The Compactness of Galaxy Groups in the Sloan Digital Sky Survey

Abstract

We use an updated version of the halo-based galaxy group catalog of Yang et al., and take surface brightness of galaxy group (µ lim ) based on projected positions and luminosities of galaxy members as a compactness proxy to divide groups into sub-systems with different compactness. By comparing various properties, including galaxy conditional luminosity function, stellar population, AGN activity, and X-ray luminosity of intra-cluster medium of carefully controlled high (HC) and low compactness (LC) group samples, we find that the group compactness plays an essential role in characterizing the detailed physical properties of group themselves and their group members, especially for low mass groups with M h 10 13.5 h −1 M . We find that the low mass HC groups have systematically lower magnitude gap ∆m 12 and X-ray luminosity than their LC counterparts, indicating that the high compactness groups are probably in the early stage of group merging. On the other hand, a higher fraction of passive galaxies is found in HC groups, which however is a result of systematically smaller halo-centric distance distribution of their satellite population. After controlling of both M h and halocentric distance, we do not find any differences on both the quenching faction and AGN activity of the member galaxies between the HC and LC groups. Therefore, we conclude that the halo quenching effect, which makes the halo-centric dependence of galaxy population, is a faster process comparing to the dynamical relaxing time-scale of galaxy groups.