Stellar population properties for a sample of hard X-ray AGNs

TL;DR

This study analyzes the stellar populations of 20 hard X-ray AGN host galaxies, revealing predominantly old stars with some young components and a wide metallicity range, using full-spectrum fitting techniques.

Contribution

It provides new stellar population data for hard X-ray AGN hosts, combining optical spectroscopy with advanced spectral fitting to enhance understanding of their stellar properties.

Findings

Most AGN hosts have old stellar populations.

Some hosts show significant young star contributions.

Metallicity varies widely, mostly above solar.

Abstract

The aim of this paper is to study the stellar population of galaxies hosting an active galactic nucleus (AGN). We studied a sub-sample of hard X-ray emitting AGNs from the INTEGRAL and Swift catalogs which were previously identified and characterized through optical spectroscopy. Our analysis provides complementary information, namely age and metallicity, which is necessary to complete the panoramic view of such interesting objects. We selected hard X-ray emitting objects identified as AGNs by checking their optical spectra in search for absorption lines suitable for the stellar population analysis. We obtained a final sample consisting of 20 objects with redshift lower than 0.3. We used the full-spectrum fitting method and, in particular, the penalized pixel one applying the PPXF code. After masking all the regions affected by emission lines, we fitted the spectra with the MILES single stellar population templates and we derived mass-weighted ages and metallicities. Most of the objects in our sample show an old stellar population, but three of them are characterized by a bimodal distribution with a non negligible contribution from young stars. The values of the mass-weighted metallicity span a large range of metallicity with most of them slightly above the solar value. No relations between the stellar population properties and the morphological ones have been found.