The Distribution of Mass in Spiral Galaxies

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Spiral galaxies contain stars, gas, and dust, concentratedin a thin disk, which slowly rotates around the center of the galaxy. This disk of luminous matter is embedded in an extendedspherical so-called `halo´ of invisible dark matter of unknowncomposition. This dark matter reveals its presence only via itsgravitational pull on the observable luminous matter like the starsand gas. The relative amounts of dark and luminous matter is largelyunknown. However, by carefully measuring the velocities of the stars,it can be inferred how much dark matter surrounds these spiralgalaxies. To measure these stellar velocities, a new instrument isbuild at the Astrophysikalisches Institut Potsdam to be used with thePotsdam Multi-Aperture Spectrophotometer (PMAS) on the 3.5m telescopeof the Calar Alto observatory in southern Spain.