Multi stage three-dimensional sweeping and annealing of disc galaxies in clusters

We present new three-dimensional, hydrodynamic simulations of the ram press ure stripping of disc galaxies via interaction with a hot intracluster medi um (ICM). The simulations were carried with the smoothed-particle hydrodyna mics, adaptive mesh 'HYDRA' code (SPH-AP M-3), with model galaxies consisti ng of gas and stellar disc components and dark haloes. The simulations also include radiative cooling, which is important for keeping the warm, diffus e gas of moderate density from being unrealistically heated by the ICM. We examine the rote that wind velocity, density and galaxy tilt play in gas st ripping. We include cases with lower ram pressures than other recent studie s. In accord with previous studies, we find that low column density gas is pro mptly removed from the outer disc. However, we also find that not all of th e gas stripped from the disc escapes immediately from the halo, some of mat erial can linger for times of order 10(8) yr. We use a simple analytic mode l to demonstrate that gas elements in the ICM wind feel an effective potent ial with a minimum displaced downstream from the halo centre. The onset of the ICM wind has a profound effect on the disc gas that is not immediately stripped. This remnant disc is displaced relative to the halo centre and compressed. This can trigger gravitational instability and the f ormation of numerous flocculent spirals. These waves transport angular mome ntum outward, resulting in further compression of the inner disc and the fo rmation of a prominent gas ring. This 'annealing' process makes the inner d isc, which contains much of the total gas mass, resistant to further stripp ing, but presumably susceptible to global starbursts. Spirals in the outer disc stretch, shear and are eventually stripped on timescales of a few time s 108 yr, after which time, mass and angular momentum loss effectly cease. For inclined galaxies, these effects are considerably modified over the sam e time-scale. The amount of mass loss is reduced. In addition, we find that a higher galaxy tilt couples the wind and the rotating disc, and produces a higher degree of angular momentum removal. Temperature and line-of-sight velocity maps from several of the simulations are presented for comparison with observation. When the mass loss and annealing processes go to completion, we find that t he total amount of mass lost from a fixed target galaxy is well-fitted by a simple power-law function of a dimensionless parameter that combines the r am pressure and internal properties of the galaxy. Ramifications for the cl uster galaxy evolution are discussed.