Galaxies can contain hundreds of billions of stars, but every one of those stars begins with cold, dense gas. Large galaxies, including the Milky Way, are surrounded by an enormous reservoir of this material known as the circumgalactic medium, or CGM.
The CGM extends roughly 10-20 times farther than the visible part of a galaxy. Over time, some of this gas can cool, flow inward, gather into dense clouds, and eventually form new stars. Because stars give rise to planets and the conditions necessary for life, the CGM plays an important role in a galaxy’s long term evolution.
Yet astronomers have faced a persistent puzzle. If galaxies are surrounded by so much potential star-forming material, why do they not produce far more stars? Something appears to be preventing much of that gas from cooling and falling inward.
Black Hole Jets May Be the Missing Piece
A new study led by astronomers Sanchayeeta Borthakur of Arizona State University (ASU) and Namrata Roy, now at the Raman Research Institute (RRI), offers evidence for one possible answer.
The researchers found that narrow jets of extremely hot plasma launched by supermassive black holes at the centers of galaxies can influence gas far beyond the visible galaxy. These jets may disturb the reservoir of material galaxies depend on for future growth, potentially affecting whether they continue producing stars. The findings were 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.”
Small Objects With Enormous Influence
When a black hole actively consumes gas, it can release tremendous amounts of energy and heat nearby material. Despite their extraordinary power, these black holes are tiny compared with their host galaxies. A supermassive black hole can be roughly comparable in scale to our solar system, while the galaxy around it may contain about 100 billion 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.
The scale of that influence can be difficult to picture. One comparison would be an ant somehow leaving a noticeable mark hundreds or thousands of kilometers away. Astronomers have known that active black holes release immense energy, but exactly how that energy travels across such enormous distances and changes along the way has remained unclear.
The new research provides evidence for how that process may work.
Searching for a Glow Along the Jets
The researchers studied active black holes that produce powerful jets. These narrow streams of hot, rapidly moving plasma can extend well beyond the visible boundaries of their galaxies.
The team searched for signs that the jets were changing the ionization state of gas in the surrounding CGM. In particular, they looked for the faint glow produced by ionized hydrogen.
That signal is too weak to detect clearly around a single galaxy. To overcome that limitation, the researchers combined observations from hundreds of galaxies with active jets. They used data from the Dark Energy Spectroscopic Instrument (DESI) survey together with radio jet measurements from the LOFAR Two-meter Sky Survey (LoTSS).
By combining measurements taken along the axes of the jets, the team searched for H-alpha, a characteristic signature of ionized hydrogen gas.
Roy, Borthkur and colleagues found that the signal remained weak when observations from every direction around the galaxies were averaged together. Along the radio jets, however, the H-alpha signal became much stronger and easier to detect.
That pattern indicates that the gas is not glowing equally in every direction. Instead, the strongest emission appears along the route traveled by the jet. Rather than behaving like a lamp illuminating everything around it, the jet acts more like a concentrated beam that causes gas to glow along its path.
Black Hole Jets Leave a Distinct Signature
The results suggest that black hole jets affect the gas surrounding galaxies in a highly directional way. Gas becomes more strongly ionized and luminous where the radio jets pass through it.
The researchers also found two regions where the ionized hydrogen glow was particularly bright. One occurs relatively close to the galaxy, where the jet first encounters the CGM. Another appears much farther away, near the outer edge of the CGM, where the jet appears to deposit much of its energy.
Together, these observations reveal how jets can alter or illuminate gas at tremendous distances from a galaxy’s center. By changing the surrounding environment, the jets can influence how a galaxy grows and evolves, including whether it continues creating 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.”
How Black Holes Can Change a Galaxy’s Future
The results provide some of the clearest evidence so far that supermassive black holes can influence galaxies far outside their central regions.
By heating, stirring, and disrupting gas throughout the CGM, black hole jets can make it harder for that material to cool and fall back toward the galaxy. Without that incoming supply of cold gas, the galaxy has less fuel available for creating new stars.
In this way, the jets can act as a brake on galaxy growth. Over time, they may help push a galaxy toward a quieter state with much lower levels of star formation.
The black hole, therefore, is doing more than simply “feeding” on matter near the galactic center. Its influence can extend throughout the galaxy’s surrounding environment and potentially alter the galaxy’s long-term fate.
Direction Was the Key to the Discovery
Earlier studies had searched for similar signals without detecting them. The new findings suggest one reason why.
The H-alpha signature becomes visible only when astronomers examine the gas specifically along the direction of the jets. If researchers had assumed that the CGM behaved the same way in every direction, this effect could easily have remained hidden.
Instead, the results show that the jets ionize hydrogen along their paths, producing a highly directional glow in H-alpha.
The discovery also demonstrates the power of combining large optical and radio surveys such as the Dark Energy Spectroscopic Instrument (DESI) survey and the LOFAR Two-meter Sky Survey (LoTss). By adding together many individually faint signals, astronomers can reveal patterns that would otherwise be impossible to see.
The findings now provide astronomers and theorists with another way to test how black hole jets interact with galaxies and influence their evolution.
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.