Probing recent star formation with absorption-line strengths in hierarchical models and observations

TL;DR

This study uses spectral line-strengths to analyze galaxy formation histories, revealing complex relationships between SSP-equivalent ages, metallicities, and galaxy properties, and comparing hierarchical model predictions with observations.

Contribution

It introduces a method for deriving stellar population parameters from synthetic spectra in hierarchical models and compares these with real galaxy data.

Findings

SSP-equivalent age is influenced by recent star formation and differs from mass-weighted age.

Observed metallicity relations closely match model predictions, validating some aspects of the hierarchical model.

Model ages and metallicities show discrepancies with observations, indicating areas for model improvement.

Abstract

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)