Merger signatures in low excitation radio galaxies

Abstract

ABSTRACT While no consensus governs our understanding of the origin of low-redshift radio galaxies, the possibility that mergers may trigger accretion from hot cluster halo gas has spurred a recent search for such signatures. Evidence for mergers is at best tenuous, however, and even when found, generates more questions than answers. With scant evidence for minor mergers, some connection to major mergers is found in isolated environments but not where one would expect, i.e. in clusters. We provide an explanation for these recent results by Gordon et al. on the relevance of major mergers in low excitation radio galaxies (LERGs) at low redshift. While LERGs are not the direct result of a merger, we describe how they form in clusters in only a few million years while that time-scale is an order of magnitude longer in field environments. As a result of these different time-scales, the average lifetime of a cluster LERG is estimated at an order of magnitude greater value than for field LERGs. Observing an LERG in the cluster environment, therefore, will tend to occur when greater time has passed since the major merger event that produced its high excitation radio galaxy ancestor, such that fewer signatures of that event remain visible. We provide simple estimates for the fraction of LERGs as a function of environment that are directly related to these time-scales, obtaining a probability of about 7 per cent that field LERGs will show merger signatures and 3 per cent for clusters, showing that theory and observation match if major merger signatures remain visible for a few hundred million years.