Galactic Rotation and the Constants Specifying the Kinematic and Linear Scales of the Milky Way; Line-broadening Mechanisms Determining Profile Shapes at Low Latitudes

Abstract

Global distances in the Milky Way are commonly based on the acceptance of a galactic rotation curve as a known, well-behaved function. The determination of the rotation curve, Θ(R), is discussed in the context of the assumption of simple, circular motion. Also discussed is the observational background of the two galactic constants, R ₀ and Θ ₀, which determine the kinematic and linear scales entering morphological investigations. The assumption of circular rotation is known to be wrong, but only at the level of about two percent of the rotation motions. Kinematic irregularities can be separated into several distinct categories, including intrinsic broadening within a single feature; random motions of individual clouds with respect to each other; systematic flows, for example along spiral arms; and large-scale distortions from circularity of the Galaxy as a whole. The perceived kinematics of the interstellar medium dominate (for example over the effects of density variations) the appearance of low-latitude tracers of galactic morphology, through the behavior of the velocity-crowding parameter |dv/dr|. The dominance of this parameter, in the known presence of substantial kinematic activity, frustrates many mapping ambitions in our own system.