The Relation between Morphological Asymmetry and Nuclear Activity in Low-redshift Galaxies

The morphology of galaxies reflects their assembly history and ongoing dynamical perturbations from the environment. Analyzing i-band images from the Pan-STARRS1 Survey, we study the optical morphological asymmetry of the host galaxies of a large, well-defined sample of nearby active galactic nuclei (AGNs) to investigate the role of mergers and interactions in triggering nuclear activity. The AGNs, comprising 245 type 1 and 4514 type 2 objects, are compared with 4537 star-forming galaxies matched in redshift and stellar mass. We develop a comprehensive masking strategy to isolate the emission of the target from foreground stars and other contaminating sources, all the while retaining projected companions of comparable brightness that may be major mergers. Among three variants of nonparametric indices, both the popular CAS asymmetry parameter and the outer asymmetry parameter (A_outer) yield robust measures of morphological distortion for star-forming galaxies and type 2 AGNs, while only A_outer is effective for type 1 AGNs. The shape asymmetry, by comparison, is affected more adversely by background noise. Asymmetry indices > 0.4 effectively trace systems that are candidate ongoing mergers. Contrary to theoretical expectations, galaxy interactions and mergers are not the main drivers of nuclear activity, at least not in our sample of low-redshift, relatively low-luminosity AGNs, whose host galaxies are significantly less asymmetric than the control sample of star-forming galaxies. Moreover, type 2 AGNs are morphologically indistinguishable from their type 1 counterparts. The level of AGN activity does not correlate with asymmetry, not even among the major merger candidates. As a by-product, we find, consistent with previous studies, that the average asymmetry of star-forming galaxies increases above the main sequence, although not all major mergers exhibit enhanced star formation.