ANDROMEDA'S DUST

Spitzer Space Telescope and Herschel Space Observatory imaging of M31 is used, with a physical dust model, to construct maps of dust surface density, dust-to-gas ratio, starlight heating intensity, and polycyclic aromatic hydrocarbon (PAH) abundance, out to R approximate to 25 kpc. The global dust mass is M-d = 5.4 x 10(7) M-circle dot, the global dust/H mass ratio is M-d/M-H = 0.0081, and the global PAH abundance is < q(PAH)> = 0.039. The dust surface density has an inner ring at R = 5.6 kpc, a maximum at R = 11.2 kpc, and an outer ring at R approximate to 15.1 kpc. The dust/gas ratio varies from M-d/M-H approximate to 0.026 at the center to similar to 0.0027 at R approximate to 25 kpc. From the dust/gas ratio, we estimate the interstellar medium metallicity to vary by a factor similar to 10, from Z/Z(circle dot) approximate to 3 at R = 0 to similar to 0.3 at R = 25 kpc. The dust heating rate parameter < U > peaks at the center, with < U > approximate to 35, declining to < U > approximate to 0.25 at R = 20 kpc. Within the central kiloparsec, the starlight heating intensity inferred from the dust modeling is close to what is estimated from the stars in the bulge. The PAH abundance reaches a peak q(PAH) approximate to 0.045 at R approximate to 11.2 kpc. When allowance is made for the different spectrum of the bulge stars, q(PAH) for the dust in the central kiloparsec is similar to the overall value of q(PAH) in the disk. The silicate-graphite-PAH dust model used here is generally able to reproduce the observed dust spectral energy distribution across M31, but overpredicts 500 mu m emission at R approximate to 2-6 kpc, suggesting that at R = 2-6 kpc, the dust opacity varies more steeply with frequency (with beta approximate to 2.3 between 200 and 600 mu m) than in the model.