Modelling the Galaxy

Most of the light in the Universe comes from stars located in galaxies like that in which we live. In this sense our own Galaxy is typical of the galaxies that dominate the universe, and it would be a large step forward in our understanding of the Universe as a whole if we had a complete knowledge how the Galaxy is structured, and how it came to be the way it is. Dynamical models of the Galaxy are the key to gaining this knowledge. It is helpful to imagine that the Galaxy is made up of components. The dominant component at the location of the Sun is the stellar disk, of which the Sun is a tiny part. Further in the Galaxy is dominated by the bulge/bar, while further out the dominant component is believed to be the dark halo, about which we know very little. The dark halo is especially problematic for a Galaxy modeller because it has so far only been detected through its gravitational field. It is thought to be made up of a kind of elementary particle that has never been detected on Earth. The properties of these particles can only be speculated about, so it is hard to design an experiment to detect them. Perhaps for this reason, efforts to detect the particles that make up the Galaxy's dark halo as they stream through the Earth have so far been unsuccessful - but they could bear fruit any day! A major goal of a Galaxy modeller is to map the gravitational field of the dark halo. (The mass distribution of the dark halo can be inferred from the gravitational field that it generates.) This has to be done by first mapping the gravitational field of the whole Galaxy, and then estimating the gravitational fields of the visible components, principally the bulge/bar and the disk. The difference between the gravitational field of the whole Galaxy and the combined fields of the visible components yields the field of the dark halo. The gravitational field has to be measured by modelling the dynamics of objects that move in the field. This is most easily done in the case of gas clouds, which may be assumed to move on closed orbits. The simplest closed orbits are circles in an axisymmetric potential, and our knowledge of the Galactic potential has mostly been obtained from observations of gas clouds on the assumption that they move on circular orbits in the Galactic plane. Notable features of the data for the Galaxy's interstellar gas are incompatible with the hypothesis that clouds move on circular orbits. These features are neatly explained if the Galaxy interior to about 4 kpc is dominated by a rotating bar (Binney et al 1991), and over the last ten years a sustained effort has been made to interpret observations of interstellar gas on the assumption that clouds move on closed orbits in the rotating non-axisymmetric field of the bar.