Synthetic properties of starburst galaxies

We present the results of an extensive grid of evolutionary synthesis models for populations of massive stars. The parameter space has been chosen to correspond to conditions typically found in objects like giant H II regions, H II galaxies, blue compact dwarf galaxies, nuclear starbursts, and infrared luminous starburst galaxies. The models are based on the most up-to-date input physics for the theory of stellar atmospheres, stellar winds, and stellar evolution. Observable properties of a population of stars are computed for the two limiting cases of an instantaneous burst and a constant star- formation rate over a time interval of 3 x 10⁸^ yr. We also investigate the effects of starformation histories which are intermediate between those two extreme cases. Three choices of the initial mass functions are studied: a Salpeter and a Miller-Scalo type IMF with upper mass limits of 100 Mₛun_, and a Salpeter IMF truncated at 30 Mₛun_. Metallicities of 0. 1 Zₛun_, 0. 25 Zₛun_, Zₛun_, and 2 Zₛun_ are considered. The model predictions include the numbers of hot stars, supernova rates, colors in the UBVRIJHKL passbands, slopes of the ultraviolet continuum observable by IUE, HST, and HUT, equivalent widths of hydrogen recombination lines, ionizing fluxes shortward of the H⁰^, He⁰^, and He^+^ ionization edges, the Lyman discontinuity, and mass- and energy- deposition rates due to stellar winds and supernovae. We discuss the contamination of the stellar ultraviolet, optical, and near-infrared continuum by nebular emission. It is found that under typical starburst conditions the nebular continuum is not negligible. Depending on the wavelength, addition of the nebular continuum leads to significantly redder or bluer broadband colors than obtained from a pure stellar continuum. A population of massive stars is not only important in terms of its output of radiation but also via its deposition of mechanical energy. The output of radiative and mechanical luminosity is compared at various starburst epochs. In a supernova dominated instantaneous starburst, the mechanical luminosity can be as large as almost 10% of the total radiative luminosity. This occurs when most massive O stars have disappeared, and the synthetic spectrum in the optical and near- ultraviolet is dominated by B and A stars. During this epoch, the output of ionizing radiation below 912 A becomes very small, as indicated by a very large Lyman discontinuity and a very small ratio of ionizing over mechanical luminosity. We discuss the relevance of these results for the interpretation of starburst galaxies, active galactic nuclei, and the energetics of the interstellar medium.