0 v 1 9 D ec 1 99 7 Star Formation Rate at z = 0 . 2 derived from H α luminosities : constraint on the reddening

We discuss the relative merits of using UV and Hα as star formation indicators from galaxy surveys. In particular, comparing UV and Hα in the CFRS gives a limit of a factor 2.5 for the UV(2800 Å) extinction from dust, using the conversion factors of Madau et al. 1998 (Salpeter IMF, 0.1−125 M⊙). Our strong correlation between B and Hα argues for a universal IMF slope. The Hα LF at z = 0.2 shows a faint end slope of α = −1.35, which is consistent with fading of short bursts of star formation. We also discuss the contribution of AGN to UV and Hα luminosities. 1. Luminosity density and star formation rate The cosmological evolution of the star formation rate (SFR) can be inferred from the luminosity density of the galaxy population seen at different redshifts (Madau et al. 1998). Indeed the luminosity density history traces the global evolution of the galaxy population, or in other words, of the whole content of galaxies (gas, stars, dust, active galactic nucleus (AGN), supernovae remnants, etc.), and not simply of the stellar population. Recovering the genuine shape of the SFR history is essential to differentiate between scenarios of galaxy evolution and formation. Extensive studies of nearby galaxies have shown that the UV, FIR, radio continuum and Hα line fluxes are closely connected (Buat 1992), even though these radiations are produced by different physical processes. Thus they originate from a common photoionizing spectrum. Retrieving the absolute photoionizing flux depends crucially on the understanding of these processes. 2. Dust extinction of UV and Hα emission Assuming there is no AGN, the galactic UV continuum radiation directly traces the UV stellar spectra. From the Lyman break (912 Å) to ∼ 3500 Å, the emissivity of the starburst galaxy population becomes less dominated by the relatively massive, young stars (O, B types, T≥10 000 K, M≥ 10 M⊙, tMS ∼10 7 yr), while it becomes more dominated by the intermediate mass stars (A type, M= 2 − 5 M⊙, tMS ∼ 10 8 yr). The emissivity of the more quiescent galaxy population (i.e. ellipticals) may be dominated by massive, old stars (t≫ 10 yr), but other possibilities, of which some are non stellar, have been proposed to account for the UV rise seen in these galaxies. Assuming there is no AGN, the Hα(λ6563 Å) fluxes come from the hydrogen gas surrounding very massive,