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 mu m. Methods. XMM observations of the ELAIS-S1 15 mu m sample down to flux limits similar to 2 x 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 similar to 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 mu m. It turns out that at least 24% of the extragalactic sources brighter than 0.6 my at 15 mu m are AGN (similar to 13% contribution to the extragalactic background produced at fluxes brighter than 0.6 mJy).
LA FRANCA, F., Puccetti, S., Sacchi, N., Feruglio, C., Fiore, F., Gruppioni, C., et al. (2007). AGN counts at 15 mu m - XMM observations of the ELAIS-S1-5 sample. ASTRONOMY & ASTROPHYSICS, 472(3), 797-U26 [10.1051/0004-6361:20077394].
AGN counts at 15 mu m - XMM observations of the ELAIS-S1-5 sample
LA FRANCA, Fabio;
2007-01-01
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 mu m. Methods. XMM observations of the ELAIS-S1 15 mu m sample down to flux limits similar to 2 x 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 similar to 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 mu m. It turns out that at least 24% of the extragalactic sources brighter than 0.6 my at 15 mu m are AGN (similar to 13% contribution to the extragalactic background produced at fluxes brighter than 0.6 mJy).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
