The 2-10 keV luminosity as a Star Formation Rate indicator

Radio and far infrared luminosities of star forming galaxies follow a tight linear relation. Making use of BeppoSAX and ASCA observations of a well-defined sample of star forming galaxies, we argue that a tight linear relation holds between the 2-10 keV X-ray luminosity and both the radio and far infrared ones. It is suggested that the hard X-ray emission is directly related to the Star Formation Rate. Preliminary results obtained from deep Chandra and radio observations of the Hubble Deep Field North show that a similar relation might hold also at high (0.2<z<1.2) redshift.