A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

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 Hα. Aims. We study the Hα tail trailing NGC 4569 in the Virgo cluster. Methods. The initial discovery was made with the deep Hα 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 Hα tail downstream of NGC 4569, the deep Hα imaging data allow the Hα tail detected to at least 230 kpc from the galaxy. More importantly, we confirmed the Hα clumps implied from the imaging data with spectroscopic data. The Hα clumps show a smooth radial velocity gradient across ∼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 Hα 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.