The abundance and spatial distribution of ultra-diffuse galaxies in nearby galaxy clusters

Abstract

Recent observations have highlighted a significant population of faint but large (r_eff>1.5 kpc) galaxies in the Coma cluster. The origin of these Ultra Diffuse Galaxies (UDGs) remains puzzling, as the interpretation of the observational results has been hindered by the subjective selection of UDGs, and the limited study of only the Coma (and some examples in the Virgo-) cluster. We extend the study of UDGs using 8 clusters in the redshift range 0.044<z<0.063 with deep g- and r-band imaging data taken with MegaCam at the CFHT. We describe an automatic selection pipeline for quantitative identification, tested for completeness using image simulations of these galaxies. We find that the abundance of the UDGs we can detect increases with cluster mass, reaching ~200 in typical haloes of M200~10^15 Msun. The cluster UDGs have colours consistent with the cluster red sequence, and have a steep size distribution that declines as n~r_eff^-3.4. Their radial distribution is significantly steeper than NFW in the outskirts, and is significantly shallower in the inner parts. They follow the same radial distribution as the more massive quiescent galaxies in the clusters, except within the core region of r<0.15XR200 (or <300 kpc). Within this region the number density of UDGs drops and is consistent with zero. These diffuse galaxies can only resist tidal forces down to this cluster-centric distance if they are highly centrally dark-matter dominated. The observation that the radial distribution of more compact dwarf galaxies (r_eff<1.0 kpc) with similar luminosities follows the same distribution as the UDGs, but exist down to a smaller distance of 100kpc from the cluster centres, indicates that they may have similarly massive sub-haloes as the UDGs. Although several scenarios can give rise to the UDG population, our results point to differences in the formation history as the most plausible explanation.