OPTICAL CONTINUUM SIGNATURE OF THE JET IN THE SPIRAL GALAXY NGC 4258

摘要

The large spiral nearby SABbc galaxy NGC 4258 has been imaged in the optical continuum using two filters at 5320 A and 7020 A with 200 A bandpasses which are free of emission lines. The color image produced by dividing the two bandpasses reveals an elongated feature at the location of the jet seen in the radio continuum and in Ha. The jet seen in the optical continuum appears as a "blue" feature compared to the galaxy bulge. We consider four possible origins for the optical continuum signature of the jet: (1) optical synchrotron radiation, (2) light from the invisible active nucleus scattered by interstellar electrons or dust particles in the channel bored in the interstellar medium by the high-energy particles of the jet, (3) continuum emission from shocks in colliding high-velocity clumps of gas, and (4) a passive mechanism: reduced extinction through the galaxy because of the absence of dust in the jet. It is shown that a combination of the last two effects may be the most likely explanation for the optical continuum observed in the jet of NGC 4258.