A Generalised Bondi Accretion Model for the Galactic Centre

Abstract

We develop an analytic, steady-state model for the gas environment in quiescent galactic nuclei. We assume that the mass is constantly supplied by a spherically symmetric distribution of wind emitting stars, and that gravity is solely due to a central supermassive black hole. We show that at some finite radius, where the Keplerian velocity is comparable to the wind velocity, the bulk velocity vanishes. Matter generated below that radius will be accreted onto the black hole, while matter outside it will escape the system. Under certain conditions, the flow may become supersonic at both domains. We obtain radial profiles of the hydrodynamic variables and verify them using a time-dependent hydrodynamic simulation. We delineate the conditions under which radiative cooling can be neglected, and predict the luminosity and spectrum of the free-free X-ray emission from such a system. We discuss applications of our solution to our own Galactic Centre and other quiescent galactic nuclei.