Measures of star formation rates from infrared (Herschel) and UV (GALEX) emissions of galaxies in the HerMES fields

The reliability of infrared (IR) and ultraviolet (UV) emissions to measure star formation rates (SFRs) in galaxies is investigated for a large sample of galaxies observed with the Spectral and Photometric Imaging Receiver (SPIRE) and the Photodetector ArrayCamera and Spectrometer (PACS) instruments on Herschel as part of the Herschel Multi-Tiered Extragalactic Survey (HerMES) project. We build flux-limited 250-mu m samples of sources at redshift z < 1, cross-matched with the Spitzer/MIPS and GALEX catalogues. About 60 per cent of the Herschel sources are detected in UV. The total IR luminosities, L-IR, of the sources are estimated using a spectral energy distribution (SED) fitting code that fits to fluxes between 24 and 500 mu m. Dust attenuation is discussed on the basis of commonly used diagnostics: the L-IR/L-UV ratio and the slope, beta, of the UV continuum. A mean dust attenuation A(UV) of similar or equal to 3 mag is measured in the samples. L-IR/L-UV is found to correlate with L-IR. Galaxies with L-IR > 10(11) L-circle dot and 0.5 < z < 1 exhibit a mean dust attenuation A(UV) of about 0.7 mag lower than that found for their local counterparts, although with a large dispersion. Our galaxy samples span a large range of beta and L-IR/L-UV values which, for the most part, are distributed between the ranges defined by the relations found locally for starburst and normal star-forming galaxies. As a consequence the recipe commonly applied to local starbursts is found to overestimate the dust attenuation correction in our galaxy sample by a factor of similar to 2-3. The SFRs deduced from L-IR are found to account for about 90 per cent of the total SFR; this percentage drops to 71 per cent for galaxies with SFR < 1M(circle dot) yr(-1) (or L-IR < 10(10) L-circle dot). For these faint objects, one needs to combine UV and IR emissions to obtain an accurate measure of the SFR.