2008MNRAS.388..165G

We present MERLIN observations of Galactic 21-cm Hi absorption at an angular resolution of ∼0.1-0.2 arcsec and a velocity resolution of 0.5km/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. Hi 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 Hi (earlier work probed Galactic Hi in the directions of the compact sources 3C 138 and 3C 147). The 0.1-arcsec scale is intermediate between Hi 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 Hi 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 Hi opacity at the level of about 1 are detected at the 2.5-3σ level in the direction of 3C 123.