Anywhere between ~10% to ~70% of the stars in a rich galaxy cluster may not belong to any galaxy at all, but be free-floating in intergalactic space. By observing their positions, metallicities, and kinematics, astronomers can perform galaxy cluster archaeology, in much the same way as they do in the Milky Way.
The Virgo cluster is likely to contain tens of thousands of planetary nebulae, whose radial velocities and metallicities can be measured with deep, careful spectroscopy.
Publications
Palladino, L.E., Holley-Bockelmann, K., Morrison, H., Durrell, P.R., Ciardullo, R., Feldmeier, J., Wade, R.A., Kirkpatrick, J.D., & Lowrance, P. 2012, "Identifying High-metallicity M Giants at Intragroup Distances with Sloan Digital Sky Survey," A. J., 143, 128 [ADS - Abstract and Full Paper] Doherty, M., Arnaboldi, M., Das, P., Gerhard, O., Aguerri, J.A.L., Ciardullo, R., Feldmeier, J.J., Freeman, K.C., Jacoby, G.H., & Murante, G. 2009, "The Edge of the M87 Halo and the Kinematics of the Diffuse Light in the Virgo Cluster Core," Astr. & Astrop. 502, 771 [ADS Entry]
Arnaboldi, M., Doherty, M., Gerhard, O., Ciardullo, R., Aguerri, J.A.L., Feldmeier, J.J., Freeman, K.C., & Jacoby, G.H. 2008, "Expansion Velocities and Core Masses of Bright Planetary Nebulae in the Virgo Cluster" Ap. J. Letters, 674, L17 [ADS Entry]
Williams, B.F., Ciardullo, R., Durrell, P.R., Vinciguerra, M., Feldmeier, J.J., Jacoby, G.H., Sigurdsson, S., von Hippel, T., Ferguson, H.C., Tanvir, N.R., Arnaboldi, M., Gerhard, O., Aguerri, J.A.L., & Freeman, K.C. 2007, "The Metallicity Distribution of Intracluster Stars in Virgo" Ap. J., 656, 756 [ADS Entry]
Williams, B.F., Ciardullo, R., Durrell, P.R., Feldmeier, J.J., Sigurdsson, S., Vinciguerra, M., Jacoby, G.H., von Hippel, T., Ferguson, H.C., Tanvir, N.R., Arnaboldi, M., Gerhard, O., Aguerri, J.A.L., & Freeman, K.C. 2007, "Virgo's Intracluster Globular Clusters as Seen by the Advanced Camera for Surveys" Ap. J., 654, 835 [ADS Entry]
Feldmeier, J.J., Ciardullo, R., Jacoby, G.H., & Durrell, P.R. 2004, "Intracluster Planetary Nebulae in the Virgo Cluster. III. Luminosity of the Intracluster Light and Tests of the Spatial Distribution" Ap. J., 615, 196 [ADS Entry]
Feldmeier, J.J. Ciardullo, R., Jacoby, G.H., & Durrell, P.R. 2003, "Intracluster Planetary Nebulae in the Virgo Cluster. II. Imaging Catalog" Ap. J. Supp., 145, 65 [ADS Entry]
Durrell, P.R., Ciardullo, R., Feldmeier, J.J., Jacoby, G.H., & Sigurdsson, S. 2002, "Intracluster Red Giant Stars in the Virgo Cluster" Ap. J., 570, 119 [ADS Entry]
Ciardullo, R., Feldmeier, J.J., Krelove, K., Jacoby, G.H., & Gronwall, C. 2002, "A Measurement of the Contamination in [O III] 5007 Surveys of Intracluster Stars and the Surface Density of z=3.13 Ly-alpha Galaxies," Ap. J., 566, 784 [ADS Entry]
Feldmeier, J.J., Ciardullo, R., & Jacoby, G.H. 1998, "Intracluster Planetary Nebulae in the Virgo Cluster. I. Initial Results," Ap. J., 503, 109 [ADS Entry]
Feldmeier, J.J., Ciardullo, R., Jacoby, G.H., & Bartlett R. 1998, "Intracluster Stars and the Galactic Halos of Virgo," published in Galactic Halos: A UC Santa Cruz Workshop, ASP Conference Series, 136, 87 [ADS Entry]
Ciardullo, R., Jacoby, G.H., Feldmeier, J.J., & Bartlett, R. 1998, "The Planetary Nebula Luminosity Function of M87 and the Intracluster Stars of Virgo," Ap. J., 492, 62 [ADS Entry]