The Consequences of the Cosmic Star-formation Rate: X-ray Number Counts

We discuss the observable consequences for the detection of galaxies in the X-ray bandpass resulting from a peak in the cosmic star-formation rate at a redshift > 1. Following White & Ghosh, we assume a large evolution in the X-ray/B luminosity ratio at z ∼ 0.5− 1.5 resulting from the X-ray binaries that have evolved from stars formed at z > 1 − 2. Using the HDF-N redshift survey data and the locally observed X-ray/B luminosity ratio as a guide, we estimate a median X-ray flux (2-10 keV) on the order of 8 × 10 erg cm s for galaxies in the HDF-N, which is consistent with a signal derived from a stacking analysis of the HDF-N Chandra data by Brandt et al. (2001). We also predict the number counts in deep X-ray surveys expected from normal galaxies at high redshift. Subject headings: evolution-galaxies:evolution–X-rays: galaxies–X-rays