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Black hole survives a galaxy wreck

Analysis of the galaxy ESO 243-49 in multiple wavelengths has detected the signature of hot stars swirling around a midsize black hole, highlighted by the white circle on this Hubble Space Telescope image. Astronomers say the readings suggest that the black hole is actually part of the leftovers from a dwarf galaxy that crashed into the bigger galaxy and disintegrated.




Astronomers have reconstructed what they think is a galactic crash scene, with a rare breed of black hole left behind amid a dwarf galaxy's wreckage. The Hubble Space Telescope played a key role in the accident investigation.

The black hole was detected three years ago in the edge-on spiral galaxy ESO 243-49, about 290 million light-years from Earth, and raised a question that's been bugging astronomers ever since.


The theoretical scenario for creating black holes through the collapse of stars is well-known. But scientists are just beginning to figure out how galaxy formation can lead to the creation of supermassive black holes that are millions or billions of times heavier than the sun. This particular black hole, designated HLX-1, was even more of a puzzler: It's about 20,000 times as massive as our sun, a kind of midsize monster that's rarely seen in our celestial neighborhood.

The astronomer who led the HLX-1 search effort, Sean Farrell of the University of Leicester and the Sydney Institute for Astronomy, took a closer look at the black hole with the aid of imagery in ultraviolet, visible and infrared wavelengths from Hubble, as well as X-ray imagery from NASA's Swift satellite. Now he and his colleagues are suggesting that the midsize black hole is a leftover from a dwarf galaxy's unfortunate encounter with the much bigger galaxy less than 200 million years earlier.

They came to that conclusion based on observations of light toward the reddish side of the spectrum — so much red light that it can't be explained just by the blaze of material falling into the black hole. Farrell and his colleagues think the light is coming from a cluster of hot stars surrounding the black hole.

"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," Farrell said in a news release from the European Space Agency's Hubble team. "The dwarf galaxy was then swallowed by the more massive galaxy."

As the dwarf galaxy was ripped apart, the black hole and some of its surrounding material would have survived.

The researchers say it's not yet clear what will happen to the black hole. It might spiral into the center of ESO 243-49, merging with the supermassive black hole that's already there. Or it might settle into a stable orbit in the bigger galaxy's outer environs. Either way, the X-ray emissions that brought the black hole to light in the first place will eventually fade away.

The findings from Farrell and his colleagues were published today by The Astrophysical Journal, and the team will continue watching HLX-1 for more clues.

Looking beyond just one intermediate-mass black hole, the astronomers say the case of HLX-1 sheds light on the bigger mysteries surrounding the formation of those supermassive, galaxy-scale black holes. Most theorists surmise that big galaxies — and the big black holes at their centers — are built up gradually through the merger of smaller galaxies. This research supports that view.

Our own Milky Way galaxy might well go through the next phase of the merger process in a few billion years, when it's due to mix it up with Andromeda and create a bigger behemoth nicknamed "Milkomeda."

More about galaxy mergers:


In addition to Farrell, authors of "A Young Stellar Population Around the Intermediate Mass Black Hole ESO 243-49 HLX-1" include M. Servillat, J. Pforr, T.J. Maccarone, C. Knigge, O. Godet, C. Maraston, N.A. Webb, D. Barret, A. Gosling, R. Belmont and K. Wiersema.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.