The effect of environment on galaxy interactions

We have investigated the effect of environment on the interaction rate of galaxies in the local universe using data drawn from the Las Campanas Redshift Survey. This data set consists of galaxies inhabiting the entire range of galactic environments, from the sparsest field to the densest clusters, thus allowing us to study environmental variations without combining multiple data sets with inhomogeneous characteristics. The interactions of the sample galaxies are characterized by the asymmetry of galactic light and the existence of apparent close companions, while the environment of the galaxies is characterized both by the three-space local galaxy density and by membership in groups and clusters, in order to illustrate the possible differential influences of various environmental scales. We find that existence of apparent companions and the distortions of galaxies are influenced differently by the surrounding environments. (1) Existence of the apparent close companions seems to be sensitive to both the local galaxy density and membership in association, which is sensitive to the environment on a larger scale than the local galaxy density. However, the sensitivity to the local density is generally low, while that to the membership is higher in some cases. In particular, galaxies with companions are distinctively more abundant in poor clusters than in the field. On the other hand, the rich cluster/field comparison shows no environmental effects. (2) For the case of the distortions, a comparison of the population ratio in the high-and low-density subsets shows that distorted galaxies are slightly more prevalent in the high-density environment. The effect is somewhat stronger for highly distorted galaxies than moderately disturbed ones. However, the "membership" comparison reveals no significant effect of clustering on the incidence of distorted galaxies. Our results show that interactions of galaxies (characterized by the distortions of galaxies and the existence of apparent companions) are influenced by the surrounding environments. The degree of the influence varies, however, depending on the measure we have used for characterizing the interactions. The degree of the influence also varies from weak to strong interactions. Generally, the stronger interacting measures show stronger correlations with the environments than the weak measures, which show weak or marginal trends. Our results on the environmental dependence of interaction are consistent with the environmental variation of strong star formation investigated in our previous study, which used the same data set and similar analyses, supporting the popular understanding of the importance of interactions on the occurrence of strong star-forming activities.