2023A&A...678L...9D

A great deal of effort has been made in recent years to probe the gas fraction evolution of massive quiescent galaxies (QGs); however, a clear picture has not yet been established. Recent spectroscopic confirmations at z > 3 offer the chance to measure the residual gas reservoirs of massive galaxies a few hundred Myr after their death and to study how fast quenching proceeds in a highly star-forming Universe. Even so, stringent constraints at z > 2 remain hardly accessible with the Atacama Large Millimeter/submillimeter Array (ALMA) when adopting molecular gas tracers commonly used for the quenched population. In this Letter we propose overcoming this impasse by using the carbon [CII] 158 μm emission line to systematically probe the gaseous budget of unlensed QGs at z > 2.8, when these galaxies could still host non-negligible star formation on an absolute scale and when the line becomes best observable with ALMA (Bands 8 and 7). Predominantly used for star-forming galaxies to date, this emission line is the best choice to probe the gas budget of spectroscopically confirmed QGs at z > 3, reaching 2-4 and 13-30 times deeper than dust continuum emission (ALMA band 7) and CO(2-1)/(1-0) (Very Large Array, VLA, K - Kα bands), respectively, at fixed integration time. Exploiting archival ALMA observations, we place conservative 3σ upper limits on the molecular gas fraction (f_mol = M_H2/M_⋆) of ADF22-QG1 (f_mol < 21%) and ZF-COS-20115 (f_mol < 3.2%), two of the best-studied high-z QGs in the literature, and GS-9209 (f_mol < 72%), the most distant massive QG discovered to date. The deep upper limit found for ZF-COS-20115 is three times lower than previously anticipated for high-z QGs suggesting, at best, the existence of a large scatter in the f_mol distribution of the first QGs. Lastly, we discuss the current limitations of the method and propose ways to mitigate some of them by exploiting ALMA bands 9 and 10.