OH absorption in the first quadrant of the Milky Way as seen by THOR
OH absorption in the first quadrant of the Milky Way as seen by THOR
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DOI：
10.1051/0004-6361/201731872
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发表时间：
2018-03
影响因子：
6.5
通讯作者：
M. Rugel;H. Beuther;S. Bihr;Yuting Wang;Juergen Ott;A. Brunthaler;Andrew Walsh;S. Glover;P. Goldsmith;L. Anderson;Nicola Schneider;K. Menten;S. Ragan;J. Urquhart;R. Klessen;J. D. Soler;N. Roy;J. Kainulainen;T. Henning;F. Bigiel;Rory J. E. Smith;F. Wyrowski;S. Longmore
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文献类型：
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作者：
M. Rugel;H. Beuther;S. Bihr;Yuting Wang;Juergen Ott;A. Brunthaler;Andrew Walsh;S. Glover;P. Goldsmith;L. Anderson;Nicola Schneider;K. Menten;S. Ragan;J. Urquhart;R. Klessen;J. D. Soler;N. Roy;J. Kainulainen;T. Henning;F. Bigiel;Rory J. E. Smith;F. Wyrowski;S. Longmore
上下文。羟基自由基（OH）存在于星际介质（ISM）的弥漫分子和部分原子相中，但其相对于氢的丰度尚不清楚。目标。我们的目标是利用银河系第一象限上cm-连续源的OH吸收来评估相对于分子氢的OH丰度。方法。这项OH研究是内银河系（THOR）的H/ OH/重组线调查的一部分。THOR是卡尔·g·扬斯基（Karl G. Jansky）的超大型阵列（VLA）大型程序，在15°≤l≤67°和bb|≤1°范围内的原子、分子和电离气体。这是迄今为止对该地区进行的最高分辨率的无偏氢氧根吸收调查。我们将这些观测得到的光学深度与文献13CO（1-0）和H I观测相结合，以确定OH丰度。结果。对于强度大于Fcont≥0.1 Jy光束−1的连续谱源，我们检测到了1665和1667 MHz跃迁的吸收，即“主”超精细结构线。在这些连续源中，约有15%的源对OH有吸收，较强的源对OH的吸收比例增加。大部分吸收发生在与银河H II区有关的分子云中。我们发现氢氧根和13CO气体具有相似的运动学性质。数据表明，OH丰度随氢柱密度的增加而降低。推导出的OH丰度相对于总氢核柱密度（原子和分子相）与AV < 10−20时的恒定丰度一致。在最低柱密度处，我们发现源表现出OH吸收，但没有13CO发射，表明OH是低柱密度分子气体的合适示踪剂。我们还研究了在突出的扩展H ii区W43上的空间分辨OH吸收。结论。THOR调查的无偏性为研究星际介质的气体特性打开了一扇新的窗口。在大范围的氢气柱密度范围内对OH丰度的表征有助于理解OH作为分子气体示踪剂，并为未来的研究提供了一个起点。
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 Fcont ≥ 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 AV < 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.