The metallicity dependence of the CO {\rightarrow} H_2 conversion factor in z>1 star forming galaxies

TL;DR

This study investigates how the CO-to-H2 conversion factor varies with metallicity in high-redshift star-forming galaxies, revealing a significant dependence that impacts molecular gas mass estimates.

Contribution

It provides the first systematic analysis of metallicity dependence of {o} in z>1 galaxies, quantifying the relation and implications for gas mass calculations.

Findings

The depletion rate is ~1 Gyr-1 for near-solar metallicity galaxies.

Depletion rate increases rapidly with decreasing metallicity below M_S.

The {o} factor strongly depends on metallicity, with a slope between -1 and -2.

Abstract

We use the first systematic samples of CO millimeter emission in z>1 'main-sequence' star forming galaxies (SFGs) to study the metallicity dependence of the conversion factor {\alpha}CO, from CO line luminosity to molecular gas mass. The molecular gas depletion rate inferred from the ratio of the star formation rate (SFR) to CO luminosity, is ~1 Gyr-1 for near-solar metallicity galaxies with stellar masses above M_S~1e11 M_sun. In this regime the depletion rate does not vary more than a factor of two to three as a function of molecular gas surface density, or redshift between z~0 and 2. Below M_S the depletion rate increases rapidly with decreasing metallicity. We argue that this trend is not caused by starburst events, by changes in the physical parameters of the molecular clouds, or by the impact of the fundamental metallicity-SFR-stellar mass relation. A more probable explanation is that the conversion factor is metallicity dependent and that star formation can occur in 'CO-dark' gas. The trend is also expected theoretically from the effect of enhanced photodissociation of CO by ultraviolet radiation at low metallicity. From the available z~0 and z~1-3 samples we constrain the slope of the log({\alpha}CO) -log (metallicity) relation to range between -1 and -2, fairly insensitive to the assumed slope of the gas-star formation rate relation. Because of the lower metallicities near the peak of the galaxy formation activity at z~1-2 compared to z~0, we suggest that molecular gas masses estimated from CO luminosities have to be substantially corrected upward for galaxies below M_S.