Galaxy rotation curve

Rotation curve of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line).[1]
Left: A simulated galaxy without dark matter. Right: Galaxy with a flat rotation curve that would be expected with dark matter.

The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy. Theories involving dark matter are the main postulated solutions to account for the variance.[2]

The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. In contrast, the orbital velocities of planets in planetary systems and moons orbiting planets decline with distance according to Kepler’s third law. This reflects the mass distributions within those systems. The mass estimations for galaxies based on the light they emit are far too low to explain the velocity observations.[3]

The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material. When mass profiles of galaxies are calculated from the distribution of stars in spirals and mass-to-light ratios in the stellar disks, they do not match with the masses derived from the observed rotation curves and the law of gravity. A solution to this conundrum is to hypothesize the existence of dark matter and to assume its distribution from the galaxy's center out to its halo. Thus the discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.[4]

Though dark matter is by far the most accepted explanation of the rotation problem, other proposals have been offered with varying degrees of success. Of the possible alternatives, one of the most notable is modified Newtonian dynamics (MOND), which involves modifying the laws of gravity.[5]

  1. ^ Corbelli, E.; Salucci, P. (2000-01-15). "The extended rotation curve and the dark matter halo of M33". Monthly Notices of the Royal Astronomical Society. 311 (2): 441–447. arXiv:astro-ph/9909252. Bibcode:2000MNRAS.311..441C. doi:10.1046/j.1365-8711.2000.03075.x. ISSN 0035-8711.
  2. ^ Hammond, Richard (May 1, 2008). The Unknown Universe: The Origin of the Universe, Quantum Gravity, Wormholes, and Other Things Science Still Can't Explain. Franklin Lakes, NJ: Career Press.
  3. ^ Bosma, A. (1978). The Distribution and Kinematics of Neutral Hydrogen in Spiral Galaxies of Various Morphological Types (PhD). Rijksuniversiteit Groningen. Retrieved December 30, 2016 – via NASA/IPAC Extragalactic Database.
  4. ^ Wechsler, Risa H.; Tinker, Jeremy L. (2018-09-14). "The Connection Between Galaxies and Their Dark Matter Halos". Annual Review of Astronomy and Astrophysics. 56 (1): 435–487. arXiv:1804.03097. doi:10.1146/annurev-astro-081817-051756. ISSN 0066-4146.
  5. ^ For an extensive discussion of the data and its fit to MOND see Milgrom, M. (2007). "The MOND Paradigm". arXiv:0801.3133 [astro-ph].