Black hole jets may hold the key to why galaxies stop forming stars
A new study finds that jets from supermassive black holes energise the circumgalactic medium, suppressing star formation in galaxies.
Astronomers have identified a mechanism by which the supermassive black holes at the centres of galaxies could influence the vast, faint gas reservoir surrounding their hosts, potentially shaping galactic evolution.
Beyond the visible disk of a galaxy lies a diffuse envelope of gas extending 10 to 20 times the galaxy's size. Known as the circumgalactic medium (CGM), this reservoir supplies the fuel for star formation. If all the gas within the CGM were to collapse into stars, galaxies would become extraordinarily luminous. Yet such galaxies are not observed.
For over a decade, researchers have proposed that energy from supermassive black holes could prevent this gas from cooling and collapsing. However, the exact process has remained unclear. The scale of the puzzle is immense: an average supermassive black hole may be the size of the solar system, while its host galaxy can contain 100 billion such systems.
A team from the Raman Research Institute (RRI), an autonomous institute of the Department of Science and Technology, and Arizona State University has now found one way this influence operates. Black holes often eject jets of hot, charged plasma. The researchers investigated whether these jets affect the CGM.
They found that when a jet first meets the CGM, it collides with the gas, creating a disturbance that energises it. This bright, ionised gas can then suppress star formation. Ionised gas emits a characteristic glow, which the team searched for in the CGM. While no signal appeared when measurements were averaged in all directions, a strong signal emerged along the jet's path.
"This told us that the jet was illuminating only the gas in its path, rather than affecting the gas equally in all directions," said Namrata Roy, assistant professor at RRI and lead author of the paper published in The Astrophysical Journal Letters.
The signal was strongest at two locations: the edge of the stellar disk, where the jet first encounters the CGM, and the CGM's outer boundary, where the slowed jet strikes the gas again. This energy deposition heats the gas, preventing it from clumping into stars.
"The wider implication is that black holes can shape the lives of galaxies far beyond the small central region where they sit," said Sanchayeeta Borthakur, associate professor at Arizona State University and a co-author. She compared it to an ant in India making a huge mark in the USA.
The findings suggest that the black hole at a galaxy's centre can govern its fate, rendering it quiet and passive by disturbing the gas along its jets' paths.