(Proposal ID) S16B-031

(PI) Koda, Jin

(Proposal Title)
Environmental dependence of ultra diffuse galaxies (UDGs) with HSC

(Abstract)
The recently-discovered ultra-diffuse galaxies (UDGs) in the Coma cluster are 
promising candidates for dark galaxies that consist dominantly of dark matter. 
Their preferred presence in dense clusters is only inferred from scarce 
circumstantial evidence. Their nature, origin, and properties have remained 
mostly unknown. Here we propose a systematic census of the UDG population in 
local galaxy clusters and groups, covering a {full} range of cluster/group 
richness (equivalently mass and density). The deep HSC r-band imaging of 
clusters/groups {out to their virial radii} will reveal the presence and 
fraction of UDGs, e.g., as a function of cluster/group richness and cluster-
centric radius. We will also obtain $g$-band images, exploring the variation 
of UDG colors and stellar populations with the environments. Several 
mechanisms have been suggested for the formation of UDGs in clusters, 
including gas removal from their dark halos by ram-pressure stripping, 
galaxy harassment, and galactic winds triggered by interaction-driven 
starbursts. The impacts of these mechanisms depend on cluster/group richness 
and radial locations within clusters. The proposed study of the environmental 
dependence and radial distribution will place an important constraint.