Astronomers may finally have the goods on a cosmic cold case: why the universe's biggest galaxies are missing so many stars. New data from the X-Ray Imaging and Spectroscopy Mission (XRISM) suggest that supermassive black holes are the culprits, blowing away the gas needed to form stars.
Current models predict that the most massive galaxies should be bursting with stellar mass, but observations show a significant shortfall. University of Michigan doctoral student Xin "Cindy" Xiang used XRISM data to investigate one leading explanation and found evidence pointing directly to black holes.
Most people know black holes as objects whose gravity is so strong that even light cannot escape. However, black holes can also create extremely bright regions around themselves. As gas and dust spiral inward, they form an accretion disk that emits enormous amounts of energy, including powerful X-rays.
Accretion disks are among the most energetic environments in the universe. Material falling toward the black hole is heated by gravity and friction until it becomes an intensely hot plasma. At the same time, the disk can launch powerful outflows of matter.
These winds can be strong enough to sweep gas out of a galaxy. Because gas is the raw material needed to make new stars, such outflows could significantly reduce future star formation.
Data from XRISM support that possibility. The mission is led by the Japanese Aerospace Exploration Agency in partnership with NASA and the European Space Agency.
"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?"
Launched in 2023, XRISM began scientific observations in fall 2024. Its energy resolution is roughly 10 times better than that of its predecessor, allowing astronomers to examine black hole environments in far greater detail.
Xiang and her collaborators have focused on NGC 4151, a bright galaxy located a little more than 50 million light-years from Earth. At its center is an active galactic nucleus (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 driven 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 showed that winds from NGC 4151's accretion disk can reach speeds high enough to eject material from the system. She also identified the likely mechanism driving these outflows - magnetocentrifugal driving, similar to what sets off solar flares.
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. The approach could help researchers identify similar outflows in other galaxies and improve understanding of AGNs throughout 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 work focused on periods when the galaxy's X-ray output brightened in flares and on how the X-ray signal evolved in the hours afterward.
In addition to measuring brightness, Xiang studied whether the detected X-rays were relatively hard or soft, a property comparable to color in visible light. She combined these measurements into a new metric called the color intensity index. Miller suggested shortening the name 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."
The analysis revealed a surprising pattern. In NGC 4151, the strongest fast winds appeared when the X-rays were hard but relatively faint. The fastest outflows did not occur during the X-ray flares themselves. Instead, they typically appeared about 10,000 seconds, or just under three hours, later. This finding provides the first direct timing connection between X-ray activity and the powerful winds flowing from the black hole's accretion disk.
By identifying when these outflows occur, astronomers now have a valuable new tool for studying how black holes influence the growth and evolution of galaxies, and possibly why some of the universe's most massive galaxies are missing so many stars.