The gas/dust ratio in spiral galaxies

Abstract

The Infrared Astronomical Satellite (IRAS) data are used to calculate warm dust masses for 58 spiral galaxies in which the radial distributions of both the atomic and the molecular gas have been measured. The warm dust masses are compared with the atomic and molecular gas masses in order to constrain the fraction and the phase of the interstellar medium that contributes to the warm dust emission measured by IRAS. The dispersion in the ratio of dust mass to total (molecular plus atomic) gas mass is larger than that expected on the basis of measurement errors. The dispersion can be measurably reduced, however, if one excludes the extended component of atomic gas that lies beyond the inner half of the optical disk. Evidently the dust that is associated with the extended H I component in spiral galaxies is either too cold or too sparse to contribute significantly to the emission measured by IRAS. Conversely, the warm dust that is detected by IRAS is primarily located in the inner region of spiral galaxy disks. Excluding the extended H I component in spiral galaxies reduces the dispersion in the gas/dust ratio to a level that is smaller than the dispersion in the remarkable infrared-radio correlation and comparable to the 30% measurement uncertainties estimated for the global gas mass determinations. Consequently the ratio of gas mass to warm dust mass is well determined; we find M_gas_/M_dust_ = 1080 +/- 70. If the gas-to-total dust ratio in external galaxies is the same as the value of 100-150 determined within the Galaxy, then the high value of 1080 that is derived from the IRAS data indicates that only ~ 10%-20% of the total dust mass is warm enough to radiate in the IRAS bands. Conversely, ~ 80%-90% of the dust mass must be radiating at λ > 100 microns and is therefore colder than 30 K. Although the warm, ~ 30 K dust temperatures that are derived from observations of the infrared continuum emission provide no direct evidence for cold dust, it can be shown that the present observations do not rule out the possibility that most of the dust mass in spiral galaxies is at a temperature of 15 K or less. It is argued, on the basis of the small dispersion in the ratio of dust mass to inner disk gas mass, that the global molecular gas masses, derived from CO observations and a constant CO-to-H_2_ conversion, are accurate to +/- 30% for luminous spiral galaxies. If this were not true, one would not observe such a small scatter in the gas/dust ratio.