Mizener, AndrewPope, AlexandraMcKinney, JedKamieneski, PatrickWhitaker, Katherine E.Battisti, AndrewMurphy, Eric2025-03-282025-03-28Scopus:85198581457https://dspace-test.anu.edu.au/handle/1885/733744066We 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.We thank the referee for constructive comments which improved the paper. The authors would also like to thank the NAASC data reduction training team, which provided valuable assistance during the data reduction process. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2021.1.00272.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work utilizes gravitational lensing models produced by PIs Bradac, Natarajan & Kneib (CATS), Merten & Zitrin, Sharon, Williams, Keeton, Bernstein and Diego, and the GLAFIC group. This lens modeling was partially funded by the HST Frontier Fields program conducted by STScI. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The lens models were obtained from the Mikulski Archive for Space Telescopes (MAST).EnglishPublisher Copyright: © 2024. The Author(s). Published by the American Astronomical Society.2024-07-0110.3847/1538-4357/ad4965http://www.scopus.com/inward/record.url?scp=85198581457&partnerID=8YFLogxK