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

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) similar to 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 x 10(10) M-circle dot. This is close to the molecular gas mass derived from previous CO(2-1) observations, and implies a CO to H-2 conversion factor a(CO) < 2.3 M-circle dot (K kms(-1) pc(2))(-1). The mass budget also constrains the stellar mass to <3.1 x 10(10) M-circle dot, and entails a gas fraction of f(gas) greater than or similar to 0.4. The diameter of the dust continuum emission is < 2 kpc, while the star-formation rate is as high as 1000 M-circle dot 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.