Abstract
This first lecture has detailed our knowledge of dark matter halos around spiral galaxies, which are characterized by rotating disks. Deriving a rotation curve requires several steps from the Doppler velocity field measured by gas emission lines in a disk projected onto the sky. The academic velocity field is represented by the spider diagram, but this diagram is most often disturbed by spiral or bar waves inside the disk, or by warping of the plane outside. These perturbations need to be interpreted and corrected before we can deduce the velocity field, and by modeling the visible mass, deduce dark matter. Statistics on thousands of rotation curves can then be used to deduce the parameters of a universal rotation curve, which reveals more dark matter the less massive and concentrated the galaxy. The case of the Milky Way is closely studied, given our more detailed knowledge. Statistics also reveal laws of scale, such as the famous baryonic Tully-Fisher law, which relates a galaxy's maximum rotation speed (and hence dark matter) to its total visible mass, or bayonics (gas and stars combined). Dark matter and visible matter are intimately linked.
