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

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/σ_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 ± 2 × 10¹⁰ M_⊙. This is close to the molecular gas mass derived from previous CO(2-1) observations, and implies a CO to H₂ conversion factor α_CO < 2.3 M_⊙ (K km s^-1 pc²)^-1. The mass budget also constrains the stellar mass to <3.1 × 10¹⁰ M_⊙, 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_⊙ yr^-1. 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.