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Session 21 - Galaxies and Clusters of Galaxies.
Oral session, Monday, June 10
Humanities 3650,

[21.06] Color Evolution in Moderate-Redshift Galaxy Clusters

S. A. Stanford (IGPP/Livermore), P. R. M. Eisenhardt (JPL), M. E. Dickinson (STScI)

We have obtained optical--IR imaging of \sim40 galaxy clusters in the range 0.02 < z < 0.9. Our sample traces cluster galaxy properties over nearly half a Hubble time. The imaging typically reaches 2 mag below K_\ast, yielding \sim50--100 galaxies per cluster. We are using this dataset in conjunction with archival WFPC2 imaging to investigate color evolution of the elliptical galaxy population. Analysis of a subset of the clusters has been carried out using IR--selected galaxy samples. After field--correction, we calculated median optical--IR colors of the morphologically--identified ellipticals. The colors are then zeropointed to the Coma cluster using our own UBVRIzJHK photometry of an IR--selected sample. The resulting observed--frame color differences represent change from the present epoch, largely independent of model--dependent k--corrections. We find a bluing trend with redshift in the optical--IR colors, in general agreement with the standard passive evolution scenario in which ellipticals are a coeval population formed at z > 5.

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