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 distant galaxies, dating back to a time when the universe was just 700 to 800 million years old. This achievement is a significant step forward in our understanding of how the first galaxies formed and evolved.
The key to this discovery lies in the use of the Atacama Large Millimeter/submillimeter Array (ALMA) and the detection of the [O I] 145 micrometer emission line. This line, originating from neutral oxygen, serves as a powerful tracer of the neutral gas that fuels star formation. By focusing on this specific line, the team was able to distinguish between neutral and ionized gas, providing a clearer picture of the star formation process.
The four target galaxies, REBELS-38, A1689-zD1, REBELS-25, and REBELS-18, were identified as bright in [C II] and subsequently observed with ALMA. The [O I] line was detected in all four galaxies, with significant results. The measured [O I]-to-[C II] luminosity ratios indicated that most of the [C II] emission likely originates from neutral gas rather than ionized regions. This finding helps settle a long-standing debate about the nature of [C II] in galaxies from the epoch of reionization.
Furthermore, the study revealed that the gas in these galaxies was remarkably dense, with hydrogen densities around 10^4 to 10^6 particles per cubic centimeter. This density is comparable to what astronomers observe in high-redshift starbursts and submillimeter galaxies, known for their intense star formation. Interestingly, the radiation field in these galaxies was more moderate, differing from the extreme conditions 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 combining these estimates 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 relative to the galaxies' stellar masses.
However, the study also highlights some uncertainties. One galaxy, REBELS-25, required a lower metallicity for the neutral gas compared to the ionized gas observed with JWST, suggesting inflowing, less enriched material. Additionally, the [O I] line appeared narrower than the [C II] line in REBELS-38, indicating that the two signals may not originate from the same interstellar regions.
Despite these uncertainties, the research marks a significant shift in our understanding of the early universe. By directly tracing neutral gas in ordinary star-forming galaxies from the epoch of reionization, astronomers can now study the elusive gas component that powers star formation. This achievement opens a new window onto the 'fuel' behind star formation and strengthens the role of ALMA alongside JWST in unraveling the mysteries of the cosmic dawn.
The practical implications of this work are far-reaching. It provides astronomers with a more direct method to study the gas that powered star formation in the early universe. By clarifying the interpretation of [C II] observations, the study can now be used more confidently to probe neutral gas in young galaxies. Over time, this may lead to better estimates of galaxy star formation rates, gas densities, and the growth of the first substantial galactic structures during cosmic reionization.
