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

Ryan L. Sanders, Alice E. Shapley, Tucker Jones, Irene Shivaei, Gergö Popping, Naveen A. Reddy, Romeel Davé, Sedona H. Price, Bahram Mobasher, Mariska Kriek, Alison L. Coil, Brian Siana

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14 Scopus citations


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-H2 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.

Original languageEnglish
Article number24
JournalAstrophysical Journal
Issue number1
StatePublished - Jan 1 2023

Bibliographical note

Publisher Copyright:
© 2023. The Author(s). Published by the American Astronomical Society.

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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