How Supermassive Black Holes Stop Stars from Being Born

Some of the largest galaxies in the known universe seem to be missing many of their stars. This is strange because growing and creating new stars is a main job of a galaxy. According to Xin "Cindy" Xiang from the University of Michigan, something is actively stopping star birth in these giant cosmic structures. She believes that supermassive black holes in the centers of these galaxies are the reason for this halted growth.

Xiang led a team of researchers who used the X-Ray Imaging and Spectroscopy Mission, known as XRISM. They studied powerful winds coming from the accretion disks around black holes. These disks are rings of material swirling into the black hole. The material creates intense X-rays because of the high energy produced. The strength of these winds may help decide how stars form in the galaxy. To understand this process, the team needed very clear studies of the emissions from the black holes.

"Previously, without XRISM, we could only see broad features of the outflows," Xiang explained. "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?"

Supermassive black holes, like smaller black holes, feed on material pulled by their gravity. They consume light, gas, dust, and even stars that come too close. This material spirals inward through an accretion disk. Around a supermassive black hole, this disk creates a very energetic environment. The activity mixes gas and dust, and the structure is covered in complex magnetic fields.

The intense motion creates friction. Gravity and friction work together to break atoms apart. If the energy is high enough, it can strip electrons from atoms. This creates a hot, bright plasma. Like a bubbling pot, the disk can throw material outward, creating powerful winds. If these winds are strong, they can blow away gas in nearby areas. This displaced gas is what galaxies need to make new stars. Therefore, black holes can have a negative effect on star birth nearby.

Xiang and her team used XRISM to study the supermassive black hole in the center of galaxy NGC 4151. The mission gave a high-resolution view of the winds flowing from the accretion disk. They measured the specific characteristics of these winds. Active galactic nuclei, or AGNs, usually happen when a supermassive black hole is growing. During this time, their energetic activities shape the evolution of the host galaxy.

These black holes grow by eating gas while influencing surrounding gas clouds. They emit powerful winds during this active growth phase. This process is happening in the core of NGC 4151 as it consumes nearby material. "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 stated.

The strongest winds, which shape the galaxy and take away the gas stars need, do not flow constantly. Xiang developed a method to understand when these winds are at their most powerful. She analyzed hundreds of days of observations of NGC 4151. She looked for peaks in X-ray brightness that indicated strong winds.

She also examined how hard or soft the X-rays were. This allowed her to connect X-ray properties with wind strength. She put these variables into a metric called the "color intensity index," or "cindicity." "Partly because my name is Cindy," Xiang said with a smile. "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 that fast winds were strongest when X-rays were hard but faint. Surprisingly, the fastest winds did not appear during major flares. Instead, they appeared about 10,000 seconds, or just under three hours, later. This discovery provided the first direct timing link to the outflows. It helps scientists predict when such events occur.

The main effect of an AGN on surrounding gas clouds is bad for star birth. The winds can blow gas away, spreading it through the galaxy or into space. If the gas is spread out enough, there is not enough density to start star formation. The winds can also break apart gas molecules. This makes it harder for clouds to collapse and form stars.

The black hole’s activity of consuming material also removes the available gas. The result is the same: no gas is left to form stars. The galaxy loses its chance to grow through star formation. This process "quenches" the galaxy, stopping its development.

Xiang's team found multiple types of disk winds in the outflows from NGC 4151. All outflows had rates equal to or greater than the mass accretion rate. This means they blew essential material away at a rate that matched or exceeded the material falling inward. The team's measurements will help astronomers predict when such outflows happen in other galaxies. This could enhance the understanding of AGNs across the Universe. It offers a clearer picture of how the largest structures evolve and why some stop growing stars entirely.