  A massive black hole exists in almost every galaxy. They occasionally radiate
a vast amount of light by releasing gravitational energy of accreting gas, with
a cumulative active period of only a few $10^8$ years, so-called the duty cycle
of the Active Galactic Nuclei. Namely, many galaxies today host a starving
massive black hole. Although galaxy collisions have been thought to enhance
nucleus activity, the origin of the duty cycle, especially the shutdown
process, is a still critical issue. 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. 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).
