Search for HI emission from superdisk candidates associated with radio
  galaxies
  Giant gaseous layers (termed "superdisk") have been hypothesized in the past
to account for the strip-like radio emission gap (or straight-edged central
brightness depression) observed between the twin radio lobes, in over a dozen
relatively nearby powerful Fanaroff-Riley Class II radio galaxies. They could
also provide a plausible alternative explanation for a range of observations.
Although a number of explanations have been proposed for the origin of the
superdisks, little is known about their material content. Some X-ray
observations of superdisk candidates indicate the presence of hot gas, but a
cool dusty medium also seems to be common in them. If they are made of entirely
or partly neutral gas, it may be directly detectable and we report here a first
attempt to detect/image any neutral hydrogen gas present in the superdisks that
are inferred to be present in four nearby radio galaxies. We have not found a
positive HI signal in any of the four sources, resulting in tight upper limits
on the HI number density in the postulated superdisks, estimated directly from
the central rms noise values of the final radio continuum subtracted image. The
estimated ranges of the upper limit on neutral hydrogen number density and
column density are $10^{-4}-10^{-3}$ atoms per cm$^3$ and $10^{19}-10^{20}$
atoms per cm$^{2}$, respectively. No positive HI signal is detected even after
combining all the four available HI images (with inverse variance weighting).
This clearly rules out an HI dominated superdisk as a viable model to explain
these structures, however, the possibility of superdisk being made of warm/hot
gas still remains open.
