Shocked molecular hydrogen emission from the centre of the Galaxy

Observations of molecular hydrogen emission lines near 2 µm show that the nucleus of the Galaxy is encircled by a ring of shocked gas; this ring has a radius of 2 parsecs, lies in the plane of the Galaxy, is symmetric about the centre of mass, and rotates in the sense of Galactic rotation. Gas is being shocked at a rate of $$\gt 10^{-2} M_\odot \enspace \text {yr}^{-1}$$ to a temperature about 2000 K, in a region of mean molecular density $$5 \times 10^3 \text {cm}^{-3}$$. The momentum needed to shock the gas cannot be provided radiatively. Mass loss from the nucleus can account naturally for the central density minimum and for the shocked gas; a mass loss rate of $$3 \times 10^{-3} M_\odot \enspace \text {yr}^{-1}$$ is required. Simple time-scale arguments suggest that observable molecular hydrogen emission from the Galactic centre may be a long-lived phenomenon. A model involving a single central engine is suggested.