Giant cosmic blob found lit from the inside

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A cosmological simulation of a Lyman-alpha blob similar to LAB-1. The simulation tracks evolution of gas and dark matter.
J Geach / D Narayanan / R Crain

A brilliant blob of gas that sprawls more than 300,000 light-years across is illuminated from the inside, thanks to two galaxies furiously churning out stars.

Observations from a fleet of telescopes showed the “Lyman-alpha blob”, which is 11.5 billion light-years from Earth, contains the large pair along with a swarm of smaller galaxies which what appears to be the early phase in the formation of a massive galaxy cluster.

The work is published in Arxiv and will appear in the Astrophysical Journal.

Lyman-alpha blobs are gigantic clouds of cool hydrogen gas which give off a certain ultraviolet wavelength of light (called Lyman-alpha radiation).

The first Lyman-alpha blob was found in 2000 and called LAB-1. But why they’re so radiant has been a mystery.

So a team of astronomers, led by Jim Geach, from the University of Hertfordshire, UK, peered into LAB-1’s depth with the Atacama Large Millimetre/submillimetre Array in Chile, which can observe light from such cool dust clouds. 

Combining data with that from the Very Large Telescope, also in Chile, they mapped the Lyman-alpha light and found the galaxies within LAB-1 are forming stars at a rate of around 150 each year – more than 100 times faster than the Milky Way galaxy.

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ESO / J Geach

The Hubble Space Telescope and Keck Observatory picked out the smaller galaxies surrounding the massive pair. These fainter counterparts could well be supplying the fuel that’s driving the star-forming frenzy.

Simulations supported this idea.

“Think of a streetlight on a foggy night – you see the diffuse glow because light is scattering off the tiny water droplets,” Geach explains. 

“A similar thing is happening here, except the streetlight is an intensely star-forming galaxy and the fog is a huge cloud of intergalactic gas. The galaxies are illuminating their surroundings.”

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