The distribution of mass in spiral galaxies

Abstract

Rotation curves for the 60 Sa, Sb, and Sc galaxies which we have previously studied indicate that galaxies of very different optical morphology and luminosity can have rotation curves of very similar form. These observations imply that the form of the mass distribution in spiral galaxies is not necessarily predicted by the distribution of either optical luminosity or morphology. Thus, we now choose to examine the dynamical mass distribution (both luminous and nonluminous) in these galaxies in a manner that is independent of optical morphology. We assume that the relationship between the observed rotation curve and the projected mass distribution is the same for all spiral galaxies. With this assumption, the derived mass distributions for 41 of these galaxies can be classified as one of three well-defined integral mass forms, where mass and radius are measured in units of a fiducial mass and a scale length, chosen so as to minimize the scatter within a mass form. The mass distributions for 12 other galaxies are intermediate in form, sharing the characteristics of more than one integral mass form, while the remaining seven galaxies have mass distributions too irregular to classify. Although the forms of mass distributions for 53 of these galaxies clearly form a continuum, the specification of three representative mass types is still useful and analogous to the use of Hubble types for the continuum of optical morphology. Galaxies of all Hubble types and luminosities are represented in each integral mass type. The forms of mass distribution do not correlate with Hubble type, bulge-to-disk ratio, luminosity, mass density, size, mass, or other known global properties of these galaxies. Some mechanism unrelated to the global property of a galaxy determines the form of mass distribution; we suggest that this mechanism is the initial environment of the protogalaxy.