Massive Reservoir of 'Star Fuel' Discovered in One of the Universe's Earliest Galaxies

The universe was still in its infancy—just 700 million years after the Big Bang, or roughly 5% of its current age—when galaxies capable of rapid growth and star formation already existed. Using two powerful radio telescopes, an international team of astronomers has directly observed the 'fuel' driving this process for the first time. Their findings, published in the Monthly Notices of the Royal Astronomical Society, offer a new perspective on the formation of the first massive galaxies.

The study focuses on the galaxy REBELS-25, observed at a redshift of z=7.3. It dates back to the Epoch of Reionization, a time when the first generations of stars and galaxies were transforming the cosmos by ionizing neutral hydrogen. Astronomers had long suspected that these early, massive structures harbored vast reservoirs of cold molecular gas—the essential building blocks for star formation. Yet direct proof remained elusive, as the signals are incredibly weak and the distant cosmic microwave background interferes with observations.

Observations from the Very Large Array (VLA) in New Mexico and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile were instrumental in this discovery. Researchers used the VLA to detect faint traces of carbon monoxide (CO), which serves as a reliable proxy for cold gas. This marks the most distant detection of its kind achieved to date. ALMA provided additional depth, allowing the team to assess the gas's temperature, density, and spatial distribution. The findings showed that REBELS-25 contains approximately 100 billion solar masses of cold molecular gas, representing about 95% of the galaxy's total baryonic matter. By comparison, significantly fewer stars have formed within the galaxy so far.

The galaxy is remarkably mature for its age. Previous studies had already revealed that it features a rotating disk, high dust content, and a relatively high concentration of heavy elements. An enormous gas reservoir has now been added to this list of features. 'Our findings demonstrate that just 700 million years after the Big Bang, galaxies possessed extensive reserves of cold gas to fuel new star formation,' says lead author Carin Cescon of Leiden University.

Manuel Aravena, a Chilean astronomer from the Center for Excellence in Astrophysics and Associated Technologies and Diego Portales University, also contributed to the study. He notes that such findings help clarify how certain systems in the early universe were able to accumulate material so rapidly. While this may not be a typical case—REBELS-25 could be a rare outlier—scientists now have the methodology to determine how common these reservoirs actually were.

This discovery represents a major milestone in our exploration of the 'Cosmic Dawn.' Future, more sensitive instruments like the Next Generation VLA will enable us to observe cold gas in a wider range of early galaxies. For now, we can see that even in the universe's infancy, conditions were already ripe for the rapid development of the structures that form the foundation of the modern cosmos. It serves as a poignant reminder of just how dynamic and surprising the early universe was.