Faintest galaxy in the early cosmos

Galaxy JD1

Galaxy JD1 (detail) and the galaxy cluster Abell 2744, which acts as a gravitational lens. © Guido Roberts-Borsani/UCLA); Original images: NASA, ESA, CSA, Swinburne University of Technology, University of Pittsburgh, STScI

In the early days of the universe, it was only the ultraviolet light of the first stars that ensured that the primordial hydrogen was ionized and the cosmic dawn began. Now astronomers have discovered one of the galaxies that played a crucial role in this development. The young, small and faint galaxy JD1 already existed 480 million years after the Big Bang and its type corresponds to what was probably the most common form of galaxy at the time. It is the faintest galaxy discovered at this distance to date. It was spotted using the James Webb Space Telescope and thanks to the magnifying effect of a galaxy cluster in the foreground.

When, around 380,000 years after the Big Bang, atomic nuclei and electrons combined to form the first atoms, the universe became transparent for the first time. Neutral hydrogen filled space and formed the basis for the first stars. With them the dark age of the cosmos ended and the "cosmic dawn" began - the age of reionization. During this phase, the intense radiation from stars and galaxies ionized the primordial hydrogen, making it transparent to ultraviolet radiation as well. But when exactly the reionization began and what kind of galaxies was primarily responsible for it has only been partially clarified so far. Astronomers have already found some galaxies that existed hundreds of millions of years after the Big Bang. At the time, however, these were very rare and did not produce enough ionizing radiation to have been decisive drivers of cosmic reionization.

Faint light from the distant cosmos

"Most of the early galaxies discovered so far are therefore not considered particularly representative of the young star clusters that populated the early Universe," explains lead author Guido Roberts-Borsani of the University of California at Los Angeles (UCLA). “Ultra-faint galaxies, on the other hand, were far more numerous back then. So we think they more likely represent the galaxies that were driving the reionization process back then.” However, until now, these faint galaxies have been difficult to observe. On the one hand, they are very far away, so that powerful telescopes are necessary, on the other hand, they are often surrounded by neutral gas, which swallows the most detectable photons in the Lyman-alpha wavelength range. But the new James Webb Space Telescope and a cosmic coincidence came to the aid of Roberts-Borsani's team. Massive galaxy cluster Abell 2744 pushed ahead of a distant early galaxy in such a way that its faint light was magnified 13-fold.

This allowed the team to locate and analyze the distant, faint galaxy JD1 in more detail. "Before the James Webb Telescope began operations, we could not have dreamed of detecting such a faint galaxy," says co-author Tommaso Treu of UCLA. Images taken with the near-infrared camera NIRCam showed that JD1 is a rather small galaxy, composed of a central clump of stars and two smaller centers. These recordings also indicated a strong red shift and a correspondingly high age of this observation. Spectral analyzes of the galaxy light using the near-infrared spectrograph NIRSpec confirmed this: The JD1 galaxy therefore has a redshift of z= 9.79 and therefore already existed when the universe was only 480 million years old. The light from their stars was on its way to us for 13.3 billion years.

Typical representative of the early “reionization actors”

The galaxy JD1 is thus the faintest galaxy from this early phase of cosmic development, as the astronomers report. The spectral data also suggest that this galaxy was only around 30 million years old at the time of observation and was therefore still very young. Their stellar mass was also significantly lower than that of most of today's galaxies and the massive, bright galaxies previously known from the early cosmos. As expected for a galaxy at the beginning of reionization, its stars only had a few heavier elements - they are significantly less metal-rich than the sun. "This confirms that this galaxy is dominated by a young, star-forming system that has just begun its chemical enrichment," write Roberts-Borsani and colleagues. In their view, JD1 corresponds exactly to the type of galaxies that are considered to be possible drivers of reionization. "The luminosity of the galaxy is similar to that from the sources that must have supplied the bulk of the UV photons at the time," the team said.

"The combination of spectroscopic analysis and gravitational lensing provides us with a unique and unprecedented insight into the physics of an ultra-faint galaxy from the Dark Ages of the Universe," the astronomers state. The James Webb telescope was built for precisely such insights.

Source: Guido Roberts-Borsani (University of California, Los Angeles) et al., Nature, doi: 10.1038/s41586-023-05994-w

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