High-Resolution IRAS Maps and Infrared Emission of M31. II. Diffuse Component and Interstellar Dust

January 1996 • 1996ApJ...456..163X

Authors • Xu, Cong • Helou, George

Abstract • 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 (<20%). This effectively excludes most of the bright arm regions from our analysis. We find the following:

1. The mean optical depth (viewed from the inclination angle of 77°) increases with radius from τ_ν ∼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 τ_ν 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 τ_ν), 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 τ_ν/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, τ_ν,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 τ_ν 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 M_sun 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 M_sun), 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.

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