The Chandra Deep Field North Survey. XIV. X-Ray–Detected Obscured AGN[CLC]s[/CLC] and Starburst Galaxies in the Bright Submillimeter Source Population

Abstract

We provide X-ray constraints and perform the first X-ray spectral analyses for bright SCUBA sources (f_{850 μm} ≥ 5 mJy; signal-to-noise ratio ≥4) in an 84 × 84 area of the 2 Ms Chandra Deep Field North survey containing the Hubble Deep Field North. X-ray emission is detected from seven of the 10 bright submillimeter sources in this region down to 0.5–8.0 keV fluxes of ≈1 × 10^-16 ergs cm^-2 s^-1, corresponding to an X-ray–detected submillimeter source density of 360 deg^-2; our analyses suggest that this equates to an X-ray–detected fraction of the bright submillimeter source population of 36%, although systematic effects may be present. Two of the X-ray–detected sources have nearby (within 3'') X-ray companions, suggesting merging/interacting sources or gravitational lensing effects, and three of the X-ray–detected sources lie within the approximate extent of the protocluster candidate CXOHDFN J123620.0+621554. Five of the X-ray–detected sources have flat effective X-ray spectral slopes (Γ < 1.0), suggesting obscured AGN activity. X-ray spectral analyses suggest that one of these AGNs may be a Compton-thick source; of the other four AGNs, three appear to be Compton-thin sources and one has poor constraints. The rest-frame unabsorbed X-ray luminosities of these AGNs are more consistent with those of Seyfert galaxies than QSOs (i.e., L_X ≈ 10⁴³–10⁴⁴ ergs s^-1). Thus, the low X-ray detection rate of bright submillimeter sources by moderately deep X-ray surveys appears to be due to the relatively low luminosities of the AGNs in these sources rather than Compton-thick absorption. A comparison of these sources with the well-studied, heavily obscured AGN NGC 6240 shows that the average AGN contribution is negligible at submillimeter wavelengths. The X-ray properties of the other two X-ray–detected sources are consistent with those expected from luminous star formation; however, we cannot rule out the possibility that low-luminosity AGNs are present. The three X-ray–undetected sources appear to lie at high redshift (z > 4) and could be either AGNs or starburst galaxies.