Dark matter halo
A dark matter halo is a hypothetical component of a galaxy that envelops the galactic disk and extends well beyond the edge of the visible galaxy. The halo's mass dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas in galaxies. Dark matter halos play a key role in current models of galaxy formation and evolution.
Rotation curves as evidence of a dark matter halo
The presence of dark matter in the halo is inferred from its gravitational effect on a spiral galaxy's rotation curve. Without large amounts of mass throughout the (roughly spherical) halo, the rotational velocity of the galaxy would decrease at large distances from the galactic center, just as the orbital speeds of the outer planets decrease with distance from the Sun. 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 flattens out, meaning that rotational velocities do not decrease with distance from the galactic center. This is a very general behaviour, evident in the Universal Rotation Curve of Spirals. The absence of any visible matter to account for these observations implies either that unobserved ("dark") matter exists or that the theory of motion under gravity (General Relativity) is incorrect.
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 visible disk of the Milky Way Galaxy is embedded in a much larger, roughly spherical halo of dark matter. The dark matter density drops off with distance from the galactic center. 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 luminous matter makes up approximately 9 x 1010 solar masses. The dark matter halo is likely to include around 6 x 1011 to 3 x 1012 solar masses of dark matter.
- Galaxy formation and evolution
- Galactic coordinate system
- Disc (galaxy)
- Bulge (astronomy)
- Galactic halo
- Spiral arm
- Dark matter
- Dark galaxy
- Universal Rotation Curve
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