The Angular Momentum of Spiral Galaxies. Methods of Rotation-Curve Analysis

Abstract

The disk method of rotation-curve analysis (Toomre) is more suitable than older "spheroid" methods for computing the angular momentum and mass distribution in spiral galaxies because (a) no fitting of the velocity curve is necessary, (b) the mass surface density, which is analogous to the surface brightness, is computed directly, and (c) less numerical labor is involved. The mass surface density probably drops off exponentially with radius in faint regions where there exists no rotation information, since the brightness distribution there is exponential and the MIL ratio does not vary rapidly (see following paper). The rapid descent of the surface density allows a good estimate of the total angular momentum to be obtained from existing rotation information and photometry for a number of galaxies. Formulae that are suitable for numerical calculations by these methods are presented. Simple mathematical models that represent the luminosity distribution in the spheroidal central bulges of spirals and in elliptical galaxies are developed. They demonstrate that the existence of spheroidal subsystems in spirals does not affect the derived angular momentum, but may be important for the total (turbulent plus rotational) kinetic energy. Subject headings: galaxies, motions in - stellar dynamics