Astronomers have identified a possible mechanism through which supermassive black holes at the centres of galaxies can influence the vast reservoirs of gas surrounding their host galaxies, potentially regulating star formation and shaping their evolution.
Beyond the visible disk of a galaxy lies a diffuse envelope of gas extending up to 10-20 times the size of the galaxy. Known as the circumgalactic medium (CGM), this reservoir provides the material needed for star formation.
However, if all the gas within the CGM cooled and collapsed into stars, galaxies would contain far more stars than are observed. Astronomers have for more than a decade proposed that energy released by supermassive black holes (SMBHs) could prevent the gas from cooling and collapsing, although the precise mechanism remains under study.
A team of researchers from the Raman Research Institute (RRI), an autonomous institute under the Department of Science and Technology (DST), and Arizona State University has now found evidence of how black hole activity could influence gas across the CGM.
Although black holes are commonly associated with consuming matter, actively feeding black holes can also launch powerful jets of extremely hot plasma, a stream of charged particles. The researchers investigated whether these jets could affect gas far beyond the immediate vicinity of the black hole.
They found that when a jet first encounters the CGM, its interaction with the surrounding gas deposits energy and excites it. The resulting heated and ionised gas can inhibit the cooling and clumping required for star formation.
To investigate this process, the researchers looked for the characteristic emission from energised and ionised gas in the CGM. They did not detect a significant signal when measurements were averaged across all directions around the galaxies. However, a strong signal emerged when they examined the regions aligned with the direction of the jets.
“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 in the Astronomy and Astrophysics Division at RRI and lead author of the study published in The Astrophysical Journal Letters.
The researchers found that the signal was strongest at two locations: near the edge of the stellar disk, where the jet first encounters the CGM, and near the outer boundary of the CGM, where the jet, having slowed down, interacts again with the surrounding gas.
The interaction deposits energy into the gas, heating and disturbing it and making it less likely to cool and clump into stars.
Cool gas is essential for star formation. However, a powerful jet from the central black hole can heat and disturb the gas along its path, suppressing the formation of new stars. In this way, activity from a relatively tiny region around the central black hole can influence the evolution of the much larger host galaxy.
“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 of the study.
The findings provide evidence that black hole jets can transfer energy to the CGM in a directional manner and influence the gas reservoir that fuels star formation, offering new insight into how galaxies evolve from active, star-forming systems to more passive ones.
