Merger signatures in low excitation radio galaxies
  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.
