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A Universe aglow

Deep observations made with the MUSE spectrograph on ESO’s Very Large Telescope have uncovered vast cosmic reservoirs of atomic hydrogen surrounding distant galaxies. The exquisite sensitivity of MUSE allowed for direct observations of dim clouds of hydrogen glowing with Lyman-alpha emission in the early Universe — revealing that almost the whole night sky is invisibly aglow.

Glowing invisibly

An international team of astronomers published their discovery in Nature, including Professor of The formation of galaxies Joop Schaye and Associate professor Jarle Brinchmann. The researchers discovered an unexpected abundance of Lyman-alpha emission in the Hubble Ultra Deep Field region with the MUSE instrument on ESO’s Very Large Telescope. Lyman-alpha emission is emitted by a hydrogen atom when its electron falls from its first excited state to its ground energy state. The discovered emission covers nearly the entire field of view — leading the team to extrapolate that almost all of the sky is invisibly glowing with Lyman-alpha emission from the early Universe.  

Understanding galaxy formation

‘I managed the Dutch contribution to the instrument MUSE and helped define the consortium's observing programmes’, Schaye explains his contribution. ‘For this paper, I helped with the interpretation of the observations and the writing of the manuscript.’ He adds that the discovery can be of great importance: ‘This will help us understand how galaxies form and grow.’

Only one dimension

‘Most of the matter in the Universe resides in between galaxies’, Schaye explains. ‘This intergalactic medium is extremely dilute and the radiation it emits is therefore very faint and difficult to detect. Previously, we could only see the gas surrounding ordinary galaxies through the light it absorbs from bright, point-like background objects. Due to the rarity of suitable background sources, such observations are however only possible along a small number of lines of sight. And each line of sight provides 1-dimensional information, rather than an image.’

MUSE to the rescue

Thanks to the MUSE instrument it is now possible to detect emission from the denser intergalactic gas, which constitutes the fuel for future star formation. ‘In particular, MUSE detects the gas via the strongest emission line of hydrogen, the most common element in the Universe’, says Schaye. ‘MUSE is so sensitive that the faint glow that it detects from gas surrounding galaxies at various distances from us covers nearly all of the sky.’

A sea of glowing gas

‘The remarkable result of these observations is that rather than thinking about galaxies as small islands of light in a sea of blackness, if we have sufficiently sensitive eyes we see that they are in truth embedded in a sea of glowing ionised gas which nearly fills the sky’, Brinchmann says. It is very important that that astronomers can now see the emission from the gas that could previously only be studied in absorption in a tiny fraction of the sky, Schaye states. ‘The emission provides us with a much more complete picture of the diffuse gas that is falling onto galaxies. This will give us insights into the formation of galaxies and how they grow.’


The international team of astronomers has tentatively identified what is causing these distant clouds of hydrogen to emit Lyman-alpha, but the precise cause remains a mystery. However, as this faint omnipresent glow is thought to be ubiquitous in the night sky, future research is expected to shed light on its origin. ‘In the future, we plan to make even more sensitive measurements’, concludes team leader Lutz Wisotzki from the Leibniz Institute for Astrophysics Potsdam. ‘We want to find out the details of how these vast cosmic reservoirs of atomic hydrogen are distributed in space.’

Based on ESO’s press release

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