The homogeneity of spheroidal populations in distant clusters

The small scatter observed for the (U-V) colors of spheroidal galaxies in nearby clusters of galaxies provides a powerful constraint on the history of star formation in dense environments. However, with local data alone, it is not possible to separate models where galaxies assembled synchronously over redshifts 0<z<1 from ones where galaxies formed stochastically at much earlier times. Here we attempt to resolve this ambiguity via high-precision rest-frame UV-optical photometry of a large sample of morphologically selected spheroidal galaxies in three z similar to 0.54 clusters that have been observed with the Hubble Space Telescope (HST). We demonstrate the robustness of using the HST to conduct the morphological separation of spheroidal and disk galaxies at this redshift and use our new data to repeat the analysis conducted locally at a significant look-back time. We find a small scatter (<0.1 mag rms) for galaxies classed as E's and E/SO's, both internally within each of the three clusters and externally from cluster to cluster. We do not find any trend for the scatter to increase with decreasing luminosity down to L similar to L-V*+3, other than can be accounted for by observational error. Neither is there evidence for a distinction between the scatter observed for galaxies classified as ellipticals and SO. Our result provides a new constraint on the star formation history of cluster spheroidals prior to z similar or equal to 0.5 confirming and considerably strengthening the earlier conclusions. Most of the star formation in the elliptical galaxies in dense clusters was completed before z similar or equal to 3 in conventional cosmologies. Although we cannot rule out the continued production of some ellipticals, our results do indicate an era of initial star formation consistent with the population of star-forming galaxies recently detected beyond z similar or equal to 3.