VLBI observations of high-opacity HI gas in NGC 5793

We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the λ=21 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of 2×10²² cm⁻² assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes (≈15 pc) and atomic gas densities (≈200 cm⁻³). Our results suggest that the HI gas is not associated with the <10 pc region which presumably contains the H₂O masers, but it is more distant from the nucleus, and is probably associated with the r1 kpc gas seen in CO.