Ringlike Structure in the Radio Lobe of MG 0248+0641

Abstract

We present radio and optical observations of MG 0248+0641, which contains a kiloparsec-scale ringlike structure in one of its radio lobes. The radio observations show a typical core-double morphology: a central core between two lobes, each of which has a hot spot. The western radio lobe appears as a nearly continuous ring, with linear polarization electric field vectors that are oriented in a radial direction from the ring's center. We consider several different interpretations for the nature of this ring, including gravitational lensing of a normal jet by a foreground galaxy. Even though simple lensing models can describe the ring morphology reasonably well, the high linear polarization seen around the ring cannot be easily explained, and no lensing object has yet been found in deep optical and infrared searches within the extent of the ring. If the radio ring is indeed caused by gravitational lensing, the implied mass-to-light ratio is typical of the very high values seen in other candidate dark gravitational lenses. The chance interposition of a Galactic supernova remnant, nova, planetary nebula, or H II region has been ruled out. The highly polarized ring of MG 0248+0641 is much like the prominent ring seen in 3C 219 and the multiple ones in 3C 310 and Hercules A, suggesting that similar physical processes are producing shell structures in these radio galaxies. The ring in MG 0248+0641 may be caused by the formation of bubbles as a result of instabilities in the energy flow down the western radio jet. It may also be possible that the required instabilities are triggered by the infall of gas, via tidal interaction of the central source with a nearby galaxy. This scenario may be indicated by our marginal detection of an optical source close to the western hot spot.