Tracing Molecular Gas Mass in z similar or equal to 6 Galaxies with [C II]

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We investigate the fine-structure [C II] line at 158 mu m as a molecular gas tracer by analyzing the relationship between molecular gas mass (M-mol) and [C II] line luminosity (L-[C (II)]) in 11,125 z similar or equal to 6 star-forming, main-sequence galaxies from the SIMBA simulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (similar to 50%-100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [C II] emission comes from the molecular phase. We find a sublinear (slope 0.78 +/- 0.01) log L-[C II]-log M-mol relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies at z similar or equal to 6. We derive a median [C II]-to-M-mol conversion factor of alpha([C )(II]) similar or equal to 18 M-circle dot/L-circle dot. This is lower than the average value of similar or equal to 30 M-circle dot/L-circle dot derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [C II] observations of low-mass galaxies. For our simulations, [C is a better tracer of the molecular gas than CO J= 1-0, especially at the lowest metallicities, where much of the gas is CO-dark. We find that L-[C (II)] is more tightly correlated with M-mol than with star formation rate (SFR), and both the log L-[C II] - log M-mol and log L-[C II]-log SFR relations arise from the Kennicutt-Schmidt relation. Our findings suggest that L-[C II] is a promising tracer of the molecular gas at the earliest cosmic epochs.

OriginalsprogEngelsk
Artikelnummer92
TidsskriftAstrophysical Journal
Vol/bind929
Udgave nummer1
Antal sider13
ISSN0004-637X
DOI
StatusUdgivet - 1 apr. 2022

ID: 304150436