Convective cores in galactic cooling flows

We use hydrodynamic simulations with adaptive grid refinement to study the dependence of hot gas flows in X-ray luminous giant elliptical galaxies on the efficiency of heat supply to the gas. We consider a number of potential heating mechanisms including Type Ia supernovae and sporadic nuclear activ ity of a central supermassive black hole. As a starting point for this rese arch we use an equilibrium hydrostatic recycling model. We show that a comp act cooling inflow develops, if the heating is slightly insufficient to cou nterbalance radiative cooling of the hot gas in the central few kiloparsecs . An excessive heating in the centre, instead, drives a convectively unstab le outflow. We model the onset of the instability and a quasi-steady convec tive regime in the core of the galaxy in two dimensions assuming axial symm etry. Provided the power of net energy supply in the core is not too high, the co nvection remains subsonic. The convective pattern is dominated by buoyancy driven large-scale mushroom-like structures. Unlike in the case of a coolin g inflow, the X-ray surface brightness of an (on average) isentropic convec tive core does not display a sharp maximum at the centre. A hybrid model, w hich combines a subsonic peripheral cooling inflow with an inner convective core, appears to be stable. We also discuss observational implications of these results.