Star Formation and X-Ray Emission in Distant Star-Forming Galaxies

Abstract

About 45% of the point sources detected in the 2 Ms Chandra exposure of the Hubble Deep Field-North (HDF-N) can be matched with moderately bright galaxies with z<1.4 that have been studied by the Caltech Faint Galaxy Redshift Survey. Although the optical spectra of these galaxies appear normal, based on their X-ray properties ~20% of them appear to contain weak active galactic nuclei (AGNs). More than 90% of the X-ray photons detected by Chandra from galaxies within the redshift regime 0.4<z<1.1 are powered by accretion onto massive black holes. For the sample of galaxies in common, we use their emitted luminosity in the 3727 Å line of [O II] to estimate their star formation rates (SFRs). The X-ray-emitting galaxies are not those with the highest rest-frame equivalent width in this emission line, but rather are among those with the highest SFRs. With SFRs corrected for inclination effects, the distant galaxies show an L_X-SFR relationship that is comparable to that of local galaxies. The HDF sample has a significantly higher median SFR and median SFR/galaxy stellar mass than does a sample of local star-forming galaxies. We demonstrate that the observed SFR for most of the galaxies at z~1 in the HDF sample, if maintained as constant over their ages, suffices to produce the stellar mass observed in these galaxies. A rise in SFR at still earlier times is not required. We provide further evidence to support the conclusion that, once AGNs are eliminated, X-ray emission in these distant star-forming galaxies is related to the SFR through the same physical mechanisms that prevail locally.

Based in part on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.