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

Massive black holes in the central region of galaxies radiate a vast amount of light as active galactic nuclei by releasing gravitational energy of accreting gas when a sufficient amount of gas accretes onto the massive black holes. Galaxy collisions have been thought to enhance nucleus activity, by triggering mass fueling to the central massive black hole through angular momentum transfer. Here we show that galaxy collisions are also capable of suppressing black hole fueling by using an analytic model and three-dimensional hydrodynamic simulations, applying the well-determined parameter sets for the galactic collision in the Andromeda galaxy. Our models demonstrate that a central collision of galaxies can strip the torus-shaped gas surrounding the massive black hole, the putative fueling source, away via momentum transfer between the torus gas and the infalling gas when the column-density of the infalling gas is higher than that of the torus. The derived condition for switching-off the black hole fueling indicates that a significant fraction of currently bright nuclei can become inactive, reminiscent of fading/dying active nucleus phenomena associated with galaxy merging events. Galaxy collisions may therefore be responsible both for switching-off and turning-on the nucleus activity, depending on the collision orbit (head-on or far-off-centre).