Magneto-hydrodynamical Numerical simulation of wind production from black hole hot accretion flows at very large radii

Abstract

Numerical simulations of black hole hot accretion flows have shown the existence of strong wind. Those works focus only on the region close to black hole thus it is unknown whether or where the wind production stops at large radii. To address this question, Bu et al. (2016) have performed hydrodynamic (HD) simulations by taking into account the gravitational potential of both the black hole and the nuclear star clusters. The latter is assumed to be $\propto σ^2 \ln(r)$, with $σ$ being the velocity dispersion of stars and $r$ be the distance from the center of the galaxy. It was found that when the gravity is dominated by nuclear stars, i.e., outside of radius $R_A\equiv GM_{\rm BH}/σ^2$, winds can no longer be produced. That work, however, neglects the magnetic field, which is believed to play a crucial dynamical role in the accretion and thus must be taken into account. In this paper, we revisit this problem by performing magneto-hydrodynamical (MHD) simulations. We confirm the result of Bu et al. (2016), namely wind can't be produced at the region of $R>R_A$. Our result, combined with the results of Yuan et al. (2015), indicates that the formula describing the mass flux of wind $\dot{M}_{\rm wind}=\dot{M}_{\rm BH}(r/20r_s)$ can only be applied to the region where the black hole potential is dominant. Here $\dot{M}_{\rm BH}$ is the mass accretion rate at the black hole horizon and the value of $R_A$ is similar to the Bondi radius.