Hierarchical Formation in Action: Characterizing Accelerated Galaxy
  Evolution in Compact Groups Using Whole-Sky WISE Data
  Compact groups provide an environment to study the growth of galaxies amid
multiple prolonged interactions. With their dense galaxy concentrations and
relatively low velocity dispersions, compact groups mimic the conditions of
hierarchical galaxy assembly. Compact group galaxies are known to show a
bimodality in $Spitzer$ IRAC infrared colorspace: galaxies are preferentially
either quiescent with low specific star formation rates, or are prolifically
forming stars---galaxies with moderate levels of specific star formation are
rare. Previous $Spitzer$ IRAC studies identifying this "canyon" have been
limited by small number statistics. We utilize whole-sky WISE data to study 163
compact groups, thereby tripling our previous sample and including more
galaxies with intermediate mid-IR colors indicative of moderate specific star
formation rates (SSFRs). We define a distinct WISE mid-IR color-space
($\log[{\frac{\rm f_{12}}{\rm f_{4.6}}}]$ vs. $\log[{\frac{\rm f_{22}}{\rm
f_{3.4}}}]$) that we use to identify canyon galaxies from the larger sample. We
confirm that compact group galaxies show a bimodal distribution in the
mid-infrared and identify 37 canyon galaxies with reliable photometry and
intermediate mid-IR colors. Morphologically, we find that the canyon harbors a
large population of both Sa-Sbc and E/S0 type galaxies, and that they fall on
the optical red sequence rather than the green valley. Finally, we provide a
catalog of WISE photometry for 567 of 652 galaxies selected from the sample of
163 compact groups.
