First Constraints on the Interstellar Medium Conditions of a Low-mass, Highly Obscured z = 4.27 Main-sequence Galaxy

We present the molecular gas content and interstellar medium conditions of MACS J0717_Az9, a strong gravitationally lensed z = 4.273, M _* ≃ 2 × 10⁹ 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 μm, 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 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[G ₀ 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.