Narrow band imaging in [OIII] and Halpha to search for ICPNe in the Virgo cluster

We have identified intracluster planetary nebulae in a Virgo cluster core field by imaging with the Subaru Suprime-Cam through two narrow band filters centered at the redshifted wavelengths of the [OIII] λ = 5007 ˚A and the H α λ = 6563 ˚A lines; broad-band 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 ([OIII] – H α ) vs. ([OIII] – (V+R)), which is calibrated using PNe in M84 (Jacoby et al. 1990). Using both [OIII] and H α allows us to distinguish bona-fide planetary nebulae from high redshift emission-line galaxies at the bright end of the [OIII] luminosity function. Spectroscopic observations of a subsample of these objects were made at the TNG and at the VLT, in a region around M84 and in an intracluster field respectively. The observations confirm the efficiency of the combined [OIII]+H α imaging to identify true PNe. We also obtained the first spectrum of an intracluster PN which shows the [OIII] doublet with S/N > 10 and its H α emission. From the results based on the spectroscopic follow-up, we derive a lower limit to the fraction of the Virgo cluster light con-tributed by the intracluster stars at the surveyed position in the cluster core: it amounts to 10%.