ALMA resolves turbulent, rotating [CII] emission in a young starburst galaxy at z=4.8

TL;DR

This study uses ALMA to spatially resolve [CII] emission in a young, high-redshift starburst galaxy, revealing turbulent, rotating gas dynamics, high star formation rates, and near-solar metallicity, indicating an early, intense star-forming phase.

Contribution

First spatially resolved [CII] observations of a z=4.8 starburst galaxy, revealing detailed gas kinematics, mass estimates, and metallicity insights in an early galaxy.

Findings

Gas kinematics dominated by rotation with high turbulence.

Dynamical mass close to molecular gas mass, constraining CO-to-H2 conversion.

High specific star formation rate (>80 Gyr^{-1}) indicating a nascent starburst.

Abstract

We present spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) [CII] observations of the z=4.7555 submillimetre galaxy, ALESS 73.1. Our 0.5" FWHM map resolves the [CII] emitting gas which is centred close to the active galactic nucleus (AGN). The gas kinematics are dominated by rotation but with high turbulence, v_rot/sigma_int~3.1, and a Toomre Q parameter <1 throughout the disk. By fitting three independent thin rotating disk models to our data, we derive a total dynamical mass of 3+-2x10^10 M_sol. This is close to the molecular gas mass derived from previous CO(2-1) observations, and implies a CO to H_2 conversion factor alpha_CO<2.3M_sol(K km/s/pc^2)^-1. The mass budget also constrains the stellar mass to <3.1x10^10 M_sol, and entails a gas fraction of f_gas>~0.4. The diameter of the dust continuum emission is <2 kpc, while the star-formation rate is as high as 1000 M_sol/yr. Combined with our stellar mass constraint, this implies an extreme specific star formation rate >80 Gyr^{-1}, especially since there are no clear indications of recent merger activity. Finally, our high signal-to-noise [CII] measurement revises the observed [NII]/[CII] ratio, which suggests a close to solar metallicity, unless the [CII] flux contains significant contributions from HII regions. Our observations suggest that ALESS73.1 is a nascent galaxy undergoing its first major burst of star formation, embedded within an unstable but metal-rich gas disk.