A Comparative Study of the Ground State Transitions of CO and [C I] as Molecular Gas Tracers at High Redshift
arxiv(2024)
Abstract
The CO(1–0) and [C1](1–0) emission lines are well-established
tracers of cold molecular gas mass in local galaxies. At high redshift, where
the interstellar medium (ISM) is likely to be denser, there have been limited
direct comparisons of both ground state transitions. Here we present a study of
CO(1–0) and [C1](1–0) emission in a sample of 20 unlensed dusty,
star-forming galaxies at z=2-5. The CO(1–0)/[C1](1–0) ratio is
constant up to at least z=5, supporting the use of [CI](1-0) as a gas mass
tracer. PDR modelling of the available data indicates a median H_2 density of
log(n [cm^-3])=4.7±0.2, and UV radiation field log(G_UV
[G_0])=3.2±0.2. We use the CO(1–0), [C1](1–0) and 3mm dust
continuum measurements to cross–calibrate the respective gas mass conversion
factors, finding no dependence of these factors on either redshift or infrared
luminosity. Assuming a variable CO conversion factor then implies [C1]
and dust conversion factors that differ from canonically assumed values but are
consistent with the solar/super-solar metallicities expected for our sources.
Radiative transfer modelling shows that the warmer CMB at high redshift can
significantly affect the [C1] as well as CO emission, which can change
the derived molecular gas masses by up to 70% for the coldest kinetic gas
temperatures expected. Nevertheless, we show that the magnitude of the effect
on the ratio of the tracers is within the known scatter of the
L'_CO-L'_[CI] relation. Further determining the absolute
decrease of individual line intensities will require well-sampled spectral line
energy distributions (SLEDs) to model the gas excitation conditions in more
detail.
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