The 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 (∼900 Å rest-frame) to non-ionizing ultraviolet (∼1500 Å 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 z ≲ 1 to about 0.1 at z ≳ 4.