Astronomy and Astrophysics, volume 508, 1141-1159 (2009/12-4)
Evolution of the early-type galaxy fraction in clusters since z = 0.8.
SIMARD L., CLOWE D., DESAI V., DALCANTON J.J., VON DER LINDEN A., POGGIANTI B.M., WHITE S.D.M., ARAGON-SALAMANCA A., DE LUCIA G., HALLIDAY C., JABLONKA P., MILVANG-JENSEN B., SAGLIA R.P., PELLO R., RUDNICK G.H. and ZARITSKY D.
Abstract (from CDS):
We study the morphological content of a large sample of high-redshift clusters to determine its dependence on cluster mass and redshift. Quantitative morphologies are based on PSF-convolved, 2D bulge+disk decompositions of cluster and field galaxies on deep Very Large Telescope FORS2 images of eighteen, optically-selected galaxy clusters at 0.45<z<0.80 observed as part of the ESO Distant Cluster Survey (``EDisCS''). Morphological content is characterized by the early-type galaxy fraction fet, and early-type galaxies are objectively selected based on their bulge fraction and image smoothness. This quantitative selection is equivalent to selecting galaxies visually classified as E or S0. Changes in early-type fractions as a function of cluster velocity dispersion, redshift and star-formation activity are studied. A set of 158 clusters extracted from the Sloan Digital Sky Survey is analyzed exactly as the distant EDisCS sample to provide a robust local comparison. We also compare our results to a set of clusters from the Millennium Simulation. Our main results are: (1) the early-type fractions of the SDSS and EDisCS clusters exhibit no clear trend as a function of cluster velocity dispersion. (2) Mid-z EDisCS clusters around σ=500km/s have fet≃0.5 whereas high-z EDisCS clusters have fet≃0.4. This represents a ∼25% increase over a time interval of 2Gyr. (3) There is a marked difference in the morphological content of EDisCS and SDSS clusters. None of the EDisCS clusters have early-type galaxy fractions greater than 0.6 whereas half of the SDSS clusters lie above this value. This difference is seen in clusters of all velocity dispersions. (4) There is a strong and clear correlation between morphology and star formation activity in SDSS and EDisCS clusters in the sense that decreasing fractions of [OII] emitters are tracked by increasing early-type fractions. This correlation holds independent of cluster velocity dispersion and redshift even though the fraction of [OII] emitters decreases from z∼0.8 to z∼0.06 in all environments. Our results pose an interesting challenge to structural transformation and star formation quenching processes that strongly depend on the global cluster environment (e.g., a dense ICM) and suggest that cluster membership may be of lesser importance than other variables in determining galaxy properties.
galaxies: fundamental parameters - galaxies: evolution - galaxies: clusters: general
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