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)
Monthly Notices of the Royal Astronomical Society: Letters – Oxford University Press
Published: May 17, 2018
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