Key points are not available for this paper at this time.
Large-scale dust heating and cooling in the diffuse medium of M31 is studied using the high-resolution (HiRes) IRAS maps in conjunction with UV, optical (UBV), and H I maps. A dust heating/cooling model is developed based on a radiative transfer model which assumes a "sandwich" configuration of dust and stars and takes fully into account the effect of scattering of dust grains. The model is applied to a complete sample of "cells" (small areas of size 2' x 2') generated from the above maps. The sample covers the M3 1 disk in the galactocentric radius range 2-14 kpc and includes only the cells for which the contribution of the discrete sources to the 60 microns surface brightness is negligible (1. The mean optical depth (viewed from the inclination angle of 77°) increases with radius from τν ∼0.7 at r = 2 kpc outward, reaches a peak of ∼1.6 near 10 kpc, and stays quite flat out to 14 kpc, where the signal falls below the 5 σ level. 2. A correlation between τν and H I surface density is suggested by the similarity between their radial profiles. Significant differences are found between the radial profiles of the H2 gas (estimated from CO) and of the dust (from τν), which are most probably caused by the large uncertainty in the CO-to-H2 conversion factor, and to the underrepresentation of H2-rich regions in the sample of cells of diffuse regions. 3. The τν/N(H I) ratio decreases with increasing radius in the disk of M31, with an exponential law fit yielding an e-folding scale length of 9.6 + 0.4 kpc. 4. The optical depth adjusted for this gradient, τν,c is strongly and linearly correlated with N(H I) over 1.5 orders of magnitude of column density, indicating that at a given radius r the dust column density is proportional to the H I gas column density, with the proportionality factor decreasing with increasing r. 5. With the assumption that the ratio of τν to dust column density is the same as that in the solar neighborhood, the portion of the M31 disk at radii between 2 and 14 kpc contains 1.9±0.6 × 107 Msun of dust, yielding a global dust-to-total gas mass ratio of 9.0±2.7 × 10-3, very close to solar neighborhood value. 6. The nonionizing UV radiation, mainly caused by B stars (4-20 Msun), contributes only 27% of the heating of the diffuse dust in M31. Throughout the M31 disk, heating of the diffuse dust is dominated by optical radiation from stars at least a billion years old.
Xu et al. (Mon,) studied this question.