A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) XIX. The discovery of a spectacular 230 kpc Hα tail following NGC 4569 in the Virgo cluster

Context. Galaxies fly inside galaxy clusters and ram pressure by the intracluster medium (ICM) can remove a large amount of the interstellar medium (ISM) from the galaxy, and deposit the gas in the ICM. The ISM decoupled from the host galaxy leaves a long trail following the moving galaxy. Such long trails track galaxy motions and they can be detected with sensitive data in H alpha. Aims. We study the H alpha tail trailing NGC 4569 in the Virgo cluster. Methods. The initial discovery was made with the deep H alpha imaging data with CFHT from the VESTIGE project. The follow-up spectroscopic observations were carried out with APO/DIS, MMT/Binospec, and CFHT/SITELLE. Results. Aside from the known 80 kpc H alpha tail downstream of NGC 4569, the deep H alpha imaging data allow the H alpha tail detected to at least 230 kpc from the galaxy. More importantly, we confirmed the H alpha clumps implied from the imaging data with spectroscopic data. The H alpha clumps show a smooth radial velocity gradient across similar to 1300 km/s, eventually reaching the velocity of the cluster. Conclusions. For the first time, this discovery offers a view of the full deceleration process of the stripped ISM. This discovery also showcases the potential with wide-field H alpha survey on galaxy clusters to discover intracluster optical emission-line clouds originated from cluster galaxies. These clouds provide kinematic tracers to the infall history of cluster galaxies and the turbulence in the ICM. They also serve as excellent multi-phase objects for studying the classical cloud-crushing problem and other important physical processes.