An ALMA CO(2-1) Survey of Nearby Palomar-Green Quasars
  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 R_21 =
L'_CO(2-1)/L'_CO(1-0) is subthermal, broadly consistent with nearby galaxies
and other quasars previously studied. No clear correlation is found between
R_21 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, linear L_IR-L_CO 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-H_2
conversion factor of quasar host galaxies significantly differs from that of
normal star-forming galaxies.
