HerMES: unveiling obscured star formation – the far-infrared luminosity function of ultraviolet-selected galaxies at z ∼ 1.5

Abstract

We study the far-infrared (IR) and sub-millimeter properties of a sample of ultraviolet (UV) selected galaxies at z\sim1.5. Using stacking at 250, 350 and 500 um from Herschel Space Observatory SPIRE imaging of the COSMOS field obtained within the HerMES key program, we derive the mean IR luminosity as a function of both UV luminosity and slope of the UV continuum beta. The IR to UV luminosity ratio is roughly constant over most of the UV luminosity range we explore. We also find that the IR to UV luminosity ratio is correlated with beta. We observe a correlation that underestimates the correlation derived from low-redshift starburst galaxies, but is in good agreement with the correlation derived from local normal star-forming galaxies. Using these results we reconstruct the IR luminosity function of our UV-selected sample. This luminosity function recovers the IR luminosity functions measured from IR selected samples at the faintest luminosities (Lir ~ 10^{11} L_sun), but might underestimate them at the bright-end (Lir > 5.10^{11} L_sun). For galaxies with 10^{11}<Lir/L_sun<10^{13}, the IR luminosity function of a UV selection recovers (given the differences in IR-based estimates) 52-65 to 89-112 per cent of the star-formation rate density derived from an IR selection. The cosmic star-formation rate density derived from this IR luminosity function is 61-76 to 100-133 per cent of the density derived from IR selections at the same epoch. Assuming the latest Herschel results and conservative stacking measurements, we use a toy model to fully reproduce the far IR luminosity function from our UV selection at z\sim 1.5. This suggests that a sample around 4 magnitudes deeper (i.e. reaching u \sim 30 mag) and a large dispersion of the IR to UV luminosity ratio are required.