Planck early results. XIX. All-sky temperature and dust optical depth from Planck and IRAS. Constraints on the “dark gas” in our Galaxy

Abstract

An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350 μm to 2 mm) and IRAS (100 μm) data. The optical depth maps are correlated with tracers of the atomic (H i) and molecular gas traced by CO. The correlation with the column density of observed gas is linear in the lowest column density regions at high Galactic latitudes. At high NH, the correlation is consistent with that of the lowest NH, for a given choice of the CO-to-H2 conversion factor. In the intermediate NH range, a departure from linearity is observed, with the dust optical depth in excess of the correlation. This excess emission is attributed to thermal emission by dust associated with a dark gas phase, undetected in the available H i and CO surveys. The 2D spatial distribution of the dark gas in the solar neighbourhood (|bII |>10◦) is shown to extend around known molecular regions traced by CO. The average dust emissivity in the H iphase in the solar neighbourhood is found to be τD /Ntot H = 5.2 ×10−26 cm2 at 857 GHz. It follows roughly a power law distribution with a spectral index β = 1.8 all the way down to 3 mm