First Results from THOR

thor.jpgTo study giant molecular clouds (GMCs) in our Galaxy, the authors initiated a large VLA program: The H i, OH, Recombination line survey of the Milky Way (THOR). THOR maps the 21 cm H I line, 4 OH lines, up to 19 Hα recombination lines and the 1-2 GHz continuum over a significant fraction of the Milky Way at ~20” angular resolution. This paper focuses on the H I emission from the W43 GMC complex. The authors analyzed strong continuum sources to measure the optical depth along the line-of-sight and correct the HI emission for optical depth effects and weak diffuse continuum emission. Their analysis reveals a lower limit for the H I mass of 6.6 × 106 M⊙, a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The H I column densities are as high as ~ 150 M⊙ pc-2, an order of magnitude higher than for low-mass star formation regions. This result challenges theoretical models that predict a H I column density threshold of ~10 M⊙ pc-2, at which the formation of molecular hydrogen should set in. For the atomic gas particle density, the authors find a linear decrease toward the center of W43 with n(HI) = 20 cm-3 near the cloud edge, and decreasing to almost 0 cm-3 at its center. The molecular hydrogen traced via dust observations with Herschel shows an exponential increase toward the center with densities increasing to n(H2) > 200 cm-3, averaged over a region of ~10 pc. While atomic and molecular hydrogen are well mixed at the cloud edge, the cloud center is strongly dominated by H2 emission. There is no sharp transition between hydrogen in atomic and molecular form. These results, which challenge current theoretical models, are an important characterization of the atomic to molecular hydrogen transition in an extreme environment.

View Paper: THOR: The HI, OH, Recombination line survey of the Milky Way: H I observations of the giant molecular cloud W43⋆, S. Bihr (MPIfA), H. Beuther (MPIfA), J. Ott (NRAO), K. G. Johnston (Leeds), A. Brunt haler (MPIfR), L. D. Anderson (WVU), F. Bigiel (Heidelberg), P. Carlhoff (Köln), E. Churchwell (Wisconsin), S. C. O. Glover (Heidelberg), P. F. Goldsmith (JPL), F. Heitsch (North Carolina), T. Henning (MPIfA), M. H. Heyer (Massachusetts), T. Hill (JAO), A. Hughes (MPIfA), R. S. Klessem (Heidelberg, Stanford, UC-Santa Cruz), H. Linz (MPIfA), S. N. Longmore (Liverpool), N. M. McClure-Griffiths (ATNF), K. M. Mentn (MPIfR), F. Motte (CEA/Saclay), Q. Nguyen-Luong (Toronto), R. Plume (Calgary), S. E. Ragan (MPIfA), N. Roy (MPIfR, Indian Institute of Technology), P. Schilke (Köln), N. Schneider (Bordeaux), R. J. Smith (Heidelberg), J. M. Stil (Calgary), J. S. Urquhart (MPIfR), A. J. Walsh (Curtin), and F. Walter (MPIfA), 2015 A&A, 580, A112 (published online 11 August 2015).

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