The motion and distribution of the gas in the central 300 pc of the galaxy as revealed by spectra of H.

Spectroscopy of absorption lines of H in the central molecular zone (CMZ) of the Galaxy show that a previously largely unknown component of the interstellar medium there, warm (T∼200 K) and diffuse (n ≲ 10 cm) gas, makes up a large fraction of the volume of the CMZ, and that this gas is moving radially outward from the centre. These discoveries upend the generally accepted understanding that the interstellar environment of the CMZ comprises almost entirely an ultra-hot plasma and dense molecular clouds. The radial momentum associated with the diffuse gas in the CMZ exceeds that of the ejecta of thousands of core-collapse supernovae and implies some extraordinary past activity in the centre, possibly associated with the supermassive black hole, Sgr A*. We speculate that within approximately 10 years, gravity could halt the expansion of the diffuse gas and that contraction towards the centre could then commence. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H, H and beyond'.