Basic properties and a discussion on age-dependent dust heating

Context. Aims. Recent observations derived from the Spitzer Space Telescope and improvements in theoretical modeling of dust emission properties are used to discuss the distribution of dust and its characteristics in the closest neighbor spiral galaxy M31. Together with GALEX FUV, NUV, and SDSS images we studied the age dependence of the dust heating process. Methods. Spitzer IRAC/MIPS maps of M31 were matched together and compared to dust emission models allowing to constrain the dust mass, the intensity of the mean radiation field, the abundance of Polycyclic Aromatic Hydrocarbons (PAH) particles. The total infrared emission (TIR) was analyzed in function of UV and Optical colors and compared to predictions of models which consider the agedependent dust heating. Results. We demonstrate that cold-dust component emission dominates the infrared spectral energy distribution of M31. The mean intensity of the radiation field heating the dust is low (typically U < 2, where U = 1 is the value in the solar surrounding). Due to the lack of submillimetric measurements the dust mass (Mdust) is only weakly constrained by the infrared spectrum, but we derived a lower limit of Mdust & 1.1×107 M⊙ with a best fit value of Mdust = 7.6×107 M⊙, in good agreement with expectations from CO and HI measurements. We show that across the spiral-ring structure of M31 a fraction > 3% of the total dust mass is in PAHs. UV and optical colors are correlated to total infrared to far ultraviolet (TIR/FUV) ratios in ∼ 670 pc-sized regions overall the disk of M31, although deviating from the relationship between infrared excess and ultraviolet spectral slope (referred as IRX-β relationship) for starburst galaxies. In particular, redder regions show lower values of the TIR/FUV ratio for a fixed color. Considering the predictions of models that account for the dust-heating age dependence we derived that in 83% of the regions analyzed across the 10kpc ring more than 50% of the energy absorbed by the dust is rediated at λ > 4000Å and that dust in M31 appears mainly heated by populations a few Gyr old even across the star-forming ring. We also found that the attenuation is varying radially peaking near 10kpc and decreasing faster in the inner regions of M31 than in the outer regions in agreement with previous studies. We finally derived the attenuation map of M31 at 6 ′′ /px resolution (∼ 100 pc/px along the plane of M31). Conclusions.