Blown Away by the Winds of NGC 4151

Astronomers have known about NGC 4151’s X-ray-emitting active galactic nucleus since the 1970s. Now, observations from the X-Ray Imaging and Spectroscopy Mission (XRISM) show it in a whole new light.

Getting Feedback

Active galactic nuclei (AGNs) are extremely luminous galactic centers powered by accretion of matter onto a supermassive black hole. These nuclei provide a venue for galaxies and their supermassive black holes to interact. This interaction goes both ways, with the galaxy providing fuel to the supermassive black hole, and the AGN injecting material, momentum, and energy back into the surrounding galaxy in a process known as feedback.

AGN feedback can dramatically reshape a galaxy’s star formation and evolution. One of the most powerful ways an accreting supermassive black hole can influence its host galaxy is through ionized winds, which can scour the surrounding galactic bulge clear of star-forming material. There is much still to learn about these winds, including how they are structured and what physical processes drive them.

New Insights from XRISM

NGC 4151 provides an excellent opportunity to learn about active galactic nucleus winds. Only about 60 million light-years away, NGC 4151 hosts an accreting 34-million-solar-mass black hole that is luminous, highly variable, and shows a complex absorption spectrum that suggests that the AGN’s intense radiation is filtering through outflowing winds.

To study the winds of NGC 4151, a team led by Xin Xiang (University of Michigan) obtained X-ray spectra of NGC 4151 from XRISM on five dates spanning a period of about 6 months.

Xiang’s team modeled the array of absorption and emission lines in the XRISM spectra to tease out the properties of NGC 4151’s winds. They found that the best match to the spectra required up to six layers of absorbing material, showing that NGC 4151’s winds are highly structured. These absorbing layers took the form of slow warm absorbers (velocities between 100 and 1,000 km/s), very fast outflows (1,000–10,000 km/s), and ultra-fast outflows (10,000–100,000 km/s; up to one-third of the speed of light). While most of these components carry away mass as fast as or faster than the black hole accretes, some appeared to lack the oomph to escape the oncoming accretion flow, suggesting that they might be “failed” winds that curl back toward the AGN.

Being Blown Away

The data showed that the winds are likely magnetocentrifugally driven, meaning that material is lifted and accelerated from the surface of the disk along magnetic field lines as the disk rotates. For some of the wind components, especially warm absorbers emerging farther from the black hole, radiation pressure may be a driver as well.

Altogether, the modeling paints a picture of a complex, asymmetric, time-variable, and clumpy set of winds that carry significant mass away from the AGN. Several fast-moving outflow components appear to exceed the threshold luminosity necessary to blow star-forming gas out of the galactic bulge, suggesting that star formation in NGC 4151’s galactic bulge may someday be halted entirely by these winds.

Citation

“XRISM Spectroscopy of Accretion-Driven Wind Feedback in NGC 4151,” Xin Xiang et al 2025 ApJL 988 L54. doi:10.3847/2041-8213/adee9b