UV and FIR selected samples of galaxies in the local Universe. Dust extinction and star formation rates

We have built two samples of galaxies selected at 0.2micron (hereafter UV) and 60micron (hereafter FIR) covering a sky area of 35.36 deg^2. The UV selected sample contains 25 galaxies brighter than AB_0.2=17mag. All of them, but one elliptical, are detected at 60micron with a flux density larger or equal to 0.2Jy. The UV counts are significantly lower than the euclidean extrapolation towards brighter fluxes of previous determinations. The FIR selected sample contains 42 galaxies brighter than f_60=0.6Jy. Excepting four galaxies, all of them have a UV counterpart at the limiting magnitude AB_0.2=20.3mag. The mean extinction derived from the analysis of the FIR to UV flux ratio is ~1mag for the UV selected sample and ~2mag for the FIR selected one. For each sample we compare several indicators of the recent star formation rate (SFR) based on the FIR and/or the UV emissions and we find linear relationships with slopes close to unity, meaning that no trend with the SFR exists when converting between each other. Various absolute calibrations for both samples are discussed in this paper. A positive correlation between extinction and SFR is found when both samples are considered together although with a considerable scatter. A similar result is obtained when using the SFR normalized to the optical surface of the galaxies.