There goes the neighborhood. On this hypothetical world near the centre of the Milky Way, the planet’s sun paints the sky purple as it descends toward the horizon at left. The bright supernova exploding at upper right lends an ominous feeling because its radiation is about to wipe out the alien life on this world. (image: David A. Aguilar, CfA)

Activity at the centres of galaxies is visible across most of the history of the universe: to understand the gravitational, radiative, dynamical, chemical and magnetohydrodynamic processes that occur in such regions we need to start with a clear picture of the centre of our own Galaxy. These processes drive galactic evolution on all scales. The stellar component is largely obscured by dust and can only be studied laboriously, if at all, at infrared wavelengths. Through radio spectroscopy, in particular millimeter-and submillimeter-wave spectral line observations, we can measure the large scale morphology, dynamics and thermodynamic properties of Galactic center gas.

Together with a group at the Harvard-Smithsonian Center for Astrophysics, the inner three square degrees of the Galaxy has been mapped at frequencies between 450 GHz and 810 GHz to trace spectral lines originating from dense atomic and molecular gas. The observations were performed during the austral winter seasons of 2001, 2002 and 2003 at the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) located at 2847m altitude at the Amundsen-Scott South Pole Station. These observations have been combined with other surveys to construct a new picture of gas structures in this interesting region of the Galaxy.

The resulting images show that emission from gas in the Galactic center region is complex, with chaotic, asymmetric, and nonplanar structures making up hundreds of clouds, shells, arcs, rings, and filaments. On scales of a few hundred to a few thousand light years the gas is loosely organized around closed orbits in the rotating potential of the underlying stellar bar. Gas on certain classes of non-intersecting orbits having a density near 3000cm^-3 is rendered marginally unstable against gravitational coagulation into a few giant molecular clouds: indeed some gas is seen to be bound into cloud complexes, while some is sheared by tidal forces into a molecular intercloud medium of a kind not seen elsewhere in the Galaxy. The multimillion-solar-mass clouds are dense, as they must be to survive in the Galactic tide, and are sinking toward the center of the Galactic gravitational well as a result of dynamical friction and hydrodynamic effects. The deposition of these massive lumps of gas upon the centre will fuel a starburst or an eruption of the central black hole. Depending on the accretion rate near the inner resonances, this process cycles with a timescale of order some tens of millions of years.

The next starburst in the Milky Way may occur within the next 10 million years. Some 30 million solar masses of matter will flood inward, overwhelming the black hole at the galactic centre. The black hole will be unable to consume most of the gas, which will instead form millions of new stars, the more massive of which will burn their fuel quickly and explode as supernovae and irradiate the surrounding space. With so many stars packed so close together as a result of the starburst, the impact on the entire Galactic centre will be dramatic enough to kill any life on an Earth-like planet.

Wilfred Walsh