In the largest galaxies in the Universe, there are far fewer stars than theory predicts. A likely reason for this deficit may be powerful winds from the accretion disk of a supermassive black hole at the center of a galaxy, which disperse the gas needed for star formation.
An Active Nucleus in the Constellation Canes Venatici
The spiral galaxy NGC 4151 is located approximately 50 million light-years from Earth and is one of the nearest objects with an active galactic nucleus (AGN). Around its central supermassive black hole, an accretion disk has formed, where matter is heated to extreme temperatures and becomes a powerful source of X-ray radiation.
Back in 1943, Carl Seyfert described NGC 4151 as one of six galaxies with bright nuclei in a paper that defined an entire class of Seyfert galaxies. University of Michigan graduate student Xin “Cindy” Xiang, together with astronomy professor Jon Miller, studied the winds from this disk in detail using the XRISM observatory, the X-Ray Imaging and Spectroscopy Mission, Universe Today reports. This joint mission of the Japan Aerospace Exploration Agency (JAXA), NASA, and ESA was launched in September 2023, and scientific observations began in the fall of 2024.
The Complex Structure of the Winds
The energy resolution of the Resolve spectrometer aboard XRISM is approximately ten times greater than that of previous instruments. This made it possible to distinguish individual components of the gas flows from the AGN accretion disk in detail.
In NGC 4151, up to six different absorption layers were detected. These include slow “warm absorbers” moving at 100 to 1,000 km/s, intermediate-range flows moving at 1,000 to 10,000 km/s, and ultra-fast outflows with speeds approaching one-tenth the speed of light.
These outflows arise through magnetocentrifugal acceleration, in which the magnetic field lifts matter from the surface of the disk and accelerates it along magnetic field lines as the disk rotates. Solar flares occur according to a similar principle. Xin “Cindy” Xiang and her colleagues published their results in the peer-reviewed journal The Astrophysical Journal Letters.
Time Diagnostics of the Outflows
At the 248th meeting of the American Astronomical Society in Pasadena, researcher Xin “Cindy” Xiang presented a new method for determining the periods of the most intense winds from the disk. She analyzed hundreds of days of observations of NGC 4151 and developed a color intensity index, abbreviated as “cindicity.”
This index takes into account the brightness of the X-ray emission and its “hardness,” meaning the ratio of high-energy to low-energy photons. The fastest outflows form not during flares, but approximately 10,000 seconds, or slightly less than three hours, after them, when the radiation becomes hard but dim. This is the first direct temporal link between a burst of nuclear activity and the appearance of an ultra-fast outflow.
Deficit of Stellar Mass
The power of the detected winds is sufficient for NGC 4151 to lose the raw material needed to form new stars. The rate of material outflow is equal to or greater than the rate of accretion, and a significant portion of the gas is simply dispersed into the surrounding space.
In combined X-ray, visible-light, and radio images, NGC 4151 so closely resembles a burning eye from The Lord of the Rings that astronomers have given it the unofficial nickname “the Eye of Sauron.” Astronomers plan to use further XRISM observations to determine whether a similar mechanism of star-formation suppression operates in other nearby AGNs.
