The Star Formation Epoch of the Most Massive Early‐Type Galaxies

We present new Keck spectroscopy of early-type galaxies in three galaxyclusters at z~0.5. We focus on the fundamental plane (FP) relation, and combinethe kinematics with structural parameters determined from HST images. Thegalaxies obey clear FP relations, which are offset from the FP of the nearbyComa cluster due to passive evolution of the stellar populations. The z~0.5data are combined with published data for 11 additional clusters at0.1810^11 M_sun, as implied by the FP. We finddlog(M/L(B))/dz = -0.555+-0.042, stronger evolution than was previouslyinferred from smaller samples. The observed evolution depends on theluminosity-weighted mean age of the stars in the galaxies, the initial massfunction (IMF), selection effects due to progenitor bias, and other parameters.Assuming a normal IMF but allowing for various other sources of uncertainty wefind z* = 2.01+-0.20 for the luminosity-weighted mean star formation epoch. Themain uncertainty is the slope of the IMF in the range 1-2 Solar masses: we findz* = 4.0 for a top-heavy IMF with slope x=0. The M/L(B) ratios of the clustergalaxies are compared to those of recently published samples of fieldearly-type galaxies at 0.32

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