Hi-GAL Observations: Emissivity Spectral Index along the Galactic Plane

Variations in the dust emissivity are critical for gas mass determination derived from far-infrared observations, but also for separating dust foreground emission from the Cosmic Microwave Background. Hi-GAL observations allow for the first time to study dust emissivity variations in the inner regions of the Galactic plane. We present emissivity spectral index maps derived from the Herschel PACS 160 and SPIRE 250, 350 and 500 micron data combined to the IRIS 100 micron data, and we analyze the spatial variations of the spectral index as a function of dust temperature and wavelength in the two Science Demonstration Phase Hi-GAL fields, centered at l=30deg and l=59deg. Applying two different methods, we determine both dust temperature and emissivity spectral index between 100 and 500 micron, at an angular resolution of 4 arcmin. Combining both fields, the results evidence variations of the emissivity spectral index in the range 1.8-2.6 for temperatures between 14 and 23 K. The median values of the spectral index are identical in both fields, i.e. 2.3 in the range 100-500 micron, for median dust temperature equal to 19.1 K and 16.0 K in the l=30deg and l=59deg field, respectively. Statistically, we do not see in the emissivity spectra any significant deviations from a power law between 100 and 500 micron. We show an inverse correlation between the emissivity spectral index and the dust temperature, found in previous analysis in various environments.