Probing recent star formation with absorption-line strengths in
  hierarchical models and observations
  Stellar population parameters derived from spectral line-strengths provide a
powerful probe of galaxy properties and formation histories. We implement the
machinery for extracting single-stellar-population-equivalent stellar
population parameters from synthetic spectra generated by a hierarchical galaxy
formation model. We find that the SSP-equivalent age is related to the
light-weighted age in a complicated fashion that reflects the influence of
recently-formed stars and is poorly correlated with the mass-weighted age. The
tendency for SSP-equivalent ages to be biased young means that archaeological
downsizing overstates the mass-weighted downsizing in age with mass. We find
that the SSP-equivalent metallicity closely tracks the mass- and light-weighted
metallicities, so that observed mass--metallicity relations for old galaxies
closely reflect the underlying trends. We construct mock catalogues of
early-type galaxies in a Coma cluster-sized halo and compare them directly to
observations of early-type galaxies in the Coma cluster. The similarity of the
SSP-equivalent ages in the observational samples and the mock catalogues gives
us confidence that the star-formation quenching implemented in the hierarchical
galaxy formation model produces roughly the correct amount of recent star
formation. The SSP-equivalent metallicities are however too low and have the
wrong slope as a function of velocity dispersion, and the SSP-equivalent ages
of the model galaxies may have an incorrect slope as a function of velocity
dispersion. (Abridged)
