A cool accretion disk around the Galactic Centre black hole

There is a supermassive black hole of mass 4 × 10⁶ solar masses at the centre of the Milky Way^1,2. A large reservoir of hot (10⁷ kelvin) and cooler (10² to 10⁴ kelvin) gas surrounds it within a few parsecs³. Although constraints on the amount of hot gas in the accretion zone of the black hole-that is, within 10⁵ Schwarzschild radii (0.04 parsecs)-have been provided by X-ray observations^4-6, the mass in cooler gas has been unconstrained. One possible way this cooler gas could be detected is by its emission in hydrogen recombination spectral lines^7,8. Here we report imaging of a 10⁴-kelvin ionized gas disk within 2 × 10⁴ Schwarzschild radii, using the 1.3-millimetre recombination line H30α. This emission line is double-peaked, with full velocity linewidth of about 2,200 kilometres per second. The emission is centred on the radio source Sagittarius A*, but the redshifted side is displaced 0.11 arcsec (0.004 parsecs at a distance of 8 kiloparsecs) to the northeast and the blueshifted side is displaced a similar distance to the southwest. We interpret these observations in terms of a rotating disk of mass 10^-5 to 10^-4 solar masses and mean hydrogen density of about 10⁵ to 10⁶ per cubic centimetre, with the values being sensitive to the assumed geometry. The emission is stronger than expected, given the upper limit on the strength of the Brγ spectral line of hydrogen. We suggest that the H30α transition is enhanced by maser emission.