X-shaped 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

Elliptical galaxy

An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...

 that is the source of the lobes, giving the radio galaxy

Radio galaxy

Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process...

 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-flip

Spin-flip

A black hole spin-flip occurs when the spin axis of a rotating black hole undergoes a sudden change in orientation due to absorption of a second black hole....

s 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

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 ejected from the center of the galaxy by jets associated with the accretion disk around the supermassive black hole

Supermassive black hole

A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...

. 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

Spectral index

In astronomy, the spectral index of a source is a measure of the dependence of radiative flux density on frequency. Given frequency \nu and radiative flux S, the spectral index \alpha is given implicitly byS\propto\nu^\alpha....

 than the active lobes and are highly polarized. With one exception, none of the X-shaped sources shows the broad, optical emission lines associated with quasar

Quasar

A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

 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 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

Radio spectrum

Radio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz ....

, 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

Spin-flip

A black hole spin-flip occurs when the spin axis of a rotating black hole undergoes a sudden change in orientation due to absorption of a second black hole....

 of the supermassive black hole. In this model, a galaxy merger

Galaxy merger

Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies...

 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 hole undergoes a sudden reorientation due to absorption of the smaller hole's orbital angular momentum

Angular momentum

In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 -- 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;; backflow of gas along the active lobes and binary-disk interactions before coalescence. 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.

External links

• Massive black hole binary evolution Review article on binary black holes
• Scientists Detect "Smoking Gun" of Colliding Black Holes Popular article on X-shaped galaxies