VLA Observations of the Inner Lobes of Centaurus A

Scaled-array VLA observations of the closest active radio galaxy Centaurus A at 1.4, 1.6, and 4.9 GHz with 4.5" x 1.2" resolution have been used to derive the spectral and polarimetric properties of the inner radio lobes of the source. The extended emission of the lobes is filamentary, similar to other well-resolved radio galaxies. Two compact radio features have been detected in the southern lobe which are roughly aligned with the axis of the main jet. This may be the first indication of a counterjet in Centaurus A. Also, new high-resolution (0.44" x 0.14")2 cm data show that portions of the inner jet may be overpressured relative to a plausible interstellar medium (ISM) by as much as a factor of 30. The rotation measure (RM) and the depolarization of the inner lobes are found to be highly correlated. In most of the northern lobe and in the southeastern half of the southern lobe, the RM is modest and consistent with the Galactic contribution. In these regions of the source, the data are consistent with no significant depolarization between 6 and 18 cm. In the central region of the source, the RM is substantially higher than that which could be ascribed to the Galaxy. Here, the depolarization is correspondingly high and the fractional polarization at 18 cm is less than half that at 6 cm. It is noted that the maximum extent of the region with high RM and depolarization corresponds to the steepest gradient in the optical emission from the stellar component of NGC 5128, and to the innermost concentration of "Malin shells." In addition, the disruption of the main jet occurs when the outflow reaches this boundary. It is suggested that a shock separating the ISM from the intergalactic medium may be responsible for the sharp gradients in the RM and depolarization, the concentration of Malin shells, and the disruption of the jet.