Galactic HI on the 50-AU scale in the direction of three extra-galactic sources observed with MERLIN

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, 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 to 10 arcsec and VLBI studies with resolutions of 10–20 milli-arcsec. 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.