We present global VLBI observations of the 21-cm transition of atomic hydrogen seen in absorption against the radio source J0855+5751 .
The foreground absorber ( SDSS J085519.05+575140.7 ) is a dwarf galaxy at z = 0.026 .
As the background source is heavily resolved by VLBI , the data allow us to map the properties of the foreground \ion Hi gas with a spatial resolution of 2 pc .
The absorbing gas corresponds to a single coherent structure with an extent > 35 pc , but we also detect significant and coherent variations , including a change in the \ion Hi optical depth by a factor of five across a distance of \la 6 pc .
The large size of the structure provides support for the \citeauthor heiles03 model of the ISM , as well as its applicability to external galaxies .
The large variations in \ion Hi optical depth also suggest that caution should be applied when interpreting T _ { S } measurements from radio-detected DLAs .
In addition , the distorted appearance of the background radio source is indicative of a strong jet-cloud interaction in its host galaxy .
We have measured its redshift ( z = 0.54186 ) using optical spectroscopy on the William Herschel Telescope and this confirms that J0855+5751 is a FRII radio source with a physical extent of < 1 kpc and supports the previous identification of this source as a Compact Symmetric Object .
These sources often show absorption associated with the host galaxy and we suggest that both \ion Hi and OH should be searched for in J0855+5751 .