Evidence for universal gas depletion in a sample of 41 luminous Type 1 quasars at <i>z</i> ∼ 2

ABSTRACT We present ALMA CO observations of the molecular gas in a sample of 41 luminous unobscured quasars at z $\sim$ 2 from the Sloan Digital Sky Survey (SDSS). 32 targets comprise the main sample observed in CO(3-2), and 9 targets have archival ALMA data of CO(3-2), CO(4-3), and CO(7-6). All quasars have rest-ultraviolet to optical spectra tracing ionized gas in the broad line region (e.g. C iv) and the narrow line region (e.g. [O iii]) and they cover the full range of outflow properties in the SDSS quasar population at these redshifts. 15 out of the 32 quasars in the main sample are detected in CO(3-2) and five out of the nine archival quasars are also detected in CO. The median gas mass for all 20 CO-detected quasars is 8.0 $\pm$ 1.5 $\times$ 10$^9$ M$_{\odot }$ with a median M$_{\rm dyn}$ of 1.4 $\pm$ 0.9 $\times$ 10$^{11}$ M$_{\odot }$. We find gas fractions in the range 0.02–0.32, which are generally lower than both inactive galaxies and obscured quasars at similar redshifts. We suggest an evolutionary trend in gas fractions of quasar host galaxies from obscured and gas rich to unobscured and gas poor. We note a tentative correlation between the gas fractions and the broad-line region properties with quasars showing high C iv blueshifts, indicating stronger broad-line region winds, having higher gas fractions. Six of the quasars corresponding to 15 per cent of the sample also show evidence for at least one companion galaxy detected in CO at the same redshift.