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

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 LX-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