4.1.2. Stellar Mass Densities
Masses and surface densities for the stellar component in galaxies are probably even more uncertain than molecular gas masses. Because of the flat rotation curves of disk galaxies, and the consequent inference that the galaxies are dominated by non-luminous, non-baryonic matter, the mass-to-light (M/L) ratio and mass surface density of the stellar component can not be derived from galaxy dynamics. Unless one can count the stars in a region directly (possible only for very nearby systems), it is necessary to infer M/L for the stellar component by indirect means. This is difficult because the luminosity and colors of composite stellar populations depend on both the star formation history and the metal enrichment history.
Nevertheless, recent work by Bell & de Jong (2001) indicates that the stellar M/L ratios of galaxies are rather robustly related to their colors. Bell & de Jong examined population synthesis models for galaxies assuming a variety of star formation histories. They found that M/L for the stellar component correlated very well with optical colors, although there is some scatter in the correlations. IR colors did not correlate as well with M/L because of the strong metallicity dependence of the IR luminosity of giants. The B-band M/L ratio shows a steep correlation with color, while the K-band M/L ratio shows a much less steep correlation (a factor three increase between B-R = 0.6 and B-R = 1.6, compared to a factor 10 increase in B-band M/L over the same color range). If the population synthesis models can reliably reproduce the colors and spectra of real galaxies, this method offers the possibility of greatly improved estimates of masses and mass surface densities for the stellar components of disk galaxies.