Luminous Red Galaxies in Simulations: Cosmic Chronometers?
  There have been a number of attempts to measure the expansion rate of the
universe at high redshift using Luminous Red Galaxies (LRGs) as "chronometers".
The method generally assumes that stars in LRGs are all formed at the same
time. In this paper, we quantify the uncertainties on the measurement of H(z)
which arise when one considers more realistic, extended star formation
histories. In selecting galaxies from the Millennium Simulation for this study,
we show that using rest-frame criteria significantly improves the homogeneity
of the sample and that H(z) can be recovered to within 3% at z~0.42 even when
extended star formation histories are considered. We demonstrate explicitly
that using Single Stellar Populations to age-date galaxies from the
semi-analytical simulations provides insufficient accuracy for this experiment
but accurate ages are obtainable if the complex star formation histories
extracted from the simulation are used. We note, however, that problems with
SSP-fitting might be overestimated since the semi-analytical models tend to
over predict the late-time star-formation in LRGs. Finally, we optimize an
observational program to carry out this experiment.
