Dust in the diffuse emission of the galactic plane - The
  Herschel/Spitzer SED 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 paper, we present the first
analysis of these data in combination with that of Spitzer Glimpse & Mipsgal.
We selected a relatively diffuse and low excitation region of the
l~59\,^{\circ} Hi-Gal Science Demonstration Phase field to perform a pixel by
pixel fitting of the 8 to 500 microns SED using the DustEM dust emission model.
We derived maps of the Very Small Grains (VSG) and PAH abundances from the
model. Our analysis allows us to illustrate that the Aromatic Infrared Bands
(AIB) intensity does not trace necessarily the PAH abundance but rather the
product of "abundance x column density x intensity of the exciting radiation
field". We show that the spatial structure of PACS70microns map resembles the
shorter wavelengths (e.g. IRAC8microns) maps, because they trace both the
intensity of exciting radiation field and column density. We also show that the
modeled VSG contribution to PACS70microns (PACS160microns) 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.
