Kinematics of the Milky Way halo Our Milky Way is a typical spiral galaxy containing a rapidly rotating disk of stars; like all known spirals, measurements of its rotation shows it to be much heavier than it appears. It seems to be embedded in very large amounts of so-called "dark matter". About 99 percent of the Galaxy's stars belong to the disk and bulge of the Galaxy (see figure at right), with the rest forming an extended, faint and roughly spherical halo in which the disk is embedded. The stars in this halo are on orbits which take them to all corners of the Galaxy --- and in this sense they are messengers about the amount of dark matter in the Galaxy, as their orbits are dominated by the gravity of the dark matter, rather than the paltry amounts of visible matter. Exactly how halo stars move has been investigated from a new large sample of observations by Chris Thom, as part of his PhD thesis. Chris is supervised by Chris Flynn at Tuorla Observatory and Brad Gibson at Swinburne University of Technology in Melbourne, Australia. About 2000 halo stars have been put together by the researchers in various directions on the sky, mainly gazing outward from the center of the Galaxy, using telescopes in Australia (UK Schmidt) as well as published samples from the Sloan Digital Sky Survey. The speeds relative to the Sun were measured, and compared to a model of how such stars have been thought to be moving in the Galaxy's dark matter. The model turned out to fit the data quite well, but to the researchers' surprise, this wasn't the only model that fit. It turned out that it was quite difficult to distinguish between their physical model, which contained a few parameters, and an exceedingly simple model in which the velocities can be fit by a single parameter only. The researchers had been expecting to be able to rule out such a simple model with confidence, with access to several thousand distant halo stars. It isn't so easy to say whence and wither the halo stars! The models are much easier to distinguish from each other for halo stars very close to the Sun -- in this case, only the simple model clearly does not fit. This suggests a test -- we should be looking at distant halo stars where the difference between the two models is greatest -- and this turns out to be in the inner parts of the Galaxy, toward the central bulge. The team is planning to follow up their work by gazing inward instead! The research has been published in the Monthly Notices of the Royal Astronomical Society. Thom et al, MNRAS, 360, 354 (2005)

The DIRBE view of our Milky Way Galaxy showing the disk (seen edge on) and the central bulge. The Sun is located some 30,000 light years from the central region, so that from our vantage point on the Earth we get a spectacular view of our own galaxy as we 'look in'. Halo stars, which were studied in this project, form a very faint roughly spherical distribution around these two basic components. They are so faint as to be essentially invisible on this map. Comparison between data (dots with error bars) and the two models. We compare the velocity dispersion of the stars in various places on the sky, and at various distances along the line of sight. Our physical model of the halo velocity dispersion is shown by the solid line for these fields, and the simple, isothermal model, as a dashed line. Distinguishing between these models by using distant halo stars only, is quite difficult!