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Astrophysics

Gravitational kick

Fusion of galaxies expels black hole from its nucleus

ESOThe M83 galaxy: with the collision of three black holes one of them was ejectedESO

Discovered in the skies of the southern hemisphere in 1752 by French astronomer Nicholas Louis de la Caille, the spiral galaxy M83 is probably the prettiest celestial formation in the Hydra constellation. From its yellowish center, two arms sprout, punctuated by blue and red stars that form a figure similar to a paper pinwheel. Because it is very bright and located relatively close to Earth, at about 15 million light-years away, it can be spotted with the aid of good binoculars. Although the contours of the galaxy are truly fascinating, a group of astronomers from Brazil, Argentina and Spain decided to examine a more hidden aspect of M83: the nature of its main sources of X-rays. Analysis of this type of emission led researchers to state that an extremely rare event probably took place there: a black hole, a kind of invisible object and one that is extremely dense that captures surrounding matter, had been ejected when approaching the nucleus of the galaxy, where two other, bigger black holes were on their way to starting a fusion process.

The expulsion of the mysterious celestial body from the center of the galaxy, also known as NGC 5236, was the result of gravitational interactions caused by an unusual triple collision. “The smaller black hole came too close to the other two and received a gravitational kick,” says astrophysicist, Horacio Dottori, from the Federal University of Rio Grande do Sul (UFRGS), the main author of an article published on July 1 in the The Astrophysical Journal Letters, in which he proposes the scenario described above. “It was ‘catapulted’ out of the optical nucleus of the galaxy at a speed of several hundred kilometers a second.” The conclusions are based on a new analysis of data obtained in 2000 by the Chandra space telescope from NASA, the American space agency, which detects X-ray emissions coming from the warmer regions of the Universe. The satellite observed M83 for 18 hours and produced 150 images. Eight photos caught what appears to be the ejection of a black hole from the galaxy.

The astrophysicists began to suspect that the above phenomenon might have occurred due to the characteristics of a mysterious source of X-rays and radio waves located in a relatively peripheral region of the galaxy, an object called J133658.3-295105. Some 3,300 light-years away from the heart of M83 according to measurements made by the Gemini telescope (in which Brazil is a partner), a distance about eight times less than that of the Earth to the center of the Milky Way, the object was apparently thrown out of the heart of the galaxy and created a trail of emissions at some wavelengths. Although a black hole does not directly furnish any sign of its presence, the so-called accretion disk formed around it is so hot (it reaches millions of degrees Kelvin) that it makes the matter about to be swallowed up, interstellar gas and dust, release massive amounts of energy, basically in the form of jets of X-rays.

Tim Jones / McDonald ObservatoryDrawing of the black hole expelled from a galaxyTim Jones / McDonald Observatory

The presence of large sources of X-ray near stars or in the center of galaxies is, therefore, the main indicator that there ought to be a black hole there. The problem is that other types of celestial bodies, such as neutron stars and quasars, also emit X-rays. However, Dottori and his Spanish and American colleagues are convinced that the object, J133658.3-295105, shows characteristics that are compatible with a super-massive black hole. According to their calculations, the mass of the black hole ejected from the nucleus of M83 is about 1 million times greater than that of the Sun and is the equivalent of a fifth of the sum of the masses of the two black holes that are moving towards becoming one in the center the galaxy. “We believe that the ejected black hole will orbit the galaxy for millions of years and that sporadically its accretion disk is going to capture matter from the interstellar medium,” says Dottori, who used computer simulations in his study that were made by Master’s degree student, Guilherme Gonçalves Ferrari. “It is unlikely that it’s going to be incorporated again by the active nucleus of M83.”

Double or triple collision
Today, most astrophysicists believe that at the center of virtually every galaxy, including the Milky Way, there is a really massive black hole endowed with a mass millions or even billions of times greater than that of the Sun. When two or more galaxies collide and begin their interactions to fuse, a relatively common process in the history of the Universe, their respective black holes also interact to unite. It is in this context, the union of galaxies, that a black hole may be expelled from the heart of the system. The scientific article of the astrophysicist from UFRGS is not the first to defend the idea of having found a black hole that has been ejected from its galaxy. Over the past five years, the works published on this subject have increased. However, in none of the cases reported do the scientists have unequivocal observational evidence on the occurrence of the elusive phenomenon. This is not necessarily a criticism of the scientific articles, but the realization of how difficult it is to prove such a finding. “The work of Horacio Dottori is good and was accepted in a good scientific journal, but is not conclusive,” says astrophysicist, John Steiner, from the Institute of Astronomy, Geophysics and Atmospheric Sciences, at the University of São Paulo (IAG / USP), a specialist in galaxies with active nuclei. The mathematical models show that the occurrence of triple collision is plausible, when the dynamic of interactions between the bodies causes the expulsion of the black hole with the smaller mass from the nucleus of a galaxy before the other two black holes merge with each other. “However, triple collisions are much rarer than double ones,” Steiner states.

There are cases in which the scientists say that a black hole was displaced from the center of a galaxy, but they are unable to indicate whether the phenomenon occurred because of a double or triple crash. On July 1 this year, the same period in which Dottori’s work was published, astrophysicists from the Harvard-Smithsonian Center for Astrophysics (CfA), published an article in the Astrophysical Journal in which they dealt with a discovery surrounded by this doubt. They found a super-massive black hole – actually a source of the emission of X-rays, called CID-42 – which had been expelled from the nucleus of a galaxy located in the Sextant constellation that is some 3.9 billion light-years away from Earth. Knowing whether the displacement results from the collision of two or three holes makes all the difference. The dynamics of events involved in one situation differ from those of the other.

Horacio Dottori, Ruben J. Dıaz, Juan F. Albacete-Colombo, Damian Mast / ChandraRadio wave and X-ray sources of M83: two black holes in the center and one ejected (trail)Horacio Dottori, Ruben J. Dıaz, Juan F. Albacete-Colombo, Damian Mast / Chandra

If CID-42 was driven from the center of the system due to the collision of three black holes, there was an expulsion process similar to that described by Dottori in M83. The smaller black hole would have been ‘catapulted’ out of the nucleus of the galaxy as it got too close to the other two. In other words, the new galaxy that is forming would have two black holes (not yet fused) at its center and a smaller one on its outer edge. If this is an encounter of just two objects that suck up matter, a type of event supposedly more common, the cause of the displacement of the black hole would be something else – and this detail would change everything.

Gravitational waves
In this second scenario, first the fusion of the two black holes would occur. Their two masses would be added together and they would generate a new and single black hole at the galaxy’s nucleus. The resulting black hole would then be thrown out of the heart of the system by the action of so-called gravitational waves, whose existence was predicted by Albert Einstein’s theory of general relativity almost a century ago, but that has not yet been experimentally proved. “In this case, after the ejection, the galaxy has no other black hole,” says astrophysicist, David Merritt, from the Rochester Institute of Technology (USA). “The detection of this phenomenon would be confirmation of Einstein’s theories.” When the ejection velocity is not very high, the black hole would just oscillate around the nucleus of the galaxy for a while and then, after a few million years, would return to its usual place. It should not be forgotten that the scenario of black holes is part of the Einstein universe in which space has four dimensions; the three in classical physics plus time.

Gravitational waves are defined as deformations caused in the curvature of space-time by the presence of large masses in motion. Just as sailing a boat produces oscillations in the ocean, the displacement of heavenly bodies would generate gravitational waves that would travel at the speed of light. In theory, a galaxy with two black holes on the way to fusing would be an ideal environment for confirming the existence of this new form of energy. “When two black holes fuse, the gravitational waves are not emitted symmetrically, but preferentially in certain directions. In the black hole this causes an effect similar to the recoil caused by the launch of a rocket,” says astrophysicist Stefanie Komossa, from the Max Planck Institute for Extraterrestrial Physics in Germany, who in 2008 found a probable case of black hole ejection in a galaxy that is 10 billion light-years from Earth. Therefore, the black hole would be thrown out of the galaxy in the opposite direction of the gravitational waves. “Only now are we seeing these events in nature,” says Komossa. “They are important to understand the formation and evolution of the galaxies.

Scientific article
DOTTORI, H. et al. The missing goliath’s slingshot: massive black hole recoil at M83. The Astrophysical Journal Letters. v. 717, n.1, p. L42-L46. 1o jul. 2010.

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