RXRGs

RXRGs, commonly referred to as X-shaped radio galaxies (XRGs), are a rare class of radio-loud active galactic nuclei whose radio emission displays an X-shaped morphology. The structure consists of a pair of active radio lobes aligned with the current jet axis and a secondary pair of wings oriented roughly orthogonal to that axis, creating an X when viewed in deep radio maps. The wings typically have lower surface brightness and may exhibit different spectral ages compared with the main lobes. Polarization measurements often show ordered magnetic fields, supporting a structured origin for the wings.

Several scenarios have been proposed to explain the X-shape. A leading idea is a rapid reorientation of the central black hole’s spin due to a major merger with another supermassive black hole, flipping the jet direction and producing a new pair of active lobes with remnant wings. Alternative explanations involve hydrodynamic backflow of jet plasma from the active lobes into the surrounding medium or jet precession caused by binary orbital motion or misaligned accretion. Some models invoke environmental asymmetries that preferentially channel plasma into wings.

RXRGs are typically hosted by giant elliptical galaxies and are found in groups or clusters. They are relatively rare, comprising a small fraction of radio galaxies, but modern wide-area surveys are increasing the known sample.

Notable examples include NGC 326, 3C 223.1, and 4C +48.48.

Significance: The study of RXRGs informs models of supermassive black hole mergers, jet dynamics, and AGN feedback on their environments.

See also: X-shaped radio galaxy.