Astronomy Tea Talk

Star-forming galaxies are widely believed to be the main drivers of hydrogen reionization. With galaxy populations now constrained out to z~9, however, it remains unclear how these sources were able to emit sufficient numbers of ionizing photons by redshift six, when reionization is known to be completed, unless reionization-era galaxies differed substantially from their lower-redshift counterparts. One approach to resolving this tension is to determine how efficiently galaxies ionize the intergalactic medium (IGM) in the period following reionization, when galaxy luminosity functions and the global ionizing emissivity can be better determined. I will present new measurements of the ionizing UV background over 2 < z < 5 derived from the physical properties of the IGM. When combined with direct observations of star-forming galaxies over these redshifts, the results point to a substantial evolution in the ionizing "efficiency" of galaxies that may help to explain how these sources were able to reionize hydrogen within one gigayear of the Big Bang.