Webb space telescope discovers faintest galaxy in the early universe

Peering back in time

Scientists examined the early years of the universe using cosmic light that has traveled for millions of years to reach us. The first billion years of the universe, in particular, have instilled curiosity among astronomers.  

It all began with a bang! It has long been assumed that the universe arose after the Big Bang – about 13.8 billion years ago. It was long after this that the universe began to expand and gradually cooled, allowing hydrogen atoms to form. 

However, the early universe experienced a completely dark period after the Big Bang known as the Cosmic Dark Ages. This nearly one billion-year period was devoid of light sources. During this time, the universe was shrouded in a “fog of neutral hydrogen that trapped the light of the first stars and galaxies.” And, almost a billion years after the Big Bang, this fog mysteriously began to dissipate. 

This marked the Epoch of Reionization, which led to the formation of the first stars and galaxies. And one crucial goal in astronomy has been to understand the first generation of galaxies, which began to populate this era of the universe’s early history. 

Using the power of Webb, astronomers are just beginning to understand this perplexing yet fascinating era. 

Ultra-faint galaxies such as JD1 are numerous

“Most of the galaxies found with JWST so far are bright galaxies that are rare and not thought to be particularly representative of the young galaxies that populated the early universe. As such, while important, they are not thought to be the main agents that burned through all of that hydrogen fog,” said Guido Roberts-Borsani, a UCLA postdoctoral researcher and the study’s first author.

Roberts-Borsani further explained: “Ultra-faint galaxies such as JD1, on the other hand, are far more numerous, which is why we believe they are more representative of the galaxies that conducted the reionization process, allowing ultraviolet light to travel unimpeded through space and time.”

Using Webb's high sensitivity, the team was able to locate JD1. It is located behind Abell 2744, a bright, large cluster of nearby galaxies. 

Fortunately, Webb was able to see this dim galaxy due to gravitational lensing. The gravitational pull of this galaxy cluster caused JD1 to appear larger and 13 times brighter than it would have otherwise. 

“The combination of JWST and the magnifying power of gravitational lensing is a revolution. We are rewriting the book on how galaxies formed and evolved in the immediate aftermath of the Big Bang,” added Treu.

NIRSpec, Webb's near-infrared spectrograph instrument, was used to obtain an infrared light spectrum of this galaxy. The data revealed many key details about this distant galaxy, such as the number of stars and the amount of dust and heavy elements. 

The findings have been reported in the journal Nature.

Study Abstract:

In the first billion years after the Big Bang, sources of ultraviolet (UV) photons are believed to have ionized intergalactic hydrogen, rendering the Universe transparent to UV radiation. Galaxies brighter than the characteristic luminosity L* do not provide enough ionizing photons to drive this cosmic reionization. Fainter galaxies are thought to dominate the photon budget; however, they are surrounded by neutral gas that prevents the escape of the Lyman-α photons, which has been the dominant way to identify them so far. JD1 was previously identified as a triply-imaged galaxy with a magnification factor of 13 provided by the foreground cluster Abell 2744, and a photometric redshift of z ≈ 10. Here we report the spectroscopic confirmation of this very low luminosity (≈0.05 L*) galaxy at z = 9.79, observed 480 Myr after the Big Bang, by means of the identification of the Lyman break and redward continuum, as well as multiple ≳4σ emission lines, with the Near-InfraRed Spectrograph (NIRSpec) and Near-InfraRed Camera (NIRCam) instruments. The combination of the James Webb Space Telescope (JWST) and gravitational lensing shows that this ultra-faint galaxy (MUV = −17.35)—with a luminosity typical of the sources responsible for cosmic reionization—has a compact (≈150 pc) and complex morphology, low stellar mass (107.19 M⊙) and subsolar (≈0.6 Z⊙) gas-phase metallicity.