Star formation and dust extinction in nearby star forming and starburst galaxies

We study the star formation rate and dust extinction properties of a sample of nearby star forming galaxies as derived from Hα and UV (∼ 2000 Å) observations and we compare them to those of a sample of starburst galaxies. The dust extinction in Hα is estimated from the Balmer decrement and the extinction in UV using the FIR to UV flux ratio or the attenuation law for starburst galaxies of Calzetti et al. (2000). The Hα and UV emissions are strongly correlated with a very low scatter for the star forming objects and with a much higher scatter for the starburst galaxies. The Hα to UV flux ratio is found larger by a factor ∼ 2 for the starburst galaxies. We compare both samples with a purely UV selected sample of galaxies and we conclude that the mean Hα and UV properties of nearby star forming galaxies are more representative of UV selected galaxies than starburst galaxies. We emphasize that the Hα to UV flux ratio is strongly dependent on the dust extinction: the positive correlation found between FHα/FUV and FFIR/FUV vanishes when the Hα and UV flux are corrected for dust extinction. The Hα to UV flux ratios converted into star formation rate and combined with the Balmer decrement measurements are tentatively used to estimate the dust extinction in UV.