A decade of study has established that the molecular gas properties of star-forming galaxies follow coherent scaling relations out to z \sim 3 , suggesting remarkable regularity of the interplay between molecular gas , star formation , and stellar growth .
Passive galaxies , however , are expected to be gas-poor and therefore faint , and thus little is known about molecular gas in passive galaxies beyond the local universe .
Here we present deep Atacama Large Millimeter/submillimeter Array ( ALMA ) observations of CO ( 2–1 ) emission in 8 massive ( M _ { star } \sim 10 ^ { 11 } M _ { \odot } ) galaxies at z \sim 0.7 selected to lie a factor of 3–10 below the star-forming sequence at this redshift , drawn from the Large Early Galaxy Astrophysics Census ( LEGA-C ) survey .
We significantly detect half the sample , finding molecular gas fractions \lesssim 0.1 .
We show that the molecular and stellar rotational axes are broadly consistent , arguing that the molecular gas was not accreted after the galaxies became quiescent .
We find that scaling relations extrapolated from the star-forming population over-predict both the gas fraction and gas depletion time for passive objects , suggesting the existence of either a break or large increase in scatter in these relations at low specific star formation rate .
Finally , we show that the gas fractions of the passive galaxies we have observed at intermediate redshifts are naturally consistent with evolution into local massive early-type galaxies by continued low-level star formation , with no need for further gas accretion or dynamical stabilization of the gas reservoirs in the intervening 6 billion years .