Astronomers have discovered the first direct evidence of star-forming gases in early galaxies

The first galaxies were already occupied when the universe was 700 to 800 million years old. Stars were forming rapidly. Structures were taking shape. In addition, huge gas reserves have been fueling this 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 powers the star formation process.

This missing piece has now emerged.

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

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

“Our results represent the most distant direct detection of neutral gas in model star-forming galaxies to date,” said Dr. Fudamoto. “This analysis opens up the wealth that exists [C II] Observations as a probe of neutral gas in the early universe.

Space telescopes such as Hubble and James Webb Space Telescope It changed views about the early universe. However, they mostly detect stars and ionized gases. The neutral gas is difficult to capture because its main signals are in the far infrared, outside the range of those observatories.

This is where Alma comes into play.

The team focused on [O I] 145 µm line because it tracks the neutral gas more cleanly than the more widely used gas [C II] line. Carbon can fluoresce from both neutral and ionized regions, making it difficult to determine its origin. To solve this problem, the researchers also examined… [N II] Line 205 µm. This line comes only from ionized gas.

This comparison turns out to be important. In three of Galaxiesthe [N II] The line was not detected, and on the fourth it appeared only as a weak and uncertain signal. The result indicates that most [C II] Emissions in these systems come from the neutral gas and not from the ionized regions. The team estimated that the prevailing portion of [C II] It arises from neutral gas. The lower limits of this fraction ranged from more than 0.74 to more than 0.96.

This helps resolve the long-standing question of what [C II] Really tracks galaxies from the era of reionization.

The four target galaxies, REBELS-38, A1689-zD1, REBELS-25, and REBELS-18, have already been identified as bright in [C II]. ALMA follow-up notes found [O I] In all four, each is better than 4 sigma. Measured [O I]-to-[C II] Luminosity ratios ranged from 0.08 to 0.33, with an average of 0.16.

These numbers allow researchers to move beyond the discovery phase and begin modeling the physical conditions within the gas itself.

Using the spectral synthesis code CLOUDY, the team combined [O I] and [C II] Measurements with infrared luminosity estimates to infer gas density and distanceUltraviolet rays power. They found that the gas was remarkably dense, with a hydrogen density of about 10^4 to 10^6 molecules per cubic centimeter. These values ​​are similar to what astronomers see in high-redshift starbursts and submillimeter galaxies. These systems are known for intense star formation.

But the radiation field was much milder. The team estimated the intensity of the far-ultraviolet field to be about G0 ~ 10^2.5 to 10^3.0. These values ​​are lower than in many extreme star explosions and quasars.

Together, these two results point to a particular type of young galaxy: compact, gas-rich, and efficient at converting dense neutral matter into stars. However, they do not necessarily blast this gas with the extreme radiation fields seen in brighter systems.

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

the [O I] The discoveries also opened up a way to estimate how much Oxygenthen hydrogen, which is present in the warm neutral gas. Assuming [O I] The emission is optically subtle and combined with oxygen abundances inferred from recent JWST spectroscopy, the researchers infer warm neutral hydrogen masses between 0.9 x 10^9 and 3.0 x 10^9 solar masses.

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

These estimates agree well with one [C II]The method also targets warm neutral gas, but was lower than some experimental calibrations based on this [O I] or [C II]. This gap indicates that the new method may only capture a portion of the neutral reservoir. In particular, it may only pick up the warmest, densest component, while the cooler gas may remain out of reach.

The study also had some warning signs. One galaxy, REBELS-25, did not fit perfectly into the preferred model’s grid unless a neutral gas with a lower metallicity than the galaxy was assigned. Ionized gas Seen with JWST. This may reflect an influx of less enriched material. On the other hand, the team did not demand a unique explanation. In Rebel-38, the [O I] The line also appeared narrower than [C II] line. This suggests that the two signals may not originate from the same interstellar regions, although the evidence is still marginal.

Even with these doubts, the result represents an important shift. Neutral gas in normal star-forming galaxies from the reionization era has been largely inferred, not directly traced. This study shows that [O I] 145 micrometers can change that.

“Our work is grounded [O I] “The emission line as a powerful tool for studying the elusive gas component of the early universe opens a new window on the ‘fuel’ behind star formation,” Dr Inoue said.

The team plans to expand the work to a larger sample and integrate it Alma With JWST and other observatories. This could help link stars, ionized gas, dust and neutral gas into a more complete history of how galaxies came together during cosmic dawn.

For now, the main progress is simple but important. Astronomers no longer just see where early galaxies rose. Instead, they began tracking down the raw material that made this light possible.

This work gives astronomers a more direct way to study the gas that fueled star formation in the early universe. By showing that [O I] The 145 µm line can track neutral gas in normal galaxies at redshifts higher than 6.5, and the study reinforces ALMA’s role alongside the JWST.

It also helps clarify how to interpret a much larger archive of [C II] observations, which can now be used with greater confidence to explore the neutral gas Young galaxies.

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