Radial Velocity Offsets Due to Mass Outflows and Extinction in Active Galactic Nuclei

TL;DR

This study investigates the causes of radial velocity offsets in Seyfert galaxies, attributing them mainly to ionized gas outflows and dust extinction, with implications for understanding active galactic nuclei at higher redshifts.

Contribution

The paper provides geometric models of NLRs and inner disks to explain velocity offsets, emphasizing mass outflows and dust extinction as primary factors, reducing the need for exotic explanations.

Findings

35% of Seyferts show significant [O III] blueshifts

Greater blueshift prevalence in Seyfert 1s compared to Seyfert 2s

Models explain velocity offsets without exotic mechanisms

Abstract

We present a study of the radial velocity offsets between narrow emission lines and host galaxy lines (stellar absorption and H I 21-cm emission) in Seyfert galaxies with observed redshifts less than 0.043. We find that 35% of the Seyferts in the sample show [O III] emission lines with blueshifts with respect to their host galaxies exceeding 50 km/s, whereas only 6% show redshifts this large, in qualitative agreement with most previous studies. We also find that a greater percentage of Seyfert 1 galaxies show blueshifts than Seyfert 2 galaxies. Using HST/STIS spatially-resolved spectra of the Seyfert 2 galaxy NGC 1068 and the Seyfert 1 galaxy NGC 4151, we generate geometric models of their narrow-line regions (NLRs) and inner galactic disks, and show how these models can explain the blueshifted [O III] emission lines in collapsed STIS spectra of these two Seyferts. We conclude that the combination of mass outflow of ionized gas in the NLR and extinction by dust in the inner disk (primarily in the form of dust spirals) is primarily responsible for the velocity offsets in Seyfert galaxies. More exotic explanations are not needed. We discuss the implications of this result for the velocity offsets found in higher redshift AGN.