A “uv+ir ” History of Star Formation at 0 <

The combination of both contributions from the observed UV emission and the absorbed radiations reprocessed in the infrared represents the ideal approach to constrain the activity of massive star formation in galaxies. Using recent results from GALEX and Spitzer, we compare the evolutions of the UV and IR energy densities with redshift as well as their contributions to the star formation history at 0 < ∼ z < ∼ 1. We find that the comoving IR luminosity is characterized by a much faster evolution than seen in the UV. Our results also indicate that ∼ 70% of the star-forming activity at z ∼ 1 is produced by the so-called IR-luminous sources (LIR ≥ 10 11 L⊙). 1 Dust extinction and the limitations of the UV window The multi-wavelength deep surveys performed in the last decade as well as the detection of the Cosmic Infrared (IR) Background by COBE revealed the dramatic effects of dust extinction in the distant Universe. This significant high-redshift reprocessing of short-wavelength radiations (e.g., UV, optical) into the thermal infrared appears to be associated very closely to the strong evolution that IR-luminous galaxies (i.e., L IR ≥ 10 11 L ⊙) have undergone with lookback time. The luminosity of such objects is mostly powered by highly-embedded star formation or by the accretion of dusty material around active nuclei. As a result they emit the bulk of their energy between 8 and 1000 µm. It is now well established that they contributed significantly to the assembly of the present-day stellar mass and to the growth of supermassive black holes (e.g., Blain et al. 1999, Chary & Elbaz 2001). This dominant contribution of IR-luminous systems in the building of structures simply demonstrates not only that the amount of radiations absorbed by dust can not be neglected when quantifying the cosmic evolution, but also that this dust extinction can not be properly quantified from the slope of the UV continuum as it has been proposed in the past few years. Based on a local sample of luminous and ultra-luminous infrared galaxies observed with the STIS camera on-board the HST, Goldader et al. (2002) have shown that the characteristic IR excess of these sources (defined as the ratio between their IR and UV luminosities) is significantly larger than what can be predicted based on the attenuation of their UV continuum and the relation initially proposed by Meurer et al. (1999). This …