Using the unprecedented capabilities of the NASA/ESA/CSA James Webb Space Telescope in exploring the Universe, an international team of scientists has obtained the first spectroscopic observations of the faintest galaxies during the first billion years of the Universe. These findings help answer a long-standing question for astronomers: what sources caused the reionization of the Universe?
Much remains to be understood about the timing of the early history of the Universe known as the era of reionization. It was a period of darkness without stars or galaxies, filled with a dense fog of hydrogen gas, until the first stars ionized the gas around them and light began to travel through them. Astronomers have spent decades trying to identify the sources that emitted radiation powerful enough to gradually remove this hydrogen fog that covered the early Universe.
The Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) program consists of images and spectroscopic observations of the Abell 2744 lensing cluster. An international team of astronomers used gravitational lenses for this purpose, also known as the Pandora Cluster, to investigate the sources of the Universe's reionization period. Gravitational lenses magnify and distort the appearance of distant galaxies, so they look very different from those in the foreground.
The 'lens' of the galaxy cluster is so massive that it deforms the structure of space itself, to the point that the light from distant galaxies passing through the deformed space also takes on a deformed appearance. The magnification effect allowed the team to study very distant light sources beyond Abell 2744, revealing eight extremely faint galaxies that would otherwise be undetectable, even for Webb.
The team found that these faint galaxies are immense producers of ultraviolet light, at levels four times higher than previously assumed. This means that most of the photons that reionized the Universe likely came from these dwarf galaxies.
"This discovery reveals the crucial role played by ultra-faint galaxies in the evolution of the early Universe," said Iryna Chemerynska, a member of the team from the Institute of Astrophysics of Paris in France. "They produce ionizing photons that transform neutral hydrogen into ionized plasma during cosmic reionization. It highlights the importance of understanding low-mass galaxies in shaping the history of the Universe.
"These cosmic powerhouses collectively emit more than enough energy to do the work," added team leader Hakim Atek, also from the Institute of Astrophysics of Paris and lead author of the paper describing this result. "Despite their small size, these low-mass galaxies are prolific producers of energetic radiation, and their abundance during this period is so substantial that their collective influence can transform the entire state of the Universe."
To reach this conclusion, the team first combined data from extremely sensitive Webb images with images of Abell 2744 from the NASA/ESA Hubble Space Telescope to select extremely faint candidate galaxies at the time of reionization.
This was followed by spectroscopy with Webb's Near-Infrared Spectrograph (NIRSpec). The instrument's multi-shutter array was used to capture multiple spectra of these faint galaxies. This is the first time scientists have reliably estimated how common faint galaxies are. The results confirm that they are the most abundant type of galaxies during the reionization era. This is also the first time that the ionizing power of these galaxies has been measured, allowing astronomers to determine that they are producing enough energetic radiation to ionize the early Universe.
"The incredible sensitivity of NIRSpec combined with the gravitational amplification provided by Abell 2744 allowed us to identify and study in detail these galaxies from the first billion years of the Universe, despite being more than 100 times fainter than our own Milky Way," said Hakim.
In an upcoming Webb observation program, called GLIMPSE, scientists will obtain the most sensitive observations ever made in the sky. By targeting another galaxy cluster, called Abell S1063, even fainter galaxies will be identified during the reionization era. This will allow scientists to verify if the dwarf galaxies from the current study are typical of the large-scale galaxy distribution. As these new results are based on observations obtained in one field, the team notes that the ionizing properties of faint galaxies may appear differently if they reside in denser regions.
Therefore, additional observations in a different field will provide more information and help verify these conclusions. GLIMPSE observations will also help astronomers investigate the period known as cosmic dawn, when the Universe was only a few million years old, to enhance our understanding of the emergence of the first galaxies.
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