What makes this strange space blob glow?

That green clump might not be much to look at, but it's one of the rarest sights in the cosmos: a Lyman-alpha blob. They're among the universe's biggest structures, and only now are we starting to understand how they work.

Lyman-alpha blobs are up to 400,000 light-years across, making them much larger than the diameter of our own galaxy - and remember, each blob is a single vast unit of hydrogen gas. These leviathan structures are very bright, but they're only found at the furthest, most ancient parts of the observable universe, which helps explain why this particular blob might not look quite as remarkable as it really is.

Just what makes these blobs so bright has long been an open question. Now, scientists at the European Southern Observatory say they've found the answer:

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: galaxies undergoing vigorous star formation, or containing voracious black holes engulfing matter. The new observations show that it is embedded galaxies, and not gas being pulled in, that power LAB-1.

The team tested the two theories by measuring whether the light from the blob was polarised. By studying how light is polarised astronomers can find out about 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 polarised and this subtle effect can be detected by a very sensitive instrument...

By observing their target for about 15 hours with the Very Large Telescope, the team found that the light from the Lyman-alpha blob LAB-1 was polarised in a ring around the central region, and that there was no polarisation in the centre. This effect is almost impossible to produce if light simply comes from the gas falling into the blob under gravity, but it is just what is expected if the light originally comes from galaxies embedded in the central region, before being scattered by the gas.

For more, check out the ESO website.