  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 4.9 < z < 6.1. After accounting
for sampling variance, we infer a sharp increase in Gamma from z~6.1 to z~5.8,
probably implying reionization at this redshift. We find a decrease in Gamma
from z~5.6 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 z~6.1.
  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 photo-heating, 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
(sub-galactic) 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.
