SG1120-1202: Mass-Quenching as Tracked by UV Emission in the Group
  Environment at z=0.37
  We use the Hubble Space Telescope to obtain WFC3/F390W imaging of the
supergroup SG1120-1202 at z=0.37, mapping the UV emission of 138
spectroscopically confirmed members. We measure total (F390W-F814W) colors and
visually classify the UV morphology of individual galaxies as "clumpy" or
"smooth." Approximately 30% of the members have pockets of UV emission (clumpy)
and we identify for the first time in the group environment galaxies with UV
morphologies similar to the jellyfish galaxies observed in massive clusters. We
stack the clumpy UV members and measure a shallow internal color gradient,
which indicates unobscured star formation is occurring throughout these
galaxies. We also stack the four galaxy groups and measure a strong trend of
decreasing UV emission with decreasing projected group distance ($R_{proj}$).
We find that the strong correlation between decreasing UV emission and
increasing stellar mass can fully account for the observed trend in
(F390W-F814W) - $R_{proj}$, i.e., mass-quenching is the dominant mechanism for
extinguishing UV emission in group galaxies. Our extensive multi-wavelength
analysis of SG1120-1202 indicates that stellar mass is the primary predictor of
UV emission, but that the increasing fraction of massive (red/smooth) galaxies
at $R_{proj}$ < 2$R_{200}$ and existence of jellyfish candidates is due to the
group environment.
