Dark matter halo

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Most of the mass of any galaxy is dominated by a component concentrated at the centre of the galaxy but dominating its dynamics throughout, known as the dark matter halo.

1 Rotation Curves as evidence of a dark matter halo
2 Theories about the nature of dark matter
3 Milky Way dark matter halo
4 See also
5 Notes
6 Resources
7 External links

[edit] Rotation Curves as evidence of a dark matter halo

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. Asserting that this dark matter does not exist would mean that the accepted theory of gravitation (General Relativity) is wrong, something that could be possible but most scientists would require extensive amounts of compelling evidence before considering it.

The general form of the density of the dark matter halo is:

Failed to parse (unknown error): ρ(r)=(constant)/((r/a)(1+r/a)^2)

with the constant and "a" chosen by observed mass distributions.

[edit] Theories about the nature of 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. It seems unlikely that the halo is composed of large quantities of gas and dust, because these are both detectable through observations. Observations of the halo of the Milky Way, in search of gravitational microlensing events, show that the number of MACHOs is not likely to be sufficient to account for the required mass.

[edit] Milky Way dark matter halo

The dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center. It is also the most mysterious part of the Galaxy. It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in anyway except through gravity. The dark matter halo is the location of nearly all of the Galaxy's dark matter, which is more than ten times as much mass as all of the visible stars, gas, and dust in the rest of the Galaxy. The luminous matter makes up approximately 90,000,000,000solar masses. The dark matter halo is likely to include around 600,000,000,000 to 3,000,000,000,000 solar masses of dark matter.^[1]