Evolution of the ionizing radiation background and star formation in the aftermath of cosmological reionization

The temporal evolution of the ionizing UV background radiation field at high redshift provides a probe of the evolution of the early star formation rate. By comparing the observed levels of absorption in the highest redshift quasar spectra to the predictions of a hydrodynamic simulation, we determine the evolution of the photoionization rate (Gamma) for neutral hydrogen in the intergalactic medium over the redshift range 4.9less than or similar tozless than or similar to6.1. After accounting for sampling variance, we infer a sharp increase in Gamma from zsimilar or equal to6.1 to zsimilar or equal to5.8, probably implying reionization at this redshift. We find a decrease in Gamma from zsimilar or equal to5.6 to 5.2 at 3sigma significance. This feature is a generic signature in the aftermath of reionization, entirely consistent with the cosmological reionization process being completed at zsimilar to6.1. This generic feature is a result of a significant change in the star formation rate subsequent to the cosmological reionization. There is an abrupt rise of the temperature of the intergalactic medium due to photoheating when it is reionized. This translates to a correspondingly sudden jump in the Jeans mass and a dramatic suppression of gas accretion onto the most abundant (subgalactic) halos at the epochs of interest. The star formation rate suffers a temporary setback in the aftermath of reionization, resulting in a temporary decrease in the amplitude of the ionizing radiation field.