Beneath the surface: >85% of z>5.9 quasi-stellar objects in massive host galaxies are UV-faint

Abstract

We used C II _ 158μ m observations of a large quasi-stellar object (QSO) sample to segregate sources by host galaxy mass to identify those in the most massive hosts. The CII luminosity is a known tracer of molecular gas. It was taken as a proxy for the host mass and was used to rank 190 QSOs at z>5.9, spanning a UV luminosity range of 6 mag (-28<M_ UV,AB <-22). Atacama Large Millimeter Array (ALMA) data from a cycle-10 program were particularly valuable. They provide CII coverage for 46 UV-faint (M_ UV,AB >-24.5) and 25 especially UV-faint (M_ UV,AB >-23.5) QSOs. This improved the statistics by five and six times, respectively. Taking massive host galaxies to be those where L_ CII $>1.8 of UV-bright QSOs), we identified 63 QSOs, including 13 that are UV-faint and 7 that are especially UV-faint. Using these selections and recent QSO luminosity functions (LFs), we present the first characterization of UV luminosity distribution for QSOs in massive host galaxies and quantify L_⊙ (median L_ CII CII LFs for the UV-bright and UV-faint QSOs. While ∼3% of the massive-host QSOs are UV-bright (M_ UV,AB <-26), ≳85% are UV-faint (M_ UV,AB >-24.5). This wide dispersion in UV luminosities reflects variations in dust obscuration, accretion efficiency, and black hole mass. Even though spectroscopy is needed for definitive conclusions, the black hole mass appears to be the dominant factor driving variations in the UV luminosity, based on 34 CII -luminous (L_ CII >1.8 ∼-23, the median extrapolated $ (M_ L_⊙) QSOs distributed across a baseline of ∼3 mag in UV luminosity and with measured M_BH. At M_ UV,AB 10 BH /M_⊙)$ is 8.1 ^ consistent with the local relation. Supermassive black holes (SMBHs) in UV-bright QSOs thus appear to be ∼15_ -9 +25 times more massive than is typical for massive host galaxies at z∼6.