  Compact galaxy groups are at the extremes of the group environment, with high
number densities and low velocity dispersions that likely affect member galaxy
evolution. To explore the impact of this environment in detail, we examine the
distribution in the mid-infrared (MIR) 3.6-8.0 micron colorspace of 42 galaxies
from 12 Hickson compact groups in comparison with several control samples,
including the LVL+SINGS galaxies, interacting galaxies, and galaxies from the
Coma Cluster. We find that the HCG galaxies are strongly bimodal, with
statistically significant evidence for a gap in their distribution. In
contrast, none of the other samples show such a marked gap, and only galaxies
in the Coma infall region have a distribution that is statistically consistent
with the HCGs in this parameter space. To further investigate the cause of the
HCG gap, we compare the galaxy morphologies of the HCG and LVL+SINGS galaxies,
and also probe the specific star formation rate (SSFR) of the HCG galaxies.
While galaxy morphology in HCG galaxies is strongly linked to position with MIR
colorspace, the more fundamental property appears to be the SSFR, or star
formation rate normalized by stellar mass. We conclude that the unusual MIR
color distribution of HCG galaxies is a direct product of their environment,
which is most similar to that of the Coma infall region. In both cases, galaxy
densities are high, but gas has not been fully processed or stripped. We
speculate that the compact group environment fosters accelerated evolution of
galaxies from star-forming and neutral gas-rich to quiescent and neutral
gas-poor, leaving few members in the MIR gap at any time.
