2014A&A...565A..59D

We present spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) [CII] observations of the z=4.7555 submillimetre galaxy, ALESS73.1. 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±2x10¹⁰M_☉. 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<2.3M_☉/(K*km/s*pc²). The mass budget also constrains the stellar mass to <3.1x10¹⁰M_☉, and entails a gas fraction of f_gas≥0.4. The diameter of the dust continuum emission is <2kpc, while the star-formation rate is as high as 1000M_☉/yr. Combined with our stellar mass constraint, this implies an extreme specific star formation rate >80G/yr, 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.