SQuIGG$\vec{L}$E: Observational Evidence of Low Ongoing Star Formation Rates in Gas-Rich Post-Starburst Galaxies

Abstract

ALMA observations have shown that candidate "post-starburst" galaxies (PSBs) at z$\sim$0.6 can retain significant molecular gas reservoirs. These results would imply that -- unlike many model predictions -- galaxies can shut down their star formation before their cold gas reservoirs are depleted. However, these studies inferred star formation rates (SFRs) either from [O II] line fluxes or from spectral energy distribution modeling, and could have missed large dust-obscured contributions to the SFRs. In this study, we present Keck/NIRES observations of 13 massive (${\mathrm{M}}_{\ast }\gtrsim \times 10^{11}{\mathrm{M}}_{\odot }$) PSBs, which allow us to estimate $\mathrm{H}\alpha$ SFRs in these gas-rich post-starburst galaxies. We confirm the previously inferred low SFRs for the majority of the sample: 11/13 targets show clear $\mathrm{H}\alpha$ absorption, with minimal infilling indicating dust-corrected SFRs of $<4.1{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. These SFRs are notably low given the large ${\mathrm{H}}_2$ reservoirs ($\sim 1-5\times 10^{10}{\mathrm{M}}_{\odot }$) present in 5/13 of these galaxies, placing them significantly offset from star-forming galaxies on the Kennicutt-Schmidt relation for star-forming galaxies. The [N II]/H$\alpha$ ratios of all 13 PSBs imply contributions from non-star-forming ionization mechanisms (e.g., AGN, shocks, or hot evolved stars) to their $\mathrm{H}\alpha$ emission, suggesting that even these low ongoing SFRs may be overestimated. These low $\mathrm{H}\alpha$ SFRs, dust-corrected using A${}_v$ estimates from SED fitting, confirm that these galaxies are very likely quiescent and, thus, that galaxies can quench before their cold gas reservoirs are fully depleted.

12 pages, 4 figures, 1 table. Accepted by the Astrophysical Journal (ApJ)

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