When Karin Cescon and her team at Leiden University pointed the NSF Very Large Array toward the ancient cosmos, they found something that rewrote what we thought was possible in the universe's infancy: a galaxy called REBELS-25, born just 700 million years after the Big Bang, already bristling with the cold molecular gas needed to forge stars at a furious pace. The discovery marks the most distant low-energy carbon monoxide detection ever recorded, a milestone that opens a window into how the first galaxies grew so massive so fast.
Understanding how galaxies assemble themselves has always been central to cosmology. Galaxies grow by converting gas into stars, and cold molecular gas is the primary fuel for this cosmic engine. Until now, astronomers suspected that the brightest, most massive galaxies in the early universe must have had enormous gas supplies, but no one had actually detected these reserves at such distant epochs. REBELS-25 sits at redshift z = 7.3, deep within the Epoch of Reionization—a transformative era when the first stars and galaxies began reshaping the dark, neutral universe into the glowing cosmos we inhabit today, roughly 13 billion years ago.
The detection itself required extraordinary observational prowess. Using the NSF Very Large Array, a radio telescope facility in Socorro County, New Mexico, the team searched for faint radio signals emitted by carbon monoxide molecules, which act as reliable tracers of molecular gas. The challenge was formidable: at such vast distances, the cosmic microwave background—the relic radiation from shortly after the Big Bang—acts as blinding background noise that becomes increasingly bright at high redshift, making cold gas emission harder to spot. The NSF VLA's deep observations managed to overcome this cosmic interference, revealing the telltale signature of cool gas. When combined with complementary data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile's Andes, the picture became clear: REBELS-25 possessed a very large reservoir of star-forming material despite being observed when the universe was merely five percent of its current age.
What makes this discovery significant is not just the detection itself, but what it tells us about galactic growth during cosmic dawn. By directly observing the actual fuel driving star formation rather than inferring it indirectly through other measurements, astronomers can now measure how quickly these ancient galaxies assembled mass. REBELS-25's enormous gas supply shows that some of the earliest galaxies were already primed for intense star formation—a critical insight into understanding how the universe's first billion years unfolded.
The implications extend beyond REBELS-25. This achievement is a proof of concept for what lies ahead. The Next-Generation Very Large Array (ngVLA), a planned facility with antennas distributed across New Mexico, West Texas, Arizona, northern Mexico, and throughout North America, will make these measurements ten times faster. Where REBELS-25 represents a tantalizing case study, the ngVLA paired with ALMA will map how entire populations of early galaxies gathered fuel and grew during the cosmic dawn. Fainter and more distant systems will finally become accessible, transforming our understanding from individual bright examples into a comprehensive portrait of galactic assembly in the universe's first billion years.