The Star Formation Histories of Local Galaxies: Continuous or Intermittent?

We explore a model in which efficient star formation in galaxies is triggered by merging satellites. We show that the merger/interaction rates predicted by hierarchical galaxy formation models depend strongly on galaxy mass. If a merger of a satellite larger than 1% the mass of the primary triggers efficient star formation, low-mass dwarf galaxies experience strong bursts separated by quiescent periods lasting several Gyr. Massive galaxies, such as our own Milky Way, are perturbed by a 1% satellite every few hundred million years, and thus have fluctuating, but relatively continuous star formation histories. We study the spectral signatures of a population of galaxies undergoing intermittent star formation by combining the models with the latest version of the Bruzual & Charlot spectral synthesis code. We concentrate on spectral indicators that are sensitive primarily to stellar age, rather than to metallicity and extinction. We show that if a population undergoes intermittent rather than continuous star formation, the signatures of the bursts should be evident from the observed dispersion in the star formation rates, H-delta equivalent widths and gas mass fractions of galaxies with 4000 Angstrom break strengths indicative of recent or ongoing star formation.