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

Abstract

The morphology of galaxies reflects their assembly history and ongoing dynamical perturbations from the environment. Analyzing stacked i-band images from the Pan-STARRS1 3π Steradian 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 (SFGs) matched in redshift (0.04 < z < 0.15) and stellar mass (M * > 1010 M ⊙). We develop a comprehensive masking strategy to isolate the emission of the target from foreground stars and other contaminating nearby 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 (A CAS) and the outer asymmetry parameter (A outer) yield robust measures of morphological distortion for SFGs and type 2 AGNs, while only A outer is effective for type 1 AGNs. The shape asymmetry (A shape), 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 actually significantly less asymmetric than the control sample of SFGs. 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 SFGs increases above the main sequence, although not all major mergers exhibit enhanced star formation.

Panchromatic Photometry of Low-redshift, Massive Galaxies Selected from SDSS Stripe 82

“…the saddle point between the flux peaks (Sazonova et al 2021;Zhao et al 2022). If the companion sources are deblended, we generate the deblend mask for them.…”

Measuring galaxy asymmetries in 3D

“…Pearson et al ( 2022 ) applied this technique to the HSC-NEP surv e y (Hyper Subprime-Cam North Ecliptic Pole; Goto et al 2017 ;Oi et al 2021 ) to generate a merger catalogue for that field. Additionally, Bellhouse et al ( 2022 ) explored the use of these parameters to analyse ram-pressure-stripping and post-starburst galaxies, as well as to investigate the connection between AGN activity, star formation, and the disturbances in the galaxies (Zhao et al 2022 ).…”