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

6.4 The Gas-to-Dust Ratio in Spiral Galaxies

Comparison of molecular gas masses with IRAS-derived dust masses leads to mean gas-to-dust mass ratios of ~ 600 (Young et al 1986a, 1989 Stark et al 1986, Devereux & Young 1990b, Sanders et al 1991), rather than the value of ~ 150, which is widely used for the Galaxy (Spitzer 1978, Hildebrand 1983, Draine & Lee 1984). The inclusion of atomic gas in spiral galaxies only accentuates the discrepancy. A plausible explanation for the high apparent gas-to-dust ratio is that the bulk of the dust mass is cold (T_dust < 30 K) and radiating beyond 100 µm. Henceforth, we refer to the dust detected by IRAS as ``warm dust.''

Devereux & Young (1990b) considered not only the molecular gas, but also that fraction of the atomic gas associated with the inner disk, i.e. the area within which most of the star formation and hence infrared emission occurs. They find a tight correlation between the warm dust mass measured by IRAS and the gas mass (HI and H₂) in the inner disk (R < R₂₅/2), as shown in Figure 6, and conclude that the warm dust detected by IRAS is located in the inner disks and mixed with both HI and H₂ gas. They find the mean inner disk gas-to-warm-dust ratio is ~ 1000 with no significant variations as a function of morphological type, dust temperature, or whether the HI or H₂ is the dominant phase of the ISM. If the true gas-to-dust ratio in spiral galaxies is the same as the value of ~ 150 measured within the Galaxy, the high value that is found from this study indicates that 80-90% of the dust mass in spiral galaxies is radiating at > 100 µm and has a dust temperature colder than ~ 30 K. Furthermore, the small scatter in the gas/dust ratio in Figure 6b is used to argue that the global H₂, masses derived from CO observations and a constant CO-to-H₂ conversion factor have a 1 uncertainty of ± 30% for luminous spiral galaxies (see Section 2.2.2).

Figure 6. (Left panel): Correlation between gas masses in the inner disk (R < R25/2) and warm dust masses derived from IRAS 60 and 100 µm flux densities (Devereux & Young 1990b). The open circles identify galaxies for which the molecular gas constitutes > 50% of the gas mass: filled circles identify spirals for which atomic gas constitutes > 50% of the gas mass. The solid line represents gas/dust = 100 and the arrows identify spirals with 2 upper limits, where the upper limits are due to nondetections of H2. (Right panel): Histogram illustrating the gas/dust ratio. The shaded portion of the histogram represents the HI-dominated galaxies. The numerical value of the dispersion is indicated by the value of , where the limits have been included in the 2 value. The similar widths for the histograms of H2 and HI-dominated galaxies suggest that the HI and H2 masses have similar accuracy.