The Optical Green Valley vs Mid-IR Canyon in Compact Groups
  Compact groups of galaxies provide conditions similar to those experienced by
galaxies in the earlier universe. Recent work on compact groups has led to the
discovery of a dearth of mid-infrared transition galaxies (MIRTGs) in IRAC (3.6
- 8.0 micron) color space (Johnson et al. 2007; Walker et al. 2012) as well as
at intermediate specific star formation rates (Tzanavaris et al. 2010).
However, we find that in compact groups these mid-infrared (mid-IR) transition
galaxies in the mid-infrared dearth have already transitioned to the optical
([g-r]) red sequence. We investigate the optical color-magnitude diagram (CMD)
of 99 compact groups containing 348 galaxies and compare the optical CMD with
mid-IR color space for compact group galaxies. Utilizing redshifts available
from SDSS, we identified new galaxy members for 6 groups. By combining optical
and mid-IR data, we obtain information on both the dust and the stellar
populations in compact group galaxies. We also compare with more isolated
galaxies and galaxies in the Coma cluster, which reveals that, similar to
clusters, compact groups are dominated by optically red galaxies. While we find
that compact group transition galaxies lie on the optical red sequence,
LVL+SINGS mid-IR transition galaxies span the range of optical colors. The
dearth of mid-IR transition galaxies in compact groups may be due to a lack of
moderately star forming low mass galaxies; the relative lack of these galaxies
could be due to their relatively small gravitational potential wells. This
makes them more susceptible to this dynamic environment, thus causing them to
more easily lose gas or be accreted by larger members.
