Probing the History of Cluster Assembly by Intracluster Planetary Nebulae

We describe the results and the current status of our program to search for intracluster planetary nebulae (ICPNe) in the Virgo and the Coma clusters of galaxies. For the Virgo cluster, ICPNe candidates were detected with narrow band imaging combined with broad band imaging. The use of two narrow band filters for H$α$ and [O III] lines tuned for the cluster redshift enabled us to detect secure ICPNe candidates. Kinematics of the detected ICPNe confirm that the Virgo cluster is in a highly unrelaxed state. In order to detect ICPNe in the Coma cluster, which is more than five times more distant than the Virgo cluster, we devised a multi-slit imaging spectroscopy technique and successfully detected 35 secure ICPNe candidates for the first time in this cluster. We have found a hint that the Coma cluster is currently in the midst of a subcluster merger.