2005ApJ...632..169L

We analyze a sample of ∼2600 Spitzer MIPS 24 µm sources brighter than ∼80 µJy and located in the Chandra Deep Field-South to characterize the evolution of the comoving infrared (IR) energy density of the universe up to z∼1. Using published ancillary optical data, we first obtain a nearly complete redshift determination for the 24 µm objects associated with R≲24 mag counterparts at z≲1. These sources represent ∼55%-60% of the total MIPS 24 µm population with f_24µm≳80 µJy, the rest of the sample likely lying at higher redshifts. We then determine an estimate of their total IR luminosities using various libraries of IR spectral energy distributions. We find that the 24 µm population at 0.5≲z≲1 is dominated by ``luminous infrared galaxies'' (i.e., 10¹¹ L_☉≤L_IR≤10¹² L_☉), the counterparts of which appear to be also luminous at optical wavelengths and tend to be more massive than the majority of optically selected galaxies. A significant number of fainter sources (5x10¹⁰ L_☉≲L_IR≤10¹¹ L_☉) are also detected at similar distances. We finally derive 15 µm and total IR luminosity functions (LFs) up to z∼1. In agreement with the previous results from the Infrared Space Observatory (ISO) and SCUBA and as expected from the MIPS source number counts, we find very strong evolution of the contribution of the IR-selected population with look-back time. Pure evolution in density is firmly excluded by the data, but we find considerable degeneracy between strict evolution in luminosity and a combination of increases in both density and luminosity [L^*_IR∝(1+z)^3.2+0.7-0.2_^, φ^*_IR∝(1+z)^0.7+0.2-0.6_^]. A significant steepening of the faint-end slope of the IR luminosity function is also unlikely, as it would overproduce the faint 24 µm source number counts. Our results imply that the comoving IR energy density of the universe evolves as (1+z)^3.9±0.4 up to z∼1 and that galaxies luminous in the infrared (i.e., L_IR≥10¹¹ L_☉) are responsible for 70%±15% of this energy density at z∼1. Taking into account the contribution of the UV luminosity evolving as (1+z)^∼2.5, we infer that these IR-luminous sources dominate the star-forming activity beyond z∼0.7. The uncertainties affecting these conclusions are largely dominated by the errors in the k-corrections used to convert 24 µm fluxes into luminosities.