The Universe at 700 million years old was a noisy construction site, just beginning to light up. Telescopes caught a signal from that distant epoch: the galaxy REBELS-25, massive and full of stellar fuel. Radio observations with the VLA and ALMA telescopes revealed a faint but confident glow from carbon monoxide (CO) molecules. This is the most reliable indicator of cold molecular hydrogen — the very substance from which stars are born. Detecting a low-energy CO transition at such a distance is the most distant ever recorded. It directly tells us that the building material for stars was stockpiled very early.
These reserves are staggering. The gas mass in REBELS-25 reaches one hundred billion solar masses, and the gas fraction in the galaxy is 95%. For comparison, in our Milky Way, gas makes up only about 5% of the mass; the rest is stars and dark matter. Here we see the opposite picture: almost all matter is unignited fuel. Dust, on the other hand, is extremely scarce: for every 600 parts of gas, there is only one part of dust. This ratio is typical of very young systems, where heavy elements have not yet accumulated from the explosions of the first stars.
The challenge of these observations is that at great distances, the cosmic microwave background (leftover from the Big Bang) is warmer than today and can 'drown out' the signal from the gas. The authors used the TUNER program, which accounts for this background and the dust continuum, and independently confirmed a gas mass close to 180 billion solar masses. They also tested an alternative method — using emission from ionized carbon [C II]—and showed that this line remains a viable tool even in the epoch of reionization. These findings pave the way for future telescopes that will routinely study gas in the first galaxies.
The galaxy REBELS-25 contains so much molecular gas that if gathered in one place, it would occupy a volume comparable to a thousand Milky Ways, but with almost no stars.
In Carl Sagan's novel 'Contact,' a signal from deep space is captured using a network of radio telescopes — much like how VLA and ALMA searched for gas traces in REBELS-25, except they were looking not for aliens but for the raw material of the first stars.