Aug. 20, 2008 -- There are massive black holes, like the one at the center of the Milky Way galaxy, and petite black holes, but few, if any contenders in the middle-class division, if indeed that population exists.
Until this week, scientists believed their best chance for finding a mid-size black hole would be in a globular cluster, a densely packed region of stars that often live inside or alongside larger, more spacious galaxies.
After analyzing tantalizing black hole signatures coming from a globular cluster roughly 50 million light-years away, a team of researchers headed by Michigan State University's Stephen Zepf has a suggestion: Look elsewhere.
The cluster, known as RZ2109, does indeed bear telltale signs of a black hole, but it's a small one, with about 10 times the mass of our sun.
Black holes are areas of space where the gravitational forces of matter are so strong that not even photons of light can escape the clutch.
"We know there are these black holes in the centers of the galaxies, and we know the bigger the black hole, the bigger the galaxy," Zepf told Discovery News.
"Since the globular clusters are smaller groups of stars, we thought the black holes could start smaller and then build up," he added.
Instead, the team found that conditions inside a common globular cluster are not the conditions needed to make a black hole grow.
There is only one other strong contender for a black-hole laced globular cluster, a body known as G1, though some scientists believe G1 is actually the remains of a larger galaxy that lost most of its real estate.
In terms of understanding the evolution of black holes, G1 may turn out to be an important player.
Dwarf galaxies, which may be a more accurate classification for G1, are the next logical place to look for mid-size black holes, but scientists are a bit stumped about how to find them. Since they emit no light of their own, black holes can only be detected by studying what they are doing to nearby stars and matter and that makes for slim pickings in the relative spaciousness of dwarf galaxies.
"In the centers of dwarf galaxies, there is not a lot of stuff to test, so it's hard to tell," said Daniel Stern, with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It's always hard to prove that something is nowhere in the universe."
Zepf's research appears in this week's Astrophysical Journal.
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