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

Abstract

We present the molecular gas content and interstellar medium conditions of MACS J0717_Az9, 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 ${}_{10}[{M}_{{{\rm{H}}}_{2}}({M}_{\odot })]=9.77$ , 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 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.

Keywords

• Molecular gas
• Dust formation
• Interstellar dust
• Photodissociation regions
• Galaxy kinematics
• Gravitational lensing