Detection of the Galactic Warm Neutral Medium in Hi 21cm absorption

Detection of the Galactic Warm Neutral Medium in Hi 21cm absorption Abstract We report a deep Giant Metrewave Radio Telescope (GMRT) search for Galactic Hi 21cm absorption towards the quasar B0438−436, yielding the detection of wide, weak Hi 21cm absorption, with a velocity-integrated Hi 21cm optical depth of 0.0188 ± 0.0036 km s−1. Comparing this with the Hi column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 ± 365 K, one of the highest measured in the Galaxy. This is consistent with most of the Hi along the sightline arising in the stable warm neutral medium (WNM). The low peak Hi 21cm optical depth towards B0438−436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the Hi 21cm absorption and emission spectra. This yields a gas kinetic temperature of $$\rm T_k \le (4910 \pm 1900)$$ K, and a spin temperature of $$\rm T_s = (1000 \pm 345)$$ K for the gas that gives rise to the Hi 21cm absorption. Our data are consistent with the Hi 21cm absorption arising from either the stable WNM, with $$\rm T_s \ll T_k$$, $$\rm T_k \approx 5000$$ K, and little penetration of the background Lyman-α radiation field into the neutral hydrogen, or from the unstable neutral medium, with $$\rm T_s \approx T_k \approx 1000\;K$$. ISM: atoms, ISM: general, radio lines: ISM © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press

Detection of the Galactic Warm Neutral Medium in Hi 21cm absorption

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Publisher
journal_eissn:11745-3933
Copyright
© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
ISSN
1745-3925
eISSN
1745-3933
D.O.I.
10.1093/mnrasl/sly087
Publisher site
See Article on Publisher Site

Abstract

Abstract We report a deep Giant Metrewave Radio Telescope (GMRT) search for Galactic Hi 21cm absorption towards the quasar B0438−436, yielding the detection of wide, weak Hi 21cm absorption, with a velocity-integrated Hi 21cm optical depth of 0.0188 ± 0.0036 km s−1. Comparing this with the Hi column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 ± 365 K, one of the highest measured in the Galaxy. This is consistent with most of the Hi along the sightline arising in the stable warm neutral medium (WNM). The low peak Hi 21cm optical depth towards B0438−436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the Hi 21cm absorption and emission spectra. This yields a gas kinetic temperature of $$\rm T_k \le (4910 \pm 1900)$$ K, and a spin temperature of $$\rm T_s = (1000 \pm 345)$$ K for the gas that gives rise to the Hi 21cm absorption. Our data are consistent with the Hi 21cm absorption arising from either the stable WNM, with $$\rm T_s \ll T_k$$, $$\rm T_k \approx 5000$$ K, and little penetration of the background Lyman-α radiation field into the neutral hydrogen, or from the unstable neutral medium, with $$\rm T_s \approx T_k \approx 1000\;K$$. ISM: atoms, ISM: general, radio lines: ISM © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Journal

Monthly Notices of the Royal Astronomical Society: LettersOxford University Press

Published: May 17, 2018

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