Shocks and Spatially Offset Active Galactic Nuclei Produce Velocity Offsets in Emission Lines

TL;DR

This study investigates the causes of velocity offsets in emission lines of active galactic nuclei, revealing that shocks from outflows or gas inflows, rather than galaxy mergers, are responsible for these offsets in most cases.

Contribution

The paper demonstrates that spatially offset emission peaks in AGNs are primarily caused by shocks from outflows or inflows, providing a new method to distinguish different types of velocity-offset AGNs.

Findings

Spatial offsets are due to shocks from AGN outflows or gas inflows.

Optical line ratios indicate shocks contribute to velocity offsets.

Shocks are a key factor in velocity-offset AGNs, not just galaxy mergers.

Abstract

While 2% of active galactic nuclei (AGNs) exhibit narrow emission lines with line-of-sight velocities that are significantly offset from the velocity of the host galaxy's stars, the nature of these velocity offsets is unknown. We investigate this question with Chandra/ACIS and Hubble Space Telescope/Wide Field Camera 3 observations of seven velocity-offset AGNs at z<0.12, and all seven galaxies have a central AGN but a peak in emission that is spatially offset by < kpc from the host galaxy's stellar centroid. These spatial offsets are responsible for the observed velocity offsets and are due to shocks, either from AGN outflows (in four galaxies) or gas inflowing along a bar (in three galaxies). We compare our results to a velocity-offset AGN whose velocity offset originates from a spatially offset AGN in a galaxy merger. The optical line flux ratios of the offset AGN are consistent with pure photoionization, while the optical line flux ratios of our sample are consistent with contributions from photoionization and shocks. We conclude that these optical line flux ratios could be efficient for separating velocity-offset AGNs into subgroups of offset AGNs -- which are important for studies of AGN fueling in galaxy mergers -- and central AGNs with shocks -- where the outflows are biased towards the most energetic outflows that are the strongest drivers of feedback.