Black hole jets pack a powerful punch into their host galaxies
[Posted on 25 September 2026]
Article by Unnati Ashar
Astronomers have discovered a possible mechanism by which the black holes at the hearts of galaxies could be influencing the humongous, faint reservoir of gas surrounding their host galaxies, ultimately shaping the fates of galaxies.
When one thinks of a galaxy, one imagines dust and stars sparkling within beautiful spirals. But beyond the galactic disk’s edge lies a diffuse, ghostly envelope spreading out as far as 10–20 times the size of the galaxy. This gaseous reservoir is called the circumgalactic medium or CGM. This CGM provides the fuel for star formation.
But if all the gas existing within the boundary of the CGM did end up clumping together into stars, extremely luminous galaxies with a huge number of stars would form. But such galaxies don’t really exist. For more than a decade now, astronomers have proposed that energy from supermassive black holes (SMBHs) sitting at the hearts of galaxies could be preventing the gas from cooling and collapsing into stars. But there’s no consensus yet on how this may be happening. Before going into how, it helps to put into context the relative sizes of the SMBH and the galaxy to understand the scale of influence being talked about.
Though an average SMBH could be the size of the solar system, it is still tiny compared to the galaxy, which could host 100 billion such stellar systems. Researchers in this study found one way in which the tiny black hole at the centre could influence the ginormous CGM of its host galaxy. “The surprising question is: how can something so small energetically impact something so enormous?” says Namrata Roy, assistant professor in the Astronomy and Astrophysics division at the Raman Research Institute, an autonomous institute of the Department of Science and Technology, and the lead author of the paper published yesterday in The Astrophysical Journal Letters.
Black holes shooting out powerful jets
While black holes conjure the image of monstrous voids gobbling up gas and dust from their surroundings, they also often burp out jets of seething hot plasma, which is a soup of charged particles. The team, composed of scientists from India and the USA, wanted to see if this jet modifies the gas all the way up to the CGM.
Energised and ionized gas — gas that has atoms with electrons added or removed which imparts the atoms charge — emits a particular glow. The team looked for this glow in the CGM. They didn’t detect a signal when they averaged the measurements over all directions around the galaxies. But when they probed along the direction of the jet, they detected a strong signal. This told them that the jet was illuminating only the gas in its path, rather than affecting the gas equally in all directions. “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,” says Roy.
Further, they found that the signal is strongest in two places — at the edge of the stellar disk, where the jet first encounters the CGM, and next at the CGM’s outer boundary, where the already slowed down jet fist-bumps the gas in the CGM. This is akin to punching something hard and getting a jerk. There’s a deposition of energy, and this energy is what kindles the gas.
Curiously, they also looked for another tracer of much cooler gas and found that this one was distributed equally in all directions rather than aligned with the jet. This meant that while the cool gas pervaded the CGM, the jet dramatically energised the gas lying only in its path, lighting it up like a glowing fire. This bright, energised gas in the CGM has far-reaching implications for star formation in the galaxy.
The ant sitting in India…
Cool gas clumps together to form stars. But the jet launched from the black hole fires up and disturbs the gas along its path, preventing it from clumping together and suppressing star formation. In this way, the black hole at the galaxy’s centre governs the galaxy’s fate, making it quiet and passive. “The wider implication is that black holes can shape the lives of galaxies far beyond the small central region where they sit,” says Sanchayeeta Borthakur, associate professor at Arizona State University and a co-author of this work. “[It’s like] the ant sitting in India is making a huge mark somewhere in the USA.”
“A major hurdle was that this gas in the CGM is extremely faint and tenuous,” says Roy. “We could not simply take a picture of one galaxy and see the effect clearly.” They combined the data from multiple galaxies using the Dark Energy Spectroscopic Instrument (DESI) survey for their observational study.
“The most exciting bit is that other studies tried to look for a similar impact, but they missed it. This is because the signal appears only when we pay careful attention in and around the jet direction,” says Roy. “If we had treated the galaxy’s CGM as the same in every direction, we would have missed the discovery.”
For now, Roy, Borthakur and colleagues can take satisfaction in the fact that they did not miss the blazing trail this ant of a black hole left on the mammoth galaxy in which it resides.
For more details, contact: outreach@rrimail.rri.res.in