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) abundanceout to R 25 kpc.The global dust mass is M _d=5.×10⁷_M,the global dust/H mass ratio is M_d/MH=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 15.1 kpc.The dust/gas ratio varies from M _d/MH 0.026 at the center to 0.0027 at R 25 kpc. From the dust/gas ratio, we estimate the interstellar medium metallicity to vary by a factor 10, from Z_Z3 at R = 0 to 0.3 at R = 25 kpc. The dust heating rate parameter U peaks at the center, with U 35,declining to U 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_PAH0.045 at R 11.2 kpc.When allowance is made for the different spectrum of the bulge stars,q_PAHfor 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 μm emission at R 2-6 kpc,suggesting that at R=2-6 kpc,the dust opacity varies more steeply with frequency (with β 2.3 between 200 and 600 μm)than in the model.