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

TL;DR

This study uses MERLIN to observe small-scale Galactic HI absorption structures at 0.1-0.2 arcsec resolution towards three extragalactic sources, revealing significant opacity variations on scales of hundreds of AU.

Contribution

It provides high-resolution measurements of Galactic HI small-scale structure, bridging the gap between previous interferometric and VLBI observations.

Findings

Detected large opacity variations (>1.5) towards 3C161.

Observed potential 1-level opacity fluctuations towards 3C123.

Achieved 10 sigma detection of opacity changes at 1 arcsec scale.

Abstract

We present MERLIN observations of Galactic 21-cm HI absorption at an angular resolution of c. 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 deg) extragalactic radio sources, 3C111, 3C123 and 3C161, all of which are heavily reddened. HI absorption is observed against resolved background emission sources up to c. 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 3C138 and 3C147). The 0.1-arcsec scale is intermediate between HI 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 HI opacity in excess of 1-1.5 are determined in the direction of 3C161 with a signal-to-noise ratio of at least 10 sigma. Possible fluctuations in the HI opacity at the level of about 1 are detected at the 2.5-3 sigma level in the direction of 3C123.