2018ApJ...853...24U

We investigate the X-ray active galactic nucleus (AGN) properties of millimeter galaxies in the Great Observatories Origins Deep Survey South (GOODS-S) field detected with the Atacama Large Millimeter/submillimeter Array (ALMA), by utilizing the Chandra 7-Ms data, the deepest X-ray survey to date. Our millimeter galaxy sample comes from the ASAGAO survey covering 26 arcmin² (12 sources at a 1.2 mm flux-density limit of ~0.6 mJy), supplemented by the deeper but narrower 1.3 mm survey of a part of the ASAGAO field by Dunlop et al. Ofthe 25 total millimeter galaxies, 14 have Chandra counterparts. The observed AGN fractions at z=1.5–3 are found to be 90_–19⁺⁸% and 57_–25⁺²³% for the ultra-luminous and luminous infrared galaxies with log L_IR/L_☉ = 12-12.8 and log L_IR/L_☉ = 11.5-12, respectively. The majority (∼2/3) of the ALMA and/or Herschel detected X-ray AGNs at z = 1.5-3 appear to be star-formation-dominant populations, having L_X/ L_IR ratios smaller than the "simultaneous evolution" value expected from the local black-hole-mass-to-stellar-mass (M_BH-M_*) relation. On the basis of the L_X and stellar mass relation, we infer that a large fraction of star-forming galaxies at z=1.5–3 have black hole masses that are smaller than those expected from the local M_BH-M_* relation. This contrasts previous reports on luminous AGNs at the same redshifts detected in wider and shallower surveys, which are subject to selection biases against lower luminosity AGNs. Our results are consistent with an evolutionary scenario in which star formation occurs first, and an AGN-dominant phase follows later, in objects that finally evolve into galaxies with classical bulges.