Populations and Mass Distributions in Spiral Galaxies

The distribution of mass in a spiral galaxy is usually inferred from its rotation curve. The curve is most conveniently measured using part of its extreme population I such as HI or HII. This has a low velocity dispersion so that the observed tangential motion is in the absence of non-circular motions (van der Kruit and Allen 1978; Bosma 1981a, b) close to the circular velocity required to balance the gravitational force. The main difficulty is that for a detailed interpretation of the rotation curve one has to make assumptions on some general properties of the mass distribution, even though it is true that one can estimate the total mass within the last measured point to an accuracy of about a factor two. That of axial symmetry is only the simplest of assumptions. On the basis of the light distribution with a usually prominent disk component one often assumes that the mass distribution is also basically highly flattened.