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 gia nt H II regions, H II galaxies, blue compact dwarf galaxies, nuclear s tarbursts, and infrared luminous starburst galaxies. The models are ba sed on the most up-to-date input physics for the theory of stellar atm ospheres, 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 in terval of 3 x 10(8) yr. We also investigate the effects of star-format ion histories which are intermediate between those two extreme cases. Three choices of the initial mass functions are studied: a Salpeter an d a Miller-Scalo type IMF with upper mass limits of 100 M(circle dot), and a Salpeter IMF truncated at 30 M(circle dot). Metallicities of 0. 1 Z(circle dot), 0.25 Z(circle dot) , Z(circle dot), and 2 Z(circle do t) are considered. The model predictions include the numbers of hot st ars, supernova rates, colors in the UBVRIJHKL passbands, slopes of the ultraviolet continuum observable by IUE, HST, and HUT, equivalent wid ths of hydrogen recombination lines, ionizing fluxes shortward of the H-0, He-0, 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 nea r-infrared continuum by nebular emission. It is found that under typic al starburst conditions the nebular continuum is not negligible. Depen ding on the wavelength, addition of the nebular continuum leads to sig nificantly 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 me chanical energy. The output of radiative and mechanical luminosity is compared at various starburst epochs. In a supernova dominated instant aneous 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, th e output of ionizing radiation below 912 Angstrom becomes very small, as indicated by a very large Lyman discontinuity and a very small rati o of ionizing over mechanical luminosity. We discuss the relevance of these results for the interpretation of starburst galaxies, active gal actic nuclei, and the energetics of the interstellar medium.