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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., Comment: 8 pages, 4 figures; LaTeX; paspconf.sty and epsf.sty included. To appear in the Proceedings of the UC Santa Cruz Workshop on Galactic Halos (ed. D. Zaritsky)

We present the results of a wide-field [O III] $\lambda 5007$ survey for planetary nebulae (PN) in M87 and its surrounding halo. We show that the planetary nebula luminosity function (PNLF) of M87's halo is unlike any PNLF observed to date, with a shape that differs from that of the empirical law at the 99.9% confidence level. In addition, we find that the PNLF of M87's outer halo differs from that of the galaxy's inner regions at a high degree of certainty ($\sim 92%$). We show that both these effects are most likely due to the existence of intracluster PN, many of which are foreground to M87. These intracluster objects explain the ``overluminous'' [O III] $\lambda 5007$ sources previously identified by Jacoby, Ciardullo, & Ford (1990), and present us with a new tool with which to probe the morphological and dynamical properties of the cluster. By modifying the maximum likelihood procedures of Ciardullo et al (1989a) and using an assumed M31 distance of 770 Kpc (Freedman & Madore 1990), we derive a distance modulus to M87 of $30.79 \pm 0.16$ ($14.4 \pm 1.1$ Mpc)., Comment: Accepted to Astrophysical Journal. 48 pages with 8 figures, Latex. Paper also available at: http://www.astro.psu.edu/users/johnf/Text/research.html

N-body simulations were conducted of pairs of galaxies with a 3:1 mass ratio on parabolic orbits, in order to quantify the effect of dynamical friction. The effects of varying the ratio of the dark matter halo size to the distance of closest approach were explored. Once the dark matter halos are fully overlapping, the more massive simulated galaxies achieve a larger maximum separation after the first encounter despite the increased dynamical friction caused by the more extended halos. Projected separation and radial velocity histograms were generated by "observing" the simulation results at various times and from various orientations. These histograms were compared with observations of galaxy pairs (Charlton & Salpeter 1991; Chengalur, Salpeter, & Terzian 1993) with the result that large halo radii (~200 - 600 kpc) and wide distances of closest approach are generally favored. It is difficult to reconcile the small radial velocity differences that have been observed (median of ~ 30 km/s, Chengalur et al. 1993) with the simulations when we sample all parts of the orbits equally. Including an additional population of wide pairs that have just recently reached "turnabout" from the Hubble flow would lower the median velocity differences. Models suggest that additional data for pairs at intermediate separations should have a somewhat larger median velocity difference than the wide pairs. Very narrow pairs include galaxies which are interacting, and whose gaseous components respond to forces other than gravity. If consistently small $\Delta v$ are measured from neutral hydrogen velocities in a larger sample of narrow pairs, pressure forces and dissipation effects on the gaseous components could be responsible., Comment: 16 pages, 10 figures, uuencoded gz-compressed tar file. To appear in the Astrophysical Journal.