Tidal Interactions and Mergers in Intermediate-redshift EDisCS Clusters

We study the fraction of tidal interactions and mergers (TIMs) with well-identified observability timescales (f _TIM) in group, cluster, and accompanying field galaxies and its dependence on redshift (z), cluster velocity dispersion (σ), and environment analyzing Hubble Space Telescope/ACS images and catalogs from the ESO Distant Cluster Survey. Our sample consists of 11 clusters, seven groups, and accompanying field galaxies at 0.4 ≤ z ≤ 0.8. We derive f _TIM using both a visual classification of galaxy morphologies and an automated method, the G - M ₂₀ method. We calibrate this method using the visual classifications that were performed on a subset of our sample. We find marginal evidence for a trend between f _TIM and z, in that higher z values correspond to higher f _TIM. However, we also cannot rule out the null hypothesis of no correlation at higher than 68% confidence. No trend is present between f _TIM and σ. We find that f _TIM shows suggestive peaks in groups, and tentatively in clusters at R > 0.5 × R ₂₀₀, implying that f _TIM gets boosted in these intermediate-density environments. However, our analysis of the local densities of our cluster sample does not reveal a trend between f _TIM and density, except for a potential enhancement at the very highest densities. We also perform an analysis of projected radius-velocity phase space for our cluster members. Our results reveal that TIM and undisturbed galaxies only have a 6% probability of having been drawn from the same parent population in their velocity distribution and 37% in radii, in agreement with the modest differences obtained in f _TIM at the clusters.