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 > 10-2 Msub(solar masses) yr-1, to a temperature about 2000 K, in a region of mean molecular density 5 x 103 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. Simple time-scale arguments suggest that observable molecular hydrogen emission from the Galactic centre may be a long-lived phenomenon. (author)