Issue |
A&A
Volume 580, August 2015
|
|
---|---|---|
Article Number | A112 | |
Number of page(s) | 17 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/201425370 | |
Published online | 11 August 2015 |
THOR: The H i, OH, Recombination line survey of the Milky Way
The pilot study: H i observations of the giant molecular cloud W43⋆
1
Max Planck Institute for Astronomy,
Königstuhl 17,
69117
Heidelberg,
Germany
e-mail:
name@mpia.de
2
National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM
87801,
USA
3
School of Physics and Astronomy, University of
Leeds, Leeds
LS2 9JT,
UK
4
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, 53121
Bonn,
Germany
5
Department of Physics and Astronomy, West Virginia
University, Morgantown, WV
26506,
USA
6
Universität Heidelberg, Zentrum für Astronomie, Institut für
Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120
Heidelberg,
Germany
7
1. Physikalisches Institut, Universität zu Köln,
Zülpicher Str. 77, 50937
Köln,
Germany
8
Department of Astronomy, University of Wisconsin,
Madison, WI
53706,
USA
9
Jet Propulsion Laboratory, California Institute of
Technology, 4800 Oak Grove
Drive, Pasadena,
CA
91109,
USA
10
Department of Physics and Astronomy, University of North
Carolina-Chapel Hill, Chapel
Hill, NC
27599-3255,
USA
11
Department of Astronomy, University of
Massachusetts, Amherst,
MA
01003-9305,
USA
12
Joint ALMA Observatory, Alonso de Cordova 3107, 763-0355 Vitacura,
Santiago,
Chile
13
Kavli Institute for Particle Astrophysics and Cosmology, Stanford
University, SLAC National Accelerator Laboratory, Menlo Park, CA
94025,
USA
14
Department of Astronomy and Astrophysics, University of
California, 1156 High
Street, Santa Cruz,
CA
95064,
USA
15
Astrophysics Research Institute, Liverpool John Moores
University, 146 Brownlow
Hill, Liverpool
L3 5RF,
UK
16
Australia Telescope National Facility, CSIRO Astronomy and Space
Science, Marsfield,
NSW
2122,
Australia
17
Laboratoire AIM, CEA/DSM-CNRS-Université Paris
Diderot, IRFU/Service
d’Astrophysique, Saclay, 91191
Gif-sur-Yvette,
France
18
Canadian Institute for Theoretical Astrophysics, University of
Toronto, 60 St. George Street,
Toronto, ON
M5S 3H8,
Canada
19
Department of Physics and Astronomy, University of Calgary, 2500
University Drive NW, Calgary, AB
T2N 1N4,
Canada
20
Department of Physics, Indian Institute of Technology
Kharagpur, 721302
Kharagpur,
India
21
Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux,
CNRS/INSU, 33270
Floirac,
France
22
International Centre for Radio Astronomy Research, Curtin
University, GPO Box
U1987, Perth
WA
6845,
Australia
Received: 19 November 2014
Accepted: 11 May 2015
To study the atomic, molecular, and ionized emission of giant molecular clouds (GMCs) in the Milky Way, we initiated a large program with the Karl G. Jansky Very Large Array (VLA): “THOR: The H i, OH, Recombination line survey of the Milky Way”. We map the 21 cm H i line, 4 OH lines, up to 19 Hα recombination lines and thecontinuum from 1 to 2 GHz of a significant fraction of the Milky Way (l = 15°−67°, | b | ≤ 1°) at an angular resolution of ~ 20″. Starting in 2012, as a pilot study we mapped 4 square degrees of the GMC associated with the W43 star formation complex. The rest of the THOR survey area was observed during 2013 and 2014. In this paper, we focus on the H i emission from the W43 GMC complex. Classically, the H i 21 cm line is treated as optically thin with properties such as the column density calculated under this assumption. This approach might yield reasonable results for regions of low-mass star formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth along the line of sight, and thus correct the H i 21 cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the H i mass of this region more accurately and our analysis reveals a lower limit for the H i mass of M = 6.6-1.8 × 106 M⊙ (vLSR = 60−120 km s-1), which is 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 NH i ~ 150 M⊙ pc-2 ≈ 1.9 × 1022 cm-2, which is an order of magnitude higher than for low-mass star formation regions. This result challenges theoretical models that predict a threshold for the H i column density of ~10 M⊙ pc-2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profile. For the atomic gas particle density, we find a linear decrease toward the center of W43 with values decreasing from nH i = 20 cm-3 near the cloud edge to almost 0 cm-3 at its center. On the other hand, the molecular hydrogen, traced via dust observations with the Herschel Space Observatory, shows an exponential increase toward the center with densities increasing to nH2> 200 cm-3, averaged over a region of ~10 pc. While atomic and molecular hydrogen are well mixed at the cloud edge, the center of the cloud is strongly dominated by H2 emission. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results, which challenge current theoretical models, are an important characterization of the atomic to molecular hydrogen transition in an extreme environment.
Key words: ISM: clouds / ISM: structure / ISM: atoms / stars: formation / radio lines: ISM / surveys
The H i data cubes are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/580/A112
© ESO, 2015
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