Insights on the Dusty Torus and Neutral Torus from Optical and X-ray Obscuration in a Complete Volume Limited Hard X-ray AGN Sample

TL;DR

This study analyzes a complete, volume-limited sample of nearby AGN to understand the properties and evolution of the dusty and neutral tori, revealing that the neutral gas torus coexists with the broad line region and remains relatively stable across luminosities.

Contribution

It provides new observational evidence that the neutral gas torus coexists with the BLR and shows minimal evolution with redshift, enhancing understanding of AGN structure.

Findings

Optically obscured and X-ray absorbed fractions are 50-65%.

X-ray unabsorbed Seyfert 2s are very rare.

The neutral gas torus likely coexists with the BLR.

Abstract

We describe a complete volume limited sample of nearby active galaxies selected by their 14-195keV luminosity, and outline its rationale for studying the mechanisms regulating gas inflow and outflow. We describe also a complementary sample of inactive galaxies, selected to match the AGN host galaxy properties. The active sample appears to have no bias in terms of AGN type, the only difference being the neutral absorbing column which is two orders of magnitude greater for the Seyfert 2s. In the luminosity range spanned by the sample, log L_{14-195keV} [erg/s] = 42.4-43.7, the optically obscured and X-ray absorbed fractions are 50-65%. The similarity of these fractions to more distant spectroscopic AGN samples, although over a limited luminosity range, suggests that the torus does not strongly evolve with redshift. Our sample confirms that X-ray unabsorbed Seyfert 2s are rare, comprising not more than a few percent of the Seyfert 2 population. At higher luminosities, the optically obscured fraction decreases (as expected for the increasing dust sublimation radius), but the X-ray absorbed fraction changes little. We argue that the cold X-ray absorption in these Seyfert 1s can be accounted for by neutral gas in clouds that also contribute to the broad line region (BLR) emission; and suggest that a geometrically thick neutral gas torus co-exists with the BLR and bridges the gap to the dusty torus.