The large molecular gas fraction of post-starburst galaxies at z > 1

Abstract

Post-starburst galaxies are sources that had the last major episode of star formation about 1 Gyr before the epoch of the observations and are on their way to quiescence. It is important to study such galaxies at redshift z > 1, during their main quenching phase, and estimate their molecular gas content to constrain the processes responsible for the cessation of star formation. We present CO(3-2) ALMA observations of two massive (M⋆ ∼ 5 × 1010 M⊙) post-starburst galaxies at z > 1. We measure their molecular gas fraction to be fH2 = MH2/M⋆ ∼ 8% - 16%, consistent with z < 1 post-starburst galaxies from the literature. The star formation efficiency of our targets is ∼10 × lower than that of star-forming galaxies at similar redshift, and they are outliers of the fH2 - specific star formation rate (sSFR) relation of star-forming galaxies, as they have larger fH2 than expected given their sSFR. The gas fraction of post-starbursts from our sample and the literature correlates with the Dn4000 spectral index, a proxy of the stellar population age. This suggests that their gas content decreases after the last major burst of star formation. Finally, one of our targets is undergoing a major merger phase with two highly star-forming companions. This hints at a picture where a perturber event (e.g., major merger) quenches star formation without completely removing the molecular gas.

Stars, Gas, and Star Formation of Distant Post-starburst Galaxies

“…The one undetected source, LEGA-C 133501, has a 3σ upper limit of 4.8 × 10 9 M e . The ratio between the masses of molecular gas and stars, f H2 ≡ M H2 /M * , range between 2% and 26% and are comparable to the values found in previous studies (Suess et al 2017;Bezanson et al 2022;Zanella et al 2023)…”

“…The correlations between molecular gas fraction and D n 4000, EW(Hδ), lookback time of the onset of the burst, and the lookback time when galaxies became quiescent. Post-starburst galaxies at similar redshifts with CO measurements in Bezanson et al (2022;z ∼ 0.6) and Zanella et al (2023;z ∼ 1.2) are also plotted.…”

“…Figure 11 shows the correlation between f H 2 and age-related parameters of our post-starburst galaxies. Samples at similar redshifts (Bezanson et al 2022;Zanella et al 2023) are also plotted. However, caution must be taken in drawing any definitive conclusions from such a small sample size.…”

The Fate of the Interstellar Medium in Early-type Galaxies. III. The Mechanism of Interstellar Medium Removal and the Quenching of Star Formation

“…The existence of gas in many passive galaxies may contradict the interstellar medium (ISM) removal as the only mechanism of quenching. Indeed, atomic and molecular gas and dust have been detected for a fraction of early-type galaxies (ETGs; Davis et al 2011Davis et al , 2019aYoung et al 2011;Rowlands et al 2012;Smith et al 2012;Alatalo et al 2013Alatalo et al , 2015adi Serego Alighieri et al 2013;Davis & Bureau 2016;Ashley et al 2017Ashley et al , 2018Ashley et al , 2019Zhang et al 2019;Richtler et al 2020;Magdis et al 2021;Donevski et al 2023) and poststarburst galaxies (French et al 2015;Rowlands et al 2015;Alatalo et al 2016;Suess et al 2017;Yesuf et al 2017;Smercina et al 2018Smercina et al , 2022Li et al 2019;Yesuf & Ho 2020;Bezanson et al 2022;Otter et al 2022;Wu et al 2023;Zanella et al 2023). Small amounts of dense gas, traced by the hydrogen cyanide (HCN) line, in poststarburst galaxies (French et al 2018b(French et al , 2023) may point at the inability of the gas reservoir to collapse and form stars.…”