“This is a great discovery!” said ESO team member Themiya Nanayakkara in fall of 2018 about the discovery that almost all of the sky is invisibly glowing with Lyman-alpha emission from the early Universe. “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.”
Lyman-alpha Emission –“First Building Block of the Universe”
An unexpected abundance of Lyman-alpha emission that covers nearly the entire field of view in the Hubble Ultra Deep Field (HUDF) region was discovered by an international team of astronomers using the MUSE instrument on ESO’s Very Large Telescope (VLT).
Astronomers have long been accustomed to the sky looking wildly different at different wavelengths, but the extent of the observed Lyman-alpha emission 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,” explained Kasper Borello Schmidt, a member of the team of astronomers behind this result.
Hubble Ultra Deep Field (HUDF)
The HUDF region the team observed is an otherwise unremarkable area in the constellation of Fornax (the Furnace), which 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. Now, the outstanding capabilities of MUSE have allowed us to peer even deeper. 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. This composite image shows the Lyman-alpha radiation in blue superimposed on the iconic HUDF image.
MUSE –Reveals “Gas Cocoons” of the Early Universe
MUSE, the instrument behind these latest observations, is a state-of-the-art integral field spectrograph installed on Unit Telescope 4 of the VLT at ESO’s Paranal Observatory that looks like a machine straight out of the movie The Matrix, with its Medusa-like hoses and connections. “MUSE has been built with the intention of studying the content and processes going on in the very early Universe, when the first stars and galaxies were forming,” explains Fernando Selman, Instrument Scientist for MUSE.
When MUSE observes the sky, it sees the distribution of wavelengths in the light striking every pixel in its detector. Looking at the full spectrum of light from astronomical objects provides us with deep insights into the astrophysical processes occurring in the Universe.
The Lyman-alpha radiation that MUSE observed originates from atomic electron transitions – in hydrogen atoms which radiate light with a wavelength of around 122 nanometers. As such, this radiation is fully absorbed by the Earth’s atmosphere. Only red-shifted Lyman-alpha emission from extremely distant galaxies has a long enough wavelength to pass through Earth’s atmosphere unimpeded and be detected using ESO’s ground-based telescopes.
“With these MUSE observations, we get a completely new view on the diffuse gas ‘cocoons’ that surround galaxies in the early Universe,” commented Philipp Richter, another member of the team.
Origin remains a mystery
The international team of astronomers who made these observations have 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,” concluded Lutz Wisotzki, leader of the team. “We want to find out the details of how these vast cosmic reservoirs of atomic hydrogen are distributed in space.”
“The Microwave Window”
In addition to Lyman-alpha, another emission phenomena is the line of cold hydrogen at a frequency of 1420.40575 MHz (wavelength of 21 cm), which corresponds to the electron spin-flip energy in neutral hydrogen atoms, the most abundant substance in space. It happens to fall in the quietest part of the radio spectrum, what’s known as the Microwave Window. Although there may not seem to be a lot of loose hydrogen atoms in the vicinity (there’s perhaps one atom per cubic centimeter of interstellar space), the interstellar medium contains a lot of cubic centimeters.
The hydrogen line is a frequency often used for observing the structure of the universe, and some of the best and most detailed Milky Way radio maps have been made on the hydrogen line. It is probably the world’s most popular radio astronomy frequency, and is protected by the International Telecommunications Union (ITU).
Hydrogen line –a likely frequency for interstellar contact
In 1959 Philip Morrison at Cornell University and Frank Drake at NRAO independently recognized that the hydrogen line would be a likely frequency for interstellar contact, reasoning that more advanced civilizations would assume that young civilizations might be listening there. Morrison went on to co-author one of the world’s first modern SETI article, “Searching for Interstellar Communications,” Nature. Drake conducted a similar SETI study, “Project Ozma,” a 21-centimeter hydrogen line search of two nearby Sun-like stars for possible artificial signals.
Over the past forty years, about three dozen other hydrogen line searches have been conducted. It was on the hydrogen line that in 1977 the Big Ear radio telescope at the Ohio State Radio Observatory detected the so-called “Wow!” signal, the most promising, intriguing and beguiling SETI candidate signal to date. The “Wow!” is also the best known SETI signal, having been featured in the “X-files.” After about 100 follow-up attempts over a twenty year period, that signal has never repeated and remains unexplained.
Image credit: ESA/Hubble & NASA, ESO/ Lutz Wisotzki et al.