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We construct an all-sky map of the apparent temperature and optical depth ofthermal dust emission using the Planck-HFI and IRAS data. The optical depthmaps are correlated to tracers of the atomic and molecular gas. The correlationis linear in the lowest column density regions at high galactic latitudes. Athigh NH, the correlation is consistent with that of the lowest NH. In theintermediate NH range, we observe departure from linearity, with the dustoptical depth in excess to the correlation. We attribute this excess emissionto thermal emission by dust associated with a Dark-Gas phase, undetected in theavailable HI and CO measurements. We show the 2D spatial distribution of theDark-Gas in the solar neighborhood and show that it extends around knownmolecular regions traced by CO. The average dust emissivity in the HI phase inthe solar neighborhood follows roughly a power law distribution with beta = 1.8all the way down to 3 mm, although the SED flattens slightly in the millimetre.The threshold for the existence of the Dark-Gas is found at NH = (8.0\pm 0.58)10^{20} Hcm-2. Assuming the same dust emissivity at high frequencies for thedust in the atomic and molecular phases leads to an average XCO = (2.54\pm0.13)10^{20} H2cm-2/(K km s-1). The mass of Dark-Gas is found to be 28% of theatomic gas and 118% of the CO emitting gas in the solar neighborhood. Apossible origin for the Dark-Gas is the existence of a dark molecular phase,where H2 survives photodissociation but CO does not. The observed transitionfor the onset of this phase in the solar neighborhood (AV = 0.4 mag) appearsconsistent with recent theoretical predictions. We also discuss the possibilitythat up to half of the Dark-Gas could be in atomic form, due to optical deptheffects in the HI measurements.

Planck Early Results: All sky temperature and dust optical depth from Planck and IRAS: Constraints on the "dark gas" in our galaxy