Black Hole Gets Kicked Out of Galaxy


Supermassive black holes are thought to lie at the center of most large galaxies. But off in a distant remote galaxy, astronomers have possibly found a giant black hole that appears to be in the process of being expelled from the galaxy at high speed. This newly-discovered object was found by Marianne Heida, a student at Utrecht University in the Netherlands, and confirmed by an international team of astronomers who say the black hole was likely kicked out of its galaxy as a result of the merger of two smaller black holes.

Heida discovered the bizarre object, called CXO J122518.6+144545 during her final undergraduate project while doing research at the SRON Netherlands Institute for Space Research. To make the discovery she had to compare hundreds of thousands of X-ray sources, picked up by chance, with the positions of millions of galaxies. X-rays are also able to penetrate the dust and gas that surround black holes, with the bright source appearing as a starlike point. This object was very bright; however, it wasn’t at the center of a galaxy.

Super-massive black holes easily weigh more than 1 billion times the mass of the sun. So how could such a heavy object be hurled away from the galaxy at such high speeds? Astronomers say the expulsion can take place under special conditions when two black holes merge. The merger process creates a new black hole, and supercomputer models suggest that the larger black hole that results is shot out away at high speed, depending on the direction and speed in which the two black holes rotate before their collision.

And, the team of astronomers say, there could be more of these “recoiling” black holes out there. “We have found even more of this strange class of X-ray sources,” said Heida. “However, for these objects we first of all need accurate measurements from NASA’s Chandra satellite to pinpoint them more precisely.”

If this object is not a recoiling black hole, other possibilities are that it could possibly be either a very blue type IIn supernova or a ULX (ultra-luminous X-ray source) with a very bright optical counterpart.

Finding more of these expelled black holes will provide a better understanding of the characteristics of black holes before they merge. In the future, astronomers hope to even observe this process with the planned LISA satellite, which will be able to measure the gravity waves that the two merging black holes emit. Further research will provide more insight into how supermassive black holes are created.

Paper: “A bright off-nuclear X-ray source: a type IIn supernova, a bright ULX or a recoiling super-massive black hole in CXO J122518.6+144545”.

Sources: SRON, Royal Astronomical Society

10 Replies to “Black Hole Gets Kicked Out of Galaxy”

  1. I am scratching my head a bit over how this recoiling mechanism works, but I think I have some possible sense of this. The explanation above seems to indicate that how the two black holes are rotating has some influence on the recoil. Yet at first this seems to imply that angular momentum is converted to linear momentum, which is not possible. When two black holes collide there is the production of gravitational radiation. This is a quadrupole wave, for dipole radiation P = m*x implies the need for a net dP/dt, which violates conservation of momentum. So if there is the production of an asymmetrical quadrupole moment wave, this might then carry off a large amount of momentum, resulting in an equal recoil of the merged BHs. For rotating black holes the conversion of the initial mass-energy of the two black holes into gravity waves has an upper limit of around 50%. For non-rotating black holes the limit is about 29%, but for rotating black hole collisions there is the prudction of a “twist” component to the wave.

    The paper by P.G. Jonker, M.A.P. Torres, A.C. Fabian, M. Heida, G. Miniutti, D. Pooley, referenced by Nancy Atkinson, does suggest alternative mechanisms for their observations. So the case for the BH recoil is not presented as completely solid, and the authors are being properly conservative on this.


  2. The black hole was probably in a cluster of stars of other massive objects. Likely the energy was generate in a slingshot effect from a very close encounter, which brutally ejected it at high velocity.The process is very similar to what happens in multiple stars all the time. i.e. The unstable Trapezia (like the mini-group at the heart of the Orion Nebula), where the components ‘gang up’ on one component and eject it from the system. Once done, the object normally turns into a triple, being a close binary with a third more distant component. The latter being more far more stable gravitationally.
    All this curiosity and nicely related to conservation of angular momentum. Gotta love celestial mechanics and the ol’ Newton!

  3. But I would assume that a more lightweight object and not the massive monster gets the kick.

    So, if the SMBH is surrounded by a “massive” cluster, I would think that some stars (be that neutron stars or maybe lightweight BHs) get ejected and not the SMBH.

    And I wonder also, why the merger of two SMBH should send one out into space. With three objects this is possible, but only with two? I second here LBC’s doubts and thoughts….

  4. So essentially the two BH’s collide creating a new SMBH. This new SMBH is carrying on the momentum of the quicker BH(the one which collides at a higher rate of speed) which eventually ejects itself away from the smaller BH? I guess when the larger BH gets ejected, it stabilizes it’s smaller brother? Some of this stuff is tough to follow. Could somewhere correct me if I’m wrong?

  5. The collision of black holes is not at all the same as two colliding billiard balls. The spacetime picture might be compared to a pair of pants, though I will then demolish this picture below, where the two legs are two black holes parameterized in spacetime. The vertical direction is time, and the radii of the two legs define the mass of the black holes and their event horizons. The two black holes merge into the larger black hole where now the waist measure is the measure of the mass of the new black hole. The subsequent black hole can’t be split back in two. Remember that the material of the pants represents the event horizon, which is a null region where the proper distance is zero and light particles are suspended. The reason why is that the crotch of the pant is a null point and the splitting of the black hole into two (turn the pants upside down with the legs pointing up) means that a region at the crotch, which again is a null region, propagates timelike information into the future. This is not permitted in relativity, for it is a transformation of a null path into a timelike path.

    The collision f two black holes results in gravity waves. Some of the mass-energy content of the original black holes is converted into gravity waves. Working this out is a difficult matter, for defining energy in any local region of spacetime is a slippery problem. I will avoid that problem here for brevity. This is analogous in a weak limit, or far from the collision region of black holes, to the scattering of charges in electromagnetism which produces electromagnetic waves. The gravitational interaction of this form has a field response in the form of gravitational waves — moving ripples in spacetime.

    Now I am going to demolish the pants model of black hole collisions. It requires that you take off that pair of pants and put on a skirt, preferably somewhat above knee length. Your legs going into and then above the hemline are the two black holes which collide, and the skirt is the rather stunning occurrence of the subsequent black hole formed by the merger, or what is called the “apparent horizon.” The two original black holes continue onwards and indeed endlessly inside this new apparent horizon. This is a rather difficult thing two work through, and Stephen Hawking was the first guy to show this, and it is astounding result in using so called global methods in general relativity. This can be found in the Ellis & Hawking “Large Scale Structure of Spacetime,” with a picture of this apparent horizon as a skirt-like structure that enshrouds the two black holes. Turning this upside down means that a spacelike region propagates timelike information into the future, which again is not permitted by general relativity.


  6. Ps, I wrote “This is a rather difficult thing two work through” where two should be to. I also wrote this because I could resist saying that since black holes have no hair (a theorem of classical gravity) that before putting on that skirt, shaving one’s legs makes for better model accuracy.


  7. I am looking at other objects in the image which look very similar to the circled object and can’t visually see anything that distinguishes it as a BH. Are there other instruments and procedures involved in this discovery?
    I thought the consensus was that gravitational effects on it’s surrounding environment was the only way to infer a BH. Hope I’ve made sense.

  8. @ wjwbudro

    Well, obviously they have some additional information about this object than just this “optical” photograph.
    I can just speculate, but this could be spectra, or photos at different wavelength – but I don’t know…

  9. The BH is inferred by signatures of accretion disks. These signatures are spectral and Doppler shift based.


  10. Ah, yes, I was considering the SMBH influencing the “visible” close in stars such as in our own Milky Way.

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