ALMA reveals a stable rotating gas disk in a paradoxical low-mass, ultra-dusty galaxy at z = 4.274

Understand

We report ALMA detections of [CII] and dust continuum in Az9, a multiply-imaged galaxy behind the Frontier Field cluster MACSJ0717.5+3745.

• The bright [CII] emission line provides a spectroscopic redshift of z = 4.274.
• This strongly lensed (mu = 7 +/- 1) galaxy has an intrinsic stellar mass of only 2e9 Msun and a total star formation rate of 26 Msun/yr (~80% of which is dust obscured).
• Using public magnification maps, we reconstruct the [CII] 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.