The X-ray emission of Lyman break galaxies

We present an analysis of the X-ray emission of a large sample of z similar to 3 Lyman break galaxies (LBGs), based on Chandra/ACIS observations of several LBG survey fields. A total of 24 LBGs are directly detected in the X-ray, approximately doubling the number of known detections. Thirteen of the LBGs have optical spectroscopic signatures of active galactic nucleus (AGN) activity, but almost all the other X-ray detections are also likely to host an accreting black hole based on their X-ray properties. The AGN exhibit a wide range in X-ray luminosity, from weak Seyferts to bright quasi-stellar objects (QSOs). An optical spectroscopy identified approximately one-third of the X-ray-detected sources as broad-line QSOs, one-third as narrow-line AGN (NLAGN) and one-third as normal star-forming LBGs. The fraction of X-ray-detected LBGs is 3 per cent, much lower than that which has been found for submillimetre-selected galaxies. Two galaxies have X-ray luminosities, spectra and f(X)/f(opt) values that are consistent with emission from star formation processes and are identified as candidate X-ray bright, pure starburst galaxies at z similar to 3. If powered solely by star formation, the sources would have star formation rates (SFRs) of 300-500 M-circle dot yr(-1). X-ray spectral analysis of the LBGs shows a mean photon index of Gamma = 1.96, similar to local AGN. There is evidence for absorption in at least 40 per cent of the objects. Significantly more absorption is evident in the NLAGN, which is consistent with AGN unification schemes. After correction for absorption, the narrow- and broad-line objects show the same average luminosity. X-ray-detected LBGs, spectroscopically classified as normal galaxies, however, are less luminous in both soft and hard X-ray bands, indicating that the host galaxy is outshining any optical AGN signature. Turning to the X-ray emission from LBGs without direct detections, stacking the X-ray flux in the two deepest Chandra fields under consideration [the Hubble Deep Field-North (HDF-N) and Groth-Westphal Strip (GWS)] produced significant detections in each, although the GWS result was marginal. The detection in the HDF-N gives an X-ray-derived SFR of 42.4 +/- 7.8 M-circle dot yr(-1) per LBG and, by comparing with the ultraviolet (UV) SFR, the implied UV extinction correction is 4.1 +/- 0.8. The LBG sample was split into three bins based on UV magnitude to examine the correlation between UV and X-ray emission: for the limited statistics available, there was no evidence of any correlation.