Many efforts have been done 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
hundreds of 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 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). So far predominantly used for
star-forming galaxies, 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 (ALMA band 7) and CO(2-1)/(1-0) (VLA
K-Kα bands), respectively, at fixed integration time. Exploiting
archival ALMA observations, we place conservative 3σ upper limits on the
molecular gas fraction (fmol​=MH2​​/M⋆​) of ADF22-QG1
(fmol​<21%), ZF-COS-20115 (fmol​<3.2%), two of the
best-studied high-z QGs in the literature, and GS-9209 (fmol​<72%),
the most distant massive QG discovered to date. The deep upper limit found for
ZF-COS-20115 is 3 times lower than previously anticipated for high-z QGs
suggesting, at best, the existence of a large scatter in the fmol​
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.Comment: 7 pages, 2 figures. A&A Letters in pres