Powerful winds from supermassive black holes may explain why some of the universe’s largest galaxies stopped making as many stars as expected.
Astronomers are gaining new insight into one of the universe’s biggest puzzles thanks to the X-Ray Imaging and Spectroscopy Mission (XRISM). Researchers at the University of Michigan have uncovered evidence that powerful winds driven by supermassive black holes may explain why the largest galaxies contain fewer stars than scientists expect.
Current theories predict these giant galaxies should hold much more stellar mass than they actually do. The new findings suggest that black holes can suppress star formation by blasting away the gas galaxies need to create new stars.
Black Hole Winds May Be Stopping New Stars From Forming
Black holes are best known for their intense gravity, which prevents even light from escaping once it crosses the event horizon. Outside that boundary, however, they can also produce an accretion disk, a swirling ring of gas and dust that shines intensely across the electromagnetic spectrum, including in X-rays.
Conditions inside the accretion disk are extraordinarily energetic. As material spirals inward, friction and gravity heat it into an extremely hot plasma by stripping electrons from atoms. The turbulent environment can also launch powerful winds that push gas away from the galaxy. If those winds are strong enough, they can remove the raw material required for future star formation.
Using observations from XRISM, a mission led by the Japan Aerospace Exploration Agency in collaboration with NASA and the European Space Agency, University of Michigan doctoral student Xin “Cindy” Xiang found evidence supporting this scenario.
“Previously, without XRISM, we could only see broad features of the outflows,” Xiang said. “But you need to be able to resolve fine features to answer important questions. What is their structure and geometry? How are the winds launched and when are they launched?”
XRISM Provides an Unprecedented View of Black Hole Outflows
Launched in 2023, XRISM began scientific observations in fall 2024. Its X-ray energy resolution is about 10 times better than that of its predecessor, allowing astronomers to study black hole environments with far greater precision.
Xiang and her colleagues focused on NGC 4151, a bright galaxy located a little more than 50 million light-years from Earth. At its center lies an active galactic nucleus (AGN), where a supermassive black hole is actively consuming surrounding material. That process creates a bright accretion disk, making the galaxy an ideal laboratory for studying high-speed outflows.
“With XRISM, we have the greatest resolution observing the brightest AGN, and we’re getting the richest information on outflows that we have observed so far for an accretion disk,” Xiang said.
Working alongside University of Michigan astronomy professor Jon Miller, Xiang previously demonstrated that the winds produced within NGC 4151’s accretion disk can reach speeds capable of blasting material away from the galaxy. Her research also points to magnetocentrifugal driving as the mechanism launching the winds, a process that resembles the forces responsible for triggering solar flares.
New Method Reveals When the Fastest Winds Appear
At the 248th meeting of the American Astronomical Society in Pasadena, California, Xiang presented a new technique for determining exactly when NGC 4151’s galaxy-shaping winds become active.
Being able to identify these periods could help astronomers know when to observe other active galaxies, improving the chances of catching similar outflows and deepening our understanding of how supermassive black holes influence galaxy evolution.
Because AGN winds can change dramatically over time, Xiang analyzed hundreds of days of XRISM observations of NGC 4151. She searched for peaks in the galaxy’s X-ray brightness, known as flares, and tracked how the signal evolved during the hours that followed.
Beyond brightness alone, she also measured whether the detected X-rays were harder or softer, a property comparable to color in visible light. She combined these measurements into a new metric called the color intensity index, which Miller suggested shortening to “cindicity.”
“Partly because my name is Cindy,” Xiang said. “But the idea is that, in the future, you could tell me the cindicity of your source at this moment and I can tell you the probability that you’re seeing a fast outflow.”
First Direct Timing Link Between X-Rays and Black Hole Winds
In NGC 4151, Xiang found that the strongest fast winds occurred when the X-rays were hard but relatively faint. Surprisingly, the fastest outflows did not appear during the X-ray flares themselves. Instead, they typically emerged about 10,000 seconds, or just under three hours, after a flare.
That delay represents the first direct timing connection between changes in X-ray emission and the powerful outflows that can reshape galaxies by limiting the formation of new stars.
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