A Model for the Galaxy with Rising Rotational Velocity

Abstract

Constructing mass models for our Milky Way is an old and honorable tradition in astronomy. Many of the scientists who have performed this exercise have attended this conference and would, I expect, agree with my assessment of the pitfalls. One begins with three kinds of information: a) certain fragmentary and biased bits of knowledge concerning the gravitational potential (e.g., interior rotation curve or local escape velocity); b) certain, directly determined kinematic facts (e.g., R₀, the distance to the galactic center); c) “direct” determinations of density in various components (e.g., from star counts of disc stars) or estimations of the density from measured values of light density in a given component multiplied by assumed “reasonable” values of the mass-to-light ratio. One adds to this a theoretical model for the galaxy which divides it into a (hopefully) small number of distinct components each defined by a few parameters. Then, by comparing the theoretical model with the observational data, one determines the free model parameters — i.e., the galactic mass model. The comparison requires two judgemental matters in addition to the straightforward use of Poisson’s equation. First, kinematic assumptions are required such as: the galaxy (is)/(is not) axisymmetric; one should (average over)/(take envelope of) rotation curve fluctuations; the observed cutoff of high velocity stars (is)/(is not) due to escape from the galaxy.