Offset Active Galactic Nuclei as Tracers of Galaxy Mergers and Supermassive Black Hole Growth

TL;DR

This study systematically identifies offset active galactic nuclei (AGNs) in Sloan Digital Sky Survey data, linking their occurrence to galaxy mergers and black hole growth, with implications for understanding high-luminosity AGN triggering.

Contribution

It presents a new method for selecting offset AGN candidates based on velocity offsets, and analyzes their prevalence across luminosities and redshifts, suggesting a connection to galaxy mergers.

Findings

4% - 8% of AGNs are offset candidates after correction.

Offset AGN fraction increases with luminosity from 0.7% to 6%.

Offset AGN candidate fraction rises from 1.9% at z=0.1 to 32% at z=0.7.

Abstract

Offset active galactic nuclei (AGNs) are AGNs that are in ongoing galaxy mergers, which produce kinematic offsets in the AGNs relative to their host galaxies. Offset AGNs are also close relatives of dual AGNs. We conduct a systematic search for offset AGNs in the Sloan Digital Sky Survey, by selecting AGN emission lines that exhibit statistically significant line-of-sight velocity offsets relative to systemic. From a parent sample of 18314 Type 2 AGNs at z<0.21, we identify 351 offset AGN candidates with velocity offsets of 50 km/s < |v| < 410 km/s. When we account for projection effects in the observed velocities, we estimate that 4% - 8% of AGNs are offset AGNs. We designed our selection criteria to bypass velocity offsets produced by rotating gas disks, AGN outflows, and gravitational recoil of supermassive black holes, but follow-up observations are still required to confirm our candidates as offset AGNs. We find that the fraction of AGNs that are offset candidates increases with AGN bolometric luminosity, from 0.7% to 6% over the luminosity range 43 < log(L_bol) [erg/s] < 46. If these candidates are shown to be bona fide offset AGNs, then this would be direct observational evidence that galaxy mergers preferentially trigger high-luminosity AGNs. Finally, we find that the fraction of AGNs that are offset AGN candidates increases from 1.9% at z=0.1 to 32% at z=0.7, in step with the growth in the galaxy merger fraction over the same redshift range.