Astronomers have discovered a substantial reservoir of cold molecular gas in REBELS-25, a massive, star-forming galaxy. This gas serves as the direct fuel for star formation. The galaxy was observed when the universe was approximately 700 million years old, which is about five percent of its current age. This period is known as the Epoch of Reionization, a critical time when the first stars and galaxies emerged.
The research team, led by scientists from Leiden University, published their findings in the journal *Monthly Notices of the Royal Astronomical Society*. They utilized the U.S. National Science Foundation Very Large Array (NSF VLA) in New Mexico and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The NSF VLA detected faint radio emissions from carbon monoxide (CO) molecules, which indicate the presence of cosmic molecular gas.
This detection represents the most distant low-energy CO observation in the universe to date. The strength of the signal suggests that REBELS-25 possessed a large supply of star-forming material early in the universe's history. Combined with ALMA's higher-energy CO data, these observations helped determine the gas's density and temperature under early-universe conditions.
Detecting such faint low-energy CO lines from this distant past is challenging. The cosmic microwave background (CMB), residual radiation from the Big Bang, makes these observations difficult. The CMB becomes significantly brighter at high redshift, reducing the contrast of cold gas emissions. The success of these observations indicates that cold molecular gas can be detected deep into the Epoch of Reionization with the right instrumentation.
These findings offer insight into how early galaxies grew so quickly after the Big Bang. By directly detecting the star-forming fuel, astronomers can now measure the gas driving this rapid growth. The large gas mass in REBELS-25 shows that some early galaxies were already prepared for intense star formation. This is a key step in understanding mass assembly in the universe's first billion years.
This discovery also provides a preview of the capabilities of the Next-Generation Very Large Array (ngVLA), a planned National Radio Astronomy Observatory telescope. The ngVLA is expected to make similar measurements ten times faster. This will enable the detection of cold molecular gas in a larger sample of early galaxies, including those from even earlier times.
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