The relationship between black holes and their host galaxies poses a chicken-and-egg conundrum for astronomers, long curious about which came first.
Now, the discovery of a supermassive black hole — a million times the mass of the sun — inside a tiny galaxy tips the scales in favor of the celestial chicken.
"This is a very strange galaxy in which to find a supermassive black hole," said astronomer Amy Reines, a doctoral candidate at the University of Virginia. "Normally, supermassive black holes are found in much larger, much more massive galaxies that have bulges."
Henize 2-10, located about 30 million light-years away in the direction of the constellation Pixes, is neither massive nor bulging with a metropolis of stars at its center. It is however radiating X-rays and radio waves at levels that match what is streaming out of much larger galaxies and which are considered key indicators of supermassive black holes at work.
While nothing, not even a photon, can escape the gravitational grip of a black hole -- a region of space inconceivably dense with matter — signs of their existence are found by looking at what happens to nearby material as it is ripped and funneled into a black hole's maw.
"These things can grow to upwards of 1 billion times the mass of the sun, but we don't know how they get started," Reines told reporters at the American Astronomical Society in Seattle, where she presented evidence for a black hole inside Henize 2-10 that she estimates is about two million times more massive than the sun.
Henize 2-10 is irregularly shaped, with no central nucleus. It spans about 3,000 light-years in diameter, compared to the Milky Way's 100,000 light-year diameter. The Milky Way's central black hole is estimated to be about 4 million times the mass of the sun.
Astrophysicist Paul Green, with the Harvard-Smithsonian Center for Astrophysics, says prevailing theories suggest galaxies and black holes formed synchronously.
"It's kind of like the chicken and the egg," Green told Discovery News. "They evolved together."
The new research suggests that might not always be the case.
"Apparently you don't have to have a bulge to form a massive black hole," said Reines. "Whether this is a general thing or not we don't know. We really need to go and look for more examples like this. It does suggest that it's at least possible that the black hole formed before the galaxy."
The research is published in this week's Nature.
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