ALMA Reveals a Stable Rotating Gas Disk in a Paradoxical Low-mass, Ultradusty Galaxy at z = 4.274

• Published in: Astrophysical journal. Letters 2023-07, Vol.951 (2), p.L46
• Main Authors: Pope, Alexandra, McKinney, Jed, Kamieneski, Patrick, Battisti, Andrew, Aretxaga, Itziar, Brammer, Gabriel, Diego, Jose M., Hughes, David H., Keller, Erica, Marchesini, Danilo, Mizener, Andrew, Montaña, Alfredo, Murphy, Eric, Whitaker, Katherine E., Wilson, Grant, Yun, Min
• Format: Article
• Language: English
• Subjects: Cosmic dust; Disk galaxies; Dust; Dust continuum emission; Emission lines; Galactic evolution; Galaxies; Galaxy evolution; Galaxy kinematics; Interstellar matter; Interstellar medium; Luminous infrared galaxies; Red shift; Rotating disks; Spectral energy distribution; Star & galaxy formation; Star formation; Star formation rate; Stars; Stars & galaxies; Stellar mass; Strong gravitational lensing
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/acdf5a

We report ALMA detections of [C ii ] and a dust continuum in Az9, a multiply imaged galaxy behind the Frontier Field cluster MACS J0717.5+3745. The bright [C ii ] emission line provides a spectroscopic redshift of z = 4.274. This strongly lensed ( μ = 7 ± 1) galaxy has an intrinsic stellar mass of only 2 × 10 9 M ⊙ and a total star formation rate of 26 M ⊙ yr −1 (∼80% of which is dust-obscured). Using public magnification maps, we reconstruct the [C ii ] emission in the source plane to reveal a stable, rotation-dominated disk with V / σ = 5.3, which is >2× higher than predicted from simulations for similarly high-redshift, low-mass galaxies. In the source plane, the [C ii ] disk has a half-light radius of 1.8 kpc and, along with the dust, is spatially offset from the peak of the stellar light by 1.4 kpc. Az9 is not deficient in [C ii ]; L [C II ] / L IR = 0.0027, consistent with local and high-redshift normal star-forming galaxies. While dust-obscured star formation is expected to dominate in higher-mass galaxies, such a large reservoir of dust and gas in a lower-mass disk galaxy 1.4 Gyr after the Big Bang challenges our picture of early galaxy evolution. Furthermore, the prevalence of such low-mass dusty galaxies has important implications for the selection of the highest-redshift dropout galaxies with JWST. As one of the lowest stellar mass galaxies at z > 4 to be detected in a dust continuum and [C ii ], Az9 is an excellent laboratory in which to study early dust enrichment in the interstellar medium.