X-shaped radio galaxy

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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

emission lines associated with quasar
activity. The host galaxies mostly exhibit high ellipticities and a number have nearby companion galaxies.

Origin

In their original catalog of 11 X-shaped galaxies, Leahy and Parma[3] proposed that the "wings were created in an earlier outburst, some tens of Myrs previous to the current renewal of nuclear activity, during which time the ejection axis has precessed." They noted that their proposal was consistent with the low surface brightness, steep radio spectrum, and high polarization of the wings, all of which are features associated with old (inactive) radio sources.

A widely discussed model for the origin of the X-shaped sources invokes a

gravitational waves. During the coalescence, the spin axis of the larger black hole undergoes a sudden reorientation due to absorption of the smaller hole's orbital angular momentum—a "spin-flip." Since the lobes are produced by jets that are launched perpendicularly to the inner accretion disk, and since the accretion disk is constrained by the Bardeen-Petterson effect
to lie perpendicular to the black hole's spin axis, a change in the spin orientation implies a change in the direction of the lobes. Even a rather small infalling black hole, with a mass approximately one-fifth that of the larger hole, could cause the spin of the latter to change by ninety degrees.

Alternative models to explain the X-shaped sources include a warping instability of the accretion disk;[5] backflow of gas along the active lobes[6] and binary-disk interactions before coalescence.[7] It is likely that all of these mechanisms are active at some level and that the time scale for realignment influences the radio source morphology, with the most rapid realignments producing the X-shaped sources, while slower realignment would cause the jet to deposit its energy into a larger volume, leading to an S-shaped FRI radio source.[4]

References

  1. ^ Murgia, M. (2001), A multi-frequency study of the radio galaxy NGC 326, Astron. Astrophys., 380, 102
  2. ^ Wang, T. et al. (2003), 4C +01.30: An X-shaped Radio Source with a Quasar Nucleus, Astron. J., 126, 113-118
  3. ^ Leahy, J. P. and Parma, P. (1992), Multiple outbursts in radio galaxies, Proc. 7th. I.A.P. Meeting: Extragalactic Radio Sources. From Beams to Jets, 307-308
  4. ^ a b Merritt, D. and Ekers, R. (2002), Tracing black hole mergers through radio lobe morphology, Science, 297, 1310
  5. ^ Pringle, J. E. (1996), Self-induced warping of accretion discs, Mon. Not. R. Astron. Soc., 281, 357-361
  6. ^ Leahy, J. P. and Williams, A. G. (1984), The bridges of classical double radio sources, Mon. Not. R. Astron. Soc., 210, 929-951
  7. ^ Liu, F. K. (2004), X-shaped radio galaxies as observational evidence for the interaction of supermassive binary black holes and accretion disc at parsec scale, Mon. Not. R. Astron. Soc., 347, 1357-1369

External links

See also

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