UNDERSTANDING THE KINEMATICS OF GALACTIC-CENTER GAS

A coherent picture is constructed of the Galaxy's H I, CO and CS emissions in the region \l\less-than-or-equal-to 10-degrees, \b\less-than-or-equal-to 0.5-degrees. The flow of gas at the Galactic Centre is dominated by a bar that has corotation at r = 2.4 +/- 0.5 kpc, which we view at an angle of theta-incl = 16 +/- 2-degrees from its major axis. The first CO emission arises where gas is obliged to switch from x1 orbits to x2 orbits in the notation of Contopoulos. This gives rise to a shock and a clear signature in the (l, v) diagram. The great Galactic Centre molecular clouds such as Sgr B are on x2 orbits. From the structure of the H I terminal velocity envelope we deduce that the central mass density scales as rho is-proportional-to r-1.75 out to at least approximately 1.2 kpc along the bar's major axis. Consequently the circular velocity curve is rising significantly through the radius range where naive analysis of the tangent velocity leads to a falling rotation curve. The great ring of molecular material at r congruent-to 3.5 kpc is probably associated with the bar's outer Lindblad resonance, and the region of low gas densities inwards from there with corotation.