Black hole jets may shape the fate of entire galaxies

Galaxies are enormous and hold hundreds of billions of stars.

These stars form from cold, dense gas. Every large galaxy, including the Milky Way, is wrapped in a huge envelope of gas called the circumgalactic medium, or CGM. This gas is a reservoir of raw material and stretches 10–20 times the size of the visible portion of the galaxy. This CGM gas eventually cools, moves inward into the galaxy and clumps together to form stars. Thus, the CGM plays a key role in shaping stars, planets and even life within a galaxy.

But astronomers have long puzzled over why — given how much star-forming gas surrounds them — galaxies don't have even more stars. What is keeping the fuel from cooling down and forming those stars?

A new study published in the Astrophysical Journal Letters and led by astronomers Sanchayeeta Borthakur of Arizona State University and Namrata Roy of the Raman Research Institute found evidence that narrow jets of heated plasma blasted out by the supermassive black holes at the centers of galaxies can have effects far beyond their visible reaches. The jets may disrupt the surrounding gas that galaxies need to form stars, potentially shaping how they grow and evolve.

“This is a pathbreaking result that solves the long-standing mystery of how black holes influence galaxies, their stars and life as we know it,” said Borthakur, associate professor in ASU’s School of Earth and Space Exploration. “This work opens a new direction to explore further the intricacies of the connection between the supermassive black holes trillions of miles from where we are to how we came to be here.”

When a black hole actively feeds on gas, it can release enormous energy that heats the surrounding gas. Even though these black holes can be powerhouses of energy, they are relatively small — roughly about the size of our solar system. On the other hand, their host galaxies can hold about 100 billion similarly sized solar systems. 

“The surprising question is: How can something so small energetically impact something so enormous?” said Roy, assistant professor at RRI and former ASU Exploration Prize Postdoctoral Fellow. 

Scientists still do not fully understand how energy from these active black holes reaches such distances and changes along the way. But the new study provides evidence as to how this could occur.

The research team focused on active black holes that emit strong jets, which are narrow streams of hot, fast-moving plasma shooting out far beyond a galaxy’s visible edge. They looked for a distinct imprint in the ionization state of the gaseous reservoir caused by these jets. The ionized gas, or the “glow” from hydrogen gas that they were looking for in the CGM, is so faint that no single galaxy would show it clearly. The team combined observations of hundreds of galaxies with active jets, using data from the Dark Energy Spectroscopic Instrument (DESI) survey and radio jet measurements from the LOFAR Two-meter Sky Survey (LoTSS). From there, they combined measurements coming from the direction of the jet axes and searched for a specific telltale sign of ionized hydrogen gas along the jet path, known as H-alpha. 

The research showed that when averaged over all directions around the galaxies, the signal was weak; however, along the radio jets, the H-alpha signal became clear and strong. This indicates that the gas does not glow uniformly everywhere but is particularly bright along the jet path. A simple analogy is that the jet acts less like a lamp shining in all directions, and more like a powerful beam that makes the gas glow where it passes through. 

These findings show that black-hole jets do not affect the gas surrounding galaxies equally in all directions; instead, they leave a distinct impression by causing the gas to shine and become ionized mainly along the radio jet's path. They also found that the jets have the biggest impact, and that the glow from ionized hydrogen is brightest in two places: close to the galaxy where the jet first hits the CGM, and much farther out near the CGM’s outer edge, where the jet releases most of its energy. This provides a clear signature of how jets can illuminate or disrupt the surrounding gas, even at great distances, all the way to the CGM. This mechanism helps shape the galaxy's environment, influence its growth and evolution — and determines whether the galaxy continues to form stars or becomes quiescent.

“What excites me most is the scale of the connection,” Roy said. “A black hole is incredibly small compared to a galaxy, but its impact can reach hundreds of thousands of light-years, far into the galaxy’s outer reaches. The jet carries the energy outward, and the gas lights up along its path."

As a check, the team also examined a tracer of cooler gas via the absorption signature of magnesium. Unlike the directional glow in H-alpha, magnesium was more widely distributed isotropically and did not show any connection to the jet direction. This implies that the cool gas component might already exist as a reservoir surrounding the galaxy uniformly on all sides. Yet the jet brightens, heats and ionizes gas along its own path, lighting up a trail and causing it to glow in H-alpha.

The findings offer some of the clearest evidence yet for how a black hole can affect galaxies far beyond its central region.

By heating, stirring and disturbing gas throughout the CGM, jets can prevent that gas from cooling down and falling inward to fuel new stars. This acts as a brake on the galaxy’s growth, reducing star formation and potentially changing how the galaxy evolves.

This means the black hole is not just “feeding” at the center of the galaxy. Its jets can also reach outward and change the galaxy’s environment, influencing whether it continues to form stars or becomes quiescent.

Past studies searched for H-alpha emission associated with black-hole jets but couldn't detect it. The new study suggests why: The signal becomes clear only when astronomers look along the direction of the jets. If the team had assumed the CGM was the same in every direction and averaged observations from all directions around the galaxies, the distinct H-alpha glow along the jet paths would have been obscured.

The finding highlights the value of combining large optical and radio surveys such as DESI and LoTSS. By combining many individually weak signals and accounting for the direction of the jets, astronomers can detect patterns that would otherwise remain hidden. The study also gives observers and theorists a new way to test how black-hole jets affect galaxies.