Intracluster Planetary Nebulae
  We review the progress of research on intracluster planetary nebulae (IPN). In the past five years, hundreds of IPN candidates have been detected in the Virgo and Fornax galaxy clusters and searches are also underway in poorer galaxy groups. From the observations to date, and applying the known properties of extragalactic planetary nebulae, the intracluster light in Virgo and Fornax: 1) is significant, at least 20% of the total cluster stellar luminosity, 2) is elongated in Virgo along our line of sight, and 3) may derive from lower-luminosity galaxies, consistent with some models of intracluster star production. A fraction of IPN candidates are not true IPN, but emission-line sources of very large observed equivalent width (> 200 A). The most likely source for these contaminating objects are Lyman-alpha galaxies at z ~ 3.1. Follow-up spectroscopy of the IPN candidates will be crucial to discriminate against high redshift galaxies and to derive the velocity field of the intracluster stellar population. 
John J. Feldmeier