At its center, nearly every large galaxy contains a supermassive black hole surrounded by a big bulge of stars. But whether the black hole or the bulge formed first has long been a chicken-and-egg question in astronomy.
New x-ray and radio observations of the dwarf galaxy Henize 2-10 show that, while the tiny galaxy lacks a bulge, it very likely contains a supermassive black hole—one about a quarter as large as the black hole at the center of our Milky Way.
Henize 2-10 is small and irregularly shaped, and it's actively forming stars, which means it's very similar to early galaxies, researchers say. The new finding therefore suggests that early galaxies also formed their central black holes before developing bulges.
"This is certainly evidence that the black holes came first," said Amy Reines, the doctoral candidate at the University of Virginia who discovered the black hole.
But "I wouldn't say it's a done deal, because we don't know for sure there's only one path to galaxy formation."
Galaxy Evolution in Action?
Reines had been studying star-forming regions in Henize 2-10 using the Very Large Array in New Mexico and the Hubble Space Telescope when she found a compact source of radio waves emanating from the galaxy.
The orbiting Chandra X-ray Observatory later revealed x-rays being emitted from the same source.
The radiation pattern is "characteristic of a black hole feeding into its environment," Reines said. But unlike smaller black holes that form from collapsed stars, this black hole is about a million times the mass of a star. (See black hole pictures.)
Such supermassive black holes have been found in small galaxies before, but those galaxies are "like scaled-down versions of normal galaxies," Reines said.
"Henize 2-10 has this irregular shape ... and the star formation really sets it apart," she said. "We think we might be witnessing the early phase of galaxy evolution."
Meg Urry, a Yale University astronomer who was not involved in the current research, called the study well done, noting that "they have a lot of data."
But as for whether Henize 2-10 is a key to unlocking the early universe, "you can never hang much on one object. They've done as well as can be done with only one object."
The black-hole research appeared in the January 9 online edition of the journal Nature.