Kinematics of galactic spiral structure

An understanding of the states of motion of the stellar and gaseous components of spiral structure is fundamental to the problem of the origin and maintenance of spiral structure. It is important to determine the relative ordering of the parameters characterizing the several components of a spiral arm. In our own Galaxy in many cases, this can better be done in kinematic terms rather than spatially, since for the stars the results are then less sensitive to assumptions concerning the intervening absorption, while for the gas the results are less dependent on distance derivations. The data are becoming complete enough that, with appropriate analysis procedures, quantitative measures are available of the differential effects across spiral arms. Our understanding of a few of the various problems is less tentative for external galaxies than for our own. A substantial amount of the observational material can be satisfactorily interpreted in terms of the gravitational theory of density-waves. Furthermore, it is just this kinematic information which is most crucial for the observational confrontation of this theory.