Webb observes faintest galaxies during first billion years of the Universe

Using the James Webb Space Telescope (JWST), an international team of scientists has obtained the first spectroscopic observations of the faintest galaxies during the first billion years of the Universe. These galaxies existed in the era of reionisation - a period of darkness without any stars or galaxies, when the Universe was blanketed by a dense hydrogen fog.

The Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) programme comprise both imaging and spectroscopic observations of the lensing cluster Abell 2744. Using gravitational lensing, the team studied very distant sources of light beyond Abell 2744, revealing eight extremely faint galaxies that would otherwise be undetectable, even to powerful space telescopes like Webb.

The team discovered that the faint galaxies are massive generators of ultraviolet light. They produce UV light at levels that are four times higher than previously estimated. This indicates that the majority of the photons that reionized the Universe probably originated from these small galaxies.

This marks the first time that scientists have reliably estimated how common faint galaxies are in the Universe, with the findings indicating that these galaxies are the most common type during the period of reionization. This is also the first time that scientists have measured the ionising power of these galaxies, enabling them to determine that they are producing sufficient energetic radiation to ionise the early Universe.

"The incredible sensitivity of NIRSpec combined with the gravitational amplification provided by Abell 2744 enabled us to identify and study these galaxies from the first billion years of the Universe in detail, despite their being over 100 times fainter than our own Milky Way," said team leader Hakim Atek, also of the Institut d’Astrophysique de Paris and lead author of the paper describing this result.

🆕 The NASA/ESA/CSA James Webb Space Telescope has just observed some of the faintest galaxies in the Universe!These dwarf galaxies existed in the earliest era of the Universe, a time where there was darkness without any stars or galaxies, filled with a dense fog of hydrogen… pic.twitter.com/fVY92LiZgi

— ESA (@esa) February 28, 2024

What's next?

In the next Webb observation, scientists will target another galaxy cluster, named Abell S1063, to identify even fainter galaxies during the epoch of reionisation.