Search for H i emission from superdisk candidates associated with radio galaxies

Abstract

Giant gaseous layers (termed “superdisks”) have been hypothesized in the past to account for the strip-like radio emission gap (or straight-edged central brightness depression) observed between 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. If they are entirely or partly composed of neutral gas, then 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 H i signal in any of the four sources, resulting in tight upper limits on the H i 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 cm3 and 1019−1020 atoms per cm2, respectively. No positive H i signal is detected even after combining all the four available H i images (with inverse variance weighting). This clearly rules out an H i dominated superdisk as a viable model to explain these structures, however, the possibility of a superdisk being composed of warm/hot gas still remains open.