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

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 0.5kms^-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. 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