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

• Published in: The Astrophysical journal Vol. 942; no. 1; p. 24
• Main Authors: Sanders, Ryan L., Shapley, Alice E., Jones, Tucker, Shivaei, Irene, Popping, Gergö, Reddy, Naveen A., Davé, Romeel, Price, Sedona H., Mobasher, Bahram, Kriek, Mariska, Coil, Alison L., Siana, Brian
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 01.01.2023
• Subjects: Astrophysics; CO line emission; Correlation; Depletion; Galaxies; Galaxy evolution; High-redshift galaxies; Luminosity; Main sequence stars; Metallicity; Molecular gas; Molecular gases; Scattering; Space telescopes; Spectroscopy; Star formation; Stars; Stars & galaxies
• DOI: 10.3847/1538-4357/aca46f
• ISSN: 0004-637X

Abstract We present observations of CO(3−2) in 13 main-sequence z = 2.0–2.5 star-forming galaxies at log ( M * / M ⊙ ) = 10.2 – 10.6 that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that L 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 ∼ 2. We constrain the CO-to-H 2 conversion factor ( α CO ) and find that α CO inversely correlates with metallicity at z ∼ 2. We derive molecular gas masses ( M mol ) and characterize the relations among M * , SFR, M mol , and metallicity. At z ∼ 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 ∼ 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 ∼ 2 galaxies with larger M mol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z ∼ 2, where O/H is inversely correlated with both SFR and M mol at fixed M * . These results suggest that the scatter of the z ∼ 2 star-forming main sequence, mass–metallicity relation, and M mol – 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 ∼ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium.