Intracluster Planetary Nebulae as Probes of Intracluster Starlight

I review the progress in research on Intracluster Planetary Nebulae (IPN). Hundreds of IPN candidates have now been found in the Virgo and Fornax galaxy clusters, and searches of two nearby galaxy groups have been made. From the results thus far, approximately 10–20% of all stars in Virgo and Fornax are in an intracluster component, but there are few such stars in galaxy groups. From the spatial distribution of IPN, it appears that the intracluster stars are clustered, in agreement with tidal-stripping scenarios. In Virgo, the IPN have a large line-of-sight depth, which implies that the bulk of intracluster stars in this cluster derive from late-type galaxies and dwarfs. I also discuss other important developments in IPN research such as the detection of intracluster H II regions, a possible detection of IPN in the Coma Cluster, and future observational and theoretical developments.