6. Summary: the canonical versus the present model
A comparison of the conventional explanation with the model explored here for the depolarization asymmetry of quasars, i.e., the cluster-size ICM core v/s the fat gaseous disk, posits several advantages in favour of the latter proposition. Firstly, even though the ionized portion of the captured disk is likely to grow in the radial direction (due to an ongoing heat input from the ambient corona of the elliptical), our model does not require the growth to be in tandom with the expansion of the radio source. This is unlike the conventional explanation which demands the ICM halo to somehow maintain a size close to the extent of the expanding radio source (Sect. 1). The present model only requires that the disk is large enough to occult the far-side radio lobe. For instance, even if a large disk has already formed while the radio source is just born and hence small in size, our explanation would still be viable. Secondly, in contrast to the observationally inferred decline in the abundance of clusters with dense ICM cores at higher redshifts (), there is a growing evidence for massive disks of dust and gas around radio galaxies and quasars, particularly at high redshifts (Sect. 1 & 3), where the depolarization asymmetry is found to be strong (Garrington & Conway 1991). Finally, the required ordering of the magnetic field on the kiloparsec scale within the Faraday screen proposed here, namely the gaseous superdisk, could arise in a fairly natural way; basically, the field inherited from the captured disk galaxy would probably get partially ordered as the disk material performs orbital motion around the accreting elliptical galaxy and, simultaneously, undergoes lateral expansion due to heating.
It may further be emphasized that the difference between the present and the conventional explanation for the depolarization asymmetry is not merely in the geometries of the depolarizing media; the two media have very different physical origin. In the near future, considerable progress in understanding the role of the gaseous superdisks invoked here should be possible by making detailed maps of the radio bridges, especially at metre wavelengths, as well as by sensitive imaging of the region of the radio continuum `gaps' in the HI line, and also in the far-infrared region using the recently launched ISO.
© European Southern Observatory (ESO) 1997
Online publication: April 20, 1998