RING-LIKE STRUCTURE IN THE RADIO LOBE OF MG-0248+0641

We present radio and optical observations of MG 0248 + 0641, which con tains a kiloparsec-scale ringlike structure in one of its radio lobes. The radio observations show a typical core-double morphology: a centr al core between two lobes, each of which has a hot spot. The western r adio lobe appears as a nearly continuous ring, with linear polarizatio n electric held vectors that are oriented in a radial direction from t he ring's center. We consider several different interpretations for th e nature of this ring, including gravitational lensing of a normal jet by a foreground galaxy. Even though simple lensing models can describ e the ring morphology reasonably well, the high linear polarization se en around the ring cannot be easily explained, and no lensing object h as yet been found in deep optical and infrared searches within the ext ent of the ring. If the radio ring is indeed caused by gravitational l ensing, the implied mass-to-light ratio is typical of the very high va lues 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 0 248+0641 is much like the prominent ring seen in 3C 219 and the multip le ones in 3C 310 and Hercules A, suggesting that similar physical pro cesses are producing shell structures in these radio galaxies. The rin g 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 triggere d by the infall of gas, via tidal interaction of the central source wi th a nearby galaxy. This scenario may be indicated by our marginal det ection of an optical source close to the western hot spot.