THE METALLICITY DEPENDENCE OF THE CO → H₂CONVERSION FACTOR IN<i>z</i>⩾ 1 STAR-FORMING GALAXIES

Abstract

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