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The bulk of the stars in a spiral galaxy are located either close to a single plane (the Galactic plane) in more or less conventional circular orbits around the center of the galaxy (the galactic core ), or in a spheroidal galactic bulge around the galactic core. However, some stars inhabit a spheroidal halo surrounding the galaxy. The orbital behaviour of these stars is as yet disputed, but they may describe retrograde and/or highly inclined orbits, or not to move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Elliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.
Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metal content than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.
The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the centre of the galaxy—if they do so at all—these stars often display unusually high proper motion.
Halo dark matter
Beyond the visible, inner portion of the galactic halo lies a much larger region, known as the dark halo, extended halo or galactic corona, which contains large amounts of dark matter.
The presence of dark matter in the halo is demonstrated by its gravitational effect on a spiral galaxy's rotation curve. Without large amounts of mass in the extended halo, the rotational velocity of the galaxy should decrease at large distance from the galactic core. However, observations of spiral galaxies, particularly radio observations of line emission from neutral atomic hydrogen (known, in astronomical parlance, as HI), show that the rotation curve of most spiral galaxies remains flat far beyond the visible matter. The absence of any visible matter to account for these observations implies the presence of unobserved (i.e. dark) matter.
The nature of dark matter in the galactic halo of spiral galaxies is still undetermined, but the most popular theories are that the dark halo is home to vast numbers of small bodies known as MACHOs and/or weakly-interacting particles known as WIMPs. Observations of the halo of the Milky Way show that the number of MACHOs is not likely to be sufficient to account for the required mass.
- Diemand, J. & Moore, B. & Stadel, J. (2005, January 27). Earth-mass dark-matter haloes as the first structures in the early universe. In Nature, 433, 389 – 391.
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