High-resolution observations using the Atacama Large Millimeter Array (ALMA) have revealed the presence of a rotating dusty gaseous torus around an active super-massive black hole at the center of the barred spiral galaxy Messier 77. The existence of such rotating donuts-shape structures was first suggested decades ago, but this is the first time one has been confirmed so clearly. This is an important step in understanding the co-evolution of super-massive black holes and their host galaxies.
The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical interferometer (telescope) in the Atacama desert of northern Chile. Consisting of sixty-six 12-metre (39 ft) and 7-metre (23 ft) diameter radio telescopes, ALMA is expected to provide insight on star birth during the early universe and detailed imaging of local star and planet formation.
Astronomers have known for a long time that the more massive the galaxy is, the more massive the central black hole will be. Messier 77’s central region is an active galactic nucleus (AGN), which means that matter is vigorously falling toward the central super-massive black hole and emitting intense light. To interpret various observational features of AGNs, astronomers have assumed rotating donut-like structures of dusty gas around active supermassive black holes. This is called the ‘unified model’ of AGN.
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.
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.