Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z ∼ 0.5) clusters observed with WFPC-2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the "morphology-density" relationship, T − Σ , for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the T − Σ relation is strong in centrally concentrated "regular" clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low redshift clusters where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction.Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters, and occurs instead in the loose-group phase or even earlier, but S0's are generated in large numbers only after cluster virialization.

We present a striking new Hubble Space T elescope (HST ) observation of the rich cluster Abell 2218 taken with the Wide Field Planetary Camera 2. HST Ïs restored image quality reveals a sizable number of gravitationally lensed features in this cluster, signiÐcantly more than had been identiÐed by using ground-based telescopes. The brightest arcs are resolved by HST and show internal features that enable us to identify multiply imaged examples, conÐrming and improving the mass models of the cluster determined from ground-based observations. Although weak lensing has been detected statistically in this and other clusters from ground-based data, the superlative resolution of HST enables us to individually identify weakly distorted images more reliably than hitherto, with important consequences for their redshift determination. Using an improved mass model for the cluster calibrated with available spectroscopy for the brightest arcs, we demonstrate how inversion of the lensing model can be used to yield the redshift distribution of D80 faint arclets to R^25. We present a new formalism for estimating the uncertainties in this inversion method and review prospects for interpreting our results and verifying the predicted redshifts.

The small scatter observed for the (U[V ) colors of spheroidal galaxies in nearby clusters of galaxies provides a powerful constraint on the history of star formation in dense environments. However, with local data alone, it is not possible to separate models where galaxies assembled synchronously over redshifts 0 \ z \ 1 from ones where galaxies formed stochastically at much earlier times. Here we attempt to resolve this ambiguity via high-precision rest-frame UVÈoptical photometry of a large sample of morphologically selected spheroidal galaxies in three z D 0.54 clusters that have been observed with the Hubble Space T elescope (HST ). We demonstrate the robustness of using the HST to conduct the morphological separation of spheroidal and disk galaxies at this redshift and use our new data to repeat the analysis conducted locally at a signiÐcant look-back time. We Ðnd a small scatter (\0.1 mag rms) for galaxies classed as EÏs and E/S0Ïs, both internally within each of the three clusters and externally from cluster to cluster. We do not Ðnd any trend for the scatter to increase with decreasing luminosity down to other than can be accounted for by observational error. Neither is there evidence for a L D L V * ] 3, distinction between the scatter observed for galaxies classiÐed as ellipticals and S0. Our result provides a new constraint on the star formation history of cluster spheroidals prior to z^0.5 conÐrming and considerably strengthening the earlier conclusions. Most of the star formation in the elliptical galaxies in dense clusters was completed before z^3 in conventional cosmologies. Although we cannot rule out the continued production of some ellipticals, our results do indicate an era of initial star formation consistent with the population of star-forming galaxies recently detected beyond z^3.

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We present the most extensive combined photometric and spectroscopic study to date of the enormous globular cluster (GC) system around M87, the central giant elliptical galaxy in the nearby Virgo cluster. Using observations from DEIMOS and LRIS at Keck, and Hectospec on the MMT, we derive new, precise radial velocities for 451 GCs around M87, with projected radii from ∼ 5 to 185 kpc. We combine these measurements with literature data for a total sample of 737 objects, which we use for a re-examination of the kinematics of the GC system of M87. The velocities are analyzed in the context of archival wide-field photometry and a novel Hubble Space Telescope catalog of half-light radii, which includes sizes for 344 spectroscopically confirmed clusters. We use this unique catalog to identify 18 new candidate ultra-compact dwarfs, and to help clarify the relationship between these objects and true GCs.We find much lower values for the outer velocity dispersion and rotation of the GC system than in earlier papers, and also differ from previous work in seeing no evidence for a transition in the inner halo to a potential dominated by the Virgo cluster, nor for a truncation of the stellar halo. We find little kinematical evidence for an intergalactic GC population. Aided by the precision of the new velocity measurements, we see significant evidence for kinematical substructure over a wide range of radii, indicating that M87 is in active assembly. A simple, scale-free analysis finds less dark matter within ∼ 85 kpc than in other recent work, reducing the tension between X-ray and optical results. In general, out to a projected radius of ∼ 150 kpc, our data are consistent with the notion that M87 is not dynamically coupled to the Virgo cluster; the core of Virgo may be in the earliest stages of assembly.29 These four GCs are S279, S348, S501, and VUCD10. The fifth, S1265, has a normal GC size, and is the closest of the five objects to vsys, but intriguingly has a very similar distance, color, magnitude, and velocity to the bona fide UCD S1629. Also, two bright transition objects that fit the same pattern are S77 and S137, which have similar positions, velocities (from Cohen 2000 andHanes et al. 2001), colors, magnitudes, and sizes to each other. These seem likely to share a common origin.