Galaxy Rotation Curve
The rotation curve of a disc galaxy (also called a velocity curve) is dependence of the rotational velocity of the visible stars or gas in the galaxy on the radial distance of the stars or gas from the center of the galaxy. This relationship can be summarized by a plot of the orbital speed (in km/s) of the stars or gas in the galaxy on the ordinate against the distance from the center of the galaxy on the abscissa.
A general feature of the galaxy rotation curves that have been measured is that the rotational velocity of the stars and gas are constant as far out as they can be measured (line B in illustration), i.e. stars are observed to revolve around the centre of these galaxies at a constant speed over a large range of distances from the centre of the galaxy. If disc galaxies had mass distributions which were similar to the observed distribution of stars and gas, the rotation curves velocities should decline at large distances (dotted line A in illustration), in the same way as other systems with most of their mass in the centre, such as the Solar System of planets or the moons of Jupiter, following the prediction of Kepler's Laws. It is also observed that galaxies with a uniform distribution of luminous matter have a rotation curves that slope up from center to edge and most low surface brightness galaxies (LSB galaxies) rotate with a rotation curve that slopes up from the center, indicating little core bulge.
The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the Keplerian prediction assuming a centrally-dominated mass associated with the observed luminous material. If masses of galaxies are derived solely from the luminosities and the mass-to-light ratios in the disk and core portions of spiral galaxies are assumed to be close to that of stars, the masses derived from the kinematics of the observed rotation and the law of gravity do not match. This discrepancy can be accounted for by a large amount of dark matter that permeates the galaxy and extends into the galaxy's halo.
Though dark matter is by far the most accepted explanation for the resolution to the galaxy rotation problem, other proposals have been offered with varying degrees of success. Of the possible alternatives, the most notable is Modified Newtonian Dynamics (MOND), which involves modifying the laws of gravity.
Read more about Galaxy Rotation Curve: History and Description of The Problem, Halo Density Profiles, Further Investigations, Alternatives To Dark Matter
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