Black Holes Whip Up Galaxy's Heart — Or Not
When galaxies merge, the supermassive black holes in their hearts should merge, too. But astronomers are having a tough time finding pairs of black holes that are in the process of merging. That may mean that such a merger happens more quickly than astronomers had expected, or that the black holes grow quiet in the final stages of the merger.
An artist’s concept shows two supermassive black holes, surrounded by disks of hot gas, in the center of a galaxy. Astronomers may or may not have detected such a system.
Earlier this year, Todd Boroson and Tod Lauer reported the discovery of a possible pair of supermassive black holes whipping up the center of a distant galaxy known as J1536+0441. But a couple of months later, another astronomer suggested that the galaxy contains only one supermassive black hole.
Boroson and Lauer, both of the National Optical Astronomy Observatory, found evidence of the binary in a catalog of 17,500 "active" galaxies compiled with the Sloan Digital Sky Survey telescope in New Mexico. Active galaxies have exceedingly bright cores which probably are powered by superhot gas and dust spiraling into a giant black hole.
In a paper published in the journal Nature, Boroson and Lauer said they found the "fingerprint" of two of these disks in a single galaxy, which would be the first direct evidence of such a system.
The astronomers suggested that one of the black holes in J1536+0441 is about 20 million times the mass of the Sun, while the other is about 1 billion times the Sun's mass. They determined that the black holes are about on-third of a light-year apart, so they should orbit each other once every century.
In a later Nature paper, though, University of Texas at Austin astronomer Martin Gaskell said that the galaxy contains only one supermassive black hole. The fingerprint that Boroson and Lauer detected in the galaxy's spectrum probably was produced by hot blobs of gas in a disk at the galaxy's center, not by orbiting black holes, he wrote.
In his paper, Gaskell suggested that supermassive black holes may merge quickly, or that the merger process strips away the bright disks of gas and dust around them, making it impossible to detect the black holes. Either way, he says, binary black holes in the hearts of galaxies will remain tough to find.
"What is actually most interesting about this is that these supermassive binaries are clearly rare," says Gaskell. "We're arguing about 1 in 17,500 or zero in 17,500, which is rare whatever way you look at it." — Damond
Benningfield
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