CO Emission, Molecular Gas, and Metallicity in Main-Sequence Star-Forming Galaxies at $z\sim 2.3$

Abstract

We present observations of CO(3-2) in 13 main-sequence $z=2.0-2.5$ star-forming galaxies at $\log (M_{\ast }/M_{\odot })=10.2-10.6$ that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that CO(3-2)/SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at $z\sim 2$. We constrain the CO-to-H${}_2$ conversion factor (${\alpha }_{\text{CO}}$) and find that ${\alpha }_{\text{CO}}$ inversely correlates with metallicity at $z\sim 2$. We derive molecular gas masses ($M_{\text{mol}}$) and characterize the relations among $M_{\ast }$, SFR, $M_{\text{mol}}$, and metallicity. At $z\sim 2$, $M_{\text{mol}}$ increases and molecular gas fraction ($M_{\text{mol}}$/$M_{\ast }$) decrease with increasing $M_{\ast }$, with a significant secondary dependence on SFR. Galaxies at $z\sim 2$ 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-$M_{\ast }$, O/H-$M_{\ast }$, and $M_{\text{mol}}$-$M_{\ast }$ relations is correlated such that, at fixed $M_{\ast }$, $z\sim 2$ galaxies with larger $M_{\text{mol}}$ have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at $z\sim 2$ where O/H is inversely correlated with both SFR and $M_{\text{mol}}$ at fixed $M_{\ast }$. These results suggest that the scatter of the $z\sim 2$ star-forming main sequence, mass-metallicity relation, and $M_{\text{mol}}$-$M_{\ast }$ 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 $z\sim 2$ star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium.

35 pages, 17 figures, submitted to ApJ

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