Galaxy interactions II. High density environments

Aims. To assess the role of dense environments in galaxy interactions, we present an analysis of close galaxy pairs in groups and clusters, obtained from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7). Methods. We identified pairs that reside in groups by cross-correlating the total galaxy pair catalog with the SDSS-DR7 group catalog from Zapata et al. (2009, MNRAS, 394, 2229). We classify pair galaxies according to the intensity of interaction inferred from the morphological appearance of the optical images. We analyzed the effect of high-density environments on different classes of galaxy-galaxy interactions and we studied the impact of the group global environment on pair galaxies. Results. We find that galaxy pairs are more concentrated towards the group centers than the other group galaxy members, and disturbed pairs are more likely to contain the brightest galaxy in the groups. The color-magnitude relation of pair galaxies differs significantly from that of the control sample (constructed with galaxy group members without close companions), consisting in color tails with a clear excess of extremely blue and red galaxies for merging systems. In addition, pair galaxies show a significant excess of young stellar populations with respect to galaxies in the control sample; this finding suggests that, in dense environments, strong interactions have an important effect in modifying galaxy properties. We find that the fraction of star-forming galaxies decreases toward the group center; however, galaxy pairs have a more efficient star formation activity than galaxies without a close companion. We also find that pair galaxies tend to reside in groups in low density global environments with respect to galaxies of the corresponding control sample. Blue, young stellar population galaxies are also more likely to reside in groups within low density global environments. We find that this behavior is mainly driven by galaxy-galaxy interactions, which are the fundamental physical mechanisms driving this process.