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 activity^1,2, 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 galaxy^4,5. 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 phenomena^6-9 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).