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

Abstract

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.

Figures (1)

• Figure 1: SFR vs. H$_2$ gas mass (left) for all CO(2--1) SQuIGG$\vec{L}$E measurements (blue diamonds) and nondetections (blue triangles), with the nine PSBs targeted in this work outlined in orange. We include for comparison star-forming galaxies at $\langle z \rangle \sim1.2$ from PHIBSS/PHIBSS2 (gray triangles; tacconi_phibss_2013) along with $z\sim0$ massive galaxies from COLDGASS (white circles; saintonge_cold_2011). Black dashed lines indicate constant molecular gas depletion timescales ($\tau_\mathrm{dep} \equiv {M}_{\mathrm{H_2}}/\mathrm{SFR}$). $M_{\rm{H}_2}$ vs. time since quenching $t_q$ (right) for the same sample. H$_2$ masses are taken from bezanson_now_2022 and setton_squigg_2025, while SFR and $t_q$ values are from suess_mathrmsquiggecle_2022. Median uncertainties for the entire SQuIGG$\vec{L}$E ALMA sample are shown for both panels (black cross). The selected systems are among the most gas-rich and youngest PSBs in the SQuIGG$\vec{L}$E sample.