Astronomers may be getting closer to solving a long-standing mystery about the largest galaxies in the universe. Observations from the X-ray Imaging and Spectroscopy Mission, known as XRISM, provide new evidence that supermassive black holes could be preventing these giant galaxies from forming as many stars as expected.
According to current models, the most massive galaxies should have more stellar mass than astronomers actually observe. The deficiency indicates that some process was preventing star formation. University of Michigan doctoral student Chen “Cindy” Xiang used XRISM data to investigate one of the main explanations and found evidence that points directly to black holes.
Most people know that black holes are objects whose gravity is so strong that even light cannot escape once it crosses a certain boundary. However, black holes can also create extremely bright regions around themselves. As the gas and dust spiral inward, they form an accretion disk that emits enormous amounts of energy, including powerful X-rays.
Black hole winds and star formation
Accretion disks are among the most active environments in the universe. Material falling toward the black hole is heated by gravity and friction until it becomes extremely hot plasma. At the same time, the disk can release powerful streams of matter.
These winds could be strong enough to blow gas out of the galaxy. Since gas is the raw material needed to make new stars, such outflows could dramatically reduce future star formation.
Data from XRISM support this possibility. The mission is led by the Japan Aerospace Exploration Agency in partnership with NASA and the European Space Agency.
“Previously, without XRISM, we could only see the general features of the outflows,” Xiang said. “But you have to be able to resolve the precise features to answer the important questions. What is its structure and geometry? How is the wind released and when is it released?”
XRISM provides clearer vision
Launched in 2023, XRISM began scientific observations in the fall of 2024. Its energy resolution is about 10 times better than its predecessor, allowing astronomers to examine black hole environments in much greater detail.
Chiang and her collaborators focused on NGC 4151, a bright galaxy located just over 50 million light-years from Earth. At its center is an active galactic nucleus, or AGN, where a supermassive black hole is actively consuming material and generating a luminous accretion disk. This makes NGC 4151 an ideal laboratory for studying black hole outflows.
“With XRISM, we have the best resolution for observing the brightest AGNs and obtain the richest information on outflows that we have observed to date for an accretion disk,” Xiang said.
Working alongside University of Michigan astronomy professor John Miller, Chiang previously showed that winds from the accretion disk of NGC 4151 can reach speeds high enough to blow material out of the system. It also identified the potential mechanism driving these outflows (which appears to be the so-called central magnetic thrust, similar to what ignites solar flares).
Track the fastest black hole flows
At the 248th meeting of the American Astronomical Society in Pasadena, California, Chiang presented a new method for determining when NGC 4151’s strong winds are active. This approach could help researchers identify similar outflows in other galaxies and improve understanding of active galactic nuclei throughout the universe.
Because AGN winds can change dramatically over time, Chiang needed a way to determine when the fastest and strongest outflows occurred. To do this, it analyzed hundreds of days of XRISM observations of the galaxy NGC 4151.
Her work focused on the periods when the galaxy’s X-ray output brightened in flares and on how the X-ray signal evolved in the following hours.
In addition to measuring brightness, Chiang studied whether the detected X-rays were relatively hard or soft, a property similar to color in visible light. She combined these measurements into a new metric called the Color Intensity Index. Miller suggested shortening the name to “theft”.
“Partly because my name is Cindy,” Chiang said. “But the idea is that in the future, you can tell me how risky your source is at this moment, and I can tell you the probability that you’re seeing a rapid outflow.”
A new timing link between black holes and galactic winds
The analysis revealed a surprising pattern. In NGC 4151, the strongest fast winds appeared when the X-rays were strong but relatively faint.
The fastest outflows did not occur during the X-ray flares themselves. Instead, they typically appeared after about 10,000 seconds, or just under three hours. This discovery provides the first direct timing connection between X-ray activity and the powerful winds streaming from a black hole’s accretion disk.
By determining when these outflows occur, astronomers now have a valuable new tool to study how black holes affect the growth and evolution of galaxies, and perhaps why some of the most massive galaxies in the universe lack many stars.
