Evidence for a Bar at the Centre of the Milky Way

Abstract

Bars profoundly affect the flow of interstellar gas in many galaxies. Grid-based hydrodynamic studies of gas flow in rotating barred potentials indicate that the bar’s pattern speed and degree of central condensation are its two most important characteristics. A bar at the galactic centre provides an attractive explanation for the occurrence of non-circular velocities in gas in the inner few degrees of our Galaxy. Recent studies imply that the Galaxy’s bar is more rapidly rotating than had been previously suggested — corotation probably lies near R = 2.4kpc — and that the circular speed associated with the Galaxy’s monopole component rises as υ_c ~ R ^1/8 in the inner kiloparsec or so. Experiments with two radically different sticky-particle schemes give results that are broadly similar to each other and to those obtained with grid-based schemes. They suggest that the gas at the centre has mostly been shed from bulge stars. Despite the successes of the bar model, some important discrepancies between theory and observation remain.