National Radio Astronomy Observatory | Dave Finley | 2011 Jan 09
Astronomers Discover Supermassive Black Hole in Center of Tiny GalaxyThe dwarf galaxy Henize 2-10, seen in visible light by the Hubble Space Telescope. The central, light-pink region shows an area of radio emission, seen with the Very Large Array. This area indicates the presence of a supermassive black hole drawing in material from its surroundings.
Credit: Amy Reines, et al., David Nidever, UVa/NRAO/AUI/NSF, NASA/STScI
Mouse over: Composite image of the dwarf galaxy Henize 2-10 with a cross marking the presumed location of the supermassive black hole in the galaxy.. Hubble Space Telescope data is colored red, green and blue, Very Large Array data is yellow and the Chandra X-Ray Observatory data is purple.
Credit: X-ray (NASA/CXC/UVa/Amy Reines et al.),
radio (NRAO/AUI/NSF/UVa/Amy Reines et al.),
optical (NASA/STScI/UVa/Amy Reines et al.) The surprising discovery of a supermassive black hole in a small nearby galaxy has given astronomers a tantalizing look at how black holes and galaxies may have grown in the early history of the Universe. Finding a black hole a million times more massive than the Sun in a star-forming dwarf galaxy is a strong indication that supermassive black holes formed before the buildup of galaxies, the astronomers said.
The galaxy, called Henize 2-10, 30 million light-years from Earth, has been studied for years, and is forming stars very rapidly. Irregularly shaped and about 3,000 light-years across (compared to 100,000 for our own Milky Way), it resembles what scientists think were some of the first galaxies to form in the early Universe.
"This galaxy gives us important clues about a very early phase of galaxy evolution that has not been observed before," said Amy Reines, a Ph.D. candidate at the University of Virginia.
Supermassive black holes lie at the cores of all "full-sized" galaxies. In the nearby Universe, there is a direct relationship -- a constant ratio -- between the masses of the black holes and that of the central "bulges" of the galaxies, leading them to conclude that the black holes and bulges affected each others' growth.
Two years ago, an international team of astronomers found that black holes in young galaxies in the early Universe were more massive than this ratio would indicate. This, they said, was strong evidence that black holes developed before their surrounding galaxies.
"Now, we have found a dwarf galaxy with no bulge at all, yet it has a supermassive black hole. This greatly strengthens the case for the black holes developing first, before the galaxy's bulge is formed," Reines said.
Reines, along with Gregory Sivakoff and Kelsey Johnson of the University of Virginia and the National Radio Astronomy Observatory (NRAO), and Crystal Brogan of the NRAO, observed Henize 2-10 with the National Science Foundation's Very Large Array radio telescope and with the Hubble Space Telescope. They found a region near the center of the galaxy that strongly emits radio waves with characteristics of those emitted by super-fast "jets" of material spewed outward from areas close to a black hole.
They then searched images from the Chandra X-Ray Observatory that showed this same, radio-bright region to be strongly emitting energetic X-rays. This combination, they said, indicates an active, black-hole-powered, galactic nucleus.
"Not many dwarf galaxies are known to have massive black holes," Sivakoff said.
While central black holes of roughly the same mass as the one in Henize 2-10 have been found in other galaxies, those galaxies all have much more regular shapes. Henize 2-10 differs not only in its irregular shape and small size but also in its furious star formation, concentrated in numerous, very dense "super star clusters."
"This galaxy probably resembles those in the very young Universe, when galaxies were just starting to form and were colliding frequently. All its properties, including the supermassive black hole, are giving us important new clues about how these black holes and galaxies formed at that time," Johnson said.
University of Virginia | Fariss Samarrai | 2011 Jan 09
Hole's on First?: New Evidence Shows Black Hole Growth Preceding Galactic FormationAstronomers have discovered a supermassive black hole in the center of a tiny low-mass galaxy, suggesting the formation of supermassive black holes may precede the growth of galaxies. This finding challenges conventional wisdom that supermassive black holes only inhabit massive galaxies with voluminous spheroidal components called "bulges."
The scientists found the supermassive black hole in a dwarf galaxy, known as Henize 2-10, which is undergoing a violent burst of star formation. This star-creating miniature galaxy is believed to be analogous in many ways to infant galaxies in the early universe.
The finding will be reported in the Jan. 9 online edition of the journal Nature and presented Jan. 10 at the annual meeting of the American Astronomical Society in Seattle.
"We never expected to find a supermassive black hole in Henize 2-10," said lead author Amy Reines, a Ph.D. candidate in astronomy in the University of Virginia's Graduate School of Arts & Sciences. "We may be witnessing an early stage of galaxy and black hole evolution that has not been observed before."
Normally, supermassive black holes are found in much larger galaxies that have a bulge or a nuclear star cluster. Henize 2-10 has neither. The discovery, Reines noted, will enable astronomers to gain insight into how the first supermassive black holes may have been formed very early in the history of the universe.
Supermassive black holes, gravitational monsters reaching masses upwards of a billion times more than our sun, are among the most exotic and enigmatic objects in the known universe – yet they commonly reside in the center of normal galaxies like our own Milky Way, as well as in even larger galaxies. More massive galaxies generally have more massive black holes, and it is thought that black holes and their host galaxies have grown synchronously over cosmic history.
The discovery of a supermassive black hole in Henize 2-10 challenges this notion and suggests that the formation of supermassive black holes may actually predate the build-up of their host galaxies. Reines will search for other examples like Henize 2-10 to help refine theories as to how supermassive black holes form.
Although no light can escape from a black hole itself, astronomers are able to determine their presence based on the behavior of matter in their vicinity. In our own galaxy, astronomers have established the presence of a supermassive black hole by observing the orbits of nearby stars. A black hole also can be identified by different types of light originating from superheated material swirling around these mysterious regions.
Though Henize 2-10 has been heavily studied for decades due to its extreme star formation activity, its central supermassive black hole went unidentified until Reines and her colleagues serendipitously found it during a study of dwarf starburst galaxies. They discovered evidence of the supermassive black hole after analyzing several kinds of light emanating from Henize 2-10 spanning radio to X-ray wavelengths. The observations of Henize 2-10 were taken with the Very Large Array, the Hubble Space Telescope and the Chandra X-Ray Observatory.
Reines' collaborators include U.Va. astronomy research associate Gregory Sivakoff, U.Va. astronomy professor Kelsey Johnson and National Radio Astronomy Observatory astronomer Crystal Brogan.
Scientific American | John Matson | 2011 Jan 09
Ginormous Black Hole May Solve Longstanding MysteryAn accidental find in a star-forming dwarf galaxy shows that black holes may mature early in galaxy evolution
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.'"
Space.com | Science and Astronomy | Clara Moskowitz | 2011 Jan 09
