Over 13,000 million years ago, when the first galaxies were just beginning to exist, huge clouds of cold gas fueled the birth of their stars. A primordial fuel has remained hidden from astronomers until now. An international team of researchers has managed to directly detect for the first time the neutral gas that fueled star formation in some of the oldest known galaxies, observed when the Universe was only between 700 and 800 million years old.
It may seem like a huge number, but in cosmic terms, it is equivalent to looking at a picture of the Universe's infancy. The Big Bang occurred about 13,800 million years ago, and after that event, the cosmos went through a long stage known as the dark ages. For hundreds of millions of years, there were no stars or galaxies as we know them. Until gradually, the first concentrations of matter began to form, and the first stars were born, illuminating the Universe and giving rise to primitive galaxies. Understanding how this process happened is one of the great challenges of modern astronomy.
The new study, led by researchers from the University of Chiba (Japan) and to be published in The Astrophysical Journal, used the ALMA observatory, located in the Chilean desert of Atacama, to study four extremely distant galaxies. What they were looking for was a very specific signal emitted by neutral oxygen atoms, a kind of luminous footprint that allows the direct location of the cold gas from which stars are born.
Until now, telescopes could observe already formed stars or the hot gas surrounding them. Even revolutionary instruments like the James Webb Space Telescope have allowed the observation of galaxies from the early cosmic times with unprecedented precision. However, the neutral gas that fuels star formation remained practically hidden.
"Our results represent the most distant direct detection of neutral gas in galaxies to date," explains researcher Yoshinobu Fudamoto, lead author of the study. According to the scientist, the finding allows for a reinterpretation of many previous observations made on primitive galaxies.
The detection was not easy. The researchers analyzed an emission line known as [OI] at 145 microns, considered one of the best indicators of neutral gas. To ensure that the signal actually came from that material, they compared the results with another spectral signature associated exclusively with ionized gas. The conclusion was clear: most of the observed signal came from huge reserves of neutral gas, precisely the material that serves as raw material for making stars.
What they found was surprising. The studied galaxies contained extremely dense regions of gas, comparable to some of the most intense galaxies in the current Universe. However, the radiation generated in those systems seemed relatively moderate. In other words, the first galaxies could have been compact star factories, much more concentrated than previously thought.
The importance of the discovery goes beyond these galaxies. For years, astronomers have tried to reconstruct how the cosmos evolved from a primordial soup of particles to become a Universe filled with galaxies, clusters, and planets. It is estimated that the first stars emerged about a hundred or two hundred million years after the Big Bang. It was precisely these stars that began to produce chemical elements heavier than hydrogen and helium, seeding space with the ingredients that millions of years later would allow the appearance of planetary systems and even life.
That is why each new observation from this remote era functions as a time machine. The light from these galaxies has taken over 13,000 million years to reach Earth, so scientists observe them as they were when our planet did not even exist, and the future Milky Way was just beginning to assemble.
"Our work establishes the [OI] emission line as an effective tool for studying a difficult-to-detect gaseous component in the early Universe, opening a new perspective on the fuel that drives star formation," says researcher Akio Inoue.
The authors are already preparing the next step. "We plan to extend these observations to a larger sample of galaxies and, combining ALMA with the James Webb and other facilities, build a complete picture of how galaxies formed and evolved from cosmic dawn to the present day," Fudamoto anticipates.
The question they are trying to answer is as old as humanity itself: how did the Universe go from being a hot and uniform cloud after the Big Bang to becoming the spectacular network of galaxies, stars, and planets that we observe today. Thanks to this discovery, astronomers have just found a fundamental piece of that puzzle. The fuel was there from the beginning; we just couldn't see it.