SPATIAL-DISTRIBUTION OF FAR-INFRARED EMISSION IN SPIRAL GALAXIES .2. HEATING SOURCES AND GAS-TO-DUST RATIO

We study the radial distribution of the temperature of the warm dust a nd gas-to-dust mass ratios in a sample of 22 spiral galaxies. The heat ing capabilities of the diffuse interstellar radiation field (ISRF), b ased on Desert et al. model, are investigated in 13 of the sample gala xies. In general, the temperature of the warm dust decreases away from the center, reaches a minimum value at the mid-disk and increases aga in in the outer parts of galaxies. Heating a mixture of small and big grains by the ISRF is able to explain the observed behavior qualitativ ely. However, ultraviolet photons from recent star formation events ar e necessary for a detailed matching of the warm dust temperature profi les. Very small grains contribute typically more than 50% to the obser ved flux at 60 mu m beyond half the disk radius in galaxies. Optical d epth (tau(60)) profiles, derived from the observed 60 mu m and warm du st temperature profiles, peak at or close to the galactic center. In 1 3 of the galaxies, where dust temperature profiles are modeled, we obt ain optical depth and dust mass profiles after correction for the cont aminating effects of very small grains. These profiles are combined wi th the gas density profiles in the literature, to generate profiles of the gas-to-dust mass ratio. The resulting gas-to-dust mass ratio decr eases by a factor of 8 from the center to the optical isophotal radius , where the value approaches the local galactic value. With the unders tanding that the dust mass is proportional to metallicity, and that th e metallicity increases towards the center of galaxies, one expects th e gas-to-dust ratio to decrease towards the center, contrary to what i s observed. We demonstrate that the observed steep gradient is a resul t of the over-estimation of the molecular mass, and can be flattened o ut to within a factor of 2, if the molecular hydrogen mass is recomput ed assuming a metallicity dependent conversion factor from CO intensit y to H-2 column density. The flattened radial profiles indicate a glob al gas-to-dust ratio of around 300, which is within a factor of two of the local galactic value. (C) 1997 American Astronomical Society.