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Extragalactic Radio Sources:

It is convincingly established that extragalactic radio sources radiate their energy by the commonly accepted incoherent synchrotron radiation process, i.e., radiation from relativistic electrons spiraling in a magnetic field. The evidence lies in the shapes of the spectra of the extended sources. The spectra are power law in agreement with synchrotron theory where the relativistic particles both gain and lose energy. Another evidence is the high degree of linear polarization which is another signature of synchrotron radiation.

The source of energy of these sources, the so-called central engine, is thought to be the associated quasar or active galactic nuclei (AGN) . Energy from the central engine appears to be transported to the outer lobes via a highly collimated beam of relativistic particles, called jets. Although many extended extragalactic radio sources have two sided large scale lobes, the radio jets in these sources are overwhelmingly one-sided. It is important to understand strongly in the powerful extended sources.

There are at least three physical interpretations of the apparent one-sidedness of the jets in these sources:

  1. the jets are intrinsically one-sided at a given time (flip-flop model)
  2. they are two sided, but the counterjet is intrinsically faint even though it might carry a similar energy as the main jet, that is, one is less dissipative.
  3. the jets have intrinsically similar emissivity, but relativistic Doppler beaming enhances the surface brightness of the jet that is closer to the line of sight.

My interest in the physics of extragalactic radio sources is related to the jet/counterjet search in powerful extended radio galaxies. This search was motivated by schemes that have been proposed to unify radio-loud FR II quasars and FR II radio galaxies as members of the same population of interacting galaxies observed in systematically different orientation to the line of sight (e.g., Bridle and Perley 1984; Barthel 1989). In Barthel's model, the parent population of intrinsically similar AGNs is randomly oriented, and the transition from radio-galaxy to quasar properties should occur around 44 degrees to the line of sight.

In order to check the scheme, I have proposed two main approaches. The first approach is to look for jet/counterjet in a sample of 13 radio galaxies and to compare that to the sample of 12 quasars observed by Bridle et al. (1992) and one other quasar by Fernini et al. (1991). The unified scheme suggests that counterjets should be easier to detect in RGs than in quasars. Initially, five radio galaxies were observed using the VLA at 6 cm in the A and B array. Long integration time were used for both arrays (3.5 hours for the A array and 2 hrs for the B array for each source ) . This long integration is necessary for the detection of any counterjet structure in RGs besides the main jet.

Preliminary results from the five RGs showed a define jet only in one source, and some probable/possible jets in three of the four remaining sources. The low (1 in 5) rate of unambiguous jet detection in this sample stands in strong contrast to the observations of a comparison group of extended 3CR QSRs by Bridle et al. (1992) and Fernini et al. (1991), in which 13 of 13 objects have unambiguous jets. One possible counterjet was found in one RGs in contrast to six faint counterjets candidates in QSRs. The much lower incidence of detectable jets in this first five sources of radio galaxies is broadly consistent with the RG/QSR unification scheme proposed by Barthel (1989) and with the trends of jet-to-lobe prominence in a larger sample of RGs and QSRs at these powers derived by Bridle (1992) These preliminary conclusions should be extended when the recent VLA data on the remaining eight sources will be fully reduced.

The second approach is to test the unification observing the sample at two other wavelengths (3.6 and 20 cm) in order to derive the polarization properties of the radio galaxies According to this scheme, the lobe that is fed by the brighter jet would also be closer to the observer. This lobe would be viewed along a shorter path through the magnetoionic medium, and would therefore depolarize at a longer wavelength than the other lobe. If the jets in FRII quasars are indeed oriented near the lines of sight than those in FRII radio galaxies, and all AGNs are surrounded by similar media, we would expect to find greater depolarization asymmetries in the quasars than in the radio galaxies.

Our preliminary results showed that there is little depolarization in these five radio galaxies as the wavelength increases from 3.6 to 20 cm, but three RGs showed significant depolarization between 6 and 20 cm. The one with the unambiguous jet has a strong depolarization asymmetry with the jetted lobe being the less depolarized at 20 cm. Combining our data with those of Garrington et al. (1991), we found no evidence for differences in the lobe depolarization asymmetry between RGs and QSRs at similar redshifts, but the sample is small. This first conclusion should be reviewed carefully once the whole sample is reduced.

I am seeking to continue the above work on the 3CR radio galaxies. With the two samples of RGs and QSRs at hand, the above conclusions should be extended and thoroughly checked. The observed properties of the two samples (core, jet prominence, depolarization asymmetry, jet/counterjet ratios, and emission-line gas asymmetry) can be used to check the unified scheme.

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