Supermassive black holes exist in the universe that are more than 100,000 times the mass of the Sun, and, although only a small part of them are luminous due to the energy of accreting material, in most galaxies, the central massive black hole is dark and quiet. The mechanism that causes the switch between black-hole activity and dormancy is unclear. For the first time in the world, Assistant Professor Yohei Miki of the University of Tokyo Information Infrastructure Center, Associate Professor Masao Mori of the Center for Computational Science, University of Tsukuba, Faculty of Economics and Information Studies, Onomichi City University / NAOJ used supercomputers including Oakforest-PACS to shed light on the mystery of galaxy collisions and how they might activate the central black hole of a galaxy center.
Galactic central black holes have always been believed to be more active after collisions between two galaxies. After the galaxy collision, the material from the galactic disk falls toward the center and into the black hole, which activates the black hole. But if the point of impact of a galaxy is near the center of a galaxy, the consequences were found to be very different. A galaxy that collides with another galaxy through its center carries away the gas around the black hole (see illustration), and the black hole that has now lost its energy source to shine stops its activity and becomes dormant.
The mechanism behind the switch between black-hole activity and dormancy that was clarified by this study can be linked to recent observations of active galactic centers that show signs of a sudden cessation of black hole activity. The results found in this work represent a milestone toward elucidating the co-evolution of galactic-center black-holes with the host galaxies, one of the biggest mysteries remaining in galaxy formation.