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).

Contents

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. [1] With one exception, [2] 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.

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 spin-flip of the supermassive black hole. [4] In this model, a galaxy merger causes a second, smaller supermassive black hole to be deposited near the center of the original radio galaxy. The smaller black hole forms a binary system with the larger black hole before the two coalesce via the emission of 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]

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X-ray binary Class of binary stars

X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor, to the other component, called the accretor, which is very compact: a neutron star or black hole. The infalling matter releases gravitational potential energy, up to several tenths of its rest mass, as X-rays. The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation.

Radio galaxy Type of active galaxy that is very luminous at radio wavelengths

A radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus. They have luminousities at radio wavelengths up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process. The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming. The host galaxies are almost exclusively large elliptical galaxies. Radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology. Recently, much work has been done on the effects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.

Supermassive black hole Largest type of black hole

A supermassive black hole is a type of black hole, with its mass being on the order of millions to billions of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way has a supermassive black hole in its Galactic Center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars.

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An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.

NGC 5548 Type I Seyfert galaxy in the constellation Boötes

NGC 5548 is a Type I Seyfert galaxy with a bright, active nucleus. This activity is caused by matter flowing onto a 65 million solar mass (M☉) supermassive black hole at the core. Morphologically, this is an unbarred lenticular galaxy with tightly-wound spiral arms, while shell and tidal tail features suggest that it has undergone a cosmologically-recent merger or interaction event. NGC 5548 is approximately 245 million light years away and appears in the constellation Boötes. The apparent visual magnitude of NGC 5548 is approximately 13.3 in the V band.

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NGC 4151 Intermediate spiral seyfert galaxy in the constellation Canes Venatici

NGC 4151 is an intermediate spiral Seyfert galaxy with weak inner ring structure located 15.8 megaparsecs from Earth in the constellation Canes Venatici. The galaxy was first mentioned by William Herschel on March 17, 1787; it was one of the six Seyfert galaxies described in the paper which defined the term. It is one of the nearest galaxies to Earth to contain an actively growing supermassive black hole. The black hole would have a mass on the order of 2.5 million to 30 million solar masses. It was speculated that the nucleus may host a binary black hole, with about 40 million and about 10 million solar masses respectively, orbiting with a 15.8-year period. This is, however, still a matter of active debate.

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Ursa Major B or 3C 244.1 is a radio galaxy located in the constellation Ursa Major.

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Astrophysical X-ray source

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Accretion disk Structure formed by diffuse material in orbital motion around a massive central body

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Anton Zensus German radio astronomer (born 1958)

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NGC 3998 Lenticular galaxy in the constellation of Ursa Major

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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. 1 2 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

See also

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