A wide field Hubble Space Telescope study of the cluster CL0024+16 at z=0.4. I: morphological distributions to 5 Mpc radius

Authors:
T. Treu (1),
R.S. Ellis (1),
J.-P. Kneib (1,2),
A. Dressler (3),
I. Smail (4),
O. Czoske (2,6),
A. Oemler (3),
P. Natarajan (5) ((1) California Institute of Technology, (2) Observatoire Midi-Pyrenees, (3) Carnegie Observatories, (4) University of Durham, (5) Yale University, (6) University of Bonn)

Comments: ApJ, in press. 26 pages including 18 figures. Version with high
resolution figures available at this http URL

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.
[Abridged] We examine both the morphology-radius (T-R) and morphology-density
(T-Sigma) 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-Sigma 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 Sigma.

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