Galactic Hi on the 50-au scale in the direction of three extragalactic sources observed with MERLIN

Abstract

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.