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^6,7,8,9 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).

几乎每个星系中都存在一个巨大的黑洞。黑洞偶尔会通过释放吸积气体的引力能量来辐射大量的光，累积活动周期只有几个 10 ⁸ 年，即所谓的活动星系核的占空比。今天的许多星系都有一个饥饿的大质量黑洞。尽管星系碰撞被认为会增强核活动^1,2，但占空比的起源，尤其是关闭过程，仍然是一个关键问题³。在这里，我们通过使用解析模型和三维流体动力学模拟以及应用仙女座星系中星系碰撞的确定参数集，证明星系碰撞也能够抑制黑洞燃料。^4,5。我们的模型表明，星系的中心碰撞可以剥离大质量黑洞周围的环形气体，这是假定的燃料来源。关闭黑洞燃料的推导条件表明，当前明亮的原子核中有相当一部分会变得不活跃，这让人想起与星系合并事件相关的衰落或垂死的活跃原子核现象^{6,7,8,9 。}因此，取决于碰撞轨道（正面或远离中心），星系碰撞可能会导致原子核活动的关闭和开启。