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

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Abstract
About 45% of the point sources detected in the 2 Ms Chandra exposure of the 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 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 A line of [OII] to estimate their star formation rate (SFR). 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 SFR. With SFR corrected for inclination effects, the distant galaxies show a 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 distantstar forming galaxies is related to the SFR through the same physical mechanisms that prevail locally.Comment: Accepted for the Astrophysical Journa
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