Molecular Gas Excitation in z ~ 0.7 Gas-Rich Post-starburst Galaxies from SQuIGGLE

Many post-starburst galaxies at $z\sim0.7$ have been shown to retain substantial molecular gas reservoirs yet host low ongoing star formation, suggesting that the remaining gas may be inefficient at forming stars during the early post-burst phase. We present new Atacama Large Millimeter/submillimeter Array CO(5-4) observations of nine gas-rich post-starburst galaxies at $z\sim0.7$ from the Studying Quenching in Intermediate-z Galaxies: Gas, angu$\vec{L}$ar momentum, and Evolution (SQuIGG$\vec{L}$E) survey, providing a view of the molecular gas excitation in these systems. Combined with existing CO(2-1) data, we detect CO(5-4) in 8/9 targets and find that most have moderate CO excitation with $r_{52}\equiv L’{\rm CO(5-4)}/L’{\rm CO(2-1)}\approx0.1-0.3$. These systems show no clear trend between $r_{52}$ and either total or surface-density of star formation. Specifically, all objects have $Σ_{\mathrm{SFR}} \sim 0.01-1\ \text{M}\odot\ \text{yr}^{-1}\ \text{kpc}^{-2}$, consistent with compact, modest star formation, even when allowing for buried activity, as these galaxies decline from their peak. One object J1448+1010, which has clear optical, mid-infrared, and radio indicators of an active galactic nucleus, is an outlier with $r{52}\approx0.6$; its elevated excitation likely requires significant non-stellar heating, with a contribution from potentially obscured star formation. Together, most gas-rich SQuIGG$\vec{L}$E post-starbursts have moderately excited molecular gas alongside little to modest star-forming activity, indicating that the remaining gas hosts relatively suppressed star formation efficiencies instead of strong buried starburst activity.

💡 Research Summary

This paper presents new ALMA Band 7 observations of the CO(5‑4) transition in nine gas‑rich post‑starburst (PSB) galaxies at redshift ≈ 0.7 selected from the SQuIGGLE survey, and combines these data with existing CO(2‑1) measurements to probe the excitation of their molecular gas. The nine targets were chosen as the most massive (log M_* ≈ 11.0–11.5 M_⊙) and gas‑rich (M_H₂ > 10¹⁰·²⁵ M_⊙) subsample of the larger SQuIGGLE ALMA CO(2‑1) program, and all have quenching ages t_quench ≲ 200 Myr, placing them in the early post‑burst phase.

Observations were carried out with an average on‑source integration of ~25 min per source, achieving ~0.5″ resolution (≈3–4 kpc) and a spectral resolution of 50 km s⁻¹. Continuum emission was subtracted in the uv plane, and line fluxes were extracted using two‑dimensional Gaussian fits to the integrated CO(5‑4) maps. Eight of the nine galaxies are detected at ≥3σ; the non‑detection (J0912+1523) yields a 3σ upper limit. For two objects (J0912+1523 and J1448+1010) ancillary CO(4‑3) data are also included.

The key diagnostic is the line‑ratio r₅₂ ≡ L′_CO(5‑4)/L′_CO(2‑1) expressed in brightness‑temperature units. The majority of the sample shows moderate excitation, r₅₂ ≈ 0.10–0.30, comparable to low‑z PSBs and to quiescent disk galaxies. One outlier, J1448+1010, exhibits a high r₅₂ ≈ 0.57 and simultaneously displays clear AGN signatures in optical, mid‑infrared, and radio bands, indicating that non‑stellar heating (likely AGN‑driven) is boosting the high‑J CO emission.

Star‑formation rates (SFRs) and surface densities (Σ_SFR) were derived from multi‑wavelength SED fitting. All galaxies have modest Σ_SFR ≈ 0.01–1 M_⊙ yr⁻¹ kpc⁻², consistent with compact, low‑level star formation. No statistically significant correlation is found between r₅₂ and either total SFR or Σ_SFR, suggesting that the excitation level is not driven by hidden, intense star formation. Molecular gas depletion times are ≈ 1 Gyr, far longer than the observed rapid decline of CO luminosity (≈ 75–85 Myr), implying that simple consumption by star formation cannot explain the gas loss. Instead, mechanisms such as AGN‑driven outflows, shock heating, or turbulence‑induced heating are likely responsible for both the elevated excitation (in the AGN case) and the suppression of star formation efficiency.

Spatial analysis shows that CO(5‑4) emission is generally co‑spatial with CO(2‑1), with similar or slightly more compact sizes, consistent with the expectation that higher‑J lines trace denser, warmer gas in the central regions. Two galaxies (J1448+1010 and J2258+2313) possess extended tidal gas reservoirs, but the analysis focuses on the central components.

In summary, the study finds that gas‑rich PSBs at z ≈ 0.7 typically host moderately excited molecular gas (r₅₂ ≈ 0.1–0.3) while maintaining low surface star‑formation rates. The lack of a strong r₅₂–SFR relation and the presence of an AGN‑driven outlier indicate that the remaining gas is not undergoing a hidden starburst but is instead being heated or dynamically disturbed, leading to a suppressed star‑formation efficiency. These results parallel findings for low‑redshift PSBs, suggesting that quenching can proceed with substantial molecular reservoirs that are rendered inefficient for star formation through changes in their physical state rather than outright removal.