H II regions, infrared dark molecular clouds and the local geometry of the Milky Way's nuclear star-forming ring

Abstract

To interpret the galactic center H II region complexes as constituents of a barred galaxy's nuclear star-forming ring, we compare 18cm VLA radiocontinuumm, $8-22\mu$ MSX IR and 2.6mm BTL and ARO12m CO emission in the inner few hundred pc. Galactic center H II regions are comparable in their IR appearance, luminosity and SED to M17 or N!0, but the IR light distribution is strongly modified by extinction at 8-22$\mu$, locally and overall. In Sgr B2 at $l > 0.6$\degr strong radio H II regions are invisible in the IR. In two favorable cases, extinction from individual galactic center molecular clouds is shown to have $\tau \ga 1$ at 8-22$\mu$ independent of wavelength. The gas kinematics are mostly rotational but with systematic $\pm 30-50$ \kms non-circular motion. Sgr B and C both show the same shell and high-velocity cap structure. The H II regions lie in a slightly-inclined ring of radius $\approx$ 180 pc (1.2\degr) whose near side appears at higher latitude and lower velocity and contains Sgr B. Sgr C is on the far side and both Sgr B and C represent collisions with material inflowing along the galactic dust lanes. Sgr E is a coincidental aggregation of field objects seen tangent to the ring's outer edge. Most of the volume interior to the ring is probably devoid of dense gas and some emission seen at v=20-70 \kms toward Sgr A lies outside it, in the ring.