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Giant space blob glows
from within

by Chloe Partridge
Posted: 18 August 2011

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A team of astronomers using ESO's (European Southern Observatory) Very Large Telescope (VLT) have found that a rare and vast glowing cloud of gas known as a Lyman-alpha blob is being powered by light emitted from galaxies that are embedded within it.

The observed blob, LAB-1, is situated some 11.5 billion light years from Earth in the constellation of Aquarius, and with a diameter of 300,000 light years, is one of the largest known, containing several primordial galaxies and one known active galaxy. It emits characteristic wavelengths of light known as Lyman-alpha radiation. This radiation is emitted from the cloud when the electrons in hydrogen atoms drop from energy level two to energy level one within the atom's orbital shells.

Lyman-alpha blob LAB-1. It is the largest known primordial cloud of hydrogen in the Universe. Its bright glow is believed to be powered centrally by galaxies embedded within it. Image: ESO/M. Hayes.

Lyman-alpha blobs like LAB-1 are vast clouds of hydrogen gas with an expanse larger than that of the Milky Way and as powerful as the brightest known galaxies. These clouds lie billions of light years away giving astronomers great insight into how the galaxies which are embedded within these clouds were formed billions of years ago and how the early Universe evolved.

“The blob that we studied is found in the distant Universe, and observed at a time when the Universe was about 15 percent its current age,” explains Matthew Hayes from the University of Toulouse, France, and lead author of the paper that is published in the 18 August issue of the journal Nature. “From that alone it is clear that our observations directly target galaxies in the early Universe, when they were in the process of assembling. Lyman-alpha blobs are rare objects although when the Universe was younger and galaxies were in the process of forming, they were more common. You can think of the Lyman-alpha blobs as representing a phase when the most massive galaxies were beginning to assemble.”

There are several competing theories as to why Lyman-alpha blobs glow, however, one proposed idea is that the clouds of hydrogen gas glow when cooler gas from the surrounding area is drawn in by the blobs' gravitational pull, resulting in an emission of light. Another proposed theory is that the clouds of gas contain galaxies undergoing vigorous star formation, which as a result emits a powerful glow from within. LAB-1 was used to test these theories.

The 15 hour observation of the blob revealed that light was polarized in a ring around the central region with no polarization in the centre. This effect is almost impossible to produce if the theory of cooler gas atoms falling onto the blob due to gravity were to be true. However, if the light originally came from galaxies embedded within the central region of the gas then astronomers would expect to see the observed polarized ring around the central region. These new observations therefore confirm the embedded galaxies theory for LAB-1.

“The light that we see is Lyman-alpha light that was produced in the central galaxies and subsequently scattered by gas that lies at large distances well outside of the galaxies themselves,” says Hayes.

However, the assumption cannot be made that this is the case for all Lyman-alpha blobs. Therefore, the team now plans to look at more blobs to see if the results obtained are true for all of them. “It is certainly true that a different Lyman-alpha blob could be powered by a different process – it could glow for a different reason,” says Hayes. “We now have to go and test that hypothesis on more blobs.”

Read more about the Universe's first stars and galaxies in the August issue of Astronomy Now.