Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

6.3 Gas Mass Fractions in Spiral Galaxies

Using the dynamical mass-to-light ratios of Rubin et al (1985) - in which the ratio M_dyn / L_B is 6.2 M L^-1 for Sa galaxies, 4.5 M L^-1 for Sb galaxies, and 2.6 M L^-1 for Sc galaxies - in conjunction with the blue luminosities from RC2, Young & Knezek (1989) computed the dynamical masses of spiral galaxies for which H and HI masses are known. The mean ratio of total neutral gas mass [HI + H₂] to dynamical mass, M_gas / M_dyn, ranges from 4% for Sa galaxies to 25% for Scd galaxies, as shown in Figure 5. The early type galaxies have thus locked up a much larger fraction of their mass in stars than late type galaxies. It is rather remarkable, then, that the present mean gas surface densities in early type spiral galaxies are only a factor of two lower than in late types (see Section 6.2).

Figure 5. Ratio of total interstellar gas mass (HI + H2) to dynamical mass for 150 galaxies (for H0 = 50 km s-1 Mpc-1). Dynamical masses were derived by using LB from RC2 and the results of Rubin et al (1985) for the ratio Mdyn / LB as a function of morphological type. The vertical tick mark in each panel represents the median value for that type.