Galaxies are the building blocks of the visible universe, inhabiting a variety of environments. Observations show that over half of the galaxies are located in group systems which have members from a few to dozens.
Compact Group of galaxies (CGs) is an extreme case of groups which contains a few member galaxies separated by projected distance at around tens of thousands of light years. They are believed to represent an environment where tidal interaction, harassment and galaxy merging are much more active than normal or loose group of galaxies.
Thus, to further shed light on the formation and evolution of the CGs, a large number of CGs with redshift measurements and well-defined selection effects are required, which is not an easy task.
Recently, a research team from Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences published a new research result on CGs, now accepted for publication in the Astrophysical Journal Supplement. In this study, they obtained the largest and most complete CG samples to date, by combining the traditional photometric selection algorithm and the new spectroscopic identification technique.
The sample include 6144 CGs and 8022 CG candidates. SHEN Shiyin, the corresponding author of this work said, “the sample is unique in the studies of the nature of the CGs and the evolution of the galaxies inside.”
ZHENG Yunliang, the first author of this study further explained, who is a Ph.D student of SHAO supervised by SHEN Shiyin, “the traditional photometric method bias against the nearby CG candidates, while spectroscopic identification technique is affected by the spectroscopic incompleteness of the sample galaxies and biases again the distant candidates. By combining the two methods, we can select the CGs almost without bias towards nearby or distant objects.”
In this work, the complementary spectroscopic data from the Goushoujing Telescope (also known as LAMOST) spectral and GAMA surveys have played an important role in overcoming the problem of the spectroscopic incompleteness. One of the important observational samples of the LAMOST spectral survey is to supplement the missed main sample galaxies in the SDSS. The combination of these surveys constitutes the largest spectroscopic identified CGs and CG candidates so far, providing the foundation of the future statistical study on the physical nature of the CGs.
ZHENG Yun-Liang, SHEN Shiyin. “Compact Group of Galaxies in Sloan Digital Sky Survey and LAMOST Spectral Survey: I. The Catalogs, Astrophysics Journal Supplement in press,