Astronomers discover spark of star birth over billions of years

Astronomers have completed the largest and most detailed study of what triggers star formation in the universe’s largest galaxies, using NASA’s Chandra X-ray Observatory and other telescopes . They were surprised to find that the conditions for stellar design in these exceptionally massive galaxies have not changed over the past ten billion years.

“What’s surprising here is that there are a lot of things that could have affected star formation over the last ten billion years,” said Michael Calzadilla of the Massachusetts Institute of Technology (MIT), who led the study. “Ultimately, however, the main driver of star formation in these huge galaxies really comes down to one thing: whether or not the hot gases around them can cool down quickly enough.”

Galaxy clusters are the largest objects in the universe held together by gravity and contain enormous amounts of hot gas visible in X-rays. The mass of this hot gas is several times greater than the total mass of all galaxies. stars from hundreds of galaxies typically found in galaxy clusters.

Calzadilla and his colleagues studied the brightest and most massive class of galaxies in the universe, called brightest galaxy clusters, at the centers of 95 galaxy clusters. The chosen galaxy clusters are themselves an extreme sample – the most massive clusters in a large survey using the South Pole Telescope (SPT) – and are located between 3.4 and 9.9 billion light years from Earth.

The team found that star formation in the galaxies studied is triggered when the amount of disordered motion in the hot gas – a physical concept called “entropy” – falls below a critical threshold. Below this threshold, hot gases inevitably cool to form new stars.

“It’s impressive to think that just one number tells us whether billions of stars and planets formed in these huge galaxies, going back ten billion years,” said co-author Michael McDonald, also of MIT.

While other attempts have been made to identify the factors responsible for star formation in such immense galaxies over cosmic time, this study is the first to combine radiological and optical observations of cluster centers over a also wide range of distances. This allows researchers to link the fuel needed for star formation (the hot gas detected with Chandra) to the actual formation of stars after the gas cools, as seen with optical telescopes, over most of the history of the universe.

The team also used radio telescopes to study jets of material coming from the supermassive black holes in these clusters. In a process called “feedback,” hot gas that cools to form stars ends up fueling black holes, leading to jets and other activity that heat and energize their surroundings, temporarily preventing further cooling. When the black hole runs out of fuel, the jets turn off and the process starts again.

“It’s like we’ve put together different chapters of the book about star formation throughout the life of the universe,” said co-author Brad Benson, of the University of Chicago and Fermilab in the ‘Illinois. “Instead of being written in words, this story is told in the form of x-rays, optics and radio.”

An unexpected aspect of this study is that previous work had suggested that factors other than the cooling of hot gases might play a more important role in star formation in the distant past. Ten billion years ago, in an era that astronomers call “cosmic noon,” collisions and mergers of galaxies into clusters were much more frequent, star formation rates were generally much higher, and holes The galaxy’s supermassive blacks were attracting matter much more quickly.

“The type of star formation we observe is remarkably consistent, even near cosmic noon when it could have been overwhelmed by other processes,” said Argonne co-author Lindsey Bleem. National Laboratory in Illinois. “Although the universe looked very different then, the trigger for star formation in these galaxies did not.”

By studying relatively nearby clusters, previous researchers have also found that a threshold level of disorder in hot gases is necessary for feedback from supermassive black holes, in the form of jets, to occur.

This new study by Calzadilla’s team found that the entropy threshold for feedback, however, does not apply to galaxies in more distant clusters, which could mean that clusters from around ten billion years ago years are not as well regulated by black hole feedback. This is plausible because it takes time for hot gas to begin to cool over the central galaxy, then even longer for this cold gas to flow toward the central galaxy’s supermassive black hole, and finally for jets to form and prevent further cooling of the gas.

It is also possible, however, that radio signals do not give a clear indication of jet activity at these early hours.

This result is based on X-ray data from NASA’s Chandra X-ray Observatory; radio data from the SPT, the Australia Telescope Compact Array and the Australian SKA Pathfinder Telescope; infrared data from NASA’s WISE satellite; and several optical telescopes. The optical telescopes used here are the 6.5 m Magellan telescopes, the Gemini South telescope, the 4 m Blanco telescope (DECam, MOSAIC-II) and the 1 m Swope telescope. In total, nearly 50 days of Chandra observation were used to obtain this result.

Caldazilla presented these results at the 243rd meeting of the American Astronomical Society in New Orleans. In addition, he is the first author of an article submitted to the Astrophysics Journal on this work, available on the preprint server arXiv.