Feeding of active galactic nuclei by dynamical perturbations

TL;DR

This paper models how collisions between gas structures near the Galactic Centre could have triggered past AGN activity, potentially explaining the origin of the Fermi bubbles.

Contribution

It introduces a numerical simulation framework to study gas collisions in the Galactic Centre and their role in fueling AGN episodes, linking dynamics to observed large-scale structures.

Findings

High-angle collisions can transport up to 40% of gas to the SMBH.

Energy from such collisions can account for 10% of the Fermi bubbles' energy.

Such impactful collisions may occur roughly once every 100 million years.

Abstract

There possibly was an AGN episode in the Galactic Centre 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 $\alpha$-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 $40\%$ of the initial matter to the accretion disc. Even with ring and cloud masses equal to $10^4 \, M_\odot$, which is the lower limit of present-day mass of the Circumnuclear ring in the Galactic Centre, the energy released over an interval of 1.5 Myr can produce $\sim 10\%$ of that required to inflate the Fermi bubbles. If the gas ring in the Galactic Centre 6 Myr ago had a mass of at least $10^5 \, M_\odot$, our proposed scenario can explain the formation of the Fermi bubbles. We estimate that such high-impact collisions might occur once every $\sim 10^8$ yr in our Galaxy.