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dark matter halo
A dark matter halo is a hypothetical component of a galaxy that envelops the galactic disc 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. 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.
The Navarro-Frenk-White profile:〔Navarro, J. et al. (1997), ( A Universal Density Profile from Hierarchical Clustering )〕
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:〔Merritt, D. ''et al.'' (2006), ( Empirical Models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison with Parametric Models )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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