Merger signatures in low excitation radio galaxies

TL;DR

This paper investigates the role of major mergers in the formation of low excitation radio galaxies (LERGs), explaining the scarcity of merger signatures due to different timescales in cluster and field environments.

Contribution

It offers a new explanation for the limited merger signatures in LERGs by analyzing environmental timescales and their impact on observable merger evidence.

Findings

Cluster LERGs have longer formation timescales than field LERGs.

Estimated merger signature visibility probability is about 7% in fields and 3% in clusters.

Theoretical predictions align with observed merger signature frequencies in different environments.

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 (2019) 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 timescale is an order of magnitude longer in field environments. As a result of these different timescales, the average lifetime of a cluster LERG is estimated at an order of magnitude greater value than for field LERGs. Observing a 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 timescales, obtaining a probability of about 7% that field LERGs will show merger signatures and 3% for clusters, showing that theory and observation match if major merger signatures remain visible for a few hundred million years.