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

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 (& mu; = 7 & PLUSMN; 1) galaxy has an intrinsic stellar mass of only 2 x 10(9) M (& ODOT;) and a total star formation rate of 26 M (& ODOT;) yr(-1) (& SIM;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/& sigma; = 5.3, which is >2x 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.