John Dreher

Multifrequency radio observations of Cygnus A - Spectral aging in powerful radio galaxies

A detailed analysis of the radio spectrum across the lobes of Cygnus A is presented in order to critically test the synchroton spectral aging theory. The results are in good agreement with the jet model for powerful radio galaxies, involving particle acceleration at the hot spots and outflow into the radio lobes, with subsequent energy loss due to synchrotron radiation. The hot spot spectra are well represented by a spectral aging model involving continuous injection of relativistic particles. Both hot spots have spectral break frequencies around 10 GHz. An injection index of 0.5 is found for both hot spots, consistent with diffusive shock acceleration at a strong nonrelativistic shock in a Newtonian fluid. The LF hot spot emission spectrum falls below the injected power law. This effect is isolated to the hot spots, and is best explained by a low-energy cutoff in the particle distribution.

Faraday rotation of Cygnus A - magnetic fields in cluster gas

Multifrequency, multiconfiguration observations with the VLA have allowed, for the first time, detailed mapping of the rotation measure (RM) of the prominent extragalactic radio source Cygnus A. It is found that this source lies behind a deep Faraday screen, with the rotation measure varying from -4000 to +3000 rad/sq m for the eastern lobe, and from -2000 to +1500 rad/sq m for the western lobe. Gradients in both lobes commonly exceed 300 rad/sq m/arcsec. The overall pattern in RM is the same in each lobe, increasing from east to west, leading to the conclusion that the magnetic field is ordered on scales of about 20 to 30 kpc. There is no evidence for internal depolarization. The authors examine, and reject, the hypothesis that this Faraday screen has a Galactic origin and propose that its origin is in the intracluster gas in which Cygnus A is embedded. 66 references.

Broad- and narrow-band imaging of the giant radio galaxy Cygnus A

Optical broadband and narrowband images of Cygnus A have been compared to VLA radio images of the source. The zero-redshift H-alpha image reveals the distribution of ionized Galactic material toward the radio source. The low emission values noted imply that the large amounts of Faraday rotation observed toward the radio lobes of the galaxy must be of extragalactic origin. It is shown that the radio-core source is located between two optical broadband peaks in the galaxy nucleus, and that the radial-surface brightness profiles follows a de Vaucouleurs r exp 1/4 law out to at least 31 kpc. It is suggested that the double optical nucleus observed is not caused by a kiloparsec-scale dust lane, but by polar radiation from a core source that is obscured by a disk. 36 refs.