Fact: Many SCUBA Galaxies Harbour AGNs

Abstract

Deep SCUBA surveys have uncovered a large population of ultra-luminous galaxies at z>1. These sources are often assumed to be starburst galaxies, but there is growing evidence that a substantial fraction host an AGN (i.e., an accreting super-massive black hole). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultra-deep X-ray observations (the 2 Ms Chandra Deep Field-North) and deep optical spectroscopic data. We argue that upward of 38% of bright (f_850μm≥ 5 mJy) SCUBA galaxies host an AGN, a fraction of which are obscured QSOs (i.e., L _X > 3 × 10⁴⁴ erg s^–1). However, using evidence from a variety of analyses, we argue that in almost all cases the AGNs are not bolometrically important (i.e., < 20%). Thus, star formation appears to dominate their bolometric output. A substantial fraction of bright SCUBA galaxies show evidence for binary AGN activity. Since these systems appear to be interacting and merging at optical/near-IR wavelengths, their super-massive black holes will eventually coalesce.