The role of feedback in accretion on low-luminosity AGN: Sgr A* case study

Abstract

We present numerical models of the gas dynamics in the inner parsec of the Galactic centre. We follow the gas from its origin as stellar winds of several observed young massive stars, until it is either captured by the central black hole, or leaves the system. Unlike our previous models, we include an outflow from the inner accretion flow. Two different kinds of outflows are modelled: (i) an instantaneous-response feedback mode, in which the outflow rate is directly proportional to the current black hole gas capture rate; and (ii) an outburst mode, which is stronger but lasts for a limited time. The latter situation may be particularly relevant to Sgr A*, since there is evidence that Sgr A* was much brighter in the recent past. We find that both types of outflow perturb the gas dynamics near the Bondi radius and the black hole capture rate significantly. The effects persist longer than the outflow itself. We also compare the effects of spherically symmetric and collimated outflows, and find that the latter are far less efficient in transferring its energy to the surrounding gas near the capture radius. Our results imply that accretion feedback is important for non-radiative accretion flows not only within but also outside the capture radius. Steady-state Bondi accretion rate estimates that do not account for feedback outflows over-predict not only the accretion rate onto the black hole but also the capture rate at the Bondi radius itself. Finally, the steady-state assumption under which non-radiative flows have been routinely studied in the literature may have to be abandoned if accretion feedback is bursty in nature.