OH absorption in the first quadrant of the Milky Way as seen by THOR^★

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: rugel@mpia.de
² National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
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 Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁷ Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
⁸ Adjunct Astronomer at the Green Bank Observatory, PO Box 2, Green Bank, WV 24944, USA
⁹ Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA
¹⁰ I. Physikalisches Institut, University of Cologne, Zülpicher Str. 77, 50937 Köln, Germany
¹¹ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK
¹² School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK
¹³ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, INF 205, 69120 Heidelberg, Germany
¹⁴ Department of Physics, Indian Institute of Science, Bangalore 560012, India
¹⁵ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁶ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 31 August 2017
Accepted: 3 March 2018

Abstract

Context. The hydroxyl radical (OH) is present in the diffuse molecular and partially atomic phases of the interstellar medium (ISM), but its abundance relative to hydrogen is not clear.

Aims. We aim to evaluate the abundance of OH with respect to molecular hydrogen using OH absorption against cm-continuum sources over the first Galactic quadrant.

Methods. This OH study is part of the H I/OH/Recombination line survey of the inner Milky Way (THOR). THOR is a Karl G. Jansky Very Large Array (VLA) large program of atomic, molecular and ionized gas in the range 15° ≤ l ≤ 67° and |b|≤ 1°. It is the highest-resolution unbiased OH absorption survey to date towards this region. We combine the optical depths derived from these observations with literature ¹³CO(1–0) and H I observations to determine the OH abundance.

Results. We detect absorption in the 1665 and 1667 MHz transitions, that is, the “main” hyperfine structure lines, for continuum sources stronger than F_cont ≥ 0.1 Jy beam−1. OH absorption is found against approximately 15% of these continuum sources with increasing fractions for stronger sources. Most of the absorption occurs in molecular clouds that are associated with Galactic H II regions. We find OH and ¹³CO gas to have similar kinematic properties. The data indicate that the OH abundance decreases with increasing hydrogen column density. The derived OH abundance with respect to the total hydrogen nuclei column density (atomic and molecular phase) is in agreement with a constant abundance for A_V < 10−20. Towards the lowest column densities, we find sources that exhibit OH absorption but no ¹³CO emission, indicating that OH is a well suited tracer of the low column density molecular gas. We also present spatially resolved OH absorption towards the prominent extended H II-region W43.

Conclusions. The unbiased nature of the THOR survey opens a new window onto the gas properties of the interstellar medium. The characterization of the OH abundance over a large range of hydrogen gas column densities contributes to the understanding of OH as a molecular gas tracer and provides a starting point for future investigations.