2012A&A...546A..22S

We present Very Long Baseline Array (VLBA) HI absorption observations of the core regions of the powerful radio galaxies 3C 236 and 4C 31.04. Broad absorption (Δv≃350km/s) straddling the systemic velocity is detected in both sources, in each case concentrated on one side of the radio core. We argue that the observations are consistent with the presence of HI absorbing disks with r≃900pc and r≥200pc in 3C 236 and 4C 31.04 respectively - although given the limited spatial coverage of the HI absorption other interpretations cannot be ruled out. The spatially resolved absorption detected against the counter-jet of 3C 236 shows a velocity gradient with an amplitude and direction supporting a rotating disk model; in contrast only a tentative velocity gradient is seen in the 4C 31.04 data set. While on larger scales both radio galaxies show evidence for interaction and merging we find no direct evidence in either source that cold gas is in-falling onto the radio core. In 3C 236 we instead find against the core blue-shifted absorption that might be part of the fast outflow of neutral gas detected in more sensitive (but lower spatial resolution) observations. Along the main jet of 3C 236 we find clumps of high opacity HI absorption concentrated to one side of the jet. We speculate that these HI clumps may be interacting with the jet, potentially even being the cause of the misalignment of the eastern jet between kiloparsec and megaparsec scales. In addition to its broad absorption 4C 31.04 shows a narrow absorption component (FWHM=23km/s) redshifted relative to the galaxy systemic velocity by 115km/s. This feature may be a residual sign of a recent accretion/merging event, however the absorbing gas cloud must be located ≫100pc from the core.