We explore whether the rest-frame near-ultraviolet spectral region , observable in high-redshift galaxies via optical spectroscopy , contains sufficient information to allow the degeneracy between age and metallicity to be lifted .
We do this by first testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-ultraviolet spectra of F stars of known ( sub-solar and super-solar ) metallicity .
F stars are of particular interest because the rest-frame near-ultraviolet spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turnoff .

We find that , in the case of the F stars , where the HST ultraviolet spectra have high signal : noise , model-fitting with metallicity allowed to vary as a free parameter is rather successful at deriving the correct metallicity .
As a result , the estimated turnoff ages of these stars yielded by the model fitting are well constrained .
Encouraged by this we have fitted these same variable-metallicity models to the deep , optical spectra of the z \simeq 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope .
While the age and metallicity are not so easily constrained for these galaxies , we find that even when metallicity is allowed as a free parameter , the best estimates of their ages are still \geq 3 Gyr , with ages younger than 2 Gyr now strongly excluded .
Furthermore , we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion .
Our results therefore continue to argue strongly against an Einstein-de Sitter universe , and favour a \Lambda -dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3 - 5 .