A major discovery for understanding the early Universe was made by astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope system. Researchers detected the presence of neutral gas in galaxies that formed just 700 million years after the Big Bang, offering the first direct evidence of the material from which the first stars were born. The discovery is considered particularly significant, as cold neutral gas is the essential "fuel" for the formation of new stars.
The crucial missing link
For decades, scientists were able to observe starlight and hot ionized gases in primordial galaxies, but not the cold neutral gas that initiated the star-formation process. The problem was that stars cannot form within hot plasma. Dense and relatively stable clouds of cold gas are required, which until today remained essentially invisible in observations. The new research fills this gap, allowing astronomers to directly observe the reservoirs of matter that fueled the birth of the first stars in the Universe.
Oxygen revealed the presence of the gas
The research team studied four galaxies that existed when the Universe was less than a billion years old. The detection of neutral oxygen ([OI]) played a decisive role. Unlike carbon, which can be found in both neutral and ionized regions, this specific type of oxygen is a reliable indicator of dense neutral gas. To rule out the possibility of erroneous conclusions, scientists compared the data with nitrogen emissions, which appear exclusively in hot and ionized regions. The comparison confirmed that the observations indeed concerned reserves of cold interstellar gas.
Ancient galaxies full of matter but not particularly bright
The analysis of the data revealed an unexpected picture. The gas in ancient galaxies exhibited densities similar to those observed today in the most active regions of star formation in the Universe. However, the overall luminosity of the galaxies was noticeably lower than scientists expected. This finding suggests that early galaxies were highly compact systems, rich in gas and ready for rapid development, but operated differently from modern spiral galaxies, such as the Milky Way.
A new window into the young Universe
According to Yoshinobu Fudamoto of Chiba University, this success turns the neutral oxygen emission line into a new powerful tool for astronomy. Researchers estimate that they will now be able to re-examine older observation archives with greater precision, looking for additional clues about the distribution of neutral gas in the first galaxies. The discovery is expected to help scientists better understand how the Universe transitioned from the so-called "dark ages" to the period during which the first stars, the first galaxies, and later the huge cosmic structures we observe today were formed. With the help of ALMA, astronomers are now gaining a new window of observation into one of the most mysterious and least understood periods of cosmic history.
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