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 $$|l|\le10^\circ,\enspace |b|\le0.5^\circ$$. 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 $$\theta_\text {incl}=16\pm2^\circ$$ 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 $$\rho \propto r^{-1.75}$$ 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 $$r \simeq 3.5$$ kpc is probably associated with the bar’s outer Lindblad resonance, and the region of low gas densities inwards from there with corotation.