Astronomers have discovered an enormous supply of cold molecular gas in a galaxy shortly after the Big Bang. The finding offers a rare glimpse of the raw material from which the first large galaxies were built. Measurements suggest that around 95 per cent of the normal matter in the galaxy studied, REBELS-25, is still in the form of gas.
The international research team, led by Leiden University, used observations from two of the world's most powerful radio telescopes: the VLA in the United States and ALMA in northern Chile. The results have been published in Monthly Notices of the Royal Astronomical Society.
'Our results show that galaxies already had vast reserves of cold gas from which new stars can form just 700 million years after the Big Bang,' says lead author Karin Cescon, a PhD candidate at Leiden University. 'This means that some galaxies had the fuel needed to grow rapidly at a very early stage.'
Astronomers had long suspected that massive galaxies in the early Universe contained large gas reservoirs, but this had never been directly confirmed.
The galaxy studied, REBELS-25, is seen as it was when the Universe was only about 700 million years old-roughly five per cent of its current age. As the Universe expands, the light from such distant objects is stretched to longer, redder wavelengths, a shift known as redshift. For REBELS-25, this value is 7.3. The galaxy lies in the middle of the era of reionisation, a period when the first stars and galaxies dramatically reshaped the young Universe.
Cold molecular gas is the direct raw material for star formation. Astronomers had long suspected that massive galaxies in the early Universe contained large gas reservoirs, but this had never been directly confirmed.
A vast supply of raw material for new stars
Using the Very Large Array (VLA) in New Mexico, the researchers searched for faint radio signals from carbon monoxide (CO), a molecule that reveals the presence of cold star-forming gas. Their observations resulted in the most distant detection so far of a low-energy form of CO emission-one of the most reliable ways to measure the amount of cold gas in a galaxy.
The data show that REBELS-25 already contained a huge supply of material for forming new stars just 700 million years after the Big Bang. Follow-up observations with the Atacama Large Millimetre/submillimetre Array (ALMA) allowed the team to determine properties of the gas, such as its temperature and density.
REBELS-25 also turns out to be surprisingly mature. Earlier studies had already shown that the galaxy contains a rotating disc, is rich in dust, and has relatively high levels of heavier elements. The new measurement adds a massive reservoir of cold gas to this picture. The total gas content is about one hundred billion times the mass of the Sun-many times greater than the total mass in stars the galaxy had formed up to that point. Together, these features paint a picture of a remarkably mature galaxy in a very young Universe.
'Until now, astronomers had to rely on indirect signs of cold gas. With the right telescopes, we can now study the fuel itself directly-even in galaxies from the era of reionisation.'
Challenging observations
Detecting cold gas at such great distances is extremely difficult. The cosmic background radiation from the Big Bang becomes brighter at higher redshifts, reducing the contrast needed to detect cold gas. For a long time, it was therefore unclear whether such faint signals could still be observed at these distances.
'With these observations, we have shown that it is possible,' says Cescon. 'Until now, astronomers had to rely on indirect signs of cold gas. With the right telescopes, we can now study the fuel itself directly-even in galaxies from the era of reionisation.'
The discovery also offers a glimpse of what future radio telescopes could achieve. The planned Next Generation Very Large Array (ngVLA) will be about ten times more sensitive than the current VLA. 'With the ngVLA, we will be able to detect and study cold gas in many more young galaxies,' says associate professor Jacqueline Hodge of Leiden University, Cescon's supervisor and co-author of the study. 'That will be crucial for understanding how the first galaxies formed and evolved.'
Scientific article
This press release originally appeared on astronomie.nl.
