Optical emission in the radio lobes of Cygnus A.

Deep optical imaging and spectroscopy of stellar objects in the radio lobes of Cygnus A have been carried out. Brighter optical objects close to the radio hotspots are either confirmed or suspected Galactic stars. Very faint optical emission is found coinciding with hotspot D (R~23.1) and hotspot B (V~25.4). The implications of these observations are discussed in the context of a double twin-jet model. In this model the radio lobes are assumed to contain a supermassive black hole each, and the twin jets emanating from the black holes are responsible for the radio emission. Assuming that the compact X-ray sources in the lobes arise from the accretion disks, the level of expected optical emission from the accretion disks is estimated. The present observations can rule out the model if the accretion disk is extensive, but they are not sensitive enough to exclude a truncated accretion disk, such as is expected in the slingshot ejection process. An approximate estimate for the cutoff frequency in hotspot D yields a magnetic field that is close to values obtained by other authors using minimun energy considerations.