FIRST "Winged" and X-Shaped Radio Source Candidates

Abstract

A small number of double-lobed radio galaxies (17 from our own census of the literature) show an additional pair of low surface brightness ‘‘wings,’’ thus forming an overallX-shaped appearance. The origin of the wings in these ra- dio sources is unclear.Theymay be the result of backflowing plasmafromthe currently active radio lobes into an asym- metric medium surrounding the active nucleus, which would make these ideal systems in which to study thermal/ nonthermal plasma interactions in extragalactic radio sources. Another possibility is that the wings are the aging radio lobes left over after a (rapid) realignment of the central supermassive black holeYaccretion disk systemdue perhaps to a merger. Generally, these models are not well tested; with the small number of known examples, previous works have focused on detailed case studies of selected sourceswith little attempt at a systematic study of a large sample.Using the VLAFIRSTsurvey database, we are compiling a large sample ofwinged and X-shaped radio sources for such studies. Asafirst step toward this goal, an initial sample of 100 newcandidate objects of this type are presented in this paper.The search process is described, and optical identifications fromavailable literature data and basic radio data are presented. From the limited-resolution FIRST images (?500), we can already confidently classify a sufficient number of these objects as having the characteristic wing lengths of >80%of the active lobes tomore than double the number of known X-shaped radio sources.We have also included as candidates radio sources with shorter wings (<80% wing-to-lobe length ratios), or simply ‘‘winged’’ sources, as it is probable that projection effects are important. Finally, among the candidates are four quasars (z ¼ 0:37Y0:84), and several havemorphologies suggestive of Fanaroff-Riley type I (low- power) radio galaxies. While follow-up observations are necessary to confirm these identifications, this stresses the importance of source orientation and imaging limitations in finding these enigmatic objects