The large - scale anomalous microwave emission revisited by WMAP ⋆

We present a new study of the high latitude galactic contributions to the millimeter sky, based on an analysis of theWMAP data combined with several templates of dust emission (DIRBE/COBE and FIRAS/COBE) and gas tracers (HI and Hα). To study the IR to millimeter properties of the diffuse sky at high galactic latitude, we concentrate on the emission correlated with the HI gas. We compute the emission spectrum of the dust/freefree/synchrotron components associated with HI gas from low to large column densities. A significant residual WMAP emission over the free-free, synchrotron and the dust contributions is found from 3.2 to 9.1 mm. We show that this residual WMAP emission (normalised to 10 atoms/cm) (1) exhibits a constant spectrum from 3.2 to 9.1 mm and (2) significantly decreases in amplitude when NHI increases, contrary to the HI-normalised far-infrared emission which stays rather constant. It is thus very likely that the residual WMAP emission is not associated with the Large Grain dust component. The decrease in amplitude with increasing opacity ressembles in fact to the decrease of the transiently heated dust grain emission observed in dense interstellar clouds. This is supported by an observed decrease of the HI-normalised 60 μm emission with HI column densities. Although this result should be interpreted with care due to zodiacal residual contamination at 60 μm, it suggests that the WMAP excess emission is associated with the small transiently heated dust particles. On the possible models of this so-called “anomalous microwave emission” linked to the small dust particles are the spinning dust and the excess millimeter emission of the small grains, due to the cold temperatures they can reach between two successive impacts with photons.