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 have found, remarkably, that the conditions for stellar conception in these exceptionally massive galaxies have not changed over the last ten billion years.
While there are lots of things that could have affected star formation over the last ten billion years, this new study suggests that 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.
Clusters of galaxies are the largest objects in the universe held together by gravity and contain huge amounts of hot gas seen in X-rays. The mass of this hot gas is several times the total mass of all the stars in all the hundreds of galaxies typically found in galaxy clusters.
The researchers 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. The clusters range in location between 3.4 and 9.9 billion light-years from Earth.
The team found that star formation in the galaxies that 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.
The type of star formation the astronomers are seeing is remarkably consistent. Even though the universe looked very different back billions of years ago, it appears that the trigger for stars to form in these galaxies does not. In the end, a single number could tell us whether billions of stars and planets formed in these huge galaxies, going back ten billion years.
Revised: January 18, 2024