Planck early results. XXI. Properties of the interstellar medium in the Galactic plane

This paper presents the first results of comparison of Planck along with IRAS data with Green Bank Telescope 21 cm observations in 14 fields covering more than 800 square degrees at high Galactic latitude. Galactic dust emission for fields with average column density lower than 2 x 10\^20 cm\^-2 is well correlated with 21 cm emission. The residual emission in these fields, once the HI-correlated emission is removed, is consistent with the expected statistical properties of the cosmic infrared background fluctuations. Fields with larger average column densities show significant excess dust emission compared to the HI column density. Regions of excess lie in organized structures that suggest the presence of hydrogen in molecular form, though they are not always correlated with CO emission. Dust emission from intermediate-velocity clouds is detected by this correlation analysis with high significance. Its spectral properties are consistent with, compared to the local ISM values, significantly hotter dust (T ~ 20 K), lower emission cross-section and a relative abundance of very small grains to large grains about four times higher. These results are consistent with expectations for clouds that are part of the Galactic fountain in which there is dust shattering and fragmentation. Correlated dust emission in high-velocity clouds is detected only marginally at the 1 to 3.5 sigma level. It has a dust emissivity a factor of ten lower compared to the local ISM, in accordance with the lower metallicity of these clouds. Unexpected anti-correlated variations of the dust temperature and emission cross section per H atom are identified in the local ISM and IVCs, a trend that continues into molecular environments. This suggests that dust growth through aggregation, seen in molecular clouds and then in circumstellar disks, is active much earlier in the cloud condensation and star formation processes.