Narrowband imaging in [O III] and Hα to search for intracluster planetary nebulae in the Virgo Cluster

We have identified intracluster planetary nebulae ( PNs) in a Virgo Cluster core field by imaging with the Subaru Suprime-Cam through two narrowband filters centered at the redshifted wavelengths of the [O III] lambda5007 and the Halpha 6563 Angstrom lines; broadband images in V and R were acquired to check for emission in the adjacent continuum. Emission-line objects in Virgo are then selected from the two-color diagram [O III]-Halpha versus [O III]-(V+R), which is calibrated using PNs in M84 ( Jacoby et al.). Using both [O III] and Halpha allows us to distinguish bona de planetary nebulae from high-redshift emission-line galaxies at the bright end of the [O III] luminosity function. Spectroscopic observations of a subsample of these objects were made at the Telescopio Nazionale Galileo and at the Very Large Telescope in a region around M84 and in an intracluster field, respectively. The observations confirm the efficiency of the combined [O III]+Halpha imaging to identify true PNs. We also obtained the first spectrum of an intracluster PN that shows the [O III] doublet with a signal-to-noise ratio greater than 10 and its H emission. From the results based on the spectroscopic follow-up, we derive a lower limit to the fraction of the Virgo Cluster light contributed by the intracluster stars at the surveyed position in the cluster core: it amounts to 10%.