Journal Article

Galactic H <span class="smallCaps">i</span> on the 50-au scale in the direction of three extragalactic sources observed with MERLIN

W. M. Goss, A. M. S. Richards, T. W. B. Muxlow and P. Thomasson

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 388, issue 1, pages 165-175
Published in print July 2008 | ISSN: 0035-8711
Published online July 2008 | e-ISSN: 1365-2966 | DOI:
Galactic H i on the 50-au scale in the direction of three extragalactic sources observed with MERLIN

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We present MERLIN observations of Galactic 21-cm H i absorption at an angular resolution of ∼0.1–0.2 arcsec and a velocity resolution of 0.5 km s−1, in the direction of three moderately low latitude (−8° < b < −12°) extragalactic radio sources, 3C 111, 3C 123 and 3C 161, all of which are heavily reddened. H i absorption is observed against resolved background emission sources up to ∼2 arcsec in extent and we distinguish details of the opacity distribution within 1–1.5 arcsec regions towards 3C 123 and 3C 161. This study is the second MERLIN investigation of small-scale structure in interstellar H i (earlier work probed Galactic H i in the directions of the compact sources 3C 138 and 3C 147). The 0.1-arcsec scale is intermediate between H i absorption studies made with other fixed element interferometers with resolution of 1–10 arcsec and very long baseline interferometry studies with resolutions of 10–20 mas. At a scale of 1 arcsec (about 500 au), prominent changes in Galactic H i opacity in excess of 1–1.5 are determined in the direction of 3C 161 with a signal-to-noise ratio of at least 10σ. Possible fluctuations in the H i opacity at the level of about 1 are detected at the 2.5–3σ level in the direction of 3C 123.

Keywords: ISM: kinematics and dynamics; ISM: structure; Galaxy: kinematics and dynamics; radio continuum: galaxies; radio continuum: ISM

Journal Article.  6738 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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