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 μm 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 μm, are required to fit all the data available. We show that the peak in the Spitzer Multiband Imaging Photometer 24 μm counts is dominated by galaxies with redshift between 1 and 2, with a nonnegligible contribution from the z ≥ 2 galaxies (∼30% at S = 0.2 mJy). The very close agreement between the model and number counts at 15 and 24 μm 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 ∼ 3 × 10¹¹ L_⊙ to ∼3 × 10¹² 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.