Astronomers have an answer for a long-running mystery in astrophysics: Why are supermassive black holes today growing slower than in the past?
A team of astronomers, led by Penn State researchers, used NASA's Chandra X-ray Observatory and other X-ray telescopes to find that supermassive black holes are unable to consume material as rapidly as they did in the distant past.
A paper describing these results appeared in the Astrophysical Journal.
Ten billion years ago, there was a period that astronomers call "cosmic noon," when the growth of supermassive black holes - those with millions to billions of times the mass of the sun - was at its peak across the entire history of the universe. Between cosmic noon and now, however, astronomers studying black holes at different distances from Earth and representing different time periods across the history of the universe have seen a major slowdown in how much black holes are growing.
"A longstanding mystery has been the cause of this big slowdown," said Zhibo Yu, a graduate student in astronomy and astrophysics in the Eberly College of Science at Penn State and lead author of the study. "With these X-ray data and supporting observations at other wavelengths, we can test different ideas and narrow down the answer."
When gas falls into a supermassive black hole, it heats up and produces large amounts of radiation across the electromagnetic spectrum. This includes X-rays - a form of high-energy electromagnetic radiation used in medical imaging that is also generated by extreme astrophysical processes - and radiation at different energy levels, or wavelengths, such as visible or infrared light. Using the fact that black holes that are growing more quickly produce more X-rays, Chandra and other X-ray telescopes have shown for decades a decline in black hole growth by looking at black holes at different distances across the universe.
A key challenge in this study is that both more massive black holes and faster-growing black holes produce brighter X-ray emission. However, the researchers used observations at other wavelengths, including optical and infrared data, to estimate black hole masses and disentangle these two factors.
