Escape fraction of ionizing photons from galaxies at z=0--6

The escape fraction of ionizing photons from galaxies is a crucial quantity controlling the cosmic ionizing background radiation and the reionization. Various estimates of this parameter can be obtained in the redshift range, z=0--6, either from direct observations or from the observed ionizing background intensities. We compare them homogeneously in terms of the observed flux density ratio of ionizing ($sim900$ AA rest-frame) to non-ionizing ultraviolet ($sim1500$ AA rest-frame) corrected for the intergalactic absorption. The escape fraction is found to increase by an order of magnitude, from a value less than 0.01 at $zla1$ to about 0.1 at $zga4$.