Beneath the Surface: >85% of z>5.9 QSOs in Massive Host Galaxies are UV-Faint

R. J. Bouwens, E. Banados, R. Decarli, J. Hennawi, D. Yang, H. Algera, M. Aravena, E. Farina, A. Gloudemans, J. Hodge, H. Inami, J. Matthee, R. Meyer, R. P. Naidu, P. Oesch, H. J. A. Rottgering, S. Schouws, R. Smit, M. Stefanon, P. van der Werf, B. Venemans, F. Walter, Y. Fudamoto

First listed 2025-06-30 | Last updated 2025-06-30

Abstract

We use [CII] observations of a large QSO sample to segregate sources by host galaxy mass, aiming to identify those in the most massive hosts. [CII] luminosity, a known tracer of molecular gas, is taken as a proxy for host mass and used to rank 190 QSOs at z>5.9, spanning a 6-mag UV luminosity range (-22<Muv<-28). Particularly valuable are ALMA data from a cycle-10 CISTERN program, providing [CII] coverage for 46 UV-faint (M_{UV,AB}>-24.5) and 25 especially UV-faint (Muv>-23.5) QSOs, improving statistics by 5x and 6x, respectively. Taking massive host galaxies to be those where L[CII]>1.8x10^9 Lsol (median L[CII] of UV-bright QSOs), we identify 61 QSOs, including 13 which are UV-faint and 7 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 [CII] LFs for both UV-bright and UV-faint QSOs. While ~3% of massive-host QSOs are UV-bright (Muv<-26), >~85% are UV-faint (Muv>-24.5). This wide dispersion in UV luminosities reflects variations in dust obscuration, accretion efficiency, and black hole mass. Though spectroscopy is needed for definitive conclusions, black hole mass appears to be the dominant factor driving variations in the UV luminosity, based on 34 [CII]-luminous (L[CII]>1.8x10^9 Lsol) QSOs distributed across a ~3-mag baseline in UV luminosity and with measured MBH. At Muv~-23, the median extrapolated log10 (MBH/Msol) is 8.1+/-0.4, consistent with the local relation. SMBHs in UV-bright QSOs thus appear to be ~15(-9)(+25)x more massive than typical for massive host galaxies at z~6.

Short digest

Ranks 190 z>5.9 QSOs by host mass using ALMA [CII] luminosity as a gas/host-mass proxy, boosted by CISTERN cycle-10 coverage of UV-faint sources, and defines “massive hosts” at L[CII]>1.8x10^9 Lsun. From the resulting UV-luminosity distribution and [CII] luminosity functions, only ~3% of massive-host QSOs are UV-bright (Muv<-26) while >85% are UV-faint (Muv>-24.5). At Muv≈-23 the median extrapolated log10(MBH/Msun)=8.1±0.4, implying UV-bright QSOs host SMBHs ~15x more massive than typical massive-host systems at z~6. This points to most SMBH growth in massive galaxies being missed by UV-bright QSO selections; spectroscopy is still needed to confirm the MBH interpretation.

Key figures to inspect

• Figure 1: Inspect the [CII]–UV plane to see which faint QSOs are [CII]-luminous and thus in massive hosts; note the stated 21% fraction among UV-faint and how CISTERN dramatically improves coverage in the faint regime.
• Figure 2: Compare median L[CII], L_IR, and [CII] FWHM versus Muv for [CII]-bright QSOs; look for the similarity of far-IR and kinematic properties between UV-faint and UV-bright systems that supports comparable host masses.
• Figure 3: Read the cumulative fraction of massive-host QSOs versus UV magnitude to verify that only ~15% are brighter than Muv=-24.5 and ~3% brighter than Muv=-26; use the lower panel to contrast the derived number densities with the Schindler et al. (2023) UV LF.
• Figure 4: Examine the [CII] luminosity functions split by UV class; confirm that [CII]-luminous UV-faint QSOs dominate the volume density by ~29x and use the top-axis gas-mass scale to interpret what L[CII] implies for host reservoirs.