Dust emissivity variations in the Milky Way

Aims. Dust properties appear to vary according to the environment in which the dust evolves. Previous observational indications of these variations in the far-infrared (FIR) and submillimeter (submm) spectral range are scarce and limited to specific regions of the sky. To determine whether these results can be generalised to larger scales, we study the evolution in dust emissivities from the FIR to millimeter (mm) wavelengths, in the atomic and molecular interstellar medium (ISM), along the Galactic plane towards the outer Galaxy. Methods. We correlate the dust FIR to mm emission with the HI and CO emission, which are taken to trace the atomic and molecular phases, respectively. The study is carried out using the DIRBE data from 100 to 240 mu m, the Archeops data from 550 mu m to 2.1 mm, and the WMAP data at 3.2 mm (Wband), in regions with Galactic latitude vertical bar b vertical bar <= 30 degrees, over the Galactic longitude range (75 degrees < 1 < 198 degrees) observed with Archeops. We estimate the average dust temperature in each phase and divide the emission spectral energy distribution (SED) by a black body at this temperature to derive the emissivity profile. A detailed verification of the impact of the implied simplification, such as temperature mixing along the line of sight, is provided. Results. In all regions studied, the emissivity spectra in both the atomic and molecular phases are steeper in the FIR (beta = 2.4) than in the submm and mm (beta = 1.5). We find significant variations in the spectral shape of the dust emissivity as a function of the dust temperature in the molecular phase. Regions of similar dust temperature in the molecular and atomic gas exhibit similar emissivity spectra. Regions where the dust is significantly colder in the molecular phase show a significant increase in emissivity for the range 100-550 mu m. We exclude the possibility of this effect being an artifact of our temperature determination or the assumptions made. This result supports the hypothesis of grain coagulation in these regions, confirming results obtained over small fractions of the sky in previous studies and allowing us to expand these results to the cold molecular environments in general of the outer MW. Possible reasons for the observed emissivity increase in the molecular phase that vanishes in the mm range are discussed by comparison with dust models, involving dust aggregation and solid state physics processes specific to amorphous material. We note that it is the first time that these effects have been demonstrated by direct measurement of the emissivity, while previous studies were based only on thermal arguments.