Astronomers may be close to solving a long-standing mystery about the universe’s largest galaxy. Observations by the X-ray Imaging Spectroscopy mission, known as XRISM, provide new evidence that supermassive black holes may be preventing these massive galaxies from forming as many stars as expected.
According to current models, the most massive galaxies should contain more stellar mass than astronomers actually observe. This deficiency suggests that some process is suppressing star formation. Shin “Cindy” Xiang, a doctoral student at the University of Michigan, used XRISM data to explore one leading explanation and found direct evidence for a black hole.
Most people know that a black hole is an object whose gravity is so strong that even light cannot escape beyond a certain boundary. However, black holes can also create very bright regions around them. As the gas and dust spiral inward, they form an accretion disk that releases vast amounts of energy, including powerful X-rays.
Black hole winds and star formation
Accretion disks are one of the most energetic environments in the universe. Material falling toward a black hole is heated by gravity and friction, turning into extremely hot plasma. At the same time, the disk can cause a powerful outflow of substances.
These winds can be strong enough to blow gas out of the galaxy. Since gas is the necessary raw material for creating new stars, such an outflow could significantly reduce future star formation.
Data from XRISM supports that possibility. The mission is being led by Japan’s Japan Aerospace Exploration Agency in collaboration with NASA and the European Space Agency.
“Previously, without XRISM, we could only see the general characteristics of a spill,” Xiang said. “But to answer important questions, we need to be able to resolve finer features: What is its structure and shape? How does the wind form and when does it form?”
XRISM provides a clearer view
XRISM was launched in 2023 and began scientific observations in fall 2024. XRISM’s energy resolution is about 10 times better than its predecessor, allowing astronomers to examine the black hole environment in greater detail.
Xiang and his colleagues focused on NGC 4151, a bright galaxy located just over 50 million light-years from Earth. At its center is an active galactic nucleus (AGN), a supermassive black hole actively consuming matter and producing a bright accretion disk. This makes NGC 4151 an ideal laboratory to study outflow by black holes.
“With XRISM, we have the highest resolution to observe the brightest AGN and the richest outflow information we have ever observed about the accretion disk,” Xiang said.
In collaboration with John Miller, a professor of astronomy at the University of Michigan, Xiang previously showed that the winds from NGC 4151’s accretion disk could reach speeds high enough to eject material from the system. She also identified a mechanism that could cause these outflows (which appears to be called magnetocentrifugal drive, similar to what causes solar flares).
Tracking the fastest black hole outflow
At the 248th meeting of the American Astronomical Society in Pasadena, California, Xiang presented a new method for determining when NGC 4151’s powerful winds are active. This approach could help researchers identify similar outflows in other galaxies and improve our understanding of AGNs across the universe.
Because AGN winds can change dramatically over time, Xiang needed a way to pinpoint when the fastest and strongest outflows occurred. To do this, she analyzed hundreds of days of XRISM observations of NGC 4151.
Her research focused on the period when the galaxy’s X-ray output brightened with a flare and how the X-ray signal evolved in the hours that followed.
In addition to measuring brightness, Xiang studied whether the detected X-rays were relatively hard or soft, a property comparable to the color of visible light. She combined these measurements to create a new index called the Color Intensity Index. Mr. Miller suggested shortening the name to “cindicity.”
“Partly because my name is Cindy,” Shan said. “But we think that in the future, if you can tell us how malicious the source is right now, you can tell us how likely we are to see a rapid exfiltration.”
New timing link between black holes and galactic winds
The analysis revealed a surprising pattern. In NGC 4151, the strongest wind speeds occurred when the X-rays were hard but relatively weak.
The fastest outflow did not occur during the X-ray flare itself. Instead, they typically appeared after about 10,000 seconds, or just under three hours. The discovery reveals for the first time a direct timing relationship between X-ray activity and the powerful winds flowing from a black hole’s accretion disk.
By identifying when these outflows occur, astronomers now have a valuable new tool to study how black holes affect the growth and evolution of galaxies, and why some of the universe’s most massive galaxies are missing so many stars.
