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We study the radial distribution of the temperature of the warm dust andgas-to-dust mass ratios in a sample of 22 spiral galaxies. The heatingcapabilities of the diffuse interstellar radiation field (ISRF), based onDesert et al. model, are investigated in 13 of the sample galaxies. In general,the temperature of the warm dust decreases away from the center, reaches aminimum value at the mid-disk and increases again in the outer parts ofgalaxies. Heating a mixture of small and big grains by the ISRF is able toexplain the observed behavior qualitatively. However, ultraviolet photons fromrecent star formation events are necessary for a detailed matching of the warmdust temperature profiles. Very small grains contribute typically more than 50%to the observed flux at 60 micron beyond half the disk radius in galaxies.Optical depth profiles, derived from the observed 60 micron and warm dusttemperature profiles, peak at or close to the galactic center. In 13 of thegalaxies, where dust temperature profiles are modeled, we obtain gas-to-dustmass ratio profiles, after correction for the contaminating effects of verysmall grains. The gas-to-dust mass ratio decreases by a factor of 8 from thecenter to the optical isophotal radius, where the value approaches the localgalactic value. We demonstrate that the observed steep gradient is a result ofthe over-estimation of the molecular mass, and can be flattened out to within afactor of 2, if the molecular hydrogen mass (H2) is recomputed assuming ametallicity dependent conversion factor from CO intensity to H2 column density.The flattened radial profiles indicate a global gas-to-dust ratio of around300, which is within a factor of two of the local galactic value.Comment: Uses aas2pp4.sty and epsfig.sty, 27 pages. To appear in Astronomical  Journal, September 199

Spatial distribution of far-infrared emission in spiral galaxies. II. Heating sources and gas-to-dust ratio.