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Black Hole Flags Galactic Collision

HLX-1 (Hyper-Luminous X-ray source 1) is thought to be a black hole, one that’s a welcome discovery for astronomers trying to puzzle out the mysteries of black hole formation. Located roughly 290 million light years from Earth and situated toward the edge of a galaxy called ESO 243-49, this black hole looks to be some 20,000 times the mass of the Sun, which makes it mid-sized when compared with the supermassive black holes at the center of many galaxies. The latter can have masses up to billions of times more than the Sun — the black hole at the center of our own galaxy is thought to comprise about four million solar masses.

Just how a supermassive black hole forms remains a subject for speculation, but study of HLX-1 is giving us clues that point in the direction of a series of mergers of small and mid-sized black holes. For it turns out that HLX-1, discovered by Sean Farrell (Sydney Institute for Astronomy in Australia and University of Leicester, UK) and team at X-ray wavelengths, shows evidence for a cluster of young, hot stars surrounding the black hole itself. Working in ultraviolet, visible and infrared light using the Hubble instrument as well as in X-rays using the Swift satellite, the team found that the accretion disk alone could not explain the emissions they were studying.

All of this leads to an interesting supposition about the black hole’s origins, as Farrell notes:

“The fact that there’s a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very low-mass dwarf galaxy. The dwarf galaxy was then swallowed by the more massive galaxy.”

Image: This spectacular edge-on galaxy, called ESO 243-49, is home to an intermediate-mass black hole that may have been purloined from a cannibalised dwarf galaxy. The black hole, with an estimated mass of 20,000 Suns, lies above the galactic plane. This is an unlikely place for such a massive back hole to exist, unless it belonged to a small galaxy that was gravitationally torn apart by ESO 243-49. The circle identifies a unique X-ray source that pinpoints the black hole. Credit: NASA, ESA, and S. Farrell (University of Sydney, Australia and University of Leicester, UK).

The cluster of young stars appears to be about 250 light years across and encircles the black hole. We may be looking, then, at the remains of a galaxy that has been effectively destroyed by its collision with another galaxy, while the black hole at the dwarf’s center interacted with enough gaseous material to form the new stars. Farrell and Mathieu Servillat (Harvard-Smithsonian Center for Astrophysics) peg the cluster’s age at less than 200 million years, meaning that most of these stars would have formed after the dwarf’s collision with the larger galaxy. Says Servillat:

“This black hole is unique in that it’s the only intermediate-mass black hole we’ve found so far. Its rarity suggests that these black holes are only visible for a short time.”

The paper on this work discusses the process in greater detail:

Tidally stripping a dwarf galaxy during a merger event could remove a large fraction of the mass from the dwarf galaxy, with star formation triggered as a result of the tidal interactions. This could result in the observed IMBH [intermediate mass black hole] embedded in the remnant of the nuclear bulge and surrounded by a young, high metallicity, stellar population. It has been proposed that such accreted dwarf galaxies may explain the origin of some globular clusters, with the remnant cluster appearing more like a classical globular cluster as its stellar population ages.

HLX-1’s X-ray signature is still relatively bright (which is how Farrell found it in 2009 using the European Space Agency’s XMM-Newton X-ray space telescope). But as it depletes the supply of gas around it, the black hole’s X-ray signature will weaken. Its ultimate fate may indeed be to spiral into the center of ESO 243-49, to merge with the supermassive black hole there. Consider intermediate-sized black holes like this one the ‘missing link’ between stellar mass black holes and their supermassive counterparts at galactic center. The nuclei of dwarf galaxies (along with globular clusters) have previously been suggested as the likely environments for their formation, ideas which are given powerful support through the study of what’s happening in ESO 243-49.

The paper is Farrell et al., “A Young Massive Stellar Population Around the Intermediate Mass Black Hole ESO 243-49 HLX-1,” accepted for publication in The Astrophysical Journal Letters (preprint).


Comments on this entry are closed.

  • ljk February 21, 2012, 10:08

    Astronomers think the massive globular star cluster known as Omega Centauri is actually the remains of a galaxy that collided with the Milky Way galaxy ages ago.

    OC has many more stars than all the other globular clusters in our galaxy and, most importantly, a black hole at its center.


  • Michael Weil February 21, 2012, 13:35

    The intermediate-size black hole described in the article above is first cited as being equivalent to 20 THOUSAND Sun masses, but in the caption under the picture of this very same entity and its host galaxy, the size of said black hole is noted to be 50 MILLION Sun masses; is there perhaps a bit of an inadvertent disconnect here?

  • Interstellar Bill February 21, 2012, 15:43

    If giant black holes are the results of mergers of smaller holes,
    then there must have been many merger events in our Galaxy’s past.

    Does anybody have references on the radiation product by said mergers?
    Is it enough for a Galactic Mass Extinction? That’s GME, folks.

    The last GME here, if any, must have been over a billion years ago,
    before Earth had any multicellular life to be affected,
    because our fossil record shows no such macro-event.

    This is a parallel to previous GME scenarios via gamma-ray bursts,
    but BH mergers must be terribly violent.

  • Paul Gilster February 21, 2012, 16:30

    Michael Weil writes:

    The intermediate-size black hole described in the article above is first cited as being equivalent to 20 THOUSAND Sun masses, but in the caption under the picture of this very same entity and its host galaxy, the size of said black hole is noted to be 50 MILLION Sun masses; is there perhaps a bit of an inadvertent disconnect here?

    Indeed, and it’s a reminder to me not to cut and paste a caption without a closer check! I’ve corrected the caption to reflect the NASA/ESA news release. Intermediate mass black holes, according to the paper, are defined as being ∼102 to 105 solar masses. I’ve emailed Sean Farrell re the caption error, which ran in the news release yesterday.

  • Ron S February 21, 2012, 17:54

    Interstellar Bill, the only radiation from the merger is gravitational radiation. The reason is that there is no matter outside the event horizons, which is needed for the production of particles or photons.

    If at least one of the BH’s has an accretion disk, or one of the BH’s is replaced by some other body, there would be more than gravitational radiation resulting from the merger. For example, a merger between a BH and a neutron star is a possible GRB source.

  • Rob Henry February 21, 2012, 22:13

    Ron S, I also thought that the product of black hole mergers was very hard to model, and thus the radiation it produced very hard to calculate due to the extreme nature of the conditions. It was my understanding that all the radiation being gravitational was more of a reasonable assumption rather than an outcome proved by theory, and if that is not still the case I would love a reference if possible.

  • Ron S February 22, 2012, 11:24

    Rob, what sort of reference do you need? There is no source of photons or particles, and therefore there is nothing to model in that regard. What you’re asking is a bit analogous to asking that if, in the vacuum of space, an explosion is sufficiently violent will you be able to hear it? No.

    Yes, BH merger is difficult to model — requiring high-precision evolution of the full non-linear EFE — but that will simply tell us the details of the merger process and the resulting gravitational radiation signature. An instrument such as LIGO is needed to detect BH mergers, unless (as I said) there is matter bound to one or both BH’s that can produce photons or particle jets.

  • Rob Henry February 22, 2012, 16:53

    Ron S, I note that the idea of Hawking radiation also doesn’t make sense through similar argument. Personally I was wondering about those extreme tidal forces on matter just a parsec away. Also (but a mutually exclusive thought from that first) I couldn’t help wondering that if some of those gamma ray bursters were as powerful as they seemed without the aid of focusing into a narrow beam, then the couple of solar masses of energy represented could be explained if a very small fraction of the energy released in such collisions was converted into gamma rays.