Mass Distribution and Rotation Curve in the Galaxy

Abstract

The mass distribution in the Galaxy is determined by dynamical and photometric methods. The dynamical method is based on the Virial theorem, and calculates the mass from kinematical data such as rotation velocities, velocity dispersions, and motions of satellite galaxies. Rotation curves are the major tool for determining the dynamical mass distribution in the Milky Way and spiral galaxies. The photometric (statistical) method utilizes luminosity profiles from optical and infrared observations, and assumes empirical values of the mass-to-luminosity (M/L) ratio to convert the luminosity to mass. This method is convenient to separate the mass components such as bulge and disk, while the uncertainty is large due to ambiguous M/L ratio arising from the variety of stellar populations. Also, the methods cannot detect the dark matter that dominates in the outer regions and central black holes.

In this chapter the dynamical method is described in detail, and rotation curves and mass distribution in the Milky Way and nearby spiral galaxies are presented. The dynamical method is further categorized into two methods: the decomposition method and direct method. The former fits the rotation curve by calculated curve assuming several mass components such as a bulge, disk, and halo, and adjusts the dynamical parameters of each component. Explanations are given of the mass profiles as the de Vaucouleurs law, exponential disk, and dark halo profiles inferred from numerical simulations. Another method is the direct method, with which the mass distribution can be directly calculated from the data of rotation velocities without employing any mass models. Some results from both methods are presented, and the Galactic structure is discussed in terms of the mass. Rotation curves and mass distributions in external galaxies are also discussed, and the fundamental mass structures are shown to be universal.