The Evolution of Red Galaxies in Clusters T z=0.5

A program was carried out at Palomar Observatory to study seven clusters of galaxies all at redshifts near 0. 5. Observations consisted of g, r, i, and z broadband Gunn photometry using 4-shooter, and ultraviolet u and violet v photometry using a CCD photometer at the prime focus of the Hale 5 m telescope. The u and v filters reach rest wavelengths of about 2300 A at z = 0. 5. Spectra were also obtained of a selection of objects in each cluster. An absolute energy distribution for the nearby giant elliptical galaxy NGC 4889 in the Coma cluster was obtained which, when combined with IUE observations of ellipticals, provided a good reference for present-day giant ellipticals. Observations were compared with models of Bruzual and Charlot. From measured fluxes above the H and K break at 4000A it is found that there is a change in the rest g- r and r-i colors of 0. 11 and 0. 07, respectively, between redshift z=0 and z=0. 5. At z=0 the age of the elliptical galaxy NGC 4889 is 14+/-2 Gyr while at z = 0. 5 the age is 10 Gyr with a possible range from 8 to 13 Gyr. Ages here are measured from the time when star formation begins in the galaxy. The ratio of these ages is just compatible with standard cosmological models with LAMBDA = 0. 0. The derived ages are consistent with ages determined for globular clusters; this is not surprising since the galaxy and globular cluster evolutionary ages are both based on stellar evolution calculations. The luminosities of brightest cluster members at z = 0. 5 are brighter than at present by about 0. 3 mag; the fitted models predict a brightening by 0. 25-0. 71 mag. It is found that for the majority of red galaxies at z = 0. 5 there is substantial color evolution between rest wavelengths of 2300 and 3000 A compared with the present epoch. This amounts to an increase in brightness of up to 2 mag at u and 1 mag at v and implies that there was still substantial star formation at z = 0. 5 compared with the present. As found in other studies, the red galaxies are more concentrated to the cluster center than bluer objects.