Understanding the kinematics of Galactic Centre gas

A coherent picture is constructed of the Galaxy’s H I, CO and CS emissions in the region ||l|10^, |b|0. 5^|⁠. 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 |_ incl=162^| 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 | r^-1. 75| out to at least ∼ 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 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.