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

Free access article

Issue		A&A Volume 472, Number 3, September IV 2007


Page(s)		797 - 803
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20077394

A&A 472, 797-803 (2007)
DOI: 10.1051/0004-6361:20077394

AGN counts at 15 $\mu{\rm m}$

XMM observations of the ELAIS-S1-5 sample

F. La Franca¹, S. Puccetti², N. Sacchi¹, C. Feruglio³, F. Fiore³, C. Gruppioni⁴, A. Lamastra¹, I. Matute⁵, G. Melini¹, and F. Pozzi^{4, 6}

¹ Dipartimento di Fisica, Università Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
e-mail: lafranca@fis.uniroma3.it
² ASI Science Data Center, via Galileo Galilei, 00044, Frascati, Italy
³ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
⁴ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy

(Received 2 March 2007 / Accepted 2 June 2007)

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 ~2 $\times$ 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 ~10⁴² erg s^-1.
Results. We find that at least about 13 $\pm$ 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 (~13% contribution to the extragalactic background produced at fluxes brighter than 0.6 mJy).

Key words: cosmology: observations -- infrared: galaxies -- galaxies: active -- surveys -- galaxies: evolution -- X-rays: galaxies

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