2002ApJ...566..784C

We present two pieces of evidence supporting the hypothesis that the bright [O III] λ5007 sources found in Virgo's intracluster space are intracluster planetary nebulae rather than [O II] λ3727 galaxies at z∼0.35 or Lyα sources at z∼3.13. First, we confirm the nature of five ``overluminous'' [O III] sources that are postulated to lie in front of M87: by examining the composite spectrum of these objects, we show that the weaker [O III] line at λ4959 is definitely present at a strength ~(1)/(3) that of [O III] λ5007. The ratio demonstrates that, at most, only one of the five objects is a background galaxy. We then estimate the surface density of background emission-line objects by conducting a wide-field (0.13 deg<sup>2</sup>) search at λ5019 for faint emission-line sources in a ``blank field'' located well away from any galaxy or cluster. We show that the density of blank-field emission-line sources is significantly lower than the density of sources detected between the galaxies of Virgo but in good agreement with the density of Lyα galaxies found by Hu, Cowie, & McMahon. The implication is that background galaxies account for only ∼20% of the planetary nebula candidates in Virgo's intracluster fields.