Direct Effects of the Environment on AGN Triggering in SDSS Spiral Galaxies: Merger-AGN connection

Abstract

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 Mr < −19.0 and 0.02 < z < 0.055 selected from the SDSS Data Release 7. To avoid incompleteness of the central velocity dispersion σ of the volume-limited sample and to fix the black hole mass affecting AGN activity, we limit the sample to a narrow σ range of 130 < σ < 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. Furthermore, we find many star-forming galaxies and only a few starburst-AGN composite hosts with 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 during the merger events.