Merging and Clustering of the Swift BAT AGN Sample

TL;DR

This study analyzes the merger rate, environment, and clustering of nearby AGN from the SWIFT BAT sample, revealing higher disruption signs and close pairs compared to control and optical-selected AGN, suggesting obscured merging activity.

Contribution

It provides new insights into the merging behavior and environment of hard X-ray selected AGN, highlighting differences from optical diagnostics and implications for AGN identification.

Findings

Higher incidence of galaxy disruption signs in AGN

More close pairs within 30 kpc compared to controls

Merging AGN may be obscured in optical diagnostics

Abstract

We discuss the merger rate, close galaxy environment, and clustering on scales up to a Mpc of the SWIFT BAT hard X-ray sample of nearby (z<0.05), moderate-luminosity active galactic nuclei (AGN). We find a higher incidence of galaxies with signs of disruption compared to a matched control sample (18% versus 1%) and of close pairs within 30 kpc (24% versus 1%). We also find a larger fraction with companions compared to normal galaxies and optical emission line selected AGN at scales up to 250 kpc. We hypothesize that these merging AGN may not be identified using optical emission line diagnostics because of optical extinction and dilution by star formation. In support of this hypothesis, in merging systems we find a higher hard X-ray to [OIII] flux ratio, as well as emission line diagnostics characteristic of composite or star-forming galaxies, and a larger IRAS 60 um to stellar mass ratio.