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Detection of the Galactic warm neutral medium in HI 21-cm absorption

Patra, Narendra Nath and Kanekar, Nissim and Chengalur, Jayaram N and Roy, Nirupam (2018) Detection of the Galactic warm neutral medium in HI 21-cm absorption. In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 479 (1). L7-L11.

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Official URL: http://dx.doi.org/10.1093/mnrasl/sly087

Abstract

We report a deep Giant Metrewave Radio Telescope search for Galactic HI 21-cm absorption towards the quasar B0438-436, yielding the detection of wide, weak HI21-cm absorption, with a velocity-integrated HI 21-cm 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. The low-peak HI 21-cm optical depth towards B0438-436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the HI 21-cm absorption and emission spectra. This yields a gas kinetic temperature of T-k <= (4910 +/- < t > 1900) K, and a spin temperature of T-s = (1000 +/- 345) K for the gas that gives rise to the HI21-cm absorption. Our data are consistent with the HI 21-cm absorption arising from either the stable WNM, with T-s << T-K, T-k approximate to 5000 K, and little penetration of the background Lyman-alpha radiation field into the neutral hydrogen, or the unstable neutral medium, with T-s approximate to T-k approximate to 1000 K.

Item Type: Journal Article
Additional Information: Copy right for this article belong to OXFORD UNIV PRESS
Keywords: ISM: atoms; ISM: general; radio lines: ISM
Department/Centre: Division of Physical & Mathematical Sciences > Physics
Depositing User: Id for Latest eprints
Date Deposited: 25 Oct 2018 14:31
Last Modified: 25 Oct 2018 14:31
URI: http://eprints.iisc.ac.in/id/eprint/60954

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