Evolution in the color-magnitude relation of early-type galaxies in clusters of galaxies at z≃1

We present a study of the color evolution of elliptical and S0 galaxies in six clusters of galaxies inside the redshift range 0.78 < z < 1.27. For each cluster, we used imaging from the Hubble Space Telescope (HST) to determine morphological types of the galaxies. These types were determined both by an automated technique and from visual inspection. We performed simulations to determine the accuracy of the automated classifications and found a success rate of similar to75% at m(L-*) or brighter magnitudes for most of our HST imaging data with the fraction of late-type galaxies identified as early-type galaxies to be similar to 10% at m(L-*) to similar to 20% at m(L-* + 2). From ground-based optical and near-infrared imaging, we measured the zero point and scatter in the color-magnitude relation of the elliptical and S0 galaxy populations, which we combine with the 1998 sample of Stanford et al., yielding a sample of cluster early-type galaxies that span a look-back time of almost 9 Gyr from the present. We see the colors of the early-type cluster members become bluer with increasing redshift, as expected from passively evolving stellar populations. We fitted a set of models to the change in the color as a function of redshift, with the best-fitting values ranging from a formation redshift of 3(-1)(+2) to 5(-3), depending on the specific model used, though we find no dependence for the formation epoch on the metallicity of the populations. The large scatter in resulting formation epochs, which depends on the details of the models used, implies that the oldest stars in the elliptical galaxies appear to have formed at redshifts of z > 3. We find possible evolution in the scatter of the colors, with some high-redshift clusters showing scatter as small as the Coma Cluster but others showing much larger scatter. Those clusters with a small scatter imply either a formation redshift of at least z similar to 3 or a smaller spread in the range of formation redshifts at lower redshifts, assuming a Gaussian distribution of star formation around the mean epoch.