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 19 mag 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 log r(e) and +/-0.12 in log[I](e). The combination log r(e) + 0.82 log[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, sigma. The typical uncertainty is +/-0.023 in log sigma. The data are used to establish the FP, log r(e) = alpha log sigma + beta log[I](e) + gamma, for the clusters. The FP for these two clusters adds important knowledge about the properties of E and SO 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 a 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 +/- 7 per 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 Delta logM/L-r = (-0.26 +/- 0.06)Delta z for q(0) = 0.5. This change in the M/L ratio is equivalent to the absolute magnitudes changing as Delta M-rT = (-0.65 +/- 0.15)Delta z. 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 SO galaxies. However, possible star formation in E and SO galaxies during the last 3-4 Gyr of the history of the Universe, as well as selection effects, complicates the interpretation of the data.