ON THE ORIGIN OF CORRELATED RADIOOPTICAL ASYMMETRIES IN DOUBLE RADIO-SOURCES

A new explanation is proposed for the striking correlation that, in po werful radio galaxies, the brighter [O II] lambda 3727 line emission i s nearly always found on the side of the (shorter) radio lobe that is closer to the nucleus. As an alternative to the conventional interpret ation, where the above result is understood by postulating a large-sca le density asymmetry about the parent elliptical galaxy, we propose th at the side of the longer lobe is marked by a greater accumulation of dust, which leads to an increased attenuation of blue/UV photons in th at region. This dust enrichment is caused by the motion of the galaxy, which not only stretches the downstream radio lobe but also results i n a net displacement of the (dusty) interstellar medium of the galaxy toward that lobe; the galaxy motion further supplements this dust accu mulation toward the longer lobe by causing a ram-pressure stripping of the dusty disk of the galaxy. The many advantages of this asymmetric obscuration model over the conventional explanation invoking an enviro nmental density asymmetry are pointed out, and observational tests for distinguishing between the two possibilities are outlined.