The Subaru Survey of Ultra-Diffuse Galaxies in Galaxy Clusters

Jin Koda

We will report the internal and statistical properties of ~1000 ultra-diffuse galaxies (UDGs) in the Coma cluster discovered by our Subaru Suprime-Cam and HSC studies (Koda et al. 2015; Yagi et al. 2016). Put simply, these low-surface brightness UDGs, mu_e(R)~25-28mag/arcsec2, have only 1/1000 of the Milky Way (MW)’s stellar mass, but are as extended as the MW in size. Such diffuse & puffy stellar structures are subject to tidal disruption in the cluster’s potential; however, they are surprisingly long-lived, indicated by their red/old stellar populations and their symmetric (relaxed) spatial distribution in the cluster. The longevity of these puffy UDGs requires an exceedingly large fraction of DM whose gravitation protects them from tidal forces. The enclosed mass is estimated by assuming that the observed size represents tidal extension, indicating the lower limit of the large DM fraction of more than 99% - excessively high for MW-sized galaxies. Therefore, this new, large population of galaxies is important in studies of galactic dark matter (DM), galaxy formation, and galaxy clusters. We are surveying UDGs in local clusters of a range of richness with Subaru HSC. We will show that their isophotal shapes are approximately round with very slight elongations, and in a statistical sense their major axes are preferentially oriented toward the cluster center. This indicates a potential influence of the cluster's tidal field on the internal shapes of UDGs, and therefore casting an upper limit on the DM fraction. If time allows, we will also discuss their environmental dependence.