A joint team of researchers from the Raman Research Institute (RRI), an autonomous institute of the Department of Science and Technology, and Arizona State University has identified a mechanism by which supermassive black holes at the centres of galaxies influence the vast gas reservoirs surrounding them. The finding, published in The Astrophysical Journal Letters, shows that jets of hot plasma ejected by black holes can energise and heat the surrounding gas, preventing it from collapsing into new stars and fundamentally shaping how galaxies evolve.
The discovery addresses a long-standing puzzle in astronomy: galaxies should be far more densely packed with stars than they actually are. If all the gas in a galaxy’s outer envelope were to clump together, it would form an extremely luminous galaxy with an enormous number of stars. Yet such galaxies do not exist. Scientists have suspected that energy from supermassive black holes prevents this, but until now lacked clear evidence of how.
Key facts
- Research led by Namrata Roy, assistant professor at RRI’s Astronomy and Astrophysics division
- Collaboration between Raman Research Institute and Arizona State University
- Results published in The Astrophysical Journal Letters (DOI 10.3847/2041-8213/ae9cbd)
- The circumgalactic medium (CGM) extends 10 to 20 times the size of a galaxy’s visible disk
- A supermassive black hole, though comparable to the size of a solar system, influences a galaxy containing up to 100 billion stars
- Energy signal detected strongest at two locations: the edge of the stellar disk and the outer boundary of the CGM
What is the circumgalactic medium and why it matters
Around every galaxy lies an invisible halo of diffuse gas known as the circumgalactic medium. This vast gaseous envelope, extending far beyond the visible stars and dust, serves as a reservoir that supplies fuel for star formation. Because it contains enormous amounts of cool gas, if left undisturbed, this gas would eventually cool further and clump together, triggering the birth of countless new stars. The fact that this does not happen on the scale predicted by theory suggested something was preventing it.
How black hole jets affect the surrounding gas
Supermassive black holes do not merely consume matter. They also expel jets of extremely hot plasma—ionised gas composed of charged particles—at tremendous speeds. The research team investigated whether these jets could travel far enough to interact with the circumgalactic medium itself.
The study found that when a jet from a black hole first collides with the gas in the circumgalactic medium, it creates a shock wave—a sudden disturbance that energises and heats the gas. This heated, ionised gas then emits a specific type of radiation that astronomers can detect. Crucially, the team discovered that this energised gas prevents cool gas from collapsing and forming stars.
The researchers searched for the characteristic glow of ionised gas in observations of galaxies. When they averaged measurements across all directions around the galaxies, they found no clear signal. However, when they specifically looked along the direction of the jet, a strong signal appeared. This indicated that the jet illuminates only the gas directly in its path, rather than heating gas uniformly in all directions.
The two-punch pattern
The team detected the strongest energy signatures at two distinct locations. The first occurs at the edge of the galactic disk, where the jet initially encounters the circumgalactic medium and creates its initial disturbance. The second occurs at the outer boundary of the circumgalactic medium, where the jet—by then slowed by its interaction with gas—delivers another energetic impact. This two-stage process is comparable to throwing a punch: the initial blow deposits energy, and the subsequent jerk or rebound transfers further energy into the surrounding material.
Scale of influence
The discovery is remarkable given the disparity in scale between the black hole and the galaxy it influences. A typical supermassive black hole is as large as the solar system, yet the galaxy it inhabits can contain 100 billion star systems. Despite this enormous size difference, the black hole’s influence extends across the entire galaxy. As Sanchayeeta Borthakur, a co-author and associate professor at Arizona State University, explained, it is comparable to a tiny ant sitting in India leaving a measurable mark on something located in the United States.
What this means for understanding galaxies
This research clarifies how black holes regulate the evolution and ultimate fate of galaxies. By heating the gas that would otherwise cool and collapse into stars, supermassive black holes prevent galaxies from becoming overly dense and star-rich. Instead, galaxies remain relatively “quiet and passive,” with fewer stars than their available gas supply would allow. Understanding this process is fundamental to explaining the structure and diversity of galaxies observed in the universe today.