The Kinematics and Masses of Galaxies
In the 1920s, several years before the spiral nebulae were even identified as entirely separate galaxies, it was discovered that they rotated. This discovery came about as a result of the inclination of absorption lines in the spectra of the central regions of, in particular, the galaxies M81 and M104. Up to the 1970s, rotation curves, or in other words the law describing how the rotation velocity V varies as a function of the distance to the galactic centre, were obtained solely at optical frequencies from the absorption lines of stars in the central regions and from the emission lines of H II regions in the outer regions. Subsequently interferometric radio observations at 21 cm (the HI line) with the Westerbork telescopes (in the 1970s) and the VLA (in the 1980s) enabled a large number of rotation curves to be quickly determined, at the same time with greater spectral resolution and an unequalled radial extension, the gaseous HI component generally exceeding the optical limit in the outer regions of galaxies. This confirmed in a striking way that the rotation curves remain flat at a large distance from the centre: does an enormous quantity of invisible mass then exist beyond the visible limits of a spiral galaxy?
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