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

“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 (MIT) 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 studied the brightest and most massive class of galaxies in the universe, called brightest cluster galaxies, in the centers of 95 clusters of galaxies. The galaxy clusters chosen are themselves an extreme sample — the most massive clusters in a large survey using the South Pole Telescope (SPT) with funding support from the U.S. National Science Foundation and Department of Energy — and are located between 3.4 and 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 the hot gas — a physical concept called “entropy” — falls below a critical threshold. Below this threshold, the hot gas inevitably cools to form new stars.

“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 co-author Michael McDonald, also of MIT.

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