AGN activity and nuclear starbursts: Sgr A* activity shapes the Central Molecular Zone

Abstract

The Central Molecular Zone (CMZ) of the Milky Way shows several peculiar properties: a large star formation rate, some of the most massive young star clusters and molecular clouds in the Galaxy, and a twisted ring morphology in molecular gas. In this paper, I use SPH simulations to show that most of these properties can be explained as due to a recent outburst of AGN activity in Sgr A*, the central supermassive black hole of the Milky Way. In particular, the narrow ring of dense gas, massive gas clouds, young star clusters and an elevated SFR can all be caused by the passage of an AGN outflow through the system, which compresses the gas and triggers fragmentation. Furthermore, I show that the asymmetric distribution of gas, as observed in the CMZ, can be produced by outflow-induced instabilities from an initially axisymmetric gas disc. Angular momentum mixing in the disc produces some low angular momentum material, which can subsequently feed Sgr A*. These processes can occur in any galaxy that experiences an AGN episode, leading to bursts of nuclear star formation much stronger than pure bar-driven mass inflows would predict.