Revealing how and when a black hole's mighty winds can squash star formation

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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 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 is 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 more than 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 on 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