Astronomers find sparks from the birth of stars a billion years ago

With the help of NASA’s Chandra X-ray Observatory and other telescopes, astronomers have completed the largest and most detailed study of what causes the birth of stars in the largest galaxies in the Universe. 

Star birth in ancient galaxies. Source:

Star birth in other galaxies

Recent research by scientists has found that the conditions for star birth in extremely massive galaxies have not changed over the past ten billion years. This is evidenced by traces of ancient events that scientists have recently discovered.

“What’s surprising here is that there are lots of things that could have affected star formation over the last ten billion years,” said Michael Calzadilla of the Massachusetts Institute of Technology, who led the study. “In the end, however, the main driver of star formation in these huge galaxies really comes down to one thing—whether or not the hot gas surrounding them can cool off quickly enough.”

Calzadilla and his colleagues have studied the brightest and most massive class of star systems in the Universe, which are called the brightest clusters of galaxies and are located at a distance of 3.4 to 9.9 billion light-years from Earth. 

The team found that star formation in the galaxies they studied is triggered when the amount of disordered motion in a hot gas — a physical concept called “entropy” — falls below a critical threshold. The hot gas inevitably cools below this threshold, forming new luminaries.

Driving forces of star formation

“It’s impressive to think that a single number tells us whether billions of stars and planets formed in these huge galaxies, going back ten billion years,” said study co-author Michael McDonald, also from the Massachusetts Institute of Technology.

Although there have been other attempts to determine the driving forces of star birth in such huge galaxies during cosmic time, this study is the first to combine X-ray and optical observations of cluster centers over such a large range of distances. This allows researchers to link the fuel needed to form the luminaries — the hot gas discovered by Chandra — to the actual formation of celestial bodies after the gas has cooled, which has been observed with optical telescopes for most of the history of the universe.

The team also used radio telescopes to study jets of material coming from supermassive black holes in these clusters. In a process called “feedback”, the hot gas that cools to form stars eventually feeds the black holes, which leads to the formation of jets and other activity that heats and energizes their surroundings, temporarily preventing further cooling. When the black hole runs out of “fuel,” the jets turn off, and the process begins again.

Search for other star formation factors

An unexpected aspect of this study is that previous work has suggested that factors other than the cooling of hot gas may have played a large role in star formation in the distant past. Ten billion years ago, during a period that astronomers call “cosmic noon,” collisions and mergers of galaxies in clusters were much more frequent, the rate of star formation in general was much higher, and the supermassive black holes of galaxies sucked in material much faster. 

“The type of star formation we’re seeing is remarkably consistent, even approaching cosmic noon when it could have been overwhelmed by other processes,” says study co-author Lindsey Bleem of Argonne National Laboratory in Illinois. “Although the universe looked very different back then, the trigger for stars to form in these galaxies does not.”

By studying relatively close clusters, previous researchers have also discovered the threshold level of chaos in the hot gas required for feedback from supermassive black holes in the form of jets. A new study by Calzadilla’s team has found that the entropy threshold for feedback does not apply to galaxies in more distant clusters, which may mean the clusters that formed about ten billion years ago are not as well regulated by black hole feedback.

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