Unseen matter and angular momentum in the Milky Way and other galaxies: Simple two-component models

Abstract

Simple two-component (dark+bright) models are built up for the Milky Way, where both the density distribution and the rotation curve are deduced from known observations. The derived dark to bright mass ratio μ turns out to be in the range μ≳10, in close agreement with the results of more refined approaches, with a weak dependence on the geometry of the model. The related angular momentum appears to be well in agreement with theoretical predictions, if proto-galaxies gain angular momentum via either gravitational interactions or peculiar velocities of their own sub-units, according to a logarithmic distribution of the squared fractional angular momenta close to a Maxwellian one. The rougher assumption that the whole system is represented by a rigidly rotating polytrope leads to dark components rounder than ɛ D ≈0.7 if proto-galaxies gain angular momentum via gravitational interactions, and to much more flattened dark components if proto-galaxies gain angular momentum via peculiar velocities of their own sub-units and few (∼4) sub-units are present at the beginning. To fit the observed positions of several galaxies on the (Oe B q B ) plane-e B representing the ellipticity andq B close to the ratio of maximum rotational to central peculiar velocity, averaged for all the inclinations to the line of sight — galaxies are modelled by two-component (dark+bright) rigidly rotating, concentric, co-polar, homogeneous spheroids and the Galaxy is assumed to be a typical system. An acceptable fit is produced only under the assumption that protogalaxies gain their angular momentum in late stages of evolution, i.e., after having decoupled from the Hubble flow.