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Title:
The Formation Epoch of Early-Type Galaxies in the z ~ 0.9 Cl 1604 Supercluster
Authors:
Homeier, N. L.; Mei, S.; Blakeslee, J. P.; Postman, M.; Holden, B.; Ford, H. C.; Bradley, L. D.; Demarco, R.; Franx, M.; Illingworth, G. D.; Jee, M. J.; Menanteau, F.; Rosati, P.; van der Wel, A.; Zirm, A.
Affiliation:
AA(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AB(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AC(Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814.), AD(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218.), AE(UCO/Lick Observatory, University of California, Santa Cruz, CA 95064.), AF(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AG(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AH(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AI(Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, Netherlands.), AJ(UCO/Lick Observatory, University of California, Santa Cruz, CA 95064.), AK(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AL(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AM(European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany.), AN(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.), AO(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.)
Publication:
The Astrophysical Journal, Volume 647, Issue 1, pp. 256-264. (ApJ Homepage)
Publication Date:
08/2006
Origin:
UCP
ApJ Keywords:
galaxies: clusters: individual (Cl 1604), Galaxies: Elliptical and Lenticular, cD, Galaxies: Evolution
DOI:
10.1086/505262
Bibliographic Code:
2006ApJ...647..256H

Abstract

We analyze the cluster color-magnitude relation (CMR) for early-type galaxies in two of the richer clusters in the z~0.9 supercluster system to derive average ages and formation redshifts for the early-type galaxy population. Both clusters were observed with the Advanced Camera for Surveys on board the Hubble Space Telescope through the F606W and F814W filters, which brackets the rest-frame 4000 Å break at the cluster redshifts of z~0.9. We fit the zero point and slope of the red cluster sequence and model the scatter about this relation to estimate average galaxy ages and formation redshifts. We find intrinsic scatters of 0.038-0.053 mag in (V606-I814) for the E and E+S0 populations, corresponding to average ages of 3.5-3.7 Gyr and formation redshifts zf=2.4-2.6. We find at least one significant difference between the Cl 1604+4304 and Cl 1604+4321 early-type CMRs. Cl 1604+4321, the less X-ray-luminous and massive of the two, lacks bright L* elliptical galaxies. We combine the galaxy samples to fit a composite CMR down to 0.15L* and find that the slope of the combined cluster CMR is significantly steeper than for RX J0152.7-1357 but consistent with MS 1054-03, both at similar redshift. The slope of the Cl 1604 CMR at the bright end (L>0.5L*) is flatter and consistent with the CMR slopes found for other high-redshift clusters. We find evidence for increasing scatter with increasing magnitude along the early-type CMR, consistent with a ``downsizing'' scenario, indicating younger mean ages with decreasing galaxy mass.
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