The evolution of cluster E and S0 galaxies measured from the Fundamental Plane

Abstract

Photometry has been obtained for magnitude-limited samples of galaxies in the rich clusters Abell 665 (37 galaxies) and Abell 2218 (61 galaxies). Both clusters have a redshift of 0.18. The limiting magnitude of the samples is 19mag in the I band. Spectroscopy has been obtained for seven galaxies in A665 and nine galaxies in A2218, all of which also have available photometry. Spectroscopy has been obtained for two additional galaxies in A2218, one of which is a background galaxy. Effective radii r_e and mean surface brightnesses <I>_e are derived from the photometry. The typical uncertainties are +/-0.078 in logr_e and +/-0.12 in log<I>_e. The combination logr_e+0.82log<I>_e that enters the Fundamental Plane (FP) has an uncertainty of only +/-0.018. The spectroscopy was used for measurements of the velocity dispersions, σ. The typical uncertainty is +/-0.023 in logσ. The data are used to establish the FP, logr_eαlogσ+βlog<I>_e+γ, for the clusters. The FP for these two clusters adds important knowledge about the properties of E and S0 galaxies in the relatively unexplored redshift interval 0.05 to 0.3. We have compared the FP for A665 and A2218 with the FP for the clusters CL0024+16, CL1358+62 and MS2053-04 with redshifts between 0.33 and 0.58, and with the FP for the Coma cluster. 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 α than found for the Coma cluster and other nearby clusters. This may be caused either by selection effects for the intermediate-redshift clusters or by differences in the evolution of low-luminosity galaxies and high-luminosity galaxies. The mass-to-light (M/L) ratios, as measured from the FP, change with redshift. At z=0.18 the M/L ratio for photometry in Gunn r in the rest frame of the clusters is 11+/-7per cent smaller than for the Coma cluster. Using the data for A665 and A2218, together with the previously published data for CL0024+16, CL1358+62 and MS2053-04, we find that the M/L ratios for photometry calibrated to Gunn r change with redshift as ΔlogML_r=(-0.26+/-0.06)Δz for q_0=0.5. This change in the M/L ratio is equivalent to the absolute magnitudes changing as ΔM_{r_T}=(-0.65+/-0.15)Δz. These new results are consistent with the previously published analysis for CL0024+16, CL1358+62 and MS2053-04. For q_0=0.5 the results are consistent with passive evolution of a stellar population which formed at a redshift larger than 5. For q_0=0.15 the formation redshift must be larger than 1.7. Our new data for A665 and A2218 confirm the gradual and slow evolution of the bright E and S0 galaxies. However, possible star formation in E and S0 galaxies during the last 3-4Gyr of the history of the Universe, as well as selection effects, complicates the interpretation of the data.