Disturbed spiral galaxies with high rates of star formation pervaded clusters of galaxies just a few billion years ago, but nearby clusters exclude spirals in favor of ellipticals. "Galaxy harassment" (frequent high speed galaxy encounters) drives the morphological transformation of galaxies in clusters, provides fuel for quasars in subluminous hosts and leaves detectable debris arcs. Simulated images of harassed galaxies are strikingly similar to the distorted spirals in clusters at z ∼ 0.4 observed by the Hubble Space Telescope.Clusters of galaxies are unique cosmological laboratories. There are several hundred galaxies moving at relative velocities up to several thousand km s −1 in regions no larger than the distance between the Milky Way and its nearest neighbor, the Andromeda galaxy (M31). Clusters of galaxies have been observed at redshifts up to 2 1 . By understanding their evolution over cosmic times, we probe the geometry of the Universe and the development of its largest structures.Nearby rich galaxy clusters are dominated by elliptical "E" and lenticular "S0" galaxies 2 , mostly low luminosity dwarfs. Twenty years ago Butcher and Oemler 3,4 discovered that clusters at z ∼ > 0.4 have a substantial population of "blue galaxies" seen only as fuzzy blobs in their ground based images. Recent Hubble Space Telescope (HST) images revealed that the "fuzzy blue blobs" are low luminosity, often disturbed, spiral galaxies "Sp" 5−8 . The HST imaging teams stress that the disturbed blue galaxies are ubiquitous, but few have other galaxies nearby 5 and there were multiple bursts of star formation spanning up to 2 Gyr 8 . The dramatic transformation of clusters (shown in Figure 1) occured during a "look-back time" of just 4-5 billion years, only a few cluster orbital times. In contrast, the morphological fraction in the field shows far less evolution 9 . Given a mechanism for distorting galaxies and promoting star formation that operates when a spiral first enters a cluster, hierarchical clustering models will naturally enhance the number of "Butcher-Oemler clusters" at z ∼ 0.4 13 . Proposed mechanisms include: mergers 14,15 , compression of gas in the high pressure cluster environment 16,17 and tidal compression by the cluster 18,19 . Each of these scenarios can produce star-bursts, but none address morphological evolution or identify the remnants of these distorted blue galaxies in present day clusters. By analyzing their HST images, Oemler et al 20 conclude that merging is implausible as the blue galaxy fraction is large and the merging probability is low. They observe disturbed spirals throughout the cluster, whereas both ram pressure stripping and global tides will only operate efficiently near the cluster's center.What mechanism drives star-bursts and rapid morphological evolution throughout a cluster of galaxies? Although direct mergers are extremely rare, every galaxy experiences a high speed close encounter with a bright galaxy once per Gyr. Here, "close" means within 50 kiloparsecs (several optical radii) ...

The violent star formation history of ``E+A'' galaxies and their detection almost exclusively in distant clusters is frequently used to link them to the ``Butcher-Oemler effect'' and to argue that cluster environment influences galaxy evolution. From 11113 spectra in the Las Campanas Redshift Survey, we have obtained a unique sample of 21 nearby ``E+A" galaxies. Surprisingly, a large fraction (about 75%) of these ``E+A''s lie in the field. Therefore, interactions with the cluster environment, in the form of the ICM or cluster potential, are not essential for ``E+A'' formation. If one mechanism is responsible for ``E+A''s, their existence in the field and the tidal features in at least 5 of the 21 argue that galaxy-galaxy interactions and mergers are that mechanism. The most likely environments for such interactions are poor groups, which have lower velocity dispersions than clusters and higher galaxy densities than the field. In hierarchical models, groups fall into clusters in greater numbers at intermediate redshifts than they do today. Thus, the Butcher-Oemler effect may reflect the typical evolution of galaxies in groups and in the field rather than the influence of clusters on star formation in galaxies. This abstract is abridged.Comment: 39 uuencoded, compressed pages (except Fig 1), complete preprint at ftp://ociw.edu/pub/aiz/eplusa.ps, ApJ, submitte

We describe a new wide field Hubble Space Telescope survey of the galaxy cluster Cl0024+16 (z ≈ 0.4) consisting of a sparse-sampled mosaic of 39 Wide Field and Planetary Camera 2 images which extends to a cluster radius of ∼ 5 Mpc. Together with extensive ground-based spectroscopy taken from the literature, augmented with over a hundred newly-determined redshifts, this unique dataset enables us to examine environmental influences on the properties of cluster members from the inner core to well beyond the virial radius (∼ 1.7 Mpc). We catalog photometric measures for 22,000 objects to I 25 and assign morphological types for 2181 to I = 22.5, of which 195 are spectroscopically-confirmed cluster members. We examine both the morphology-radius (T-R) and morphology-density (T-Σ) relations and demonstrate sensitivities adequate for measures from the core to a radius of ∼ 5 Mpc, spanning over 3 decades in local projected density. The fraction of early-type galaxies declines steeply from the cluster center to 1 Mpc radius and more gradually thereafter, asymptoting towards the field value at the periphery. We discuss our results in the context of three distinct cluster zones, defined according to different physical processes that may be effective in transforming galaxy morphology in each. By treating infalling galaxies as isolated test particles, we deduce that the most likely processes responsible for the mild gradient in the morphological mix outside the virial radius are harassment and starvation. Although more data are needed to pin down the exact mechanisms, starvation seems more promising in that it would naturally explain the stellar and dynamical homogeneity of cluster E/S0s. However, we find significant scatter in the local density at any given radius outside ∼0.5 Mpc, and that the same T-Σ relation holds in subregions of the cluster, independent of location. In this hitherto unprobed region, where the potential of the cluster is weak, galaxies apparently retain their identities as members of infalling sub-groups whose characteristic morphological properties remain intact. Only upon arrival in the central regions is the substructure erased, as indicated by the tight correlation between cluster radius and Σ.

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.