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

Abstract

We present new spatially resolved Keck spectroscopy of early-type galaxies in three galaxy clusters at z ≈ 0.5. We focus on the fundamental plane (FP) relation and combine the kinematics with structural parameters determined from HST images. The galaxies obey clear FP relations, which are offset from the FP of the nearby Coma Cluster due to passive evolution of the stellar populations. The z ≈ 0.5 data are combined with published data for 11 additional clusters at 0.18 ≤ z ≤ 1.28, to determine the evolution of the mean M/L_B ratio of cluster galaxies with masses M ≳ 10^(11) M_☉, as implied by the FP. We find d log(M/L_B)/dz = -0.555 ± 0.042, stronger evolution than was previously inferred from smaller samples. The observed evolution depends on the luminosity-weighted mean age of the stars in the galaxies, the IMF, selection effects due to progenitor bias, and other parameters. Assuming a normal IMF but allowing for various other sources of uncertainty, we find z_* = 2.01^(+0.22)_(-0.17) for the luminosity-weighted mean star formation epoch. The main uncertainty is the slope of the IMF in the range 1-2 M_☉: we find z_* = 4.0 for a top-heavy IMF with slope x = 0. The M/L_B ratios of the cluster galaxies are compared to those of field early-type galaxies at 0.32 ≤ z ≤ 1.14. Assuming that progenitor bias and the IMF do not depend on environment, we find that the present-day age of stars in massive field galaxies is 4.1% ± 2.0% (≈0.4 Gyr) less than that of stars in massive cluster galaxies. This relatively small age difference is surprising in the context of expectations from "standard" hierarchical galaxy formation models and provides a constraint on the physical processes that are responsible for halting star formation in the progenitors of today's most massive galaxies.

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