A decade ago, astronomers surveying the distant universe discovered giant blobs of shining hydrogen gas bigger than anything ever seen in the cosmos. Since then, researchers have wondered what makes these structures--known as Lyman-alpha blobs (LABs)--glow. Now, a team of astronomers claims to have found evidence that the blobs are illuminated by radiation and heat from supermassive black holes at their center. The finding supports an emerging idea of how a growing black hole ultimately limits a galaxy's size.
Researchers led by James Geach, an astronomer at the University of Durham in the United Kingdom, used NASA's Chandra X-ray observatory to look at 29 blobs in a patch of sky known as SSA 22. A few hundred thousand light-years across in size, the blobs date back to when the universe was less than a sixth of its current age, or about 2 billion years old.
X-ray emissions observed over 5 days allowed the researchers to see through to the center of five of the blobs. The spectral signature of each one revealed a supermassive black hole. Following up with the Spitzer Space Telescope, the researchers saw signs of star formation within the blobs. Given the presence of the black holes, they concluded that the most likely explanation for the illumination of the blobs is heat emitted by black holes as they gobble up material falling into them, as well as energy released during star formation. The conclusion runs counter to the alternate theory that the gas lights up as it cools to release gravitational energy--with no black holes in the picture.
"What we are seeing could be the signatures of galaxies coming of age," Geach said at a Wednesday press conference to announce the results, which will be published in The Astrophysical Journal. Geach says the blobs could represent a phase in galaxy evolution when the black hole at the center exerts a brake on the growing host galaxy. By emitting heat that pushes away the galactic gas, the black hole prevents any new stars from being formed.
Astronomers invoke such a "feedback" scenario involving black holes to explain the paradox of why galaxies haven't grown to sizes much bigger than those seen today. "Massive galaxies must go through a [blob] stage, or they would form too many stars and end up ridiculously large," says study co-author Bret Lehmer, also of Durham.
Martin Rees, an astronomer at Cambridge University in the U.K., says the results offer an unprecedented look at the mechanisms that regulate galaxy formation. Given another 1 or 2 billion years, he says, any gas in the blobs that isn't converted into stars should be completely blown away.