Abstract
We describe the X-ray properties of a large sample of z ~ 3 Lyman break galaxies (LBGs) in the region of the Hubble Deep Field-North, derived from the 1 Ms public Chandra observation. Of our sample of 148 LBGs, four are detected individually. This immediately gives a measure of the bright AGN fraction in these galaxies of ~3%, which is in agreement with that derived from the UV spectra. The X-ray color of the detected sources indicates that they are probably moderately obscured. Stacking of the remainder shows a significant detection (6 σ) with an average luminosity of 3.4 × 1041 ergs s-1 per galaxy in the rest-frame 2-10 keV band. We have also studied a comparison sample of 95 z ~ 1 "Balmer break" galaxies. Eight of these are detected directly, with at least two clear AGNs based on their high X-ray luminosity and very hard X-ray spectra. The remainder are of relatively low luminosity (<1042 ergs s-1), and the X-rays could arise from either AGNs or rapid star formation. The X-ray colors and evidence from other wave bands favor the latter interpretation. Excluding the clear AGNs, we deduce a mean X-ray luminosity of 6.6 × 1040 ergs s-1, a factor of ~5 lower than the LBGs. The average ratio of the UV and X-ray luminosities of these star-forming galaxies LUV/LX, however, is approximately the same at z = 1 as it is at z = 3. This scaling implies that the X-ray emission follows the current star formation rate, as measured by the UV luminosity. We use our results to constrain the star formation rate at z ~ 3 from an X-ray perspective. Assuming the locally established correlation between X-ray and far-IR luminosity, the average inferred star formation rate in each LBG is found to be approximately 60 M☉ yr-1, in excellent agreement with the extinction-corrected UV estimates. This provides an external check on the UV estimates of the star formation rates and on the use of X-ray luminosities to infer these rates in rapidly star-forming galaxies at high redshift.
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