Detecting and Characterizing Young Quasars. III. The Impact of Gravitational Lensing Magnification

Abstract

We test the impact of gravitational lensing on the lifetime estimates of seven high-redshift quasars at redshift z ≳ 6. The targeted quasars are identified by their small observed proximity zone sizes, which indicate extremely short quasar lifetimes (t Q ≲ 105yr). However, these estimates of quasar lifetimes rely on the assumption that the observed luminosities of the quasars are intrinsic and not magnified by gravitational lensing, which would bias the lifetime estimates toward younger ages. In order to test the possible effects of gravitational lensing, we obtain high-resolution images of the seven quasars with the Hubble Space Telescope and look for signs of strong lensing. We do not find any evidence of strong lensing, i.e., all quasars are well described by point sources, and no foreground lensing galaxy is detected. We estimate that the strong-lensing probabilities for these quasars are extremely small (∼1.4 × 10−5) and show that weak lensing changes the estimated quasar lifetimes by only ≲0.2 dex. We thus confirm that the short lifetimes of these quasars are intrinsic. The existence of young quasars indicates a high obscured fraction, radiatively inefficient accretion, and/or flickering lightcurves for high-redshift quasars. We further discuss the impact of lensing magnification on measurements of black hole masses and Eddington ratios of quasars.

The cold molecular gas in z ≳ 6 quasar host galaxies

“…Our finding that the molecular gas comprises a large fraction of the total mass, at least in the central regions, is consistent with earlier findings for some of the literature sample and for J2348-3054 based on dynamical modelling of [C ii] 158 µm versus CO(6-5)-or [C ii] 158 µm-derived gas masses (Yue et al 2021;Izumi et al 2021;Walter et al 2022). However, it appears to be in tension with the mean stellar masses of ≥10 10 M recently inferred for six z 6 QSOs by modelling the spectral energy distribution of their JWST photometry (Yue et al 2023). The only QSO host with both a dynamical mass (from ) and stellar mass (from their study) is J0100+2802, for which the dynamical mass is 1.2 +0.6 −0.7 ×10 10 , the CO(2-1)-derived molecular gas mass is (0.95± 0.32) × 10 10 and the stellar mass derived by Yue et al ( 2023) is <30 × 10 10 .…”