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

TL;DR

This study presents the first morpho-kinematic analysis of a galaxy at redshift 9.1, revealing potential systematic rotation and a mature stellar population formed around redshift 15, using high-resolution ALMA observations.

Contribution

It provides the earliest evidence of a rotating galaxy with a mature stellar population at such a high redshift, demonstrating the capability of ALMA for detailed galaxy dynamics studies in the early universe.

Findings

Detection of smooth [O III] emission with a velocity gradient suggestive of rotation.

Estimated disk mass consistent with a 300 Myr-old stellar population.

Most dynamical mass associated with mature stars formed at z~15.

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-$\mu$m emission is detected at 10$\sigma$ with a spatial resolution of $\sim0.3$ 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 $\Delta V_{\rm obs}/2\sigma_{\rm tot}=0.84\pm0.23$, where $\Delta V_{\rm obs}$ is the observed maximum velocity difference and $\sigma_{\rm tot}$ 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 $V_{\rm rot}/\sigma_V=0.67_{-0.26}^{+0.73}$, where $V_{\rm rot}$ and $\sigma_V$ are the rotation velocity and velocity dispersion, respectively, still consistent with rotation. The resulting disk mass of $0.65_{-0.40}^{+1.37}\times10^{9}$ 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 $z\sim15$.