Revisited cold gas content with atomic carbon [C I] in z=2.5
  protocluster galaxies
  We revisit the cold gas contents of galaxies in a protocluster at z=2.49
using the lowest neutral atomic carbon transition [CI]$^3$P$_1$-$^3$P$_0$ from
Atacama Large Millimeter/submillimeter Array observations. We aim to test if
the same gas mass calibration applied in field galaxies can be applied to
protocluster galaxies. Five galaxies out of sixteen targeted galaxies are
detected in the [CI] line, and these are all previously detected in CO(3-2) and
CO(4-3) and three in 1.1 mm dust continuum. We investigate the line luminosity
relations between CO and [CI] in the protocluster and compare them with other
previous studies. We then compare the gas mass based on three gas tracers of
[CI], CO(3-2), and dust if at least one of the last two tracers are available.
Using the calibration adopted for field main-sequence galaxies, the [CI]-based
gas measurements are lower than or comparable to the CO-based gas measurements
by -0.35 dex at the lowest with the mean deviation of -0.14 dex. The
differences between [CI]- and the dust- based measurements are relatively mild
by up to 0.16 dex with the mean difference of 0.02 dex. Taking these all
together with calibration uncertainties, with the [CI] line, we reconfirm our
previous findings that the mean gas fraction is comparable to field galaxies
for a stellar-mass range of $\log(M_{\rm star}/M_\odot = [10.6, 11.3]$.
However, at least for these secure five detections, the depletion time scale
decreases more rapidly with stellar mass than field galaxies that might be
related to earlier quenching in dense environments.
