Galaxies are enormous and hold hundreds of billions of stars. These stars form from cold, dense gas. Indeed, 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 led by astronomers Sanchayeeta Borthakur of Arizona State University (ASU) and Namrata Roy, now at the Raman Research Institute (RRI), found evidence that narrow jets of heated plasma, blasted out by the supermassive black holes at galaxies' centers, can affect the whole galaxy even beyond what we can see, and may shape its future. The jets may reach out and disrupt the gas that galaxies need to keep growing. Their research has been published in the Astrophysical Journal Letters.
"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."
Little in size, big in influence
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 quite small and are roughly about the size of our solar system. On the other hand, their host galaxies can hold about 100 billion such 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.
One way to envision it is to think of an ant leaving its impression hundreds or thousands of kilometers away. Scientists still do not fully understand how energy from these active black holes reaches such distances and changes along the way.
The team's study provides evidence as to how this could occur. They 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 they were looking for in the CGM, is so faint that no single galaxy would show it clearly. Hence, 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). 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.
Roy, Borthkur and colleagues 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 - it 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 isotopically 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.
Black holes determine a galaxy's fate.
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, changes its fate, and makes it much less active in star formation.
This means the black hole is not just "feeding" at the center of the galaxy, as some might think. It also reaches out and changes the galaxy's environment, which can eventually change its fate.
Specific Directions: Look both ways on the path
Past studies searched for the signal but couldn't detect it, making this directional discovery an exciting breakthrough. The signal only appears when we look along the jet direction. If the astronomers had thought the CGM was the same in every direction, the team would have missed this discovery. The jet ionizes hydrogen gas along its path, making it glow in H-alpha in a very specific direction.
This study shows the value of large optical and radio surveys like the Dark Energy Spectroscopic Instrument (DESI) survey and the LOFAR Two-meter Sky Survey (LoTss). By combining many weak signals, astronomers can now study the behavior of galaxies that would otherwise stay hidden. The study also gives astronomers and theorists a new way to test how black-hole jets affect galaxies.
Contributing co-authors include Timothy Heckman at Johns Hopkins University and Tanmay Singh at Arizona State University.
This work is supported by NASA, STScI, and NSF.
