Hydrodynamical numerical simulation of wind production from black hole hot accretion flows at very large radii

Abstract

Previous works show strong winds exist in hot accretion flows around black holes. Those works focus only on the region close to the black hole thus it is unknown whether or where the wind production stops at large radii. In this paper, we investigate this problem by hydrodynamical simulations. We take into account gravities of both the black hole and the nuclear star clusters. For the latter, we assume that the velocity dispersion of stars is a constant and its gravitational potential $\propto σ^2 \ln (r)$, where $σ$ is the velocity dispersion of stars and $r$ is the distance from the center of the galaxy. We focus on the region where the gravitational potential is dominated by the star cluster. We find, same as the accretion flow at small radii, the mass inflow rate decreases inward and the flow is convectively unstable. However, trajectory analysis shows that there is very few wind launched from the flow. Our result, combined with the results of Yuan et al. (2015), indicates that the mass flux of wind launched from hot accretion flow $\dot{M}_{\rm wind}=\dot{M}_{\rm BH}(r/20r_s)$, with $r\la R_A\equiv GM_{\rm BH}/σ^2$. Here $\dot{M}_{\rm BH}$ is accretion rate at black hole horizon. $R_A$ is similar to Bondi radius. We argue that the inward decrease of inflow rate is not because of mass loss via wind, but because of convective motion. The disappearance of wind outside of $R_A$ must be because of the change of the gravitational potential, but the exact reason remains to be probed.