A BACKWARD EVOLUTION MODEL FOR INFRARED SURVEYS: THE ROLE OF AGN- AND COLOR-LTIR DISTRIBUTIONS

Empirical "backward" galaxy evolution models for infrared (IR) bright galaxies are constrained using multiband IR surveys. We developed a new Monte Carlo algorithm for this task, implementing luminosity-dependent distribution functions for the galaxies' IR spectral energy distributions (SEDs) and for the active galactic nucleus (AGN) contribution, allowing for evolution of these quantities. The adopted SEDs take into account the contributions of both starbursts and AGN to the IR emission, for the first time in a coherent treatment rather than invoking separate AGN and star-forming populations. In the first part of the paper we consider the quanti. cation of the AGN contribution for local universe galaxies, as a function of the total IR luminosity. It is made using a large sample of luminous infrared galaxies and ultraluminous infrared galaxies for which mid-IR spectra are available in the Spitzer archive. We find the ratio of AGN 6 mu m luminosity and the total IR luminosity to rise with L-TIR(1.4) over the IR luminosity range 10(11)-10(13) L-circle dot and estimate its spread. Judging from the modest number of distant sources with Spitzer spectroscopy, the relation changes at high-z. In the second part we present the model. Our best-fit model adopts very strong luminosity evolution, L = L-0(1 + z)(3.4), up to z = 2.3, and density evolution, rho = rho(0)(1 + z)(2), up to z = 1, for the population of IR galaxies. At higher z, the evolution rates drop as (1 + z)(-1) and (1 + z)(-1.5), respectively. To reproduce mid-IR to submillimeter number counts and redshift distributions, it is necessary to introduce both an evolution in the AGN contribution and an evolution in the luminosity-temperature relation. At a given total IR luminosity, high-redshift IR galaxies have typically smaller AGN contributions to the rest-frame mid-IR, and colder far-IR dust temperatures than locally. We also suggest an extension of the local IR galaxy population toward lower dust temperatures. Our models are in plausible agreement with current photometry-based estimates of the typical AGN contribution as a function of mid-IR flux, and well placed to be compared to upcoming Spitzer spectroscopic results. As an example of future applications, we use our best-fitting model to make predictions for surveys with Herschel.