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

Context. The counts of galaxies and AGN in the mid infra-red (MIR) bands are important instruments for studying their cosmologica l evolution. However, the classic spectral line ratios techn iques 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 a nd to derive updated AGN1 and (Compton thin) AGN2 counts at 15� m. Methods. XMM observations of the ELAIS-S1 15� m sample down to flux limits∼ 2×10 −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 −1 . Results. We find that at least about 13±6% 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� m. It turns out that at least 24% of the extragalactic sources brighter than 0.6 my at 15� m are AGN (∼13% contribution to the extragalactic background produced at fluxes brighter than 0.6 mJy).