Intracluster Red Giant Stars in the Virgo Cluster

We have used the WFPC2 camera of the Hubble Space Telescope to obtain deepF814W images of a blank field in the Virgo Cluster located 41 arcmin northwestof M87. We perform star counts in that field, and in another Virgo fieldobserved by Ferguson, Tanvir & von Hippel (1998), and show that, when comparedto the Hubble Deep Field North and South, the Virgo Cluster contains an excessof objects with magnitudes I > 27. We attribute this excess to a population ofintracluster red-giant branch (IC-RGB) stars. By modeling the luminosityfunction of these stars, we show that the tip of the Virgo RGB is at I = 27.31+0.27/-0.17 and that the cluster contains a small, but significant, excess ofstars that are up to ~1 mag brighter than this tip. If this luminous componentis due entirely to stars on the asymptotic giant branch (AGB), it implies anage for the population of > 2 Gyr; if foreground RGB stars contribute to theluminous tail, then the derived age for the stars is older still. Theluminosity function also suggests that most of the intracluster stars aremoderately metal-rich (-0.8 < [Fe/H] <-0.2), a result consistent with thatexpected from stars that have been tidally stripped from intermediateluminosity galaxies. Additionally, a comparison with the planetary nebulae inour field also supports this view, although the existence of a more metal-poorpopulation (from stripped dwarfs) cannot be ruled out. Our derived averagesurface brightness, mu_I = 27.9 +0.3/-0.5 mag/arcsec^2 for Virgo's diffusecomponent suggests that intracluster stars contribute 10% to 20% of thecluster's total I-band luminosity.Comment: 21 pages, 8 figures included, accepted for publication in the Astrophysical Journa