Astronomers have made a groundbreaking discovery, shedding light on the early universe's star formation process. They have detected the first direct evidence of star-forming gas in early galaxies, a crucial component that was previously elusive. This achievement is significant because it provides a clearer understanding of the composition of stars and the role of neutral gas in the early universe.
The study, led by Assistant Professor Yoshinobu Fudamoto and Professor Masamune Oguri of Chiba University, utilized the Atacama Large Millimeter/submillimeter Array (ALMA) to observe four distant galaxies at redshifts above 6.5. These galaxies, REBELS-38, A1689-zD1, REBELS-25, and REBELS-18, were already known for their bright [C II] emissions, which indicate the presence of ionized gas. However, the team's focus was on the [O I] 145 micrometer emission line, which traces neutral oxygen and serves as a direct tracer of neutral gas.
The results were remarkable. In three out of the four galaxies, the [N II] line, which comes only from ionized gas, was not detected. This finding suggests that most of the [C II] emission in these systems originates from neutral gas rather than ionized regions. The team estimated that the dominant fraction of [C II] arises from neutral gas, with lower limits ranging from more than 0.74 to more than 0.96.
This discovery helps settle a long-standing question about the nature of [C II] emissions in galaxies from the epoch of reionization. It also revealed that the gas in these early galaxies was remarkably dense, with hydrogen densities around 10^4 to 10^6 particles per cubic centimeter, similar to what is observed in high-redshift starbursts and submillimeter galaxies.
The radiation field in these galaxies was more moderate compared to extreme starbursts and quasars, indicating a particular type of young galaxy: compact, gas-rich, and efficient at converting dense neutral material into stars. However, these galaxies do not exhibit 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. They derived warm neutral hydrogen masses between 0.9 × 10^9 and 3.0 × 10^9 solar masses, which translates to gas mass fractions of about 0.2 to 0.4 when compared with the galaxies' stellar masses. These estimates aligned with [C II]-based methods but were lower than some empirical calibrations based on [O I] or [C II].
The study also raised 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 the James Webb Space Telescope (JWST). This could reflect inflowing, less enriched material. Additionally, the [O I] line appeared narrower than the [C II] line in REBELS-38, suggesting that the two signals may not arise from the same interstellar regions.
Despite these uncertainties, the study marks a significant shift. Neutral gas in ordinary star-forming galaxies from the epoch of reionization has been largely inferred, but this research demonstrates that the [O I] 145 micrometer line can directly trace this elusive gas component. This achievement opens a new window onto the 'fuel' behind star formation and strengthens ALMA's role alongside JWST.
The team plans to expand their work to a larger sample and combine ALMA with JWST and other observatories. This will help connect stars, ionized gas, dust, and neutral gas into a more comprehensive history of how galaxies assembled during cosmic dawn. Ultimately, this research provides astronomers with a more direct way to study the gas that powered star formation in the early universe, leading to better estimates of galactic structures and the rate of star formation during cosmic reionization.
