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Researchers Discover that Giant Space Blob Glows from Within

A Lyman-alpha blob

University of Minnesota physics and astronomy professor Claudia Scarlata in the College of Science and Engineering is part of an international collaboration that has shed light on the power source of a rare vast cloud of glowing gas in the early Universe. The observations show for the first time that this giant “Lyman-alpha blob”—one of the largest single objects known—must be powered by galaxies embedded within it. The results appear in the August 18 issue of the journal Nature.

Scarlata and her collaborators, Matthew Hayes (Université de Toulouse, France and Observatory of Geneva, Switzerland) and Brian Siana (University of California, Riverside) have used the European Southern Observatory’s Very Large Telescope (VLT) to study the blob that holds clues about how galaxies form and evolve in the early Universe. The team found that the light from the blob was polarized.
In everyday life, for example, polarized light is used to create 3D effects in movies. This is the first time that polarization has ever been found in a Lyman-alpha blob, and this observation helps to unlock the mystery of how the blobs shine.

“We have shown for the first time that the glow of this enigmatic object is scattered light from brilliant galaxies hidden within, rather than the gas throughout the cloud itself shining,” explains Hayes, the lead author of the paper.

Lyman-alpha blobs are some of the biggest objects in the Universe: gigantic clouds of hydrogen gas that can reach diameters of a few hundred thousand light-years (a few times larger than the size of the Milky Way), and are as powerful as the brightest galaxies. They are typically found at large distances, appearing as they were when the Universe was only a few billion years old. Therefore, they provide an important glimpse at how galaxies formed and evolved when the Universe was younger. But the power source for their extreme luminosity, and the precise nature of the blobs, has remained unclear.

“This discovery is significant because it tells us something about how galaxies are able to acquire fresh gas, in other words, the fuel needed to form new stars and grow bigger,” said Scarlata, a professor in the Minnesota Institute for Astrophysics in the University of Minnesota’s School of Physics and Astronomy.

The team studied one of the first and brightest blobs, called LAB-1, it was discovered in 2000, and is so far away that its light has taken about 11.5 billion years to reach Earth. With a diameter of about 300,000 light-years it is also one of the largest, and is big enough to have several primordial galaxies inside it, including an active galaxy (a galaxy with black hole at its center).

There are several competing theories to explain Lyman-alpha blobs. One idea is that they shine when cool gas is pulled in by the blob’s powerful gravity, and heats up. Another is that they are shining because of brilliant objects inside them such as galaxies undergoing vigorous star formation, or containing voracious black holes engulfing matter. The new observations show that it is in fact the embedded galaxies, and not gas being pulled in, that powers LAB-1.

The team tested the two theories by measuring whether the light from the blob was polarized. When light waves are polarized, their electric and magnetic fields have a specific orientation. By studying how light is polarized, astronomers can determine the physical processes that produced the light, or what has happened to it between its origin and its arrival at Earth. If it is reflected or scattered, it becomes polarized, and a very sensitive instrument can detect this subtle effect.

The team found that the light from the Lyman-alpha blob was polarized in a ring around the central region, and that there was no polarization in the center. This effect could not have been produced by light from the gas falling into the blob under gravity. Instead, it is a clear signature that the light originally comes from the galaxies embedded in the central region, before being scattered by the gas.

The astronomers now plan to look at more of these objects to see if the results obtained for LAB-1 are true of other blobs.
Read the entire research paper “Central Powering of the Largest Lyman-alpha Nebula is Revealed by Polarized Radiation,” on the Nature website.

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