A new study led by researchers from the School of Astronomy and Space Science, Nanjing University has found a direct link between the mass of galaxies' central black holes and the atomic hydrogen gas content of their host galaxies. This provides important observational evidence for the long-suspected link between central black holes and the cool gas content in massive galaxies. This result, entitled "Black holes regulate cool gas accretion in massive galaxies," is published in Nature on August 14, 2024. (https://www.nature.com/articles/s41586-024-07821-2)
In the present-day Universe, there are two families of galaxies: those that form stars and those that do not, generally called "star-forming galaxies" and "passive galaxies" respectively. Unlike star-forming galaxies, passive galaxies happen to have little or no cool gas, which is also the raw material for star formation. It is therefore generally assumed that the difference between the two populations comes from the presence or absence of cool gas reservoirs. The key question, then, is what causes the transition of galaxies from star-forming to passive. It could be that passive galaxies are more massive, since they are indeed on average more massive, or that something has prevented galaxies from acquiring cool gas reservoirs. Since the 1970s, theorists have proposed that the central supermassive black holes have enough power during their rapid accretion to disrupt the cool gas reservoirs of their host galaxies. However, until now a direct link between central black holes and the cool gas reservoirs of their host galaxies has never been observed.
In this paper, the authors present an analysis of a statistical sample of nearby galaxies for which a measurement of their stellar mass, the mass of their central black holes and their reservoir of atomic hydrogen gas (HI) is available. Theye show that the gas reservoir decreases with a much better correlation with the mass of the central black holes, compare to other major galactic parameters. In fact, they show that it's not the mass of galaxies (or other galactic parameters) that plays a role in determining how much gas they have, but rather the mass of the central black hole. The more massive the black hole in a galaxy, the smaller its reservoir of atomic gas.
The cool gas comes essentially from the accretion of matter from the intergalactic space. This analysis therefore suggests that black holes have the power to prevent galaxies from receiving matter from outside. The other possibility is that the black hole ejects some of the gas, which is not impossible but may play a lesser role since previous studies have shown that even galaxies hosting the most active black holes do not show a reduction in their cool gas content or star formation compared to similarly massive star-forming galaxies. In short, the paper shows concrete evidence on central black holes affecting the growth and quenching of their host galaxies, by regulating the cool gas accretion in galaxies.
The paper also reveals that the two basic types of galaxies, star-forming and passive ones, actually follow the same cool gas content-black hole mass relation. This suggests that the cool gas content-black hole mass relation provides a novel, and probably more fundamental framework to study galaxy (trans)formation, compared to the commonly assumed relation between gas content and stellar masses, or other galactic parameters.
The first and corresponding author of the paper is Prof. Tao Wang from Nanjing University. Two graduate students from Nanjing University, Ke Xu and YuXuan Wu make significant contributions. Prof. Yong Shi, Prof. Qiusheng Gu, Prof. Zhi-Yu Zhang, and Dr. Yijun Wang from Nanjing University also provide valuable inputs. Other coauthors include researchers from University Paris-Saclay, Peking University, Shanghai Normal University, Tianjin Normal University, and Fudan University. This work is supported by the National Natural Science Foundation of China, the National Key R&D Program, and the China Manned Space Project.
Figure 1. The relation between the cool gas to stellar mass ratio and the mass of central black holes in galaxies. Galaxies with more massive central black holes exhibit smaller cool gas content, and higher probabilities of being passive galaxies.
Source: School of Astronomy and Space Science, Office of Science and Technology
Correspondent: Wang Feihao
