2020ApJS..247...15S

The properties of the molecular gas can shed light on the physical conditions of quasar host galaxies and the effect of feedback from accreting supermassive black holes. We present a new CO(2-1) survey of 23 z< 0.1 Palomar-Green quasars conducted with the Atacama Large Millimeter/submillimeter Array. CO emission was successfully detected in 91% (21/23) of the objects, from which we derive CO luminosities, molecular gas masses, and velocity line widths. Together with CO(1-0) measurements in the literature for 32 quasars (detection rate 53%), there are 15 quasars with both CO(1-0) and CO(2-1) measurements and, in total, 40 sources with CO measurements. We find that the line ratio of R₂₁≡L_CO(2–1)^′^/LCO(1-0)_^′^ is subthermal and broadly consistent with nearby galaxies and other quasars previously studied. No clear correlation is found between R₂₁ and the intensity of the interstellar radiation field or the luminosity of the active nucleus. As with the general galaxy population, quasar host galaxies exhibit a strong, tight, and linear L_IR-L_CO(1–0)^′^ relation, with a normalization consistent with that of starburst systems. We investigate the molecular-to-total-gas mass fraction with the aid of total gas masses inferred from dust masses previously derived from infrared observations. Although the scatter is considerable, the current data do not suggest that the CO-to-H₂ conversion factor of quasar host galaxies significantly differs from that of normal star-forming galaxies.