Polycyclic aromatic hydrocarbon contribution to the infrared output energy of the universe at z ≃ 2

We present an updated phenomenological galaxy evolution model to fit the Spitzer 24, 70, and 160 mum number counts, as well as all the previous mid- and far-infrared observations. Only a minor change of the comoving luminosity density distribution in the previous model (Lagache, Dole, & Puget), combined with a slight modification of the starburst template spectra mainly between 12 and 30 mum, are required to fit all the data available. We show that the peak in the Spitzer Multiband Imaging Photometer 24 mum counts is dominated by galaxies with redshift between 1 and 2, with a nonnegligible contribution from the z greater than or equal to 2 galaxies ( similar to 30% at S = 0.2 mJy). The very close agreement between the model and number counts at 15 and 24 mum strikingly implies that ( 1) the polycyclic aromatic hydrocarbon features remain prominent in the redshift band 0.5 - 2.5 and (2) the IR energy output has to be dominated by similar to3 x 10(11) L-. to similar to 3 x 10(12) L-. galaxies from redshift 0.5 to 2.5. Combining Spitzer with Infrared Space Observatory deep cosmological surveys gives for the first time an unbiased view of the infrared universe from z = 0 to 2.5.