The Circumgalactic Medium of Submillimeter Galaxies. II. Unobscured QSOs within Dusty Starbursts and QSO Sightlines with Impact Parameters below 100 Kiloparsec

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) 870\(\mu\)m observations of 29 Herschel-selected submillimeter galaxies (SMGs) near QSO sightlines. We detect a total of 39 sources with 870\(\mu\)m flux densities between \(0.7 < S_{870} < 15\) mJy in 27 of the 29 fields. Ten Herschel sources have multiple ALMA counterparts. The intrinsic source sizes range between 0.2" and 0.7", with a mean at \(0.29\pm0.03\) arcsec. The Herschel-QSO separations for six of the pairs are less than 10", comparable to the sizes of the Herschel beam. We find that four of these six Herschel-detected QSOs are embedded in SMGs with \(3.0 \leq S_{870} \leq 14.2\) mJy, although the rest-frame ultraviolet spectra of the QSOs show no evidence of significant reddening. No additional mJy-level submillimeter companions are detected around these QSOs. Black hole accretion and star formation contribute almost equally in bolometric luminosity in these galaxies. The SMGs hosting QSOs show similar source sizes, dust surface densities, and SFR surface densities as other SMGs in the sample. We find that the black holes are growing \(\sim\)3\(\times\) faster than the galaxies when compared to the present-day black-hole-galaxy mass ratio, suggesting a QSO duty cycle of \(\lesssim\)30% in SMGs at \(z \sim 3\). The remaining two Herschel-detected QSOs are undetected at 870\(\mu\)m but each has a bright submillimeter "companion" only 9" and 12" away (71 and 95 kpc at z = 3). They could be either merging or projected pairs. If the former, they would represent a rare class of "wet-dry" mergers. If the latter, the QSOs would, for the first time, probe the circumgalactic medium of SMGs at impact parameters below 100 kpc.