Reaching for the Stars

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Science  23 Feb 2001:
Vol. 291, Issue 5508, pp. 1449
DOI: 10.1126/science.291.5508.1449d

If you could look at the Milky Way Galaxy (a spiral galaxy) edge-on, you would see a thin disk of luminous stars. The halo would be harder to see because its mass is mostly dark matter, with a few clumps of stars here and there. Although we don't know what dark matter is or how much there is in the Galaxy, we can detect its influence by measuring the spatial distribution and relative velocities of stars. A standard hierarchical galaxy formation model would suggest a lumpiness to halo star distribution; in a pair of papers, Ibata et al. suggest using a newly modeled and observed stream of stars to refine galaxy formation models.

The stream of stars comes from the Sagittarius dwarf galaxy, which is the closest galaxy to the Milky Way. A tail of stars is torn away from the dwarf galaxy as it passes close to the galactic center and is pulled into a great circle path within the Milky Way halo. Ibata et al. numerically simulated multiple galaxy interactions in order to model a stream of stars along a great circle path. Over time, stars move off the path (in part due to interactions with dark matter within the halo), and the simulations show that the amount of movement is dependent on the mass distribution model of the halo. Their results indicate that the halo has a more nearly spherical distribution of dark matter than previous models would suggest. — LR

Astrophys. J., in press (astro-ph/0004011); Astrophys. J.547, L133 (2001).

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