On the origin of correlated radio-optical asymmetries in double radio sources

Abstract

A new explanation is proposed for the striking correlation that, in powerful radio galaxies, the brighter [O II] λ3727 line emission is nearly always found on the side of the (shorter) radio lobe that is closer to the nucleus. As an alternative to the conventional interpretation, where the above result is understood by postulating a large-scale density asymmetry about the parent elliptical galaxy, we propose that 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 that region. This dust enrichment is caused by the motion of the galaxy, which not only stretches the downstream radio lobe but also results in a net displacement of the (dusty) interstellar medium of the galaxy toward that lobe; the galaxy motion further supplements this dust accumulation 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 environmental density asymmetry are pointed out, and observational tests for distinguishing between the two possibilities are outlined.