AGN counts at 15um. XMM observations of the ELAIS-S1-5 sample

TL;DR

This study uses X-ray observations to identify AGN in MIR-selected galaxies, providing updated counts of AGN types at 15um and revealing a significant AGN presence among starburst galaxies.

Contribution

It introduces a method combining X-ray data with MIR classification to accurately identify AGN and updates the counts of AGN at 15um, improving previous estimates.

Findings

At least 13% of MIR starburst galaxies host AGN.

24% of bright 15um sources are AGN, contributing 13% to the extragalactic background.

Updated counts of AGN1 and AGN2 at 15um are provided.

Abstract

Context: The counts of galaxies and AGN in the mid infra-red (MIR) bands are important instruments for studying their cosmological evolution. However, the classic spectral line ratios techniques can become misleading when trying to properly separate AGN from starbursts or even from apparently normal galaxies. Aims: We use X-ray band observations to discriminate AGN activity in previously classified MIR-selected starburst galaxies and to derive updated AGN1 and (Compton thin) AGN2 counts at 15 um. Methods: XMM observations of the ELAIS-S1 15um sample down to flux limits ~2x10^-15 erg cm^-2 s^-1 (2-10 keV band) were used. We classified as AGN all those MIR sources with a unabsorbed 2-10 keV X-ray luminosity higher that ~10^42 erg/s. Results: We find that at least about 13(+/-6) per cent of the previously classified starburst galaxies harbor an AGN. According to these figures, we provide an updated estimate of the counts of AGN1 and (Compton thin) AGN2 at 15 um. It turns out that at least 24% of the extragalactic sources brighter than 0.6 my at 15 um are AGN (~13% contribution to the extragalactic background produced at fluxes brighter than 0.6 mJy).