A Spectroscopic Study on the Sloan Digital Sky Survey Compact Groups

Abstract

Compact groups of galaxies are the densest among the known systems of galaxies and they have a few member galaxies within tens of kiloparsec. Previous studies suggest that compact groups are favored regions for galaxy interaction and may suffer from merging instability. Frequent galaxy interactions in compact groups make them an ideal target to study the effects of galaxy interactions in the evolution of galaxies. Furthermore, it is a challenge to explain the existence of compact groups because they are dynamically unstable systems that can collapse in a very short time scale (< 2 Gyr).

To study environmental effects of the interaction-frequent environments of com- pact groups, I investigate the nuclear activity of galaxies in local compact groups. A spectroscopic sample of 238 galaxies in 58 compact groups from the Sloan Digital Sky Survey (SDSS) data release 7 (DR7) is used to estimate the fraction of active galactic nucleus (AGN) host galaxies in compact groups, and to compare it with those in cluster and field regions. The AGN host galaxies are identified using the emission-line ratio diagrams and the AGN fraction of compact group galaxies is estimated to be 17-42% depending on the AGN classification method. The AGN fraction in compact groups is not the highest among the galaxy environments. This trend remains in the subsamples segregated by galaxy morphology and optical luminosity. The AGN fraction for early-type galaxies decreases with increasing galaxy number density, but the fraction for late-type galaxies changes little. There is no mid-infrared detected AGN host galaxies in Wide-field Infrared Survey Explorer data for our sample of compact groups. These results suggest that the nuclear activity of compact group galaxies (mostly early types) is not strong because of the lack of gas supply even though they may experience frequent galaxy?galaxy interactions and mergers that could trigger nuclear activity.

Dynamical analysis of compact groups provides important tests of models of compact group formation and evolution. To study this issue, a catalog of compact groups with complete spectroscopic redshifts in the redshift range 0.01 < z < 0.22 is constructed by identifying 2037 redshifts from FLWO/FAST, from the literature, and from SDSS DR12 in the fields of compact groups in McConnachie et al. (2009). A secure sample of 332 compact groups includes 192 groups with N ≥ 4 member galaxies and 140 groups with 3 members. The physical properties of our sample compact groups including size, number density, velocity dispersion, and local environment are similar to those derived for the original Hickson compact groups, but they differ from the compact groups in the second Palomar Observatory Sky Survey. Differences result from subtle differences in the way the group candidates were originally selected. The velocity dispersion and space density of the compact groups change little with redshift over the range covered by this sample. The approximate constancy of the space density for this sample is potential constraint on the evolution of compact groups on a few Gigayear timescale.

To have a compact group sample without using previous selection criteria based only on the photometric information, a new catalog of compact groups selected from a complete, magnitude-limited redshift survey of SDSS DR12 is constructed by applying the friend-of-friend algorithm. This is the largest catalog of compact groups with complete redshifts. The identification of compact groups from redshift surveys without using Hicksons criteria results in a complete sample of compact groups without bias against the groups in nearby universe and in high-density regions. The surface galaxy number density (Σ5) distribution of compact groups show that the local environments of them are diverse. The physical properties of compact groups (e.g. redshift, size, density, velocity dispersion) in different environments are similar to each other. However, the compact groups in the denser region contain more early- type galaxies and redder galaxies, suggesting that embedded compact groups consist of more evolved galaxy populations.