On the origin of X-shaped radio galaxies

• Published in: Research in astronomy and astrophysics Vol. 12; no. 2; pp. 127 - 146
• Main Authors: Biermann, Peter L., Gergely, László Á., Wiita, Paul J.
• Format: Journal Article
• Language: English
• Published: Department of Physics & Astronomy, University of Bonn, 53115 Bonn, Germany 01.02.2012; Department of Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA; National Centre for Radio Astrophysics, TIFR, Pune University Campus, Post Bag 3, Ganeshkhind, Pune 411007, India%Max Planck Institute for Radioastronomy, Auf dem Hüge 169, 53121 Bonn, Germany; KIT-Karlsruhe Institute for Technology, Institute for Experimental Nuclear P%Department of Theoretical Physics, University of Szeged, Tisza Lajos k(o)rút 84-86, 6720 Szeged, Hungary; Department of Physics, University of Alabama at Huntsville, Huntsville, AL 35899, USA; Department of Experimental Physics, University of Szeged, Dóm tér 9,6720 Szeged, Hungary%Department of Physics, The College of New Jersey, P.O. Box 7718, Ewing, NJ 08628-0718,USA
• Subjects: 半人马座; 多波段观测; 射电星系; 广义模型; 流体力学; 相互作用; 起源; ISM; gravitational waves; galaxies: jets; galaxies: active; radio continuum: galaxies
• ISSN: 1674-4527; 2397-6209
• DOI: 10.1088/1674-4527/12/2/002

After a brief, critical review of the leading explanations proposed for the small but important subset of radio galaxies showing an X-shaped morphology （XRGs） we propose a generalized model, based on the jet-shell interaction and spin- flip hypotheses. The most popular scenarios for this intriguing phenomenon invoke either hydrodynamical backflows and over-pressured cocoons or rapid jet reorienta- tions, presumably from the spin-flips of central engines following the mergers of pairs of galaxies, each of which contains a supermassive black hole. We confront these models with a number of key observations, and thus argue that none of the models is capable of explaining the entire range of the salient observational properties of XRGs, although some of the arguments raised in the literature against the spin-flip scenario are probably not tenable. We then propose a new scenario which also involves galac- tic mergers but would allow the spin of the central engine to maintain its direction. Motivated by detailed multi-band observations of the nearest radio galaxy, Centaurus A, this new model emphasizes the role of the interactions between the jets and the shells of stars and gas that form and rotate around the merged galaxy and can cause temporary deflections of the jets, occasionally giving rise to an X-shaped radio struc- ture. Although each model is likely to be relevant to a subset of XRGs, the bulk of the evidence indicates that most of them are best explained by the jet-shell interaction or spin-flip hypotheses.