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Session 3 - Evolution of Galaxies & Clusters of Galaxies.
Display session, Monday, June 09
South Main Hall,
We present the Fundamental Plane (FP), \log r_e =\alpha \log \sigma + \beta \log <\!I\!>_e + \gamma, for E and S0 galaxies in the rich clusters Abell 665 and Abell 2218. Both clusters have a redshift of 0.18. The FP for A665 and A2218 adds important information about the galaxy evolution during the last \approx20% of the age of the Universe. The FP has not previously been established for clusters with redshifts from 0.05 to 0.3, while recent studies have been published for the clusters CL0024+16, CL1358+62 and MS2053-04 with redshifts between 0.3 and 0.6 (equivalent to the last 35% and 50% of the age of the Universe). We have compared the FP for A665 and A2218 to that of the clusters CL0024+16, CL1358+62, MS2053-04 and Coma. The scatter around the FP is similar for all six clusters. We find that the FP for the intermediate redshift clusters has a smaller coefficient ``\alpha'' than found for Coma and other nearby clusters. This may either be due to severe selection effects for the intermediate redshift clusters, differences in the FP related to the cluster environment, or differences in the evolution of low luminosity galaxies and high luminosity galaxies. The mass-to-light (M/L) ratio as measured by the FP changes with redshift. For photometry corrected to Gunn r in the rest frames of the clusters we find \Delta \log M/L_r \approx -0.3 \Delta z; in agreement with previous results based on Coma and clusters with redshifts from 0.3 to 0.6. For q_0=0.15 the results are consistent with passive evolution of a stellar population formed at a redshift larger than two. For q_0=0.5 the formation redshift must be considerably larger. However, the possible presence of more recent brusts of star formation as well as the selection effects complicates the interpretation of the data.
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