Astronomers have long known that more massive the galaxy is, the more massive the central black hole is, but they haven’t been able to develop an explanation around it.
Aiming to solve this shadowy problem, a team of astronomers utilized the high resolution of ALMA to observe the center of spiral galaxy M77. The central region of M77 is an “active galactic nucleus,” or AGN, which means that matter is vigorously falling toward the central supermassive black hole and emitting intense light. AGNs can strongly affect the surrounding environment, therefore they are important objects for solving the mystery of the co-evolution of galaxies and black holes.
The team imaged the area around the supermassive black hole in M77 and resolved a compact gaseous structure with a radius of 20 light-years. And, the astronomers found that the compact structure is rotating around the black hole, as expected.
Many astronomers have observed the center of M77 before, but never has the rotation of the gas donut around the black hole been seen so clearly. Besides the superior resolution of ALMA, the selection of molecular emission lines to observe was key to revealing the structure. The team observed specific microwave emission from hydrogen cyanide molecules (HCN) and formyl ions (HCO+). These molecules emit microwaves only in dense gas, whereas the more frequently observed carbon monoxide (CO) emits microwaves under a variety of conditions. The torus around the AGN is assumed to be very dense, and the team’s strategy was right on the mark.
Interestingly, the distribution of gas around the supermassive black hole is much more complicated than what a simple unified model suggests. The torus seems to have an asymmetry and the rotation is not just following the gravity of the black hole but also contains highly random motion. These facts could indicate the AGN had a violent history, possibly including a merger with a small galaxy. Nevertheless, the identification of the rotating torus is an important step.
The Milky Way Galaxy, where we live, also has a supermassive black hole at its center. This black hole is, however, in a very quiet state. Only a tiny amount of gas is accreting onto it. Therefore, to investigate an AGN in detail, astronomers need to observe the centers of distant galaxies. M77 is one of the nearest AGN and a suitable object for peering into the very center in detail.