The temperature of the neutral interstellar medium in the Galaxy

Patra, NN and Roy, N (2024) The temperature of the neutral interstellar medium in the Galaxy. In: Monthly Notices of the Royal Astronomical Society, 529 (4). pp. 4037-4049.

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Abstract

Atomic Hydrogen-21 cm transition (H I) is an excellent tracer to study and understand the properties of the atomic gas in the Galaxy. Using the Westerbork Synthesis Radio Telescope, we observed 12 quasar sightlines to detect galactic H I in absorption. We achieve an optical depth rms of �1-2 � 10-3, essential to detect the warm neutral medium (WNM). We detect H I absorption in all our sightlines except along 1006+349, for which we set a strict upper limit on the spin temperature as �Ts� > 570 K. We find around 50 per cent of our sightlines have �Ts� > 500 K, indicating a WNM dominance. Further, we calculate an upper limit of the cold neutral medium (CNM) fraction along our sightlines and find a median CNM fraction of �0.12. With our observations, we reconfirm the existence of a threshold column density of �2 � 1020 atoms cm-2 to form CNM in the interstellar medium (ISM). Using a two-temperature model of the H I disc, we explore the distribution of spin temperature in the Galactic ISM. We find that a simple fixed axisymmetric two-temperature model could not produce either the observed column density or the integral optical depth. This indicates the existence of a more complex distribution of spin temperatures in the Galaxy. © 2024 The Author(s).

Item Type:	Journal Article
Publication:	Monthly Notices of the Royal Astronomical Society
Publisher:	Oxford University Press
Additional Information:	The copyright for this article belongs to authors.
Keywords:	Atoms; High frequency telecommunication lines; Optical properties, Galaxies: spirals; Galaxy: interstellar medium; Interstellar medias; Interstellar medium: atom; Interstellar medium: kinematic and dynamic; Kinematics and dynamics; Medium kinematics; Radio line: interstellar medium; Radio lines:galaxies, Galaxies
Department/Centre:	Division of Physical & Mathematical Sciences > Physics
Date Deposited:	11 Jul 2024 05:49
Last Modified:	11 Jul 2024 05:49
URI:	http://eprints.iisc.ac.in/id/eprint/84804

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