Farid Qamar

The cause of varying galaxy interaction outcomes in different environments is investigated using a sample of over 505,000 Sloan Digital Sky Survey Data Release 7 galaxies at 0.01 < z < 0.20. The aim is to distinguish the large-scale environmental effects on galaxies from the small-scale interaction effects in order to identify whether interaction mechanisms vary in different environments, or whether different galaxy populations produce the various outcomes in different environments. To study the properties of paired galaxies, the enhancements in star formation rates (SFRs) and asymmetries are investigated as functions of projected pair separations in different environments. The paired galaxies and control samples were selected using a new technique to distinguish between the influence of nearby neighbours and larger scale environment, and we define four different environmental classes using their dark matter halo masses (the field, poor groups, rich groups, and clusters).

We find the strongest SFR enhancements at projected separations (rp) < 40 h^{−1}_70 kpc in field paired galaxies and the weakest (but statistically significant) in cluster paired galaxies, while the largest asymmetry enhancements are found in cluster paired galaxies, and the least in field paired galaxies. Using the asymmetry offset rather than enhancements reveals no dependence on environmental membership. Mean SFR suppression is found at separations 100 < rp < 400 kpc for paired galaxies with mass ratio < 1 in all environments, but is absent for paired galaxies with mass ratio > 1. For paired galaxies with mass ratio < 1, this suppression is accompanied by a lack of mean asymmetry offset or enhancement at similar separations. Possible explanations of this result include suppression due to an earlier encounter whereby star bursts consume the star forming gas, and given sufficient time the galaxy settles into a symmetric shape. At close separations (rp < 40 h^{−1}_70 kpc), reduced mean SFR enhancements in all environments are found together with increased asymmetry enhancement in comparison with paired galaxies with mass ratio > 1. We hypothesize that the suppression in lower-mass paired galaxies is due to the depletion of their gas reservoirs prior to the interaction, and conclude that interactions in different environments produce various results due to assembly bias rather than differing interaction mechanisms in various environments.