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.
