Gas excitation of post-starburst galaxies at 0.6 < z < 1.3

TL;DR

This study investigates the excitation states of molecular gas in post-starburst galaxies at redshifts 0.6 to 1.3, revealing diverse gas conditions and implications for galaxy quenching mechanisms.

Contribution

First high-J CO transition observations in quiescent galaxies beyond the local universe, providing insights into gas excitation and quenching processes at intermediate redshifts.

Findings

Post-starburst galaxies show low gas excitation with R52=0.28 on average.

Galaxies without interaction signs have R52<0.10, similar to local star-forming galaxies.

Interacting galaxies exhibit higher excitation, with R52=0.40 and SLED peaking at J>4-5.

Abstract

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.