Hydrodynamics and stability of galactic cooling flows

• Published in: Monthly notices of the Royal Astronomical Society Vol. 301; no. 2; pp. 343 - 356
• Main Authors: Kritsuk, Alexei, Böhringer, Hans, Müller, Ewald
• Format: Journal Article
• Language: English
• Published: Oxford, U.K. and Cambridge, USA Blackwell Science Ltd 01.12.1998
• Subjects: cooling flows; dark matter; galaxies: ISM; hydrodynamics; instabilities; X-rays: ISM; cooling flows; hydrodynamics; instabilities; X-rays: ISM; galaxies: ISM; dark matter
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1046/j.1365-8711.1998.01897.x

Using numerical techniques we study the global stability of cooling flows in X-ray luminous giant elliptical galaxies. As an unperturbed equilibrium state we choose the hydrostatic gas recycling model. Non-equilibrium radiative cooling, stellar mass loss, heating by type Ia supernovae, distributed mass deposition and thermal conductivity are included. Although the recycling model reproduces the basic X-ray observables, it appears to be unstable with respect to the development of inflow or outflow. In spherical symmetry the inflows are subject to a central cooling catastrophe, while the outflows saturate in a form of a subsonic galactic wind. Two-dimensional axisymmetric random velocity perturbations of the equilibrium model trigger the onset of a cooling catastrophe, which develops in an essentially non-spherical way. The simulations show a patchy pattern of mass deposition and the formation of hollow gas jets, which penetrate through the outflow down to the galaxy core. The X-ray observables of such a hybrid gas flow mimic those of the equilibrium recycling model, but the gas temperature exhibits a central depression. The mass deposition rate M consists of two contributions of similar size: (i) a hydrostatic one resembling that of the equilibrium model, and (ii) a dynamical one which is related to the jets and is more concentrated towards the centre. For a model galaxy, like NGC 4472, our 2D simulations predict M ≈ 2 M⊙ yr−1 within the cooling radius for the advanced non-linear stage of the instability. We discuss the implications of these results to Hα nebulae and star formation in cooling flow galaxies and emphasize the need for high-resolution 3D simulations.