# The redshift evolution of major merger triggering of luminous AGN: a slight enhancement at z$\sim$2

## TL;DR

This study investigates the role of galaxy mergers in triggering luminous AGN across cosmic time, finding a slight increase at z~2 but overall mergers are a subdominant factor in AGN activation.

## Contribution

The paper introduces a novel residual-based technique for identifying merger features and provides the first quantitative analysis of merger influence on AGN activity over a broad redshift range.

## Key findings

- No increase in merger features with AGN luminosity.
- A fourfold increase in merger-associated AGN at z~2.
- Major mergers contribute modestly to AGN fueling, especially at high redshift.

## Abstract

Active galactic nuclei (AGN), particularly the most luminous AGN, are commonly assumed to be triggered through major mergers, however observational evidence for this scenario is mixed. To investigate any influence of galaxy mergers on AGN triggering and luminosities through cosmic time, we present a sample of 106 luminous X-ray selected type 1 AGN from the COSMOS survey. These AGN occupy a large redshift range (0.5 < z < 2.2) and two orders of magnitude in X-ray luminosity ($\sim$10$^{43}$ - 10$^{45}$ erg s$^{-1}$). AGN hosts are carefully mass and redshift matched to 486 control galaxies. A novel technique for identifying and quantifying merger features in galaxies is developed, subtracting GALFIT galaxy models and quantifying the residuals. Comparison to visual classification confirms this measure reliably picks out disturbance features in galaxies. No enhancement of merger features with increasing AGN luminosity is found with this metric, or by visual inspection. We analyse the redshift evolution of AGN associated with galaxy mergers and find no merger enhancement in lower redshift bins. Contrarily, in the highest redshift bin (z$\sim$2) AGN are $\sim$4 times more likely to be in galaxies exhibiting evidence of morphological disturbance compared to control galaxies, at 99% confidence level ($\sim$2.4$\sigma$) from visual inspection. Since only $\sim$15% of these AGN are found to be in morphologically disturbed galaxies, it is implied that major mergers at high redshift make a noticeable but subdominant contribution to AGN fuelling. At low redshifts other processes dominate and mergers become a less significant triggering mechanism.