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International audience; 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.

This is the first in a series of papers on the properties of ultra-diffuse galaxies (UDGs) in clusters of galaxies. We present an updated catalog of UDGs in the Coma cluster using \textit{g}- and \textit{r}-band images obtained with Hyper Suprime-Cam (HSC) of the Subaru telescope. We develop a method to find UDGs even in the presence of contaminating objects, such as halos and background galaxies. This study expands upon our previous works that covered about half the area of the Coma cluster. The HSC observations covered the whole Coma cluster up to the virial radius and beyond (an area twice larger than the previous studies) and doubled the numbers of UDGs ($r_{\rm eff, r} \geq 1.5$ kpc) and sub-UDGs ($1.0 \leq r_{\rm eff, r} < 1.5$ kpc) to 774 and 729 respectively. The new UDGs show internal properties consistent with those of the previous studies (e.g., Sérsic index of approximately 1), and are distributed across the cluster, with a concentration around the cluster center. The whole cluster coverage clearly revealed an excess of their distribution toward the east to south-west direction along the cluster center, where Coma connects to the large scale structure, and where a known substructure exists (the NGC4839 subgroup). The alignment of the UDG distribution along the large scale structure around Coma supports the interpretation that most of them lie at the distance of the Coma cluster and the NGC4839 subgroup.

We report the discovery of 854 ultra diffuse galaxies (UDGs) in the Coma cluster using deep R band images, with partial B, i, and Halpha band coverage, obtained with the Subaru telescope. Many of them (332) are Milky Way-sized with very large effective radii of r_e>1.5kpc. This study was motivated by the recent discovery of 47 UDGs by van-Dokkum et al. (2015); our discovery suggests >1,000 UDGs after accounting for the smaller Subaru field. The new UDGs show a distribution concentrated around the cluster center, strongly suggesting that the great majority are (likely longtime) cluster members. They are a passively evolving population, lying along the red sequence in the CM diagram with no Halpha signature. Star formation was, therefore, quenched in the past. They have exponential light profiles, effective radii re ~ 800 pc- 5 kpc, effective surface brightnesses mu_e(R)=25-28 mag arcsec-2, and stellar masses ~1x10^7 - 5x10^8Msun. There is also a population of nucleated UDGs. Some MW-sized UDGs appear closer to the cluster center than previously reported; their survival in the strong tidal field, despite their large sizes, possibly indicates a large dark matter fraction protecting the diffuse stellar component. The indicated baryon fraction ~, Accepted for publication in ApJ Letters. 7 pages, 5 figures