# Dark matter halo

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The dark matter halo is the hypothetical gravitational core of a galaxy, consisting of dark matter.

## Rotation curves as evidence of a dark matter haloEdit

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, and while that could be possible, most scientists would require extensive amounts of compelling evidence before considering it.

$\rho(r)=\frac{\rm constant}{(r/a)(1+r/a)^2}$

is often used to model the distribution of mass in dark matter halos. Theoretical dark matter halos produced in computer simulations are best described by the Einasto profile:[5]

$\rho(r) = \rho_0 e^{-\alpha r^n}.$

## Theories about the nature of dark matterEdit

The nature of dark matter in the galactic halo of spiral galaxies is still undetermined, but there are two popular theories: either the halo is composed of weakly-interacting elementary particles known as WIMPs, or it is home to large numbers of small, dark bodies known as MACHOs. It seems unlikely that the halo is composed of large quantities of gas and dust, because both ought to be detectable through observations. Searches for gravitational microlensing events in the halo of the Milky Way show that the number of MACHOs is likely not sufficient to account for the required mass.

## Milky Way dark matter haloEdit

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 any way 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 90,000,000,000 solar 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.[6]

## References Edit

1. Peter Schneider (2006). Extragalactic Astronomy and Cosmology, Springer. p. 4, Figure 1.4. ISBN 3540331743.
2. Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe, Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879.
3. Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology, Cambridge University Press. p. 21; Figure 1.13. ISBN 0521546230.
4. Navarro, J. et al. (1997), A Universal Density Profile from Hierarchical Clustering