Direct effects of the environment on AGN triggering in SDSS spiral galaxies: merger-AGN connection

TL;DR

This study investigates how galaxy environments influence AGN triggering in SDSS spiral galaxies, revealing that mergers and interactions in group and cluster outskirts can activate AGN despite limited central gas.

Contribution

It demonstrates the direct impact of environment, especially mergers and interactions, on AGN activity in spiral galaxies, considering the effects of pre-processing and gas dynamics.

Findings

Red spiral galaxies in rich clusters show increased AGN fractions.

Mergers and encounters in group environments trigger AGN activity.

AGN signatures may be obscured after merger events.

Abstract

We examine whether galaxy environments directly affect triggering nuclear activity in Sloan Digital Sky Survey (SDSS) local spiral galaxies using a volume-limited sample with the $r$-band absolute magnitude $M_{r} < -19.0$ and $0.02 < z < 0.055$ selected from the SDSS Data Release 7. To avoid incompleteness of the central velocity dispersion $\sigma$ of the volume-limited sample and to fix the black hole mass affecting AGN activity, we limit the sample to a narrow $\sigma$ range of $130$ km s$^{-1}<\sigma<200$ km s$^{-1}$. We define a variety of environments as a combination of neighbour interactions and local density on a galaxy. After the central star formation rate (which is closely related to AGN activity level) is additionally restricted, the direct impact of the environment is unveiled. In the outskirts of rich clusters, red spiral galaxies show a significant excess of the AGN fraction despite the lack of central gas. We argue that they have been pre-processed before entering the rich clusters, and due to mergers or strong encounters in the in-fall region, their remaining gases efficiently lose angular momentum. We investigate an environment in which many star-forming galaxies coexist with a few starburst-AGN composite hosts having the highest [OIII] luminosity. We claim that they are a gas-rich merger product in groups or are group galaxies in-falling into clusters, indicating that many AGN signatures may be obscured following the merger events.