NGC 4654: Gravitational interaction or ram pressure stripping?

The Virgo cluster spiral galaxy NGC 4654 is supposed to be a good candidate for ongoing ram pressure stripping based on its very asymmetric HI distribution. However, this galaxy also shows an asymmetric stellar distribution. Numerical simulations using ram pressure as the only perturbation can produce a tail structure of the gas content, but cannot account for its kinematical structure. It is shown that a strong edge-on stripping event can produce an asymmetric stellar distribution up to 800 Myr after the stripping event, i.e. the galaxyʹs closest passage to the cluster center. Simulations using a gravitational interaction with the companion galaxy NGC 4639 can account for the asymmetric stellar distribution of NGC 4654, but cannot reproduce the observed extended gas tail. Only a mixed interaction, gravitational and ram pressure, can reproduce all observed properties of NGC 4654. It is concluded that NGC 4654 had a tidal interaction ~500 Myr ago and is continuing to experience ram pressure until the present day.