No abstract

Compact group (CG) is a kind of special galaxy system where the galaxy members are separated at the distances of the order of galaxy size. The strong interaction between the galaxy members makes CGs ideal labs for studying the environmental effects on galaxy evolution. Traditional photometric selection algorithm biases against the CG candidates at low redshifts, while spectroscopic identification technique is affected by the spectroscopic incompleteness of sample galaxies and typically biases against the high redshift candidates. In this study, we combine these two methods and select CGs in the main galaxy sample of the Sloan Digital Sky Survey, where we also have taken the advantages of the complementary redshift measurements from the LAMOST spectral and GAMA surveys. We have obtained the largest and most complete CG samples to date. Our samples include 6,144 CGs and 8,022 CG candidates, which are unique in the studies of the nature of the CGs and the evolution of the galaxies inside.

Compact groups (CGs) of galaxies appear to be the densest galaxy systems containing a few luminous galaxies in close proximity to each other, which have a typical size of a few tens of kiloparsec in observation. On the other hand, in the modern hierarchical structure formation paradigm, galaxies are assembled and grouped in dark matter halos, which have a typical size of a few hundreds of kiloparsec. Few studies have explored the physical connection between the observation-based CGs and halo model-based galaxy groups to date. In this study, by matching the largest local CG catalog of Zheng & Shen to the halo-based group catalog of Yang et al., we find that CGs are physically heterogeneous systems and can be mainly separated into two categories, the isolated systems and those embedded in rich groups or clusters. By examining the dynamical features of CGs, we find that isolated CGs have systematically lower dynamical masses than noncompact ones at the same group luminosity, indicating a more evolved stage of isolated CGs. On the other hand, the embedded CGs are mixtures of chance alignments in poor clusters and recent infalling groups (substructures) of rich clusters.