Interpreting the Cosmic Infrared Background: Constraints on the Evolution of the Dust-enshrouded Star Formation Rate

The mid-infrared local luminosity function is evolved with redshift to fit the spectrum of the cosmic infrared background (CIRB) at λ>5 μm and the galaxy counts from various surveys at mid-infrared, far-infrared, and submillimeter wavelengths. A variety of evolutionary models provide satisfactory fits to the CIRB and the number counts. The degeneracy in the range of models cannot be broken by current observations. However, the different evolutionary models yield approximately the same comoving number density of infrared luminous galaxies as a function of redshift. Since the spectrum of the cosmic background at λ>200 μm is quite sensitive to the evolution at high redshift, i.e., z>1, all models that fit the counts require a flattening at z~0.8 to avoid overproducing the CIRB. About 80% of the 140 μm CIRB is produced at 0<z<1.5, while only about 30% of the 850 μm background is produced within the same redshift range. The nature of the evolution is then translated into a measure of the dust-enshrouded star formation rate (SFR) density as a function of redshift and compared with estimates from rest-frame optical/ultraviolet surveys. The dust-enshrouded SFR density appears to peak at z=0.8+/-0.1, much sooner than previously thought, with a value of 0.25^+0.12_-0.1 M_solar yr^-1 Mpc^-3, and remains almost constant up to z~2. At least 70% of this star formation takes place in infrared luminous galaxies with L_IR>10¹¹ L_solar. The long-wavelength observations that constrain our evolutionary models do not strongly trace the evolution at z>2 and a drop-off in the dust-enshrouded SFR density is consistent with both the CIRB spectrum and the number counts. However, a comparison with the infrared luminosity function derived from extinction-corrected rest-frame optical/ultraviolet observations of the Lyman break galaxy population at z~3 suggests that the almost flat comoving SFR density seen between redshifts of 0.8 and 2 extends up to a redshift of z~4.