First Constraints on the ISM Conditions of a Low Mass, Highly Obscured z=4.27 Main Sequence Galaxy

We present the molecular gas content and ISM conditions of MACSJ0717 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]205μm, with ALMA. We derive a molecular gas mass of log₁₀[M_H₂ (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] 158μm line and total infrared luminosity to model the PDR conditions. We find PDR conditions similar to local star-forming galaxies, with a mean hydrogen density log₁₀[nH 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[Mdust (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 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 due to insufficiently deep existing millimeter surveys.