Astronomers using NASA’s Chandra X-ray Observatory and other telescopes have observed what can happen when a giant black hole fails to do its job – possibly tying the behavior of one such passive black hole to a remarkable torrent of star formation occurring in a distant cluster of galaxies.
Galaxy clusters contain hundreds or thousands of galaxies pervaded by hot, X-ray emitting gas that outweighs the combined mass of all the galaxies. Ejections of material powered by a supermassive black hole at the center of the cluster usually prevent this hot gas from cooling to form vast numbers of stars. This heating allows supermassive black holes to influence or control the activity and evolution of their host cluster.
But what happens if that black hole stops being active? The galaxy cluster SpARCS104922.6+564032.5 – or SpARCS1049 for short – may have supplied an answer, per researchers’ findings published in The Astrophysical Journal Letters.
Based on past observations from NASA’s Hubble Space Telescope and Spitzer Space Telescope, astronomers had discovered that stars were forming at an extraordinary rate of about 900 new Suns’ worth of mass per year in SpARCS1049, which is some 9.9 billion light-years from Earth’s solar system.
That’s more than 300 times faster than the rate at which the Milky Way galaxy forms stars. At the rate seen in SpARCS1049, all of the stars in the Milky Way could form in just 100 million years – far less time than the 10 billion years it took for Earth’s home galaxy to reach its current age and number of stars.
“It reminds me of the old expression, ‘When the cat’s away, the mice will play,’” study leader Julie Hlavacek-Larrondo of the University of Montreal said. “Here the cat, or black hole, is quiet and the mice, or stars, are very busy.”
This furious star formation is happening about 80,000 light-years from the center of SpARCS1049, in a region outside any of the cluster’s galaxies – leading astronomers to ask: What is causing this prodigious cycle of star birth?
The answer may come from new Chandra data revealing the behavior of the hot gas in SpARCS1049. In most of the cluster, the temperature of the gas is about 65 million degrees Fahrenheit. At the site of star formation, however, the gas is denser than average and has cooled to a temperature of only about 10 million degrees Fahrenheit. The presence of this cooler gas suggests other undetected gas reservoirs have cooled to even lower temperatures that enable huge numbers of stars to form.
“Without the black hole actively pumping energy into its surroundings, the gas can cool enough so this impressive rate of star formation can happen,” co-author Carter Rhea, also of the University of Montreal, said. “This kind of black hole shutdown may be a crucial way for stars to form in the early universe.”
Researchers have documented numerous cases in which energy ejected by black holes into their surroundings reduces the rate of star formation by factors of tens of thousands or more – but such clusters are typically only a few hundred million light-years from Earth and are much older than SpARCS1049.
Astronomers do not see any signs that a supermassive black hole in SpARCS1049’s central galaxy is actively pulling in matter. They’ve documented no evidence of jets of material being expelled from the black hole at radio wavelengths, or of an X-ray source from the galaxy cluster’s center indicating that matter was heated as it fell toward a black hole.
“Many astronomers have thought that without the intervention from a black hole, the formation of stars would run out of control. Now we have observational proof that this is indeed what takes place,” said Tracy Webb of McGill University in Montreal, co-author of the new paper and part of the Spitzer team that first discovered SpARCS1049 in 2015.
Why is the black hole so quiet? The observed difference in position between the densest gas and the central galaxy might be the cause. This would mean the supermassive black hole at the galaxy’s center is being starved for fuel, preventing outbursts and allowing the gas to cool without impediment. Scientists theorize this offset may have results from the collision of two smaller galaxy clusters sometime in the past, moving the densest gas away from the central galaxy and creating SpARCS1049.
Marshall Space Flight Center man ages the Chandra program. The Smithsonian Astrophysical Observatory controls science and flight operations from its facilities in Cambridge and Burlington, Massachusetts.