Mapping HI absorption at z=0.026 against a resolved background CSO

Observing atomic hydrogen in absorption is an extremely powerful probe of galaxies at all red-shifts: the detection sensitivity is not dependent on many of the properties of the system under study, but rather depends on the unrelated characteristics of the background source. The H I hyper-fine 21-cm line enables a determination of the kinematics and gas distribution in the intervening absorbers and presents several advantages over Ly-α as it does not saturate and is unaffected by dust. One application is the study of small-scale structure in the ISM of external galaxies as, if the background radio source is resolved into several components, different independent sight lines through the absorbing gas layer can be studied. Here we present Global VLBI observations of J0855+5751, a Jy-strength Compact Symmetric Object against which H I absorption at a red-shift of 0.026 was previously discovered using observations with the Green Bank Telescope. The combination of high angular resolution and sensitivity has allowed us to map the H I absorption across the lobes and to probe the spatial distribution of cold gas in the foreground galaxy on scales ranging from 3 to 30 pc.