SIMBAD references

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. 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 15µm. XMM observations of the ELAIS-S1 15µm sample down to flux limits ∼2x10^–15erg/cm2/s (2-10keV band) were used. We classified as AGN all those MIR sources with a unabsorbed 2-10keV X-ray luminosity higher that ∼10⁴²erg/s. 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 15µm. It turns out that at least 24% of the extragalactic sources brighter than 0.6mJy at 15µm are AGN (∼13% contribution to the extragalactic background produced at fluxes brighter than 0.6mJy).