The Evolution of Molecular Gas Fraction Traced by the CO Tully-Fisher Relation. (arXiv:1811.03107v1 [astro-ph.GA])

The Evolution of Molecular Gas Fraction Traced by the CO Tully-Fisher Relation. (arXiv:1811.03107v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Isbell_J/0/1/0/all/0/1">Jacob W. Isbell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Xue_R/0/1/0/all/0/1">Rui Xue</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fu_H/0/1/0/all/0/1">Hai Fu</a>

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 ($f_{rm mol}$) and constrain the CO$-$H$_2$
conversion factor ($alpha_{rm CO}$). We compile from the literature spatially
integrated low-$J$ CO observations of six galaxy populations, including a total
of 480 galaxies between $0.01 leq z leq 3.26$. The CO data of each population
provide an estimate of the $alpha_{rm CO}$-normalized mean molecular gas
fraction ($f_{rm mol}/alpha_{rm CO}$). The redshift evolution of the
luminosity$-$line-width correlation thus indicates an evolution of $f_{rm
mol}/alpha_{rm CO}$. We use a Bayesian-based Monte-Carlo Markov Chain sampler
to derive the posterior probability distribution functions of $f_{rm
mol}/alpha_{rm CO}$ 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, $f_{rm mol}
propto (1+z)^beta$ with $beta simeq 2$, for both normal star-forming and
starburst galaxies. Furthermore, the evolution trend agrees well with that
inferred from the Kennicutt-Schmidt law and the star-forming main sequence.
Finally, at $z < 0.1$ normal star-forming galaxies require a $sim5times$ larger $alpha_{rm CO}$ than starburst galaxies to match their molecular gas fractions, but at $z > 1$ both star-forming types exhibit sub-Galactic
$alpha_{rm CO}$ 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.

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 ($f_{rm mol}$) and constrain the CO$-$H$_2$
conversion factor ($alpha_{rm CO}$). We compile from the literature spatially
integrated low-$J$ CO observations of six galaxy populations, including a total
of 480 galaxies between $0.01 leq z leq 3.26$. The CO data of each population
provide an estimate of the $alpha_{rm CO}$-normalized mean molecular gas
fraction ($f_{rm mol}/alpha_{rm CO}$). The redshift evolution of the
luminosity$-$line-width correlation thus indicates an evolution of $f_{rm
mol}/alpha_{rm CO}$. We use a Bayesian-based Monte-Carlo Markov Chain sampler
to derive the posterior probability distribution functions of $f_{rm
mol}/alpha_{rm CO}$ 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, $f_{rm mol}
propto (1+z)^beta$ with $beta simeq 2$, for both normal star-forming and
starburst galaxies. Furthermore, the evolution trend agrees well with that
inferred from the Kennicutt-Schmidt law and the star-forming main sequence.
Finally, at $z < 0.1$ normal star-forming galaxies require a $sim5times$
larger $alpha_{rm CO}$ than starburst galaxies to match their molecular gas
fractions, but at $z > 1$ both star-forming types exhibit sub-Galactic
$alpha_{rm CO}$ 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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