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

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.