Dust properties of nearby disks: M31 case

Several properties of the M31 disk, namely: opacity, extinction law and gas-to-dust ratio are studied by means of optical and near-infrared photometry of ten globular clusters and galaxies seen through the disk. The individual extinctions of these objects were estimated with respect to several sets of theoretical spectral energy distributions for globulars and galaxies. Seven targets are consistent with reddened globulars, two - with starburst galaxies and one - with an elliptical. The extinction estimates agree with semi-transparent disk (Av ~ 1) in the inter-arm regions. The total-to-selective extinction ratio in those regions 2.75 is lower on average than the typical Galactic value of Rv=3.1. We also obtained a gas-to-dust ratio, similar to that in the the Milky way. It shows no correlation with the distance from the M31 center.