Possible Systematic Rotation in the Mature Stellar Population of a z = 9.1 Galaxy

We present new observations with the Atacama Large Millimeter/submillimeter Array for a gravitationally lensed galaxy at z = 9.1, MACS1149-JD1. [O iii ] 88 μ m emission is detected at 10 σ with a spatial resolution of ∼0.3 kpc in the source plane, enabling the most distant morphokinematic study of a galaxy. The [O iii ] emission is distributed smoothly without any resolved clumps and shows a clear velocity gradient with Δ V obs /2 σ tot = 0.84 ± 0.23, where Δ V obs is the observed maximum velocity difference and σ tot is the velocity dispersion measured in the spatially integrated line profile, suggesting a rotating system. Assuming a geometrically thin self-gravitating rotation disk model, we obtain V rot / σ V = 0.67 − 0.26 + 0.73 , where V rot and σ V are the rotation velocity and velocity dispersion, respectively, still consistent with rotation. The resulting disk mass of 0.65 − 0.40 + 1.37 × 10 9 M ⊙ is consistent with being associated with the stellar mass identified with a 300 Myr old stellar population independently indicated by a Balmer break in the spectral energy distribution. We conclude that the most of the dynamical mass is associated with the previously identified mature stellar population that formed at z ∼ 15.