A Wide‐Field Hubble Space Telescope Study of the Cluster Cl 0024+16 at z= 0.4. I. Morphological Distributions to 5 Mpc Radius

Abstract

We describe a new wide-field Hubble Space Telescope survey of the galaxy cluster Cl 0024+16 ( z ≈ 0.4) consisting of a sparsely sampled mosaic of 39 Wide Field Planetary Camera 2 images that 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 data set 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 toward 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 subgroups 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.