Dust in the Diffuse Emission of the Galactic Plane: The Herschel/Spitzer Spectral Energy Distribution Fitting

The first Herschel Hi-Gal images of the Galactic plane unveil the far-infrared diffuse emission of the interstellar medium with an unprecedented angular resolution and sensitivity. In this Letter, we present the first analysis of these data in combination with those of Spitzer GLIMPSE and MIPSGAL. We selected a relatively diffuse and low excitation region of the l ~ 59° Hi-Gal Science Demonstration Phase field to perform a pixel-by-pixel fitting of the 8 to 500 μm spectral energy distribution (SED) using the DustEM dust emission model. We derived maps of the very small grain (VSG) and polycyclic aromatic hydrocarbon (PAH) abundances from the model. Our analysis allows us to illustrate that the aromatic infrared band intensity does not necessarily trace the PAH abundance but rather the product of "abundance × column density × intensity of the exciting radiation field." We show that the spatial structure of PACS 70 μm maps resemble the shorter wavelengths (e.g., IRAC 8 μm) maps, because they trace both the intensity of exciting radiation field and column density. We also show that the modeled VSG contribution to PACS 70 μm (PACS 160 μm) band intensity can be up to 50% (7%). The interpretation of diffuse emission spectra at these wavelengths must take stochastically heated particles into account. Finally, this preliminary study emphasizes the potential of analyzing the full dust SED sampled by Herschel and Spitzer data, with a physical dust model (DustEM) to reach the properties of the dust at simultaneously large and small scales