The HI/OH/Recombination line survey of the inner Milky Way (THOR)

Survey overview and data release 1^⋆,⋆⋆

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: beuther@mpia.de
² School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
³ Max Planck Institute for Radioastronomy, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth WA 6845, Australia
⁵ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
⁶ Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary AB, T2N 1N4, Canada
⁷ Department of Astronomy, University of Massachusetts, Amherst, MA 01003-9305, USA
⁸ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁹ Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
¹⁰ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹¹ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
¹² I. Physikalisches Institut, University of Cologne, Zülpicher Str. 77, 50937 Köln, Germany
¹³ Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT, Australia
¹⁴ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
¹⁵ Department of Physics, Indian Institute of Science, 560012 Bangalore, India
¹⁶ National Astronomical Observatory of Japan, Chile Observatory, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
¹⁷ School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK

Received: 17 June 2016
Accepted: 4 August 2016

Abstract

Context. The past decade has witnessed a large number of Galactic plane surveys at angular resolutions below 20′′. However, no comparable high-resolution survey exists at long radio wavelengths around 21 cm in line and continuum emission.

Aims. We remedy this situation by studying the northern Galactic plane at ~20′′ resolution in emission of atomic, molecular, and ionized gas.

Methods. Employing the Karl G. Jansky Very Large Array (VLA) in the C-array configuration and a large program, we observe the HI 21 cm line, four OH lines, nineteen Hnα radio recombination lines as well as the continuum emission from 1 to 2 GHz in full polarization over a large part of the first Galactic quadrant.

Results. Covering Galactic longitudes from 14.5 to 67.4 deg and latitudes between ± 1.25 deg, we image all of these lines and the continuum at ~20′′ resolution. These data allow us to study the various components of the interstellar medium (ISM): from the atomic phase, traced by the HI line, to the molecular phase, observed by the OH transitions, to the ionized medium, revealed by the cm continuum and the Hnα radio recombination lines. Furthermore, the polarized continuum emission enables magnetic field studies. In this overview paper, we discuss the survey outline and present the first data release as well as early results from the different datasets. We now release the first half of the survey; the second half will follow later after the ongoing data processing has been completed. The data in fits format (continuum images and line data cubes) can be accessed through the project web-page.

Conclusions. The HI/OH/Recombination line survey of the Milky Way (THOR) opens a new window to the different parts of the ISM. It enables detailed studies of molecular cloud formation, conversion of atomic to molecular gas, and feedback from Hii regions as well as the magnetic field in the Milky Way. It is highly complementary to other surveys of our Galaxy, and comparing the different datasets will allow us to address many open questions.