CO Emission, Molecular Gas, and Metallicity in Main-sequence Star-forming Galaxies at z ∼ 2.3

We present observations of CO(3-2) in 13 main-sequence z = 2.0-2.5 star-forming galaxies at log(M-* /M-circle dot) = 10.2-10.6 that span a wide range in metallicity (O/H) based on rest-optical spectrosc/opy. We find that LCO (3-2) /SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at z similar to 2. We constrain the CO-to-H-2 conversion factor (alpha(CO)) and find that alpha(CO) inversely correlates with metallicity at z similar to 2. We derive molecular gas masses (M-mol) and characterize the relations among M-*, SFR, M-mol, and metallicity. At z similar to 2, M-mol increases and the molecular gas fraction (M-mol/M-*) decreases with increasing M*, with a significant secondary dependence on SFR. Galaxies at z similar to 2 lie on a near -linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR-M-*, O/H-M-*, and M-mol-M-* relations is correlated such that, at fixed M-*, z similar to 2 galaxies with larger Mmol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z similar to 2, where O/H is inversely correlated with both SFR and Mmol at fixed M*. These results suggest that the scatter of the z similar to 2 star-forming main sequence, mass-metallicity relation, and Mmol-M* relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive z similar to 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium.