The X-ray spectral properties of scuba galaxies

Deep SCUBA surveys have uncovered a large population of massive submillimeter-emitting galaxies (SMGs; f(850 mu m) >= 4 mJy) at z >= 1. Although it is generally believed that these galaxies host intense star formation activity, there is growing evidence that a substantial fraction also harbor an active galactic nucleus (AGN; i.e., an accreting super-massive black hole [SMBH]). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultradeep X-ray observations ( the 2 Ms Chandra Deep Field-North) and deep Keck spectroscopic data of SMGs with radio counterparts. We find that the majority (approximate to 75%) of these radio-selected spectroscopically identified SMGs host AGN activity; the other approximate to 25% have X-ray properties consistent with star formation (X-ray derived star formation rates of approximate to 1300-2700 M-circle dot yr(-1)). The AGNs have properties generally consistent with those of nearby luminous AGNs (Gamma approximate to 1.8 +/- 0.5, N-H approximate to 10(20)-10(24) cm(-2), and L-X approximate to 10(43)-10(44.5) ergs s(-1)), and the majority (approximate to 80%) are heavily obscured (NH >= 10(23) cm(-2)). We construct composite rest-frame 2-20 keV spectra for three different obscuration classes [N-H < 10(23) cm(-2), N-H = (1-5) x 10(23) cm(-2), and N-H > 5 x 10(23) cm(-2)], which reveal features not seen in the individual X-ray spectra. An approximate to 1 keV equivalent width Fe K alpha emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs, suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray to far-IR luminosity ratio of the AGN-classified SMGs (L-X/L-FIR = 0.004) is approximately 1 order of magnitude below that found for typical quasars. This result suggests that intense star formation activity ( of order approximate to 1000 M-circle dot yr(-1)) dominates the bolometric output of these SMGs. However, we also explore the possibility that the X-ray to far-IR luminosity ratio of the AGN components is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the implications of our results for the growth of massive black holes, discuss the prospects for deeper X-ray observations, and explore the scientific potential offered by the next generation of X-ray observatories.