Evolution of the velocity distribution in galactic disks

Abstract

This paper presents a numerical investigation of the evolution of the three-dimensional velocity distribution and of the vertical density distribution in galactic disks. The physical mechanism is the scattering of stars off giant molecular clouds (GMCs) which heat up the stellar populations. A population of GMCs is embedded in the fixed potential of a constant scale height exponential disk and a nearly isothermal halo. The stars orbit in the fixed axisymmetric potential and are scattered by the GMCs which are modeled as soft particles. The equations of motion are integrated directly. Initially, the three velocity dispersions each grow as sigma(t)proportional(1+(t/tau))/sup 0.38/. Asymptotically the velocity dispersions in the plane grow as the quarter power of time while the vertical dispersion grows as sigma/sub z/(t)proportionalt/sup 0.31/. The velocity ellipsoid grows slowly rounder with time. Typically it has an axial ratio of about 0.6. The functional form and growth rate for the velocity dispersions are in fairly good agreement with published theoretical predictions based on the Fokker-Planck equation. The numerical growth rates are sufficient to explain the velocity dispersions of young stellar populations in the solar neighborhood. It is, however, necessary to postulate higher masses or larger numbers of GMCs at earlier timesmore » in order to explain the velocity dispersion of the hottest disk populations. The velocity distribution is vertically isothermal and Gaussian. The vertical density distribution is also Gaussian.« less