Institute for Advanced Study/Princeton University Joint Astrophysics Colloquium

Infrared observations from Spitzer Space Telescope and Herschel Space Observatory are used to study the interstellar dust in M31. A physical dust model is used to map the dust surface density, dust/gas ratio, starlight heating intensity, and the PAH abundance, out to R=25 kpc from the center of M31. The dust/gas ratio declines smoothly with increasing galactocentric radius, decreasing by almost a factor of ten from R=0 to R=20 kpc, following the metallicity of the gas. The dust serves as a photometer to measure the intensity of the starlight heating the dust. Within the central kpc the starlight intensity estimated from the observed dust SED is in good agreement with the directly-observed bulge starlight -- a validation of the dust model. The dust model predicts the extinction through the disk. Nature has kindly placed an AGN behind the dusty disk of M31. Dust-scattered X-ray photons from this AGN could be used to get a geometric distance to M31.