DWARF GALAXY, BIG BLACK HOLE: The star-forming dwarf galaxy Henize 2-10 in optical light (red, green and blue), radio waves (yellow), and x-rays (purple). The suspected location of its massive black hole is marked with a cross. 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.)
The co-evolution of black holes, almost unfathomable in their bulk, and the even more massive galaxies that host them remains poorly understood—a kind of chicken-and-egg problem on mammoth scales. Do black holes, such as the lunker in our own Milky Way Galaxy, which contains the mass of four million suns (that's about eight undecillion, or 8 x 10^36 kilograms), drive the evolution of galaxies around them; or do galaxies naturally nurture the gravitational gobblers at their centers; or perhaps do they come into being together, as a matched pair?
A serendipitous discovery in a relatively close-by dwarf galaxy may help answer that question. Amy Reines, a graduate student in astronomy at the University of Virginia (U.V.A.), was looking at bursts of star formation in a galaxy known as Henize 2-10, which serves as a kind of observational proxy for galaxies that existed in the early universe. She noticed a suspicious radio wave source coming from a small region of the galaxy, a good distance removed from the active stellar nurseries. A comparison with archival data showed x-ray radiation from the same location within Henize 2-10; the balance of radiation levels in different wavelengths pointed to the presence of a giant black hole accreting material from its surroundings.
That is notable because Henize 2-10 lacks a detectable spheroid, or galactic bulge, in its center, which is usually directly related to the mass of a galaxy's black hole. "That suggests that you just don't need one to make a black hole," Reines says. "People have thought that galaxies and their black holes have grown synchronously," she adds. "This really challenges this notion and suggests that a massive black hole could form ahead of its galaxy." Reines and her colleagues from U.V.A. and the National Radio Astronomy Observatory, headquartered in Charlottesville, Va., reported the finding online January 9 in Nature. (Scientific American is part of Nature Publishing Group.)
The presence of the black hole is also of interest because the galaxy, about 30 million light-years from Earth, is still forming stars at a rapid clip and is thought to resemble galaxies that were prevalent many billions of years ago. "We think we may be witnessing an early stage of galaxy formation and black hole evolution," Reines says.
Based on its luminosity in x-rays and radio waves, the newfound black hole seems to have the mass of a million suns, about one quarter the mass of the black hole in the Milky Way's center. But considering that our galaxy may have more than 10 times the stellar mass of Henize 2-10, the dwarf galaxy's black hole is nothing to sneeze at.
Without a telltale galactic bulge it can be difficult to locate a black hole, which may be why Henize 2-10 and similar galaxies have not been known to harbor massive black holes. "We've been avoiding galaxies like this, because where's the center?" says Jenny Greene, an astronomer at the University of Texas at Austin who wrote a commentary to accompany the research in Nature. "We've just avoided them like the plague because you just don't know where to look for a black hole."
But if giant black holes in star-forming dwarf galaxies prove to be common—that is, if Henize 2-10 is not an outlier but a representative of a larger population—they may have much to tell about the formation of primordial black holes and galaxies in the early universe. "There are all kinds of interesting relationships" between black holes and their host galaxies, says astronomer James Ulvestad, director of the National Science Foundation's Division of Astronomical Sciences. "But we don't really know very well how that happens or how these things get started." (Ulvestad commented on the research as an astronomer in the field, not as an NSF representative.)
There are reasons to think that diminutive star-formers such as Henize 2-10 were prevalent in the early universe, before mergers incorporated those dwarfs into larger galaxies. "The early galaxies in the universe were all kind of like this," Ulvestad says. But the kinds of objects that astronomers can actually see in the early universe, by peering far across the cosmos, all give off far more radiation than the black hole found in Henize 2-10, so the question of how many black holes of that ilk existed early on remains open.
The key to the new discovery, Greene says, "is really opening a new realm for us to search." There exist many more dwarf galaxies that may also have black holes, which would hold even more clues to the history and evolution of black holes and their galaxies. "If you can find a few more of them nearby then that tells you that it's common," Ulvestad says. "Then you can say by extrapolation, 'okay, we're looking at some common phenomenon that was happening early in the universe.'"