# The Effect of Minor and Major Mergers on the Evolution of Low Excitation Radio Galaxies

## TL;DR

This study investigates how minor and major galaxy mergers influence the evolution of Low Excitation Radio Galaxies (LERGs) using deep imaging, revealing that minor mergers are not a primary fueling mechanism, while major mergers are more common in less massive LERGs.

## Contribution

It provides the first detailed analysis of merger activity in LERGs across different environments and stellar masses, highlighting the role of major mergers in their evolution.

## Key findings

- Minor mergers are not more frequent in LERGs than controls in groups and fields.
- Cluster LERGs show a deficit of minor mergers compared to controls.
- Major mergers are significantly more common in lower-mass LERGs, decreasing with stellar mass.

## Abstract

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.