Astronomers detect first direct evidence of star-forming gas in early galaxies

The first galaxies were already busy by the time the universe was 700 to 800 million years old. Stars were forming fast. Structures were taking shape. Additionally, huge stores of gas were feeding that growth. What astronomers have struggled to see clearly is the neutral gas at the center of that process. It is the cooler material that directly supplies star formation.

That missing piece has now come into view.

An international team led by Assistant Professor Yoshinobu Fudamoto and Professor Masamune Oguri of Chiba University used the Atacama Large Millimeter/submillimeter Array, or ALMA, to detect the [O I] 145 micrometer emission line in four distant galaxies. The signal comes from neutral oxygen and serves as a direct tracer of neutral gas. As a result, it is a powerful way to study the material that fuels early star formation.

The galaxies were seen as they existed more than 13 billion years ago, at redshifts above 6.5. According to the team, this is the most distant direct detection yet of neutral gas in typical star-forming galaxies.

“Our results represent the most distant direct detection of neutral gas in typical star-forming galaxies to date,” Dr. Fudamoto said. “This analysis unlocks the wealth of existing [C II] observations as a probe of neutral gas in the early Universe.”

Space telescopes such as Hubble and the James Webb Space Telescope have transformed views of the early universe. However, they mostly reveal stars and ionized gas. Neutral gas is harder to catch because its key signals fall in the far-infrared, beyond the range of those observatories.

That is where ALMA comes in.

The team focused on the [O I] 145 micrometer line because it traces neutral gas more cleanly than the widely used [C II] line. Carbon can shine from both neutral and ionized regions, which makes its origin harder to pin down. To sort that out, the researchers also examined the [N II] 205 micrometer line. This line comes only from ionized gas.

That comparison turned out to matter. In three of the galaxies, the [N II] line was not detected, and in the fourth it appeared only as a weak, uncertain signal. The result suggests that most of the [C II] emission in these systems comes from neutral gas rather than ionized regions. The team estimated that the dominant fraction of [C II] arises from neutral gas. The lower limits on that fraction ranged from more than 0.74 to more than 0.96.

That helps settle a long-running question about what [C II] is really tracing in galaxies from the epoch of reionization.

The four target galaxies, REBELS-38, A1689-zD1, REBELS-25, and REBELS-18, had already been identified as bright in [C II]. ALMA follow-up observations found [O I] in all four, each at better than 4 sigma significance. The measured [O I]-to-[C II] luminosity ratios ranged from 0.08 to 0.33, with a median of 0.16.

Those numbers let the researchers go beyond detection and start modeling physical conditions inside the gas itself.

Using the spectral synthesis code CLOUDY, the team combined the [O I] and [C II] measurements with infrared luminosity estimates to infer gas density and far-ultraviolet radiation strength. They found that the gas was remarkably dense, with hydrogen densities around 10^4 to 10^6 particles per cubic centimeter. Those values are similar to what astronomers see in high-redshift starbursts and submillimeter galaxies. These are systems known for intense star formation.

The radiation field, however, was more moderate. The team estimated far-ultraviolet field strengths of about G0 ~ 10^2.5 to 10^3.0. These values are lower than in many extreme starbursts and quasars.

Together, those two results point to a particular kind of young galaxy: compact, gas-rich, and efficient at turning dense neutral material into stars. Yet they are not necessarily blasting that gas with the most extreme radiation fields seen in more luminous systems.

The authors describe these galaxies as, in effect, a lower-radiation version of the intense dusty starbursts already studied at somewhat later times.

The [O I] detections also opened a way to estimate how much oxygen, and then hydrogen, sits in the warm neutral gas. Assuming the [O I] emission is optically thin and combining it with oxygen abundances inferred from recent JWST spectroscopy, the researchers derived warm neutral hydrogen masses between 0.9 × 10^9 and 3.0 × 10^9 solar masses.

That translates to gas mass fractions of about 0.2 to 0.4 when compared with the galaxies’ stellar masses.

Those estimates lined up well with one [C II]-based method that also targets warm neutral gas, but they were lower than some empirical calibrations based on [O I] or [C II]. The gap suggests the new method may be capturing only part of the neutral reservoir. Especially, it may only capture the warmer, denser component, while colder gas may still sit out of reach.

The study also carried some caution flags. One galaxy, REBELS-25, did not fit neatly into the preferred model grid unless the neutral gas was assigned a lower metallicity than the ionized gas seen with JWST. That could reflect inflowing, less enriched material. On the other hand, the team did not claim a unique explanation. In REBELS-38, the [O I] line also appeared narrower than the [C II] line. This hints that the two signals may not arise from exactly the same interstellar regions, though the evidence remains marginal.

Even with those uncertainties, the result marks an important shift. Neutral gas in ordinary star-forming galaxies from the epoch of reionization has been largely inferred, not directly traced. This study shows that [O I] 145 micrometers can change that.

“Our work establishes the [O I] emission line as an effective tool for studying an elusive gas component in the early Universe, opening a new window onto the ‘fuel’ behind star formation,” Dr. Inoue said.

The team plans to expand the work to a larger sample and combine ALMA with JWST and other observatories. That could help connect stars, ionized gas, dust, and neutral gas into a more complete history of how galaxies assembled during cosmic dawn.

For now, the key advance is simple but important. Astronomers are no longer just seeing where early galaxies shone. Instead, they are beginning to trace the raw material that made that light possible.

This work gives astronomers a more direct way to study the gas that powered star formation in the early universe. By showing that the [O I] 145 micrometer line can trace neutral gas in ordinary galaxies at redshifts above 6.5, the study strengthens ALMA’s role alongside JWST.

It also helps clarify how to interpret the much larger archive of [C II] observations, which could now be used more confidently to probe neutral gas in young galaxies.

Over time, that may lead to better estimates of how quickly galaxies built stars, how dense their gas was, and how the first substantial galactic structures grew during cosmic reionization.

Research findings are available online in The Astrophysical Journal.

The original story "Astronomers detect first direct evidence of star-forming gas in early galaxies" is published in The Brighter Side of News.

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