Gas excitation of post-starburst galaxies at 0.6 < z < 1.3
Molecular gas traces the fuel for star formation and the processes that regulate it. Observing its physical state (e.g. excitation) reveals when and why galaxies quench. We observed the CO(5-4) emission of 8 post-starburst (SB) galaxies at z~0.6-1.3. To our knowledge, this is the first time that high-J transitions are probed for quiescent galaxies beyond the local Universe. All targets are detected in CO(2-1) or CO(3-2) and have gas fractions up to 20%. Using the ratio R52=L'CO(5-4)/L'CO(2-1) as a proxy for gas excitation, we distinguish among mechanisms responsible for the low SFE of post-SBs. In the first scenario, the molecular gas is predominantly diffuse and cold, implying a low fraction of dense star-forming gas and low R52 values. In the second scenario, elevated gas temperatures at moderate densities, e.g. due to AGN activity, shocks, or turbulence, produce high R52 values. On average our post-SBs have R52=0.28, comparable to high-redshift galaxies. However, CO(5-4) non-detections, corresponding to galaxies without signs of interaction, yield R52<0.10, 2 times lower than local star-forming galaxies. The average CO Spectral Line Energy Distribution (SLED) peaks at J=3, similar to the Milky Way. Three galaxies show signs of ongoing mergers and have R52 = 0.40 and CO SLEDs peaking at J > 4-5, similar to high-redshift galaxies. At least one requires additional mechanisms (AGN, shocks) to explain the rise of the SLED up to J=5. Our results favor a scenario in which most systems are dominated by low-density molecular gas with low excitation, consistent with quenching driven by gas stabilization, feedback regulation, or stripping. In interacting systems instead, enhanced excitation is likely driven by heating processes not related to star-formation (e.g., AGN, turbulence, shocks). Residual star formation is insufficient to exhaust the remaining molecular gas in the majority of post-SBs.