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A tiny galaxy has been caught with a monstrously huge black hole at its center. The galactic oddity could be a transition case between young, small galaxies and stately spirals like our Milky Way, and suggests that galaxies grow around central black holes, not the other way around.

Most large galaxies, including the Milky Way, spiral around a central supermassive black hole millions of times more massive than the sun. In general, bigger galaxies host bigger black holes, suggesting a link between the two. But the nature of the link — whether black holes form first and gather galaxies around them, whether preformed galaxies crush the material in their centers into black holes, or whether the two grow in tandem — is unclear.

A newly discovered black hole in a nearby dwarf galaxy may hint at an answer.

“We might be witnessing an early stage of black-hole and galaxy evolution,” said astronomer Amy Reines, a graduate student at the University of Virginia, in a press conference here at the meeting of theAmerican Astronomical Society.

 

The galaxy, called Henize 2-10, is a blob-shaped dwarf galaxy 30 million light-years away. It’s about 3,000 light-years across, one-thirtieth the width of the Milky Way, and is famous among galactic astronomers for its rapid bursts of star formation. Some astronomers think that with its small, blobby and starbursting form, Henize 2-10 could be a nearby analogue of some of the first galaxies ever formed in the universe.

Reines and her colleagues observed the galaxy with the Hubble Space Telescope and the Very Large Array of radio telescopes in New Mexico. They found a region near the center of the galaxy, between two bright regions of gas, that was spitting out more radio waves than would be expected from a dwarf-sized black hole.

The team searched images from the Chandra X-Ray Observatory and found that same region glowed brightly in energetic X-rays.

The combination of radio waves and X-rays “is a signpost for a supermassive black hole feeding on its surroundings,” Reines said. The results were published in the Jan. 9 Nature.

Based on the amount of radiation, the black hole is about 2 million times the mass of the sun, Reines estimated. That’s comparable to the size of the black hole at the center of the Milky Way, even though Henize 2-10 is only about the size of a satellite galaxy of the Milky Way called the Magellanic Clouds.

The object is too bright in radio to be a smaller black hole, and too bright in X-rays to be a supernova remnant, Reines said. “It’s really this ratio of X-ray–to–radio emission that rules out other objects.”

Because Henize 2-10 doesn’t have a big bulge of stars at its center like other supermassive-black-hole–hosting galaxies, these big, bulging galaxies may form around their black holes.

“It’s possible that the black hole formed before the galaxy,” Reines said.

“If it really exists, it’s very significant,” said astronomer Hans Zinnecker of the SOFIA airborne observatory, who studied Henize 2-10 in the 1980s. “It surely was a surprise.”

Zinnecker wondered why this dwarf galaxy hosts such a large black hole while others of similar size do not. Maybe two clouds of gas colliding at Henize 2-10’s center collapsed into the large black hole after the galaxy had already formed.

“I find it hard to imagine that a black hole of a million solar masses could have been built from a merger of little black holes,” he said. “There must be something specific about that galaxy that could explain it.”