roteringscurve

Roteringscurve, commonly referred to in English as a rotation curve, is a plot that shows how the orbital speed of material in a rotating system varies with distance from the rotation axis. In astronomy, rotation curves are most often used for disk galaxies, where they map the circular velocity of stars and gas as a function of radius from the galactic center. The curve provides a diagnostic of how mass is distributed within the system.

Rotation curves are obtained from Doppler measurements of spectral lines. In galaxies, neutral hydrogen (HI) emission at 21 cm and optical emission lines like H-alpha are used to trace rotation at different radii. Correcting for inclination, distance, and noncircular motions is essential for accurate curves. Early expectations, based on visible matter alone, predicted a Keplerian decline in velocity at large radii. However, many observed rotation curves remain flat or slowly rising far from the center, implying more mass at large radii than visible matter accounts for.

The flattened rotation curves are a central argument for the existence of dark matter halos surrounding galaxies. They also motivate alternative theories of gravity, such as Modified Newtonian Dynamics (MOND), though dark matter remains the broadly favored explanation. Rotation curves can be decomposed into contributions from the stellar disk, bulge, gas, and a dark matter halo, with common halo models including NFW and cored profiles. The shape and amplitude of curves vary with galaxy type, mass, and surface brightness; dwarf and low-surface-brightness galaxies often show strong dark matter dominance even near the center.

Limitations of rotation curves arise from noncircular motions, bars, warps, and uncertainties in distance and inclination. Despite these, they remain a foundational tool for studying galactic mass distribution and the nature of dark matter.