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

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 Spitzer at 24 μm. Methods: We used a sample of type-1 and type-2 AGN selected at 15 μm (ISO) and 12 μm (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 μm 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∼ 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 μm is (4.2-12.1) × 10^-11 W m^{-2 sr^{-1}}, while the type-2 AGN contribution is (5.5-11.0) × 10^-11 W m^{-2 sr^{-1}}. We expect that Spitzer will observe, down to a flux limit of S_{24 μ 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 Spitzer counts should be considered as lower limits, because of a possible loss of type-2 sources caused by the optical classification.