Astronomers have completed the largest and most detailed study to date of the causes of star formation in the universe’s largest galaxy. The researchers used NASA’s Chandra X-ray Observatory and other telescopes. Scientists have discovered that the conditions for star formation in giant galaxies have remained unchanged for the past 10 billion years. “What’s surprising here is that there are a number of factors that could have influenced star formation over the past 10 billion years,” said study author Michael Calzadilla. “Ultimately, he says, star formation in these massive galaxies comes down to one thing: whether the hot gas surrounding the galaxies can cool quickly enough.” Galaxy clusters contain large amounts of hot gas that can be seen in X-rays. The mass of this hot gas is many times greater than the combined mass of all the stars in the hundreds of galaxies typically found in clusters. Calzadilla and his colleagues studied the brightest and most massive class of galaxies in the universe. The studied galaxies are located at the center of a 95-galaxy cluster, the most massive galaxy cluster studied using the South Pole Telescope (SPT). These star clusters are located between 3.4 billion light years and 9.9 billion light years from Earth.
The researchers found that when the amount of disordered motion in the hot gas falls below a critical threshold, star formation occurs in the galaxies they studied. Below this threshold, the hot gas inevitably cools and new stars form. This study is the first to combine X-ray and optical observations of cluster centers over such a wide range of distances. This allows researchers to link the fuel needed for star formation – the hot gas discovered by the Chandra telescope – to the actual formation of stars as the gas cools, as observed by optical telescopes. The research team also used radio telescopes to study jets emitted by supermassive black holes within these clusters. In a process called “feedback,” the black hole is fed hot gas that cools and forms stars. This creates jets and other activities that heat the surrounding area and provide energy, temporarily preventing further cooling. When the black hole runs out of fuel, the jets are shut down and the process begins again. An unexpected aspect of this study is that previous studies have suggested that factors other than cooling of hot gas may have played a large role in star formation in the distant past. Ten billion years ago, galaxy collisions and mergers into galaxy clusters were much more common, star formation rates were generally much higher, and galaxies’ supermassive black holes sucked in material much faster.
“The types of star formation we observe are surprisingly consistent, even toward the cosmic noon, which may be suppressed by other processes,” co-authors Argonne National Laboratory in Illinois said Lindsey Brim. “The universe looked very different then, but there is no trigger for star formation in these galaxies.” By studying relatively close clusters, previous researchers also found that a threshold level of entropy in the hot gas is required for jet feedback from a supermassive black hole to occur. This new study by Calzadilla’s team finds that the feedback entropy threshold does not apply to galaxies in more distant galaxy clusters, which were not fully covered by black hole feedback about 10 billion years ago. It could mean that it wasn’t under control. This is possible because it takes time for the hot gas in the central galaxy to begin to cool, and then this cold gas reaches the central galaxy’s supermassive black hole, which eventually creates a jet that prevents further cooling of the gas. This is because it takes more time to form.