Thanks to the X-Ray Imaging and Spectroscopy Mission, or XRISM, University of Michigan researchers are helping chip away at one of astronomy's cosmic mysteries: The universe's most massive galaxies appear to be missing stars.
Compared with theoretical expectations, these galaxies contain less stellar mass than anticipated, suggesting something has suppressed star formation. Working with data collected by the XRISM spacecraft, U-M doctoral student Xin "Cindy" Xiang has found evidence that backs one explanation for this discrepancy. Namely, black holes are at the core of it.
Black holes are famous for trapping anything, including massless particles of light, that gets too close. But beyond that threshold, a black hole's immense gravity can also create what's known as an accretion disk, which emits oodles of light, including X-rays.
The disk is an incredibly energetic environment where the black hole collects and stirs infalling gas and dust. Friction and gravity atomize the material and can even peel electrons off of those atoms, creating a very hot, very bright plasma. Like a bubbling cauldron, this disk can also fling out material, creating winds so powerful they could blow away the gas that galaxies need to form new stars.
Based on what Xiang has seen from XRISM, a mission led by the Japanese Aerospace Exploration Agency in partnership with NASA and the European Space Agency, that explanation holds up.
"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, which launched in 2023 and started making observations in fall 2024, is uniquely suited to help find answers with an energy resolution that's about 10 times that of its predecessor. Xiang and her colleagues have been using XRISM to study NGC 4151, a particularly bright galaxy a little over 50 million light years away. The galaxy has what's called an active galactic nucleus, or AGN, which means it has a black hole at its center actively gorging material and creating an accretion disk that's a great place to study these winds or 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 with Jon Miller, U-M professor of astronomy, Xiang has already shown that the winds inside NGC 4151's accretion disk can reach the necessary speeds to blast out material. She's also been able to zero in on what gives rise to the winds in the first place (that appears to be what's called magnetocentrifugal driving and it's similar to what sets off solar flares).
Presenting at the 248th meeting of the American Astronomical Society in Pasadena, California, Xiang has now shared how to analyze XRISM's data to reveal when NGC 4151's galaxy-shaping winds have kicked up. This knowledge could help astronomers predict when such outflows are happening in other galaxies, leading to more observations and a deeper understanding of AGNs across the universe.
But AGN winds are highly variable and Xiang had to develop a way of looking at XRISM's data to spot when the fastest ones were blowing and when they were at their strongest. Her method involved examining hundreds of days worth of NGC 4151 observations, looking for peaks in the X-ray brightness, or flares, as well as how the signal changed in the hours that followed.
In addition to the X-ray brightness, Xiang analyzed how hard or soft the X-rays were that XRISM detected, a feature akin to color for visible light. She rolled these variables into a metric she 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."
For NGC 4151, Xiang found the fast winds were strongest when the X-rays were hard but faint. The fastest winds were not seen during flares, but typically about 10,000 seconds-or just under 3 hours later-providing the first direct timing link to the outflows.
