2006A&A...447..489V

Localise and study the redshifted 21 cm HI absorption and the radio morphology of 2050+364. European VLBI Network UHF band (1049 MHz) spectral imaging, and examination of VLBI continuum images at multiple frequencies. 2050+364 may well be a one-sided core-jet source, which appears as a double (the well-known East-West component pair E, W) over a limited frequency range. W would then be the innermost visible portion of the jet, and could be at or adjacent to the canonical radio core. E is probably related to shocks at a sudden bend of the jet, towards an additional Northern extended steep-spectrum area (N), visible in our 1049MHz image. Our observations have resolved the HI absorbing region. A remarkably deep and narrow line component is present over the entire projected extent of 2050+364. It coincides in velocity with the [OIII] optical doublet lines to within 10km/s. This HI absorption could arise in the atomic cores of NLR clouds, and the motion in the NLR is then remarkably coherent both along the line-of-sight and across a projected distance of >300pc on the plane of the sky. Broader, shallower HI absorption at lower velocities covers only the plausible core area W. This absorption could be due to gas which is either being entrained by the inner jet or is flowing out from the accretion region; it could be related to the BLR.