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X-shaped radio galaxy
X-shaped (or "winged") radio galaxies are a class of extragalactic radio source that exhibit two, low-surface-brightness radio lobes (the "wings") oriented at an angle to the active, or high-surface-brightness, lobes. Both sets of lobes pass symmetrically through the center of the elliptical galaxy that is the source of the lobes, giving the radio galaxy an X-shaped morphology as seen on radio maps (see figure).
X-shaped sources were first described by J. P. Leahy and P. Parma in 1992, who presented a list of 11 such objects. The X-shaped galaxies have received much attention following the suggestion in 2002 that they might be the sites of spin-flips associated with the recent coalescence of two supermassive black holes.
==Properties==
X-shaped galaxies are a sub-class of Fanaroff-Riley Type II (FRII) radio galaxies. FRII objects exhibit a pair of large (kiloparsec) scale radio lobes that straddle the parent elliptical galaxy; the lobes are believed to consist of plasma ejected from the center of the galaxy by jets associated with the accretion disk around the supermassive black hole. Unlike the classical FRII sources, the X-shaped galaxies exhibit ''two'', misaligned pairs of radio lobes of comparable extent. One pair of lobes, the "active" lobes, have a relatively high surface brightness and appear to be generated by ongoing emission from the center of the galaxy. The second set, the "wings", have a lower surface brightness, and appear to consist of plasma that was ejected along a different axis than that associated with the active lobes. The wings are also observed to have a higher spectral index than the active lobes and are highly polarized.〔Murgia, M. (2001), ( A multi-frequency study of the radio galaxy NGC 326 ), ''Astron. Astrophys''., 380, 102〕 With one exception,〔Wang, T. et al. (2003), ( 4C +01.30: An X-shaped Radio Source with a Quasar Nucleus ), ''Astron. J''., 126, 113-118〕 none of the X-shaped sources shows the broad, optical emission lines associated with quasar activity. The host galaxies mostly exhibit high ellipticities and a number have nearby companion galaxies.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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