2024ApJ...970...30M

We present the molecular gas content and interstellar medium conditions of MACS J0717_Az9, a strong gravitationally lensed z = 4.273, M_* ≃ 2 × 10⁹M_⊙ 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 μm, with the Atacama Large Millimeter Array. We derive a molecular gas mass of log${}_{10}[{M}_{{{\rm{H}}}_{2}}({M}_{\odot })]=9.77$, 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 μm 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 log₁₀[n_H cm^–3] = 4.80 ± 0.39 and a mean radiation field strength log₁₀[G₀ Habing] = 2.83 ± 0.26. Based on Band 3 continuum data, we derive an upper limit on the intrinsic dust mass of log₁₀[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.