Unveiling the Origins of the Molecular Gas Reservoirs in Recently Quenched Massive Galaxies

Observations of quenched galaxies across cosmic time have revealed systems containing substantial molecular gas reservoirs, challenging the traditional picture of gas-poor, red and dead galaxies. Using the Simba cosmological simulation, we investigate three possible hypotheses for the origin of molecular gas-rich quenched systems: (i) quiescence is temporary and the galaxy will rejuvenate, (ii) molecular gas masses are overestimated due to an underestimated CO-to-H${}_2$ conversion factor, or (iii) star formation is ongoing but heavily obscured by dust. Across $M_{\ast }>3\times {10}^9{M}_{\odot }$ galaxies from $z=0-2$, we find that all three mechanisms are plausible. Many seemingly quiescent, molecular gas-rich galaxies host dust-obscured star formation, with visual optical depths increasing on average from ${\tau }_V\sim 0.5-2.3$ from $z=0-2$. Assuming a Milky Way-like CO-to-H${}_2$ conversion factor can overestimate molecular gas masses by a factor of $\sim \text{\hspace{0.17em}⁣}5$ in non-star-forming galaxies, compared to $\sim \text{\hspace{0.17em}⁣}1.45$ in star-forming galaxies. Temporary quiescence can also occur in gas-rich transitioning galaxies on timescales ranging from $100\text{Myr}-2\text{Gyr}$, although this is most prominent at high redshift and is relatively rare overall. Further work is needed to disentangle the relative contributions of these mechanisms across cosmic time.