AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 73. AGN: Radio Galaxies, QSO's and Blazars
Display, Wednesday, June 2, 1999, 10:00am-6:30pm, Southeast Exhibit Hall

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[73.05] Plasma Properties in the Lobes of the FRI Radio Galaxy 0755+37

S. Spangler, P. Parma, M. Bondi (Istituto di Radioastronomia del CNR, Bologna)

The lobes of radio galaxies radiate by synchrotron radiation, indicating the presence of relativistic electrons and magnetic fields. An understanding of the astrophysics of these objects also requires knowledge of the thermal plasma as well as the degree to which the magnetic field is turbulent. Thermal plasma can be revealed by Faraday rotation or depolarization, but it is difficult to distinguish between internal Faraday effects and ``false depolarization'' due to an external, Faraday rotating screen. We present scaled array, dual frequency Very Large Array observations of the Fanaroff-Riley Class I radio galaxy 0755+37 (z=0.0413). Observations at 1415 MHz were made with the A and C arrays, and at 4835 MHz with the B and D arrays. The resulting images have resolution of 1.90 arcseconds on a source with an angular size of 2.25 arcminutes. The high resolution assists in distinguishing between internal Faraday effects and those due to an external screen. From a detailed examination of the depolarization and position angle rotation maps, we argue that the effects of an external screen are negligible, and the polarization observables constrain plasma properties in the lobes. Plots of depolarization versus position angle rotation have been compared with the classic relationship for a uniform synchrotron source with internal thermal plasma. Our observations show less rotation for the observed depolarization than predicted by the uniform source model. This observation indicates that the magnetic field is turbulent, with several magnetic field cells along the line of sight. This interpretation is in reasonable agreement with the observed degree of linear polarization. Our main results are the apparent detection of thermal plasma in the lobes of this source and indication of the turbulent nature of the magnetic field.

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