Understanding the kinematics of Galactic centre gas

Binney, J., Gerhard, O.E., Stark, A.A., Bally, J., & Uchida, K.I. 1991, MNRAS 252, 210.

A coherent picture is constructed of the Galaxy's H I, CO and CS emissions in the region l below 10 deg, b below 0.5 deg. The flow of gas at the Galactic center is dominated by a bar that has corotation at r = 2.4 + or - 0.5 kpc, which is viewed at an angle of 16 + or - 2 deg from its major axis. The first CO emission arises where gas is obliged to switch from x(1) orbits to x(2) orbits, in the notation of Contopoulos. This gives rise to a shock and a clear signature in the (l, v) diagram. The great Galactic center molecular clouds such as Sgr B, are on x(2) orbits. From the structure of the H I terminal velocity envelope, it is deduced that the central mass density scales as rho varies with r to the -1.75 power out to at least about 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 of about 3.5 kpc is probably associated with the bar's outer Lindblad resonance, and the region of low gas densities inward from there with corotation.