The Lyman continuum radiation escaping from galaxies

The Hα luminosity density of galaxies is used to calculate the local diffuse radiation field at the Lyman limit as resulting from star formation in the universe. The fraction of Lyman continuum radiation leaking out from galaxies through their neutral hydrogen is the most uncertain parameter of the calculation. A comparison of the diffuse radiation predicted from galaxies with that measured from all sources of ionization shows that the average Lyman continuum escape fraction should be lower than 1%. This number is lower than the upper limits reported so far in a few objects. It is also shown that the luminosity density at the Lyman limit resulting from star formation is consistent with the luminosity density and the diffuse background measured in the far (non-ionizing) ultraviolet.