# Rotation in [CII]-emitting gas in two galaxies at a redshift of 6.8

## TL;DR

This study presents the first ALMA spectroscopic confirmation and velocity structure analysis of two galaxies at z > 6.8 using [C II] emission, revealing rotation-dominated, turbulent disks early in cosmic history.

## Contribution

It provides the first measurements of [C II] velocity structures in galaxies during Reionization, demonstrating the potential of ALMA for studying early galaxy dynamics.

## Key findings

- Detected high [C II] luminosity in galaxies at z > 6.8.
- Observed velocity gradients suggest rotation-dominated disks.
- Galaxies have similar stellar-to-dynamical mass ratios as later epoch galaxies.

## Abstract

The earliest galaxies are expected to emerge in the first billion years of the Universe during the Epoch of Reionization. However, both the spectroscopic confirmation of photometrically-selected galaxies at this epoch and the characterization of their early dynamical state has been hindered by the lack of bright, accessible lines to probe the velocity structure of their interstellar medium. We present the first ALMA spectroscopic confirmation of such sources at z > 6 using the far-infrared [C II]{\lambda}157.74{\mu}m emission line, and, for the first time, measurement of the velocity structure, for two galaxies at z = 6.8540+/-0.0003 and z = 6.8076+/-0.0002. Remarkably, the [C II] line luminosity from these galaxies is higher than previously found in `normal' star-forming galaxies at z > 6.5. This suggests that we are sampling a part of the galaxy population different from the galaxies found through detection of the Ly{\alpha} line. The luminous and extended [C II] detections reveal clear velocity gradients that, if interpreted as rotation, would suggest these galaxies have turbulent, yet rotation-dominated disks, with similar stellar-to-dynamical mass fractions as observed for H{\alpha} emitting galaxies 2 Gyr later at cosmic noon. Our novel approach for confirming galaxies during Reionization paves the way for larger studies of distant galaxies with spectroscopic redshifts from ALMA. Particularly important, this opens up opportunities for high angular-resolution [C II] dynamics in galaxies less than one billion years after the Big Bang.