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

    Ade, P.A.R. and Aghanim, N. and Arnaud, M. and Ashdown, M. and Aumont, J. and Baccigalupi, C. and Balbi, A. and Banday, A.J. and Barreiro, R.B. and Bartlett, J.G. and Battaner, E. and Benabed, K. and Benoit, A. and Bernard, J.-P. and Bersanelli, M. and Bhatia, R. and Bock, J.J. and Bonaldi, A. and Bond, J.R. and Borrill, J. and Bouchet, F.R. and Boulanger, F. and Bucher, M. and Burigana, C. and Cabella, P. and Cardoso, J.-F. and Catalano, A. and Cayon, L. and Challinor, A. and Chamballu, A. and Chiang, L.-Y. and Chiang, C. and Christensen, P.R. and Clements, D.L. and Colombi, S. and Couchot, F. and Coulais, A. and Crill, B.P. and Cuttaia, F. and Dame, T.M. and Danese, L. and Davies, R.D. and Davis, R.J. and De Bernardis, P. and de Gasperis, G. and de Rosa, A. and de Zotti, G. and Delabrouille, J. and Delouis, J.-M. and Desert, F.-X. and Dickinson, C. and Dobashi, K. and Donzelli, S. and Dore, O. and Dorl, U. and Douspis, M. and Dupac, X. and Efstathiou, G. and Enßlin, T.A. and Eriksen, H.K. and Falgarone, E. and Finelli, F. and Forni, O. and Fosalba, P. and Frailis, M. and Franceschi, E. and Fukui, Y. and Galeotta, S. and Ganga, K. and Giard, M. and Giardino, G. and Giraud-Héraud, Y. and Gonzalez-Nuevo, J. and Gorski, K.M. and Gratton, S. and Gregorio, A. and Grenier, I.A. and Grupposo, A. and Hansen, F.K. and Harrison, D. and Helou, G. and Henrot-Versille, S. and Herranz, D. and Hildebrandt, S.R. and Hivon, E. and Hobson, M. and Holmes, W.A. and Hovest, W. and Hoyland, R.J. and Huffenberger, K.M. and Jaffe, A.H. and Jones, W.C. and Juvela, M. and Kawamura, A. and Keihanen, E. and Keskitalo, R. and Kisner, T.S. and Kneissl, R. and Knox, L. and Kurki-Suonio, H. and Lagache, G. and Lamarre, J.-M. and Lasenby, A. and Laureijs, R.J. and Lawrence, C.R. and Leach, S. and Leonardi, R. and Leroy, C. and Lilje, P.B. and Linden-Vornle, M. and Lopez-Caniego, M. and Lubin, P.M. and Macias-Perez, J.F. and MacTavish, C.J. and Maffei, B. and Maino, D. and Mandolesi, N. and Mann, R. and Maris, M. and Martin, P. and Martinez-Gonzalez, E. and Masi, S. and Matarrese, S. and Matthai, F. and Mazzotta, P. and McGehee, P. and Meinhold, P.R. and Melchiorri, A. and Mendes, L. and Mennella, A. and Miville-Deschenes, M.-A. and Moneti, A. and Montier, L. and Morgante, G. and Mortlock, D. and Munshi, D. and Murphy, J.Anthony and Naselsky, P. and Natoli, P. and Netterfield, C.B. and Norgaard-Nielsen, H.U. and Noviello, F. and Novikov, D. and Novikov, I. and O'Dwyer, I.J. and Onishi, T. and Osborne, S. and Pajot, F. and Paladini, R. and Paradis, D. and Pasian, F. and Patanchon, G. and Perdereau, O. and Perotto, L. and Perrotta, F. and Piacentini, F. and Piat, M. and Plaszczynski, S. and Pointecouteau, E. and Polenta, G. and Ponthieu, N. and Poutanen, T. and Prezeau, G. and Prunet, S. and Puget, J.-L. and Reach, W.T. and Reinecke, M. and Renault, C. and Ricciardi, S. and Riller, T. and Ristorcelli, I. and Rocha, G. and Rosset, C. and Rowan-Robinson, M. and Rubino-Martin, J.A. and Rusholme, B. and Sandri, M. and Santos, D. and Savini, G. and Scott, D. and Seiffert, M.D. and Shellard, P. and Smoot, G.F. and Starck, J.-L. and Stivoli, F. and Stolyarov, V. and Stompor, R. and Sudiwala, R. and Sygnet, J.-F. and Tauber, J.A. and Terenzi, L. and Toffolatti, L. and Tomasi, M. and Torre, J.-P. and Tristram, M. and Tuovinen, J. and Umana, G. and Valenziano, L. and Vielva, P. and Villa, F. and Vittorio, N. and Wade, L.A. and Wandelt, B.D. and Wilkinson, A. and Yvon, D. and Zacchei, A. and Zonca, A. (2011) Planck Early Results: All sky temperature and dust optical depth from Planck and IRAS: Constraints on the "dark gas" in our Galaxy. Astronomy & Astrophysics, 536 (A19). pp. 1-16. ISSN 0004-6361

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    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 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 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, 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 from FIRAS measurements at high latitudes within 10%. The threshold for the existence of the dark gas is found at Ntot H = (8.0±0.58)×1020 Hcm−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 kms−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 onset of 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.

    Item Type: Article
    Additional Information: © ESO 2011 The original publication is available at DOI: 10.1051/0004-6361/201116479
    Keywords: dust; extinction; ISM; clouds; evolution; solar neighborhood; Galaxy; submillimeter; Planck Collaboration;
    Academic Unit: Faculty of Science and Engineering > Experimental Physics
    Item ID: 4045
    Depositing User: Dr. Anthony Murphy
    Date Deposited: 19 Dec 2012 17:16
    Journal or Publication Title: Astronomy & Astrophysics
    Publisher: EDP Sciences
    Refereed: Yes

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