Are there larger stellar velocity dispersions in low redshift Type-1 AGN than in Type-2 AGN?

TL;DR

This study compares stellar velocity dispersions between low-redshift Type-1 and Type-2 AGN, finding a statistically significant but small difference that challenges the AGN Unified Model.

Contribution

It provides the largest direct comparison of stellar velocity dispersions between Type-1 and Type-2 AGN, revealing a small but significant difference inconsistent with the Unified Model.

Findings

Type-1 AGN have about 9% larger stellar velocity dispersions than Type-2 AGN.

The difference is statistically significant at over 10 sigma confidence level.

Results challenge the assumption of similar host galaxy properties in the Unified Model.

Abstract

The main objective is to check the Unified Model (UM) expected similar stellar velocity dispersions between Type-1 AGN and Type-2 AGN, then to provide further clues on BH mass properties. Not similar as previous comparisons of BH masses estimated by \msig relations to Type-2 AGN but Virial BH masses in Type-1 AGN, reliable stellar velocity dispersions $\sigma$ measured through absorption features around 4000\AA~ are directly compared between so-far the largest samples of 6260 low redshift ($z~<~0.3$) Type-1 AGN and almost all the Type-2 AGN in SDSS DR12. Although half of Type-1 AGN do not have measured $\sigma$ due to unapparent absorption features overwhelmed by AGN activities, both properties of mean spectra of Type-1 AGN with and without measured $\sigma$ and positive dependence of $\sigma$ on [O~{\sc iii}] luminosity can lead to statistically larger $\sigma$ of all the Type-1 AGN than the 6260 Type-1 AGN with measured stellar velocity dispersions. Then, direct $\sigma$ comparisons can lead to statistically larger $\sigma$ in Type-1 AGN, with confidence level higher than 10sigma, after considering necessary effects of different redshift and different central AGN activities. Although Type-1 AGN have $\sigma$ only about $(9\pm3)$\% larger than Type-2 AGN, the difference cannot be well explained at current stage. Unless there was strong evidence to support different \msig relations or to support quite different evolution histories between Type-1 AGN and Type-2 AGN, the statistically larger $\sigma$ in Type-1 AGN provides a strong challenge to the Unified model of AGN.