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

The Navarro-Frenk-White profile:^[4]

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

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 matter

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 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 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

^ Peter Schneider (2006). Extragalactic Astronomy and Cosmology. Springer. p. 4, Figure 1.4. ISBN 3540331743.
^ Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879.
^ 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.{{cite book}}: CS1 maint: multiple names: authors list (link)
^ Navarro, J. et al. (1997), A Universal Density Profile from Hierarchical Clustering
^ Merritt, D. et al. (2006), Empirical Models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison with Parametric Models
^ Battaglia et al. (2005, The radial velocity dispersion profile of the Galactic halo: constraining the density profile of the dark halo of the Milky Way

External links

Rare Blob Unveiled: Evidence For Hydrogen Gas Falling Onto A Dark Matter Clump? European Southern Observatory (ScienceDaily) July 3, 2006

This galaxy-related article is a stub. You can help Wikipedia by expanding it.

[Schneider-1] Peter Schneider (2006). Extragalactic Astronomy and Cosmology. Springer. p. 4, Figure 1.4. ISBN 3540331743.

[Koupelis-2] Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879.

[Jones-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.{{cite book}}: CS1 maint: multiple names: authors list (link)

[4] Navarro, J. et al. (1997), A Universal Density Profile from Hierarchical Clustering

[5] Merritt, D. et al. (2006), Empirical Models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison with Parametric Models

[6] Battaglia et al. (2005, The radial velocity dispersion profile of the Galactic halo: constraining the density profile of the dark halo of the Milky Way

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@@ Line 1: / Line 1: @@
 [[Image:Dark matter halo.png|thumb|240px|Simulated dark matter halo from a [[cosmology|cosmological]] [[N-body simulation]]]]
+The '''dark matter halo''' is the [[hypothetical]] gravitational core of a galaxy, consisting of [[dark matter]].
-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'''.
 [[File:Rotation curve (Milky Way).JPG |thumb |300px |[[Galaxy rotation curve]] for the Milky Way. Vertical axis is speed of rotation about the galactic center. Horizontal axis is distance from the galactic center. The sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to [[dark matter]] or perhaps a modification of the [[MOND|law of gravity]].<ref name=Schneider>{{cite book |title=Extragalactic Astronomy and Cosmology |url=http://books.google.com/books?id=uP1Hz-6sHaMC&pg=PA100&dq=rotation+Milky+way&lr=&as_brr=0&as_pt=ALLTYPES#PPA5,M1 |author=Peter Schneider |publisher=Springer |isbn=3540331743 |year=2006 |page= 4, Figure 1.4}}</ref><ref name=Koupelis>{{cite book |title=In Quest of the Universe |author=Theo Koupelis, Karl F Kuhn |page=492; Figure 16-13  |url=http://books.google.com/books?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Way+%22rotation+curve%22&lr=&as_brr=0&as_pt=ALLTYPES#PPA492,M1 |isbn=0763743879 |year=2007 |publisher=Jones & Bartlett Publishers}}</ref><ref name=Jones>{{cite book |title=An Introduction to Galaxies and Cosmology |author=Mark H. Jones, Robert J. Lambourne, David John Adams |page = 21; Figure 1.13 |url=http://books.google.com/books?id=36K1PfetZegC&pg=PA20&dq=Milky+Way+%22rotation+curve%22&lr=&as_brr=0&as_pt=ALLTYPES#PPA21,M1 |isbn=0521546230 |year=2004 |publisher=Cambridge University Press}}</ref>  ]]