Intracluster Stars and the Galactic Halos of Virgo

Planetary nebulae can be useful dynamical probes of elliptical galaxy halos, but progress thus far has been hampered by the small data samples available for study. In order to obtain a large sample of planetary nebulae for dynamical analysis, we have re-observed the giant elliptical galaxy M87 in the center of the Virgo Cluster. Surprisingly, we find that the M87 sample is contaminated by a significant fraction of intracluster objects. Follow up observations of three blank fields in the cluster confirm that a significant fraction of Virgo’s starlight is intracluster. We discuss the implications of intracluster stars for studies of galaxy halos in clusters. 1. Planetary Nebulae as probes of galactic halos: Measuring the dark matter halos of elliptical galaxies is a difficult task. Unlike disk galaxies, there are few dynamical tracers in ellipticals that can be observed over large ranges in distance. Stellar absorption features and emission line gas disks cannot be used much beyond two effective radii (re), and although X-ray mass determinations can be invaluable at large radii, currently X-ray observations cannot reach the centers of ellipticals, due to instrumental resolution, and potential complications due to cooling flows. Planetary nebulae (PN) are objects that are extremely well suited to bridge the gap between inner and outer dynamical methods. Individual PN can be easily detected at several re with 4-m class telescopes out to distances of ∼ 20 Mpc., and samples already exist for over twenty elliptical galaxies. Since PN emit a large fraction (∼ 15%) of their flux in a single narrow emission line at 5007 Å, their velocities can be determined with a short duration spectrum at moderate, λ/∆λ ∼ 5000 resolution. Finally, PN are excellent distance indicators Visiting Astronomer, Kitt Peak National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. NSF Young Investigator