Typical AGN at intermediate redshifts

TL;DR

This study examines typical X-ray selected AGN at intermediate redshifts, revealing their host galaxies are massive, with black hole properties similar to quasars, and showing low accretion rates, suggesting a downsizing trend.

Contribution

It provides new insights into the host galaxy properties and black hole characteristics of typical AGN at intermediate redshifts, emphasizing their role in cosmic downsizing.

Findings

AGN reside in massive galaxies with intermediate stellar masses.

Black hole masses are comparable to optically identified quasars.

AGN show low Eddington ratios, indicating less efficient accretion.

Abstract

We study the host galaxies and black holes of typical X-ray selected AGN at intermediate redshifts (z~0.5-1.4). The AGN are selected such that their spectral energy distributions are dominated by stellar emission, i.e., they show a prominent 1.6micron bump thus minimizing the AGN emission contamination. This AGN population comprises approximately 50% of the X-ray selected AGN at these redshifts. AGN reside in the most massive galaxies at the redshifts probed here, with characteristic stellar masses that are intermediate between those of local type 2 AGN and high redshift (z~2) AGN. The inferred black hole masses of typical AGN are similar to those of optically identified quasars at similar redshifts. Since the AGN in our sample are much less luminous than quasars, typical AGN have low Eddington ratios. This suggests that, at least at intermediate redshifts, the cosmic AGN 'downsizing' is due to both a decrease in the characteristic stellar mass of the host galaxies, and less efficient accretion. Finally there is no strong evidence in AGN host galaxies for either highly suppressed star formation, expected if AGN played a role in quenching star formation, or elevated star formation when compared to mass selected galaxies of similar stellar masses and redshifts.