The Effect of Minor and Major Mergers on the Evolution of Low Excitation
  Radio Galaxies
  We use deep, $\mu_{r} \lesssim 28\,\text{mag}\,\text{arcsec}^{-2}$, $r$-band
imaging from the Dark Energy Camera Legacy Survey (DECaLS) to search for past,
or ongoing, merger activity in a sample of $282$ Low Excitation Radio Galaxies
(LERGs) at $z<0.07$. Our principle aim is to assess the the role of mergers in
the evolution of LERGs. Exploiting the imaging depth, we classify tidal
remnants around galaxies as both minor and major morphological disturbances for
our LERG sample and $1,622$ control galaxies matched in redshift, stellar mass,
and environment. In groups and in the field, the LERG minor merger fraction is
consistent with the control population. In galaxy clusters, $8.8 \pm 2.9\,$ %
of LERGs show evidence of recent minor mergers in contrast to $23.0\pm 2.0\,$ %
of controls. This $\sim 4 \sigma $ deficit of minor mergers in cluster LERGs
suggests these events may inhibit this type of nuclear activity for galaxies
within the cluster environment. We observe a $> 4\sigma$ excess of major
mergers in the LERGs with $M_{*} \lesssim 10^{11}\,\text{M}_{\odot}$, with $10
\pm 1.5\,$ % of these AGN involved in such large-scale interactions compared to
$3.2 \pm 0.4\,$% of control galaxies. This excess of major mergers in LERGs
decreases with increasing stellar mass, vanishing by $M_{*} >
10^{11.3}\,\text{M}_{\odot}$. These observations show that minor mergers do not
fuel LERGs, and are consistent with typical LERGs being powered by accretion of
matter from their halo. Where LERGs are associated with major mergers, these
objects may evolve into more efficiently accreting active galactic nuclei as
the merger progresses and more gas falls on to the central engine.
