Using the unprecedented capabilities of the James Webb Space Telescope, an international team of scientists has obtained the first spectroscopic observations of the faintest galaxies that existed during the first billion years of the Universe.
These findings help answer a long-standing question of astronomers: what sources caused the reionization of the Universe? These results have actually demonstrated that small dwarf galaxies are likely producers of huge amounts of energetic radiation.
The early universe
The study of the evolution of the early Universe is an important aspect of modern astronomy. Much remains to be understood about a period in the early history of the Universe known as the reionization epoch. This was a period of darkness without stars or galaxies, filled with a thick fog of hydrogen gas, until the first stars ionized the gas around them and light began to penetrate it. Astronomers have spent decades trying to identify sources that emitted radiation powerful enough to gradually dissipate this hydrogen fog that covered the early Universe.
The Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization program (UNCOVER) consists of visual and spectroscopic observations of the lensed cluster Abell 2744. An international team of astronomers used gravitational lensing of this object, also known as the Pandora cluster, to investigate the sources of the Universe’s reionization period.
Gravitational lensing
Gravitational lensing magnifies and distorts the appearance of distant galaxies, so they look completely different from those in the foreground. The “lens” of a galaxy cluster is so massive that it distorts the fabric of space itself to such an extent that light from distant galaxies passing through the distorted space also takes on a distorted appearance.
The magnification effect allowed the team to study very distant light sources beyond Abell 2744, revealing eight extremely faint galaxies that would otherwise have been invisible even to Webb.
The results exceeded expectations
The team found that these faint galaxies are huge producers of ionizing radiation at levels four times higher than previously thought. This means that most of the photons that reionized the Universe probably came from these dwarf galaxies.
“This discovery reveals the crucial role of ultra-faint galaxies in the evolution of the early Universe. They produce ionizing photons that convert neutral hydrogen into ionized plasma during cosmic reionization,” says team member Irina Chemerinskaya from the Paris Astrophysical Institute in France.
According to her, this emphasizes the importance of understanding low-mass galaxies in shaping the history of the Universe.
“Despite their tiny size, these low-mass galaxies are powerful producers of energetic radiation, and their number during this period is so significant that their collective impact could transform the entire state of the Universe,” adds team leader Hakim Atek from the Paris Astrophysical Institute, CNRS, Sorbonne University, France.
In-depth research
To arrive at this conclusion, the team first combined the Webb ultra-deep imaging data with auxiliary images of Abell 2744 from the Hubble Space Telescope to select extremely faint candidate galaxies in the reionization era. This was followed by spectroscopy using the Webb Near Infrared Spectrograph (NIRSpec).
A multi-shutter assembly of the instrument was used to obtain multi-object spectroscopy of these faint galaxies. This is the first time that scientists have reliably measured the density of these faint galaxies, and they have successfully confirmed that they are the most abundant population in the reionization era. It is also the first time that the ionizing power of these galaxies has been measured, allowing astronomers to determine that they produce enough energetic radiation to ionize the early Universe.
“The incredible sensitivity of NIRSpec, combined with the gravitational enhancement provided by Abell 2744, has allowed us to identify and study in detail these galaxies from the first billion years of the Universe’s existence, despite being more than 100 times fainter than our own Milky Way,” Atek says.
Future research
In Webb’s upcoming observing program, called GLIMPSE, scientists will obtain the deepest observations of the sky ever recorded. Targeting another cluster of galaxies, called Abell S1063, scientists will detect even fainter galaxies in the reionization era to see if this population is representative of the large-scale distribution of galaxies.
Because these new results are based on observations obtained in a single field, the team notes that the ionization properties of faint galaxies may look different if they are in superdense regions. Therefore, additional observations in an independent field will provide additional information to help verify these findings.
The GLIMPSE observations will also help astronomers explore the period known as the Cosmic Dawn, when the Universe was only a few million years old, to develop our understanding of the emergence of the first galaxies.
