Kinematics and Distribution of Neutral Interstellar Gas in the Galactic Bulge Region

Abstract

The distribution and motions of the neutral gas layer in the inner few kpc of the Galaxy, corresponding roughly to the radial extent encompassing the bulge, show properties which distinguish the gas layer in this region from that at larger radii. The inner region is pervaded by a combination of circular and noncircular motions of approximately the same amplitude. This combination is recognized most easily near b = 0° when adequate account is taken of contamination of the HI data by absorption against continuum radiation associated with the Sagittarius source complex in the galactic core. The abrupt kinematic shift in the HI velocities centered on l = 0° is a consequence of distortion of the HI pattern by effects of absorption against sources of continuum radiation confined to the galactic nucleus. This is demonstrated by considering tracers largely unaffected by absorption, like CO, and by considering tracers which follow the absorption directly, like OH and H2CO. The envelope of HI emission around the galactic core is thus not a reliable guide to the run of rotation velocity within the bulge. The noncircular component disqualifies use of the terminal-velocity method to derive the circular-velocity rotation curve for gas lying within the bulge region. The inner-Galaxy gas layer is both tilted, or warped, and flared. The fundamental plane of kinematic symmetry is tilted some 20° with respect to the plane defined by b = 0°. This tilt is revealed by the atomic and by the molecular gas, as well as by the distribution of infrared emission from stars. Lines of sight through the tilted and flared gas layer sample a much more complicated structural and kinematic geometry than prevails in the Galaxy at large. This complicated sampling geometry can, however, be held responsible for a number of characteristics of the observations, including the large line widths observed for the spectral-line tracers and the isolated, but only apparently anomalous, structures occurring in position, velocity maps of the region.