Active galactic nuclei in the mid-IR

Free access article

Issue		A&A Volume 451, Number 2, May IV 2006


Page(s)		443 - 456
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20053710

A&A 451, 443-456 (2006)
DOI: 10.1051/0004-6361:20053710

Evolution and contribution to the cosmic infrared background

I. Matute^{1, 2}, F. La Franca², F. Pozzi^{3, 4}, C. Gruppioni⁴, C. Lari⁵ and G. Zamorani⁴

¹ Max-Planck Institut für extraterrestrische Physik (MPE), Giessenbachstra $\beta$ e, Postfach 1312, 85741 Garching, Germany
e-mail: matute@mpe.mpg.de
² Dipartimento di Fisica, Università degli Studi "Roma Tre", via della Vasca Navale 84, 00146 Roma, Italy
e-mail: lafranca@fis.uniroma3.it
³ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INAF, Istituto di Radioastronomia (IRA), via Gobetti 101, 40129 Bologna, Italy

(Received 28 June 2005 / Accepted 30 December 2005 )

Abstract
Aims.We study the evolution of the luminosity function (LF) of type-1 and type-2 Active Galactic Nuclei (AGN) in the mid-infrared, derive the contribution of the AGN to the Cosmic InfraRed Background (CIRB) and the expected source counts to be observed by $S\!pitzer$ at 24 $\mu$ m.
Methods.We used a sample of type-1 and type-2 AGN selected at 15 $\,\mu$ m (ISO) and 12 $\,\mu$ m (IRAS), and classified on the basis of their optical spectra. Local spectral templates of type-1 and type-2 AGN have been used to derive the intrinsic 15 $\,\mu$ m luminosities. We adopted an evolving smooth two-power law shape of the LF, whose parameters have been derived using an un-binned maximum likelihood method.
Results.We find that the LF of type-1 AGN is compatible with a pure luminosity evolution ( L(z)=L(0)(1+z)^k_L) model where $k_L\sim 2.9$ . A small flattening of the faint ( L₁₅<L^*₁₅) slope of the LF with increasing redshift is favoured by the data. A similar evolutionary scenario is found for the type-2 population with a rate k_L ranging from ~1.8 to 2.6, depending significantly on the adopted mid-infrared spectral energy distribution. For type-2 AGN a flattening of the LF with increasing redshift is suggested by the data, possibly caused by the loss of a fraction of type-2 AGN hidden within the optically classified starburst and normal galaxies. The type-1 AGN contribution to the CIRB at 15 $\,\mu$ m is (4.2-12.1) $\times\,10^{-11}\;\mathrm{W\, m^{-2} \,sr^{-1}}$ , while the type-2 AGN contribution is (5.5-11.0) $\times\,10^{-11}\;\mathrm{W\, m^{-2} \,sr^{-1}}$ . We expect that $S\!pitzer$ will observe, down to a flux limit of $S_{24\,\mu \mathrm{m}}=0.01$ mJy, a density of ~1200 deg^-2 type-1 and ~1000 deg^-2 type-2 optically classified AGN.
Conclusions.AGN evolve in the mid-infrared with a rate similar to the ones found in the optical and X-rays bands. The derived total contribution of the AGN to the CIRB (4-10%) and $S\!pitzer$ counts should be considered as lower limits, because of a possible loss of type-2 sources caused by the optical classification.

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

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