X-Ray Properties of Lyman Break Galaxies in the Great Observatories Origins Deep Survey

Abstract

We constrain the X-ray emission properties of Lyman break galaxies (LBGs) at z 3–6 using the 2 Ms Chandra Deep Field North and 1 Ms Chandra Deep Field South. Large samples of LBGs were discovered using the Hubble Space Telescope as part of the Great Observatories Origins Deep Survey (GOODS). Deep optical and X-ray imaging over the GOODS fields have allowed us to place the most significant constraints on the X-ray properties of LBGs to date. Mean X-ray properties of 449, 1734, 629, and 247 LBGs with z ~ 3, 4, 5, and 6, respectively, were determined using stacking techniques. When stacked, we detect X-ray emission from LBGs at z ~ 3 (~7 σ) and from an optically bright subset (brightest 25%) of LBGs at z ~ 4 (~3 σ); the latter is the highest redshift detection yet for "normal" galaxies in the X-ray band. The effective exposure times for these stacked observations are 0.7 and 0.5 Gs, respectively. The derived average rest-frame 2.0–8.0 keV luminosities are 1.5 × 1041 and 1.4 × 1041 ergs s-1, respectively. X-ray emission from these LBGs is likely due to high-mass X-ray binaries and Type II supernovae; the corresponding star formation rates are 10–30 M yr-1. The X-ray–to–B-band mean luminosity ratio (LX/LB) at z ~ 3 is somewhat elevated with respect to that measured for starburst galaxies in the local universe (significance ~3 σ). When stacking full samples of LBGs at z ~ 4, 5, and 6, we do not obtain significant detections (<3 σ) and derive rest-frame 2.0–8.0 keV luminosity upper limits (3 σ) of 0.9, 2.8, and 7.1 × 1041 ergs s-1, respectively. These upper limits constrain any widespread active galactic nucleus (AGN) activity in these objects to be modest at best. Furthermore, we find that ~0.5% of our LBGs from z 3 to z 6 are detected individually in the X-ray band. These LBGs have spectral shapes and luminosities characteristic of moderate-power AGNs (e.g., Seyfert galaxies and quasars).