Effects of Tidal Interactions on the Gas Flows of Elliptical Galaxies

Abstract

During a Hubble time, cluster galaxies may undergo several mutual encounters close enough to gravitationally perturb their hot, X-ray-emitting gas flows. We ran several two-dimensional, time-dependent hydrodynamical models to investigate the effects of such perturbations on the gas flow inside elliptical galaxies, focusing on the expected X-ray properties. In particular, we studied in detail the modifications occurring in the scenario proposed in 1989 by A. D'Ercole and coworkers, in which the galactic interstellar medium produced by the aging galactic stellar population is heated by Type Ia supernovae (SNe Ia) at a decreasing rate. We find that although the tidal interaction in our models lasts less than 1 Gyr, its effect extends over several Gyr. The tidally induced turbulent flows create dense filaments which cool quickly and accrete onto the galactic center, producing large spikes in the global X-ray luminosity, LX. Once this mechanism starts, it is fed by gravity and amplified by SNe Ia. This evolution is found to be virtually independent of the dynamical state of the gas flow at the beginning of the interaction. To better understand the role of SN Ia heating, we also considered a cooling flow model without supernovae; in this case the amplitude of the LX fluctuations due to the tidal interaction is substantially reduced. We conclude that if SNe Ia significantly contribute to the energetics of the gas flows in elliptical galaxies, then the observed spread in the LX-LB diagram at any fixed optical galaxy luminosity LB 3 × 1010 L☉ may be caused, at least in part, by this mechanism. On the contrary, tidal interactions cannot be responsible for the observed spread if the pure cooling flow scenario applies.