Dust extinction law in nearby galaxies. I. M31

The observed spectral energy distributions (SEDs) for each tracer are constructed with multiband photometry from the LGGS, PS1 Survey, UKIRT, PHAT Survey, Swift/UVOT, and XMM-SUSS. We model the SED for each tracer in combination with the intrinsic spectrum obtained from the stellar model atmosphere extinguished by the model extinction curves.

Instead of mathematically parameterizing the extinction functions, the model extinction curves in this work are directly derived from the silicate-graphite dust model with a dust size distribution of dn/da~a^-{alpha}^exp(-a/0.25), 0.005<a<5um. The extinction tracers are distributed along the arms in M31, with the derived Milky Way (MW)-type extinction curves covering a wide range of R_V_ (~2-6), indicating the complexity of the interstellar environment and the inhomogeneous distribution of interstellar dust in M31.