Minimum Velocity Dispersion in Stable Stellar Disks. Numerical Simulations

Abstract

N-body dynamical simulations are used to analyze the conditions for the gravitational stability of a three-dimensional stellar disk in the gravitational field of two rigid spherical components--a bulge and a halo whose central concentrations and relative masses vary over wide ranges. The number of point masses N in the simulations varies from 40 to 500 thousands and the evolution of the simulated models is followed over 10--20 rotation periods of the outer edge of the disk. The initially unstable disks are heated and, as a rule, reach a quasi-stationary equilibrium with a steady-state radial-velocity dispersion $c_r$ over five to eight periods of rotation. The radial behavior of the Toomre stability parameter $Q_T (r)$ for the final state of the disk is estimated. Numerical models are used to analyze the dependence of the gravitational stability of the disk on the relative masses of the spherical components, disk thickness, degree of differential rotation, and initial state of the disk. Formal application of existing, analytical, local criteria for marginal stability of the disk can lead to errors in radial velocity dispersion $c_r$ of more than a factor of 1.5. It is suggested that the approximate constancy of $Q_T \simeq 1.2 -- 1.5$ for $r\simeq (1÷2)\times L$ (where L is the radial scale of disk surface density), valid for a wide range of models, can be used to estimate upper limits for the mass and density of a disk based on the observed distributions of the rotational velocity of the gaseous component and of the stellar velocity dispersion.