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

TL;DR

This paper demonstrates through models and simulations that head-on galaxy collisions can strip the gas around black holes, effectively shutting down their activity and contributing to the active nucleus duty cycle.

Contribution

It introduces an analytic model and hydrodynamic simulations showing galaxy collisions can suppress black hole fueling by stripping surrounding gas, explaining the shutdown of active galactic nuclei.

Findings

Head-on galaxy collisions can strip the gas torus around black holes.

Collision conditions can switch off black hole activity in many galaxies.

Galaxy mergers may both activate and deactivate nucleus activity depending on collision orbit.

Abstract

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).