaxies.…”

“…non-group or cluster) UDG candidates. Some studies (Das 2013;Leisman et al 2017;Papastergis et al 2017;Greco et al 2018a;Zaritsky et al 2019) have shown that a field population of UDGs does indeed exist, yet the global properties of these galaxies are poorly understood.…”

Ultra-diffuse Galaxies at Ultraviolet Wavelengths

“…This empirical finding has led to speculation that the evolution of these galaxies is guided by tidal effects, ram pressure gas loss, environmental "strangulation," and basically all of the phenomena that are speculated to alter highbrightness galaxies in clusters (see Boselli & Gavazzi 2006). In contrast, the small number of UDGs spectroscopically confirmed to lie in the field are optically bluer (Zaritsky et al 2019) and sometimes have associated HI (Leisman et al 2017;Spekkens & Karunakaran 2018). In fact, several UDG origin scenarios predict an absence of red UDGs in the field (e.g., Di Cintio et al 2017;Chan et al 2018).…”

The formation and evolution of low-surface-brightness galaxies

“…unquenched) possibly reflecting a wide range of formation scenarios across different environments (e.g. Román & Trujillo 2017b;Zaritsky et al 2019). LSBGs are typically extremely extended systems for their stellar mass, with low (n 1) Sérsic indices (Koda et al 2015).…”