Destruction of the central black hole gas reservoir through head-on galaxy collisions

A massive black hole exists in almost every galaxy. Black holes occasionally radiate a vast amount of light by releasing gravitational energy of accreting gas with a cumulative active period of only a few 108 yr, the so-called duty cycle of the active galactic nuclei. Many galaxies today host a starving massive black hole. Although galaxy collisions have been thought to enhance nucleus activity1,2, the origin of the duty cycle, especially the shutdown process, is still a critical issue3. Here, we show that galaxy collisions are also capable of suppressing black hole fuelling, by using an analytic model and three-dimensional hydrodynamic simulations and by applying the well-determined parameter sets for the galactic collision in the Andromeda galaxy4,5. Our models demonstrate that a central collision of galaxies can strip the torus-shaped gas surrounding the massive black hole, the putative fuelling source. The derived condition for switching off the black hole fuelling indicates that a notable fraction of currently bright nuclei can become inactive, which is reminiscent of the fading or dying active nucleus phenomena6–9 that are associated with galaxy merging events. Galaxy collisions may therefore be responsible for both switching off and turning on the nucleus activity, depending on the collision orbit (head-on or far-off-centre). Three-dimensional hydrodynamic simulations show that head-on galaxy collisions can suppress black hole fuelling by stripping the torus-shaped gas surrounding the massive black hole. Galaxy collisions could therefore either switch off or turn on nuclear activity, depending on the collision orbit.