Identifying the Reionization Redshift from the Cosmic Star Formation Rate

We show that the cosmic star formation rate per comoving volume should exhibit a distinct drop around the reionization redshift, when the HII regions in the intergalactic medium around individual ionizing sources first overlapped. The drop results from the increase in the temperature of the intergalactic medium as it was photo-ionized, and the consequent suppression of the formation of low-mass galaxies. We show quantitatively that the detection of this drop, which is marked by a corresponding fall in the number counts of faint galaxies, should become feasible over the coming decade with the Next Generation Space Telescope.