A giant blob of gas in space emits an intense glow, and now astronomers think they know why: The cloud is filled with galaxies.
Often several times larger than our own Milky Way galaxy, Lyman-alpha blobs are some of the largest known objects in the universe. (See "Giant 'Blob' Is Largest Thing in Universe.")
The blobs are also unusually bright, shining with the same intensity as the brightest known galaxies. Still, the objects tend to exist in very distant reaches of the universe, so their exact power source has been a mystery.
For the new study, astronomers took a long, hard look at LAB-1, one of the largest known Lyman-alpha blobs. Discovered in 2000, LAB-1 is about 300,000 light-years across and is so far away that its light takes almost 11.5 billion years to reach us.
The team found that the light from LAB-1 is polarized—that is, its magnetic and electric components are oriented in a specific direction relative to the direction in which the light travels.
The discovery shows for the first time that the blob must be lighted from within, as the glow of hidden galaxies gets scattered—and thus polarized—by the bubble of gas.
Space Blob Has Central Light Source
Before the new study, there were two main theories for how Lyman-alpha blobs generated their glow. One idea was that cool, mostly hydrogen gas gets pulled in from surrounding space by the blob's powerful gravity, and this action heats up the gas.
In this scenario "the gas itself is emitting the photons," or particles of light, explained study co-author Claudia Scarlata, an astrophysicist at the University of Minnesota.
"The hydrogen atoms are emitting light as they're cooling. It's like a hot iron in a fire glowing red as it cools."
The other idea is that the glow comes from stars, or perhaps active black holes, in a galaxy or galaxies inside the blobs, like fireflies in a mason jar.
To test the two theories, the team used the European Southern Observatory's Very Large Telescope in Chile to examine the light from LAB-1 for 15 hours.
The team's observations showed that the light from the blob is polarized in a ring around the central region.
"In order for the light to be polarized, the photons must have directionality," Scarlata said. "They have to come from the same place—in this case, the center of the blob." As light travels from this central source, it gets scattered by the surrounding gas cloud, producing polarization.
This effect would be difficult to produce if the blob's light was coming from infalling gas, the team says, since the light source would be spread across the entire structure and wouldn't get locally scattered.
Do Galaxies Power Other Blobs?
For now, though, LAB-1 is the only Lyman-alpha blob known to shine with polarized light, and it's unclear if this finding can be extended to other blobs.
"We don't know yet," Scarlata said. "This was a difficult experiment with one of the largest telescopes on Earth, but we are now planning to obtain more data on other [blobs]."
The space-blob research is detailed in this week's issue of the journal Nature.