The radio source PKS 0400-181 was originally identified with a 16th magnitude spiral galaxy (Savage & Wall, 1976). Subsequent VLA observations, however, revealed it to be an extreme Cygnus-A type edge-brightened, double-lobed radio source (Shaver et al., 1983). Such sources had in the past always been associated with elliptical galaxies, so this apparent identification of a classical radio double with a spiral galaxy generated considerable interest. There were two possible interpretations: either this was the first case of a classical radio double physically associated with a spiral galaxy, or it was a mis-identification with an undetected background elliptical galaxy. As no radio core was detected down to relatively low flux densities, a definitive identification based on astrometry was not possible, and a variety of other observations also failed to solve the puzzle.
CCD R and I -band images finally provided a tentative answer. They revealed the presence of a very red and slightly extended object superimposed on the image of the spiral, just 24 from its centre and close to the radio centroid. It was concluded that this is probably a background elliptical, and that its properties (and those of the radio source) are consistent with a redshift of (Shaver et al., 1983).
Some years later the radical suggestion was made that spiral galaxies may be completely opaque (Valentijn (1990) - although this has been the subject of considerable controversy; e.g. Huizinga & van Albada (1992); White & Keel (1992) - it has even been suggested that distant quasars may be totally obscured by intervening galaxies (Ostriker & Heisler, 1994)). If that were the case, the above interpretation of PKS 0400-181 would have to be completely re-considered. We have tested this interpretation by obtaining a spectrum of the red object, and here we present the results of these observations.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998