(Proposal ID) S16A-045

(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 cluster environment has been only inferred 
from scarce circumstantial evidence, and their nature, origin, and properties 
have remained mostly unknown. Here we propose a systematic census of the UDG 
population in {all} nearby (z<0.03) galaxy clusters observable in the S16A 
visibitily, covering a range of cluster internal velocity dispersion 
(equivalently mass, density, and richness). The deep HSC r-band imaging of 
the 6 clusters {out to their virial radii} will reveal the presence and 
fraction of UDGs, e.g., as a function of cluster richness. We will also obtain 
g-band images of 2 of the 6 clusters, exploring the variation of UDG colors 
with cluster-centric radius. 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 richness and UDGs' radial locations within clusters. The proposed 
study of the environmental dependence and radial distribution will place an 
important constraint.