Active galactic nuclei in the mid-IR - Evolution and contribution to the cosmic infrared background

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 Spitzer 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)(kL)) model where kL similar to 2.9. A small flattening of the faint ( L-15 < 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 kL ranging from similar to 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) x 10(-11) W m(-2) sr(-1), while the type-2 AGN contribution is ( 5.5-11.0) x 10-11 W m-2 sr-1. We expect that Spitzer will observe, down to a flux limit of S 24 mu m = 0.01 mJy, a density of similar to 1200 deg(-2) type(-1) and similar to 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 Spitzer counts should be considered as lower limits, because of a possible loss of type-2 sources caused by the optical classification