Dust temperature maps of the Galactic plane: The Herschel spectral energy distribution fitting with Cloudy predictions

Context. Dust grains absorb the interstellar far ultra-violet and visible photons and re-emit them in far-infrared (FIR) wavebands. The dust FIR continuum can be predicted by a grid of models using various values of the interstellar radiation field. Aims. We analyze the dust continuum emission in two Hi-GAL science-demonstration phase (SDP) fields using both the radiative transfer code. Cloudy, and the DustEM dust model, to explore the effect of radiative transfer on dust temperature. The 500 mu m submillimeter excess emission and the very small grain (VSG) contribution to the 70 mu m intensity are investigated by spectral energy distribution (SED) fitting using the Cloudy model. Methods. By comparing the observation with the model prediction, we derive dust temperature maps of the two SDP fields by fitting the dust SED with 4-band data (SPIRE bands plus PACS 160 mu m) using both Cloudy and DustEM models. Considering radiative transfer and grain physics simultaneously, we investigate the existence of a 500 mu m excess and estimate the VSG contribution to the 70 mu m intensity by fitting the dust SED with 3-band data (160, 250, and 350 mu m) and 5-band data (SPIRE and PACS bands), respectively. Results. We confirm that the field with star formation activities have a higher temperature (18.7 +/- 0.9 K) than the quiescent region (15.2 +/- 0.6 K). We find that the radiative transfer affects the FIR SED of the SDP fields and results in a higher temperature distribution than the dust-only model fit. There is no significant detection of a 500 mu m excess in the two SDP fields. The relative contribution from the VSGs to the 70 mu m intensity can be up to 50%.