Papers › The Evolution of Molecular Gas Fraction Traced by the CO Tully-Fisher Relation
The Evolution of Molecular Gas Fraction Traced by the CO Tully-Fisher Relation
Jacob W. Isbell, Rui Xue, Hai Fu
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Carbon monoxide (CO) observations show a luminosity$-$line-width correlation that evolves with redshift. We present a method to use CO measurements alone to infer the molecular gas fraction (fₘₒₗ) and constrain the CO$-$H₂ conversion factor (α_(CO)). We compile from the literature spatially integrated low-J CO observations of six galaxy populations, including a total of 449 galaxies between 0.01 ≤z ≤3.26. The CO data of each population provide an estimate of the α_(CO)-normalized mean molecular gas fraction (fₘₒₗ/α_(CO)). The redshift evolution of the luminosity$-$line-width correlation thus indicates an evolution of fₘₒₗ/α_(CO). We use a Bayesian-based Monte-Carlo Markov Chain sampler to derive the posterior probability distribution functions of fₘₒₗ/α_(CO) for these galaxy populations, accounting for random inclination angles and measurement errors in the likelihood function. We find that the molecular gas fraction evolves rapidly with redshift, fₘₒₗ ∝(1+z)^β with β≃2, for both normal star-forming and starburst galaxies. Furthermore, the evolution trend agrees well with that inferred from the Kennicutt-Schmidt relation and the star-forming main sequence. Finally, at z < 0.1 normal star-forming galaxies require a ∼5× larger α_(CO) than starburst galaxies to match their molecular gas fractions, but at z > 1 both star-forming types exhibit sub-Galactic α_(CO) values and normal star-forming galaxies appear more gas-rich than starbursts. Future applications of this method include calibrating Tully-Fisher relations without inclination correction and inferring the evolution of the atomic gas fraction with HI observations.
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