Annu. Rev. Astron. Astrophys. 1979. 17: 135-87
Copyright © 1979 by . All rights reserved

Next Contents Previous

3. MASS-TO-LIGHT RATIOS OF SPIRAL GALAXIES

The rotation of spiral nebulae 256s first noticed by Wolf (1914) and Slipher (1914) fully a decade before astronomers discovered the true nature of galaxies. Pease (1916, 1918) made the first measurements of what we now call the stellar ``rotation curve'' in the nuclear regions of M31 and the Sombrero galaxy (M104). These observations required truly heroic dedication; both exposures lasted 80 hours spread out over a period of 3 months!

From these modest beginnings, the study of rotation curves has since matured to become our single most powerful tool for determining mass distributions inside galaxies. For many decades, optical spectroscopists dominated the field by measuring velocities of HII regions. Lately, it has been discovered that neutral atomic hydrogen extends outward past the optically bright regions of most spiral galaxies, and the 21-cm line is now being fully exploited to follow the dynamics to radii far beyond the last observable H II region.

The fundamental theory of inferring mass distributions from rotation curves has been discussed by Burbidge & Burbidge (1975), de Vaucouleurs & Freeman (1972), Freeman (1975), and Schmidt (1965), among others. Brosche et al. (1974) give a bibliography of dynamical measurements for all galaxies complete through May 1973, and van der Kruit (1978) reviews recent observational results.