  We study the radial distribution of the temperature of the warm dust and
gas-to-dust mass ratios in a sample of 22 spiral galaxies. The heating
capabilities of the diffuse interstellar radiation field (ISRF), based on
Desert 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 a
minimum value at the mid-disk and increases again in the outer parts of
galaxies. Heating a mixture of small and big grains by the ISRF is able to
explain the observed behavior qualitatively. However, ultraviolet photons from
recent star formation events are necessary for a detailed matching of the warm
dust 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 dust
temperature profiles, peak at or close to the galactic center. In 13 of the
galaxies, where dust temperature profiles are modeled, we obtain gas-to-dust
mass ratio profiles, after correction for the contaminating effects of very
small grains. The gas-to-dust mass ratio decreases by a factor of 8 from the
center to the optical isophotal radius, where the value approaches the local
galactic value. We demonstrate that the observed steep gradient is a result of
the over-estimation of the molecular mass, and can be flattened out to within a
factor of 2, if the molecular hydrogen mass (H2) is recomputed assuming a
metallicity dependent conversion factor from CO intensity to H2 column density.
The flattened radial profiles indicate a global gas-to-dust ratio of around
300, which is within a factor of two of the local galactic value.
