The recent discovery of star-forming gas in early galaxies has opened a new window into the universe's cosmic dawn. Astronomers have long sought to understand the processes that fueled the rapid star formation in the early universe, and this breakthrough provides a crucial piece of the puzzle. The detection of neutral oxygen gas, traced by the [O I] 145 micrometer emission line, offers a direct view into the material that directly supplies star formation. This is a significant advancement, as previous observations primarily revealed stars and ionized gas, making it challenging to study the cooler, neutral gas component. The international team, led by Assistant Professor Yoshinobu Fudamoto and Professor Masamune Oguri, utilized the Atacama Large Millimeter/submillimeter Array (ALMA) to make this groundbreaking discovery.
One of the key findings is that the four target galaxies, REBELS-38, A1689-zD1, REBELS-25, and REBELS-18, were found to have remarkably dense gas, with hydrogen densities around 10^4 to 10^6 particles per cubic centimeter. This is similar to what astronomers observe in high-redshift starbursts and submillimeter galaxies, known for their intense star formation. However, the radiation field was more moderate, suggesting these galaxies are a lower-radiation version of the intense dusty starbursts studied at later times. This discovery challenges the notion that these early galaxies were necessarily blasting their gas with the most extreme radiation fields seen in more luminous systems.
The [O I] detections also allowed the researchers to estimate the amount of oxygen and hydrogen in the warm neutral gas. By assuming optically thin [O I] emission and combining it with oxygen abundances from JWST spectroscopy, they derived warm neutral hydrogen masses between 0.9 × 10^9 and 3.0 × 10^9 solar masses. This translates to gas mass fractions of about 0.2 to 0.4 when compared with the galaxies' stellar masses. These estimates align with [C II]-based methods targeting warm neutral gas but are lower than some empirical calibrations based on [O I] or [C II]. This gap suggests that the new method may only capture part of the neutral reservoir, particularly the warmer, denser component, while colder gas remains out of reach.
The study also raises intriguing questions. 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. This could reflect inflowing, less enriched material. Additionally, the [O I] line appeared narrower than the [C II] line in REBELS-38, hinting that the two signals may not arise from exactly the same interstellar regions, though the evidence is marginal. Despite these uncertainties, the result marks an important shift, as neutral gas in ordinary star-forming galaxies from the epoch of reionization has been largely inferred, not directly traced. The [O I] 145 micrometer line has now been established as an effective tool for studying this elusive gas component, opening a new window onto the 'fuel' behind star formation.
This discovery has practical implications for astronomers, providing a more direct way to study the gas that powered star formation in the early universe. By strengthening ALMA's role alongside JWST, it helps clarify how to interpret the vast archive of [C II] observations, which can now be used more confidently to probe neutral gas in young galaxies. Over time, this 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. The study also paves the way for future research, with the team planning to expand the work to a larger sample and combine ALMA with JWST and other observatories, aiming to connect stars, ionized gas, dust, and neutral gas into a more complete history of how galaxies assembled during cosmic dawn.
