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Abstract We present the molecular gas content and interstellar medium conditions of MACS J0717Az9, a strong gravitationally lensed z = 4. 273, M * ≃ 2 × 10 9 M ⊙ star-forming galaxy with an unusually high (∼80%) obscured star formation fraction. We detect CO (4–3) in two independent lensed images, as well as N ii 205 μ m, with the Atacama Large Millimeter Array. We derive a molecular gas mass of log 10 M H 2 (M ⊙) = 9. 77, making it moderately deficient in molecular gas compared to the lower-redshift gas fraction scaling relation. Leveraging photodissociation region (PDR) models, we combine our CO (4–3) measurements with existing measurements of the C ii 158 μ m line and total infrared luminosity to model the PDR conditions. We find PDR conditions similar to those in local star-forming galaxies, with a mean hydrogen density log 10 n H cm −3 = 4. 80 ± 0. 39 and a mean radiation field strength log 10 G 0 Habing = 2. 83 ± 0. 26. Based on Band 3 continuum data, we derive an upper limit on the intrinsic dust mass of log 10 M dust (M ⊙) < 7. 73, consistent with existing estimates. We use the 3D tilted-ring model fitting code 3D-Barolo to determine the kinematic properties of the CO (4–3) emitting gas. We find that it is rotationally dominated, with a V / σ = 4. 6 ± 1. 7, consistent with the kinematics of the C ii. With PDR conditions remarkably similar to those in normal dusty star-forming galaxies at z < 0. 2 and a stable molecular disk, our observations of Az9 suggest that the dust-obscured phase for a low-mass galaxy at z ∼ 4 is relatively long. Thus, Az9 may be representative of a more widespread population that has been missed owing to insufficiently deep existing millimeter surveys.
Mizener et al. (Mon,) studied this question.