Galaxy Evolution in Compact Groups

Abstract

The processes by which galaxies formed is one of the most important open questions in astrophysics. Most generally, it is now thought that galaxies formed through hierarchical formation, where galaxies grow by merging with similarly-sized galaxies and accreting smaller galaxies. Subsequently, clusters formed when galaxies would accumulate in gravitational potential wells. However, the detailed astrophysical processes involved are poorly constrained. Progress has been largely limited due to the lack of suitably high sensitivity and resolution data sets of these high redshift systems. In the absence of detailed observations of these environments in the earlier universe, the best current path forward is through relatively nearby analogs. Compact groups of galaxies provide a nearby environment with conditions similar to those in the earlier universe when galaxies were assembled. In particular, these groups give us the opportunity to witness hierarchical formation in progress. My studies of compact groups aim to determine how this intense environment - one of high galaxy density and constant interactions - affects the evolution of member galaxies. I compare the mid-IR colorspace distribution of compact group galaxies to several other environments, and find that the compact group galaxies occupy mid-IR colorspace in a unique way; there is a statistically significant canyon between quiescent galaxies and active galaxies not seen in a field sample, interacting pairs, or the center of Coma. However, the infall region of Coma shows a similar colorspace distribution, indicating similar environments; both have high densities and still contain neutral gas. An analysis of compact group galaxies' SEDs reveals that galaxies in different regions of mid-IR colorspace contain dust with varying properties. I compare mid-IR and optical colors of compact group galaxies and find that the mid-IR canyon galaxies do not occupy the optical green valley as expected, rather they fall on the optical red sequence, indicating that the mid-IR properties transition on a different timescale than the optical properties. I obtain HI observations of Redshift Survey compact groups and place them in context with those of Hickson compact groups as well as compare with mid-IR and optical properties of member galaxies.