2020MNRAS.492..603T

There possibly was an active galactic nuclei (AGN) episode in the Galactic Centre (GC) about 6 Myr ago, powerful enough to produce the Fermi bubbles. We present numerical simulations of a possible scenario giving rise to an activity episode: a collision between a central gas ring surrounding the supermassive black hole (SMBH) and an infalling molecular cloud. We investigate different initial collision angles between the cloud and the ring. We follow the hydrodynamical evolution of the system following the collision using GADGET-3 hybrid N-body/SPH code and calculate the feeding rate of the SMBH accretion disc. This rate is then used as an input for a 1D thin α-disc model in order to calculate the AGN luminosity. By varying the disc feeding radii, we determine the limiting values for possible AGN accretion disc luminosity. Small angle collisions do not result in significant mass transport to the centre of the system, while models with highest collision angles transport close to 40{{ per cent}} of the initial matter to the accretion disc. Even with ring and cloud masses equal to 10^4 M_{☉}, which is the lower limit of present-day mass of the circumnuclear ring in the GC, the energy released over an interval of 1.5 Myr can produce ∼ 10{{ per cent}} of that required to inflate the Fermi bubbles. If the gas ring in the GC 6 Myr ago had a mass of at least 10^5 M_{☉}, our proposed scenario can explain the formation of the Fermi bubbles. We estimate that such high-impact collisions might occur once every ∼108 yr in our Galaxy.