A Measurement of the Contamination in [ O III ] λ 5007 Surveys of Intracluster Stars and the Surface Density of z = 3 . 13 Ly α Galaxies

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) 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 (1998). The implication is that background galaxies only account for ∼ 20% of the planetary nebula candidates in Virgo’s intracluster fields. Subject headings: galaxies: clusters: general — galaxies: interactions — planetary nebulae: general — cosmology: observations — galaxies: evolution — galaxies: formation