The Hubble Ultra Deep Field is an amazingly deep look at a tiny piece of heaven in the constellation Fornax, a 270-hour observation made in 2004 that revealed thousands of galaxies, some of which were observed shortly after their formation in the aftermath of the Big Bang.
Now astronomers using the MUSE spectrograph on the Very Large Telescope of the European Southern Observatory have found enormous clouds of atomic hydrogen surrounding many of the same galaxies, a discovery that indicates that almost the entire night sky shines with an invisible hydrogen glow.
"Realizing that the whole sky was optically light when observing Lyman alpha emission from distant hydrogen clouds was a literally dazzling surprise," said Kasper Borello Schmidt, a member of the team of astronomers behind this result.
Team member Themiya Nanayakkara said it like this: "Next time you think of the moonless night sky and see the stars, imagine the invisible glow of hydrogen: the first building block of the universe, which illuminates the whole night sky."
Lyman-alpha emissions are generated when electrons in atomic hydrogen fall to the lowest permissible energy state, producing ultraviolet radiation that is not visible to the human eye but within the range of ESO's MUSE spectrograph
The European Southern Observatory has compiled a composite image showing the Lyman alpha emissions on top of the Hubble Ultra Deep Field image.
Philipp Richter, another member of the team, said that the MUSE observations provide a "completely new picture of the diffuse gas cocoons" surrounding galaxies in the early universe. "
It is not yet clear which process is responsible for the hydrogen emissions in the