Astronomers using the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope have discovered an unexpected abundance of Lyman-alpha emission in the Hubble Ultra Deep Field, a galaxy-rich patch of the southern constellation Fornax. The discovered emission covers nearly the entire field of view, leading the scientists to extrapolate that nearly all the sky in the early Universe is invisibly glowing with Lyman-alpha emission.
The exquisite sensitivity of the MUSE spectrograph on ESO’s Very Large Telescope allowed for direct observations of dim clouds of hydrogen glowing with Lyman-alpha emission in the early Universe. Image credit: NASA / ESA / Hubble / ESO / Lutz Wisotzki et al.
Astronomers have long been accustomed to the sky looking wildly different at different wavelengths, but the extent of the observed Lyman-alpha emission — a type of emission that originates from atomic electron transitions in hydrogen atoms which radiate light with a wavelength of around 122 nm — was still surprising.
“Realizing that the whole sky glows in optical when observing the Lyman-alpha emission from distant clouds of hydrogen was a literally eye-opening surprise,” said co-author Dr. Kasper Borello Schmidt, an astronomer at the Leibniz-Institut für Astrophysik Potsdam in Germany.
“This is a great discovery,” added co-authors Dr. Themiya Nanayakkara, an astronomer at Leiden Observatory in the Netherlands.
“Next time you look at the moonless night sky and see the stars, imagine the unseen glow of hydrogen: the first building block of the Universe, illuminating the whole night sky.”
The Hubble Ultra Deep Field (HUDF) region the researchers observed was famously mapped by the NASA/ESA Hubble Space Telescope in 2004, when Hubble spent more than 270 hours of precious observing time looking deeper than ever before into this region of space.
The HUDF observations revealed thousands of galaxies scattered across what appeared to be a dark patch of sky, giving us a humbling view of the scale of the Universe.
The detection of Lyman-alpha emission in the HUDF is the first time astronomers have been able to see this faint emission from the gaseous envelopes of the earliest galaxies.
“We tentatively identified what is causing these distant clouds of hydrogen to emit Lyman-alpha, but the precise cause remains a mystery,” the scientists said.
“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,” said lead author Dr. Lutz Wisotzki, an astronomer at the Leibniz-Institut für Astrophysik Potsdam.
“We want to find out the details of how these vast cosmic reservoirs of atomic hydrogen are distributed in space.”
The study was published online this week in the journal Nature.
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L. Wisotzki et al. Nearly all the sky is covered by Lyman-α emission around high-redshift galaxies. Nature, published online October 1, 2018; doi: 10.1038/s41586-018-0564-6
