Color of Ultra-Diffuse Galaxies in the Coma Cluster

We investigate the color distribution of ultra-diffuse galaxies (UDGs) across the entire Coma cluster using the Hyper-Suprime Cam images from the Subaru telescope. We detect UDGs down to an effective surface brightness of 29 mag/arcsec² in a 2 deg-radius circular region around the cluster center, out to the virial radius of Coma. This increases the number of Coma UDGs found with Suprime-cam in Yagi et al. 2016 by a factor of 2. With this new catalog, in conjunction with rigorous testing of the photometric zero-point, we confirm the presence of a radial color-gradient around the cluster center reported previously in the Suprime-cam catalog by Alabi et al. 2020. Within the inner 1 Mpc, we find that the color distributions of UDGs are slightly, but systematically, redder than those of UDGs outside this region. On the other hand, these central UDGs have magnitude distributions similar to UDGs away from the center; hence this is not a color shift along the color-magnitude relation. The central UDGs are redder than the red sequence of Coma. We consider the possibility that shift to red colors traces a difference in quenching time of star formation, by comparing the new observations with a model of ram-pressure stripping in extended galaxies.