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Dwarf galaxy hides a cosmic ‘Little Big Man’


Dwarf galaxies are usually thought to be too small to contain supermassive black holes yet Henize 2-10, located about 30 million light years from Earth, appears to be an exception. Work by Amy Reines, a graduate student in astronomy at the University of Virginia in Charlottesville reveals that the little galaxy harbors a big secret: a black hole more than 2 million times the mass of our Sun.

“There is thought to be a correlation between black hole mass and galaxy mass,” says Amy Reines, who presented the finding as part of her doctoral thesis at the annual meeting of the American Astronomical Society (AAS) in Seattle, Washington, on 10 January. The new discovery undermines this assumption, she adds.

For instance, our Milky Way galaxy has a black hole at its center that weighs in at roughly 4 million times the mass of the Sun. The black hole in the center of Henize 2-10 is only half this mass, yet the dwarf galaxy is less than 10% the mass of the Milky Way. The finding suggests that something is missing in the standard picture of supermassive black hole formation.

The case of Henize 2-10 could also have implications for the evolution of galaxies, Reines says. All galaxies that have supermassive black holes also contain a central concentration of stars known as a bulge. Astronomers have long debated which came first: the black hole or the bulge. But Henize 2-10 lacks this central structure, indicating that huge black holes can form in the absence of a bulge, says Reines. She suggests that Henize 2-10 is a galaxy in its earliest stages with it’s defining features still unformed.

The discovery is extremely puzzling, says Kevin Schawinski, an astrophysicist from Yale University in New Haven, Connecticut, who was not connected with the work. “It’s like a poke by the Universe telling us: ‘Ha’ you don’t get this,” he adds.

Larger spiral galaxies are traditionally thought to form from the merger of smaller, dwarf galaxies. This would imply that their small, central black holes would have to merge, says Schawinski. But if other dwarf galaxies are found to contain central black holes that are already so massive then astronomers will need to rethink their view of galaxy formation, he says.

This work was also published in a recent issue of Nature

Image: X-ray (NASA/CXC/Virginia/A.Reines et al.), radio (NRAO/AUI/NSF/Virginia/A.Reines et al.), optical (NASA/STScI/Virginia/A.Reines et al.)


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    Marc Benhamou said:

    And what if the galaxy was not so dwarf when the black hole first appeared, and what if its present dwarfness is only the result of the black hole having eaten up most of the original galaxy?

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