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

¹ Istituto Nazionale di Astrofisica (INAF), Via Gobetti 93/3, 40129 Bologna, Italy
² Dipartimento di Fisica e Astronomia, Università di Bologna, Italy
³ Cosmic Dawn Center (DAWN), Denmark
⁴ DTU Space, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany
⁶ INAF – IASF Milano, Via A. Corti 12, I-20133 Milano, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 November 2025
Accepted: 12 March 2026

Abstract

Context. Molecular gas in galaxies traces both the fuel for star formation and the processes that enhance or suppress it. Observing its physical state (e.g., excitation) can reveal when and why galaxies stop forming stars.

Aims. We observed the CO(5–4) emission of eight post-SB galaxies at z ∼ 0.6 − 1.3. To our knowledge, this is the first time that high-J transitions have been probed for post-SB or quiescent galaxies beyond the local Universe. All of them have detections in lower-J CO transitions (either CO(2–1) or CO(3–2)) and molecular gas fractions up to ∼20%. By studying the ratio R₅₂ = L′CO(5 − 4)/L′CO(2 − 1), a proxy for the gas excitation, we aim to constrain the physical state of the gas.

Methods. The CO excitation helps to distinguish among different mechanisms responsible for the low star formation efficiency (SFE) of post-SB galaxies. In the first scenario, the molecular gas is predominantly diffuse and cold, implying a low fraction of dense star-forming gas and in turn low R₅₂ values. In the second scenario, elevated gas temperatures at moderate densities, for example due to active galactic nucleus (AGN) activity, shocks, or enhanced turbulence, would instead produce high R₅₂ values.

Results. Our post-SBs have on average R₅₂ = 0.28, comparable to high-redshift main-sequence galaxies. However, when considering only the CO(5–4) non-detections, which also coincide with post-starbusts that do not show signs of interaction, we obtain R₅₂ < 0.10, twice lower than local star-forming galaxies and more than 2.5 times lower than high-redshift sources. The average CO spectral line energy distribution (SLED) peaks at J = 3, similar to the Milky Way. Three galaxies show signs of interactions (tidal features, companions). They have R₅₂ = 0.40 and SLEDs peaking at J ≳ 4 − 5. In at least one case additional mechanisms (e.g., AGNs, shocks) are needed to explain the steep rise of the SLED up to J = 5.

Conclusions. 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., AGNs, turbulence, shocks). Residual star formation is insufficient to rapidly exhaust the remaining molecular gas in the majority of post-SB galaxies.