Evolution of the far-infrared luminosity functions in the Spitzer Wide-area Infrared Extragalactic Legacy Survey

We present new observational determination of the evolution of the rest-frame 70 and 160 mu m and total infrared (TIR) galaxy luminosity functions (LFs) using 70 and 160 mu m data from the Spitzer Wide-area Infrared Extragalactic (SWIRE) Legacy survey. The LFs were constructed for sources with spectroscopic redshifts only in the XMM-LSS and Lockman Hole fields from the SWIRE photometric redshift catalogue. The 70 mu m and TIR LFs were constructed in the redshift range 0 < z < 1.2 and the 160 mu m LF was constructed in the redshift range 0 < z < 0.5 using a parametric Bayesian and the 1/V-max methods. We assume in our models that the faint-end power-law index of the LF does not evolve with redshift. We find that the double power-law model is a better representation of the infrared (IR) LF than the more commonly used power-law and Gaussian model. We model the evolution of the far-IR LFs as a function of redshift where the characteristic luminosity L* evolve as proportional to (1 + z)L-alpha. The rest-frame 70 mu m LF shows a strong luminosity evolution out to z = 1.2 with alpha L = 3.41(-0.25)(+0.18). The rest-frame 160 mu m LF also showed rapid luminosity evolution with alpha L = 5.53(-0.23)(+0.28) out to z = 0.5. The rate of evolution in luminosity is consistent with values estimated from previous studies using data from IRAS, ISO and Spitzer. The TIR LF evolves in luminosity with alpha L = 3.82(-0.16)(+0.28) which is in agreement with previous results from Spitzer 24 mu m which find strong luminosity evolution. By integrating the LF we calculated the comoving IR luminosity density out to z = 1.2, which confirms the rapid evolution in number density of luminous IR galaxies which contribute similar to 68(-07)(+10) per cent to the comoving star formation rate density at z = 1.2. Our results based on 70 mu m data confirm that the bulk of the star formation at z = 1 takes place in dust-obscured objects.