Vertical structure and dynamics of a galactic disk

Abstract

Most of the visible mass in a typical spiral galaxy is distributed in a thin disk, with a radial extent much larger than its thickness. While the planar disk structure, including non-axisymmetric features such as spiral structure, has been studied extensively, the vertical structure normal to the disk plane has not received comparable attention. This review aims to give a comprehensive, pedagogic introduction to the rich topic of vertical structure of a galactic disk in hydrostatic equilibrium, and discuss the theoretical developments in this field in the context of recent observations. A realistic multi-component disk plus halo model of a galaxy has been developed and studied by us in detail. This takes account of both stars and interstellar gas, treated as isothermal components with different velocity dispersions, which are gravitationally coupled; further, the disk is in the gravitational field of the dark matter halo. This review focuses on this model and the results from it in different physical cases.

The gas and halo crucially affect the resulting self-consistent stellar distribution such that it is vertically constrained to be closer to the mid-plane, and has a steeper profile than in the standard one-component case, in agreement with modern observations. The scale height of a typical stellar disk increases with galactocentric radius; that is, the disk flares, by a factor of few within the visible radial extent of the disk. These robust results question the sech${}^2$ profile and a constant scale height, routinely used in the literature for convenience. In an important application, the observed HI gas scale height is used as a constraint on the model which helps determine the shape and the density profile of the dark matter halo for galaxies. Finally, we outline some key, open questions which can be addressed in the near future using the above model, and new observational data — for example, from the IFU surveys and JWST. These promise to give a better understanding of this topic.