NONLINEAR EVOLUTION OF INTERNAL GRAVITY-WAVES IN CLUSTER COOLING FLOWS

We investigate the formation and evolution of finite-amplitude and finite-wavelength disturbances through time-dependent axisymmetric numerical simulations of model cluster atmospheres perturbed by a single galaxy orbiting along the symmetry axis. For density and temperature profiles typical of a large cluster cooling flow, the perturbing galaxy excites nonlinear g-modes which result in local density enhancements of order unity and fluid velocities in excess of 500 km s(-1). The modes are trapped within the resonant radius, where the local buoyant oscillation frequency exceeds the orbital frequency of the perturbing galaxy. The resulting perturbed velocity field is characterized by nonzero vorticity. By contrast, acoustic disturbances travel out of the cluster center and generally correspond to an irrotational velocity field. In cases where modes are resonantly excited, a global dipole mode dominates after several orbits of the perturbing galaxy. These results suggest that in the presence of a temperature gradient due to cooling in the cluster center, an orbiting galaxy can generate local inhomogeneities via the excitation of internal gravity modes. This process may be important for understanding the clumpy X-ray structure now becoming apparent in ROSAT images.