Far infrared and Far ultraviolet emissions of galaxies: luminosity functions and selection effects

The FIR(60 microns) and UV (0.2 microns) emissions of individual star forming galaxies are compared to the mean properties of the local Universe. Almost all the galaxies exhibit a FIR to UV flux ratio larger than the ratio of the FIR and UV luminosity densities. It is likely to be due to the contribution of low luminosity galaxies to the UV luminosity density: these galaxies are deficient in any survey. As the galaxies become brighter in FIR their FIR to UV flux ratio increases: (around 0.2 microns) it implies an increase of 0.5 mag of extinction (around 0.2 microns) per decade of FIR (60 microns) luminosity. This trend is extended and amplified in Ultra Luminous Infrared Galaxies observed at low and high redshift. The Lyman Break galaxies detected by their U dropout in the HDF seem to have less extreme properties more compatible with the trend found in the nearby star forming galaxies. The detection limits of the NGST and of the future large millimeter array (ALMA) in terms of FIR to UV ratios are presented