The evolution of type 1 active galactic nuclei in the infrared (15 mu m): the view from ELAIS-S1

We present the 15-mum luminosity function of type 1 active galactic nuclei (AGN1; quasi-stellar object+Seyfert 1). Our sample of 21 high-redshift sources is selected from the Preliminary Analysis catalogue in the S1 field of the European Large Area ISO Survey (ELAIS). To study the cosmological evolution of the AGN1 luminosity function, our sample has been combined with a local sample of 41 sources observed by IRAS . We find that the luminosity function of AGN1 at 15 mum is fairly well represented by a double-power-law-function. There is evidence for significant cosmological evolution consistent with a pure luminosity evolution (PLE) model L(z )proportional to(1+z ) (kL), with k(L) =3.0-3.3. The value of k(L) depends on the existence or non-existence of an evolution cut-off at redshift similar to2, and on the adopted cosmology. From the luminosity function and its evolution we estimate a contribution of AGN1 to the cosmic infrared background (CIRB) of nuI(nu) similar to 6x10(-11) W m(-2) sr(-1) at 15 mum. This corresponds to similar to2-3 per cent of the total observed CIRB at this wavelength. Under the usual assumptions of unified models for AGN, the expected contribution of the whole AGN population to the CIRB at 15 mum is 10-15 per cent.