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

Abstract

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-$\hat{I}¼$m emission is detected at 10$\ddot{I}ƒ$ with a spatial resolution of $\sim 0.3$ kpc in the source plane, enabling the most distant morpho-kinematic study of a galaxy. The [O III] emission is distributed smoothly without any resolved clumps and shows a clear velocity gradient with $\hat{I}”V_{\mathrm{obs}}/2\ddot{I}{ƒ}_{\mathrm{tot}}=0.84\pm 0.23$, where $\hat{I}”V_{\mathrm{obs}}$ is the observed maximum velocity difference and $\ddot{I}{ƒ}_{\mathrm{tot}}$ 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 $V_{\mathrm{rot}}/\ddot{I}{ƒ}_V=0.67_{-0.26}^{+0.73}$, where $V_{\mathrm{rot}}$ and $\ddot{I}{ƒ}_V$ are the rotation velocity and velocity dispersion, respectively, still consistent with rotation. The resulting disk mass of $0.65_{-0.40}^{+1.37}\times 10^9$ M${}_{\odot }$ 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 $z\sim 15$.

ApJL accepted