Hierarchical Formation in Action: Characterizing Accelerated Galaxy Evolution in Compact Groups Using Whole-Sky WISE Data

TL;DR

This study uses whole-sky WISE data to analyze galaxy evolution in compact groups, revealing a bimodal distribution in mid-infrared colors and identifying a significant population of galaxies with moderate star formation, advancing understanding of hierarchical galaxy assembly.

Contribution

It expands previous infrared studies by analyzing a larger sample of compact groups and introduces a new WISE mid-IR color-space to identify galaxies with intermediate star formation rates.

Findings

Bimodal distribution of galaxies in mid-IR colors.

Identification of 37 canyon galaxies with intermediate star formation.

Large population of early-type galaxies on the red sequence.

Abstract

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.