Abstract: An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350?m to 2 mm) andIRAS(100?m) data. The optical depth maps are correlated with tracers of the atomic (Hi) 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 Hi 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 Hi phase in the solar neighbourhood is found to be ?D/NHtot = 5.2×10-26 cm2 at 857 GHz. It follows roughly a power law distribution with a spectral index ? = 1.8 all the way down to 3 mm, although the SED flattens slightly in the millimetre. Taking into account the spectral shape of the dust optical depth, the emissivity is consistent with previous values derived fromFIRAS measurements at high latitudes within 10%. The threshold for the existence of the dark gas is found at NHtot = (8.0±0.58)×1020 H cm?2 (AV = 0.4mag). Assuming the same high frequency emissivity for the dust in the atomic and the molecular phases leads to an average XCO = (2.54 ± 0.13) × 1020 H2 cm-2/(K km s-1). The mass of dark gas is found to be 28% of the atomic gas and 118% of the CO emitting gas in the solar neighbourhood. The Galactic latitude distribution shows that its mass fraction is relatively constant down to a few degrees from the Galactic plane. A possible explanation for the dark gas lies in a dark molecular phase, where H2 survives photodissociation but CO does not. The observed transition for the onsetof this phase in the solar neighbourhood (AV = 0.4mag) appears consistent with recent theoretical predictions. It is also possible that up to half of the dark gas could be in atomic form, due to optical depth effects in the Hi measurements.

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