Models for Multiband Infrared Surveys

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Abstract
Empirical "backward" galaxy evolution models for IR bright galaxies are constrained using multiband IR surveys. A new Monte Carlo algorithm is developed for this task. It exploits a large library of realistic spectral energy distributions (SEDs) of 837 local IR galaxies (IRAS 25 μm selected) from the UV (1000 Å) to the radio (20 cm), including Infrared Space Observatory (ISO) measured 3-13 μm unidentified broad features (UIBs). The basic assumption is that the local correlation between SEDs and mid-infrared (MIR) luminosities can be applied to earlier epochs of the universe, an assumption that will be strongly tested by SIRTF. By attaching an SED appropriately drawn from the SED library to every source predicted by a given model, the algorithm enables simultaneous comparisons with multiple surveys in a wide range of wave bands. Three populations of IR sources are considered in the evolution models. These include (1) starburst galaxies, (2) normal late-type galaxies, and (3) galaxies with active galactic nuclei (AGNs). Constrained by data from the literature, our best-fit model ("peak model") predicts that since z = 1.5 the population of starburst galaxies undergoes a very strong luminosity evolution [L = L0(1 + z)4.2] and also strong density evolution [ρ = ρ0(1 + z)2], the normal late-type galaxy population undergoes a passive luminosity evolution [L = L0(1 + z)1.5], and the galaxies with an AGN undergo a pure luminosity evolution similar to that of optical QSOs [L = L0(1 + z)3.5]. Prior at z ≥ 1.5 all evolution rates drop as (1 + z)-3. The luminosity evolution results in evolution of SEDs of IR bright sources because of the luminosity dependence of the SEDs. Predictions for number counts, confusion limits, redshift distributions, and color-color diagrams are made for multiband surveys using the upcoming SIRTF satellite. A Λ cosmology (ΩΛ = 0.7, Ωm = 0.3, H0 = 75 km s-1 Mpc-1) is assumed throughout the paper.