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HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and H I overview

Wang, Y. and Beuther, H. and Rugel, M. R. and Soler, J. D. and Stil, J. M. and Ott, J. and Bihr, S. and McClure-Griffiths, N. M. and Anderson, L. D. and Klessen, R. S. and Goldsmith, P. F. and Roy, N. and Glover, S. C. O. and Urquhart, J. S. and Heyer, M. and Linz, H. and Smith, R. J. and Bigiel, F. and Dempsey, J. and Henning, T. (2020) HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and H I overview. Astronomy and Astrophysics, 634 . Art. No. A83. ISSN 0004-6361.

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Aims. The first half of THOR data (l = 14.0°−37.9°, and l = 47.1°−51.2°, |b|≤ 1.25°) has been published in our data release 1 paper. With this data release 2 paper, we publish all the remaining spectral line data and Stokes I continuum data with high angular resolution (10′′–40′′), including a new H I dataset for the whole THOR survey region (l = 14.0−67.4° and |b|≤ 1.25°). As we published the results of OH lines and continuum emission elsewhere, we concentrate on the H I analysis in this paper. Methods. With the Karl G. Jansky Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant, achieving an angular resolution of ≤40′′. At L Band, the WIDAR correlator at the VLA was set to cover the 21 cm H I line, four OH transitions, a series of Hnα radio recombination lines (RRLs; n = 151 to 186), and eight 128 MHz-wide continuum spectral windows, simultaneously. Results. We publish all OH and RRL data from the C-configuration observations, and a new H I dataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey) for the whole survey. The H I emission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out, likely due to higher spatial and velocity crowding of structures at the positive velocities. Compared to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well, but with significant material in the inter-arm regions. With the C-configuration-only H I+continuum data, we produce an H I optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this τ map, we corrected the H I emission for optical depth, and the derived column density is 38% higher than the column density with optically thin assumption. The total H I mass with optical depth correction in the survey region is 4.7 × 10⁸ M⊙, 31% more than the mass derived assuming the emission is optically thin. If we applied this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4–10.5 × 10⁹ M⊙. Comparing the H I with existing CO data, we find a significant increase in the atomic-to-molecular gas ratio from the spiral arms to the inter-arm regions. Conclusions. The high-sensitivity and resolution THOR H I dataset provides an important new window on the physical and kinematic properties of gas in the inner Galaxy. Although the optical depth we derive is a lower limit, our study shows that the optical depth correction issignificant for H I column density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H I data provide the basis for high-angular-resolution studies of the interstellar medium in different phases.

Item Type:Article
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URLURL TypeDescription Paper
Wang, Y.0000-0003-2226-4384
Beuther, H.0000-0002-1700-090X
Ott, J.0000-0001-8224-1956
Goldsmith, P. F.0000-0002-6622-8396
Additional Information:© 2020 Y. Wang et al. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society. Received 9 November 2019; Accepted 16 December 2019; Published online 12 February 2020. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Y.W., H.B., S.B., and J.D.S. acknowledge support from the European Research Council under the Horizon 2020 Framework Program via the ERC Consolidator Grant CSF-648505. H.B., S.C.O.G., and R.S.K. acknowledge support from the Deutsche Forschungsgemeinschaft in the Collaborative Research Center (SFB 881) “The Milky Way System” (subproject B1, B2, B8). This work was carried out in part at the Jet Propulsion Laboratory which is operated for NASA by the California Institute of Technology. R.J.S. acknowledges an STFC Rutherford fellowship (grant ST/N00485X/1). N.R. acknowledges support from the Max Planck Society through the Max Planck India Partner Group grant. F.B. acknowledges funding from the European Union’s Horizon 2020 research and innovation program (grant agreement No 726384). This research made use of Astropy and affiliated packages, a community-developed core Python package for Astronomy (Astropy Collaboration 2018), Python package SciPy25, APLpy, an open-source plotting package for Python (Robitaille & Bressert 2012), and software TOPCAT (Taylor 2005). The authors thank the anonymous referee for the detailed and constructive comments that improve the paper.
Funding AgencyGrant Number
European Research Council (ERC)648505
Deutsche Forschungsgemeinschaft (DFG)SFB 881
Science and Technology Facilities Council (STFC)ST/N00485X/1
Max Planck SocietyUNSPECIFIED
European Research Council (ERC)726384
Subject Keywords:ISM: clouds – ISM: atoms – ISM: molecules – radio lines: ISM – stars: formation
Record Number:CaltechAUTHORS:20200309-085129467
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Official Citation:The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and H I overview. Y. Wang, H. Beuther, M. R. Rugel, J. D. Soler, J. M. Stil, J. Ott, S. Bihr, N. M. McClure-Griffiths, L. D. Anderson, R. S. Klessen, P. F. Goldsmith, N. Roy, S. C. O. Glover, J. S. Urquhart, M. Heyer, H. Linz, R. J. Smith, F. Bigiel, J. Dempsey and T. Henning. A&A, 634 (2020) A83; DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101767
Deposited By: Tony Diaz
Deposited On:09 Mar 2020 16:26
Last Modified:09 Mar 2020 16:26

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