Everything about Dark Matter Halo totally explained
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.
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 (for example dark) matter. Asserting that this dark matter doesn't 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 distribution of mass within the dark matter halo may follow the
Navarro-Frenk-White profile, with density given by
with the constant and "a" chosen by observed mass distributions.
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's 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 doesn't 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 90,000,000,000
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.
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