Dark matter in spiral galaxies. III - The SA galaxies

Luminosity profiles have been obtained for 14 Sa galaxies with optical rotation curves measured by Rubin et al. In about half the galaxies, the rotation curves can be explained by a constant M/L ratio stellar component alone; the curves do not extend far enough to reveal unequivocally the presence of any significant dark halo. One object may have a central, dark mass concentration. Six objects show rotation curves that rise too slowly for a constant M/L ratio; if the rotation curves are measuring the true circular velocity, then they must be explained in terms of a very low M/L bulge embedded in a high M/L disk. The bulge dynamics are used to explore the problem further. Central bulge velocity dispersions are computed by using the measured bulge light distribution and a mass profile given by the rotation curves. For the "normal" galaxies, the predicted dispersions match the observed values reason- ably well. For the "massless bulge" galaxies, the predicted dispersion is always low, by a factor of 3 in the worst case. Models with an anisotropic velocity distribution do not resolve the discrepancy. Most likely, the rotation curves in bulge-dominated regions are not measuring the true circular velocity.