  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~0.54 clusters which have been observed with HST. We
demonstrate the robustness of using 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 Es and E/S0s, 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 = L*+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 S0. Our result provides a new constraint on the
star formation history of cluster spheroidals prior to z=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~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~3.
