Starburst99: Synthesis models for galaxies with active star formation

Starburst99 is a comprehensive set of model predictions for spectrophotomet ric and related properties of galaxies with active star formation. The mode ls are an improved and extended version of the data set previously publishe d by Leitherer & Heckman. We have upgraded our code by implementing the lat est set of stellar evolution models of the Geneva group and the model atmos phere grid compiled by Lejeune et al. Several predictions which were not in cluded in the previous publication are shown here for the first time. The m odels are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age co verage is 10(6)-10(9) yr. We also show the spectral energy distributions wh ich are used to compute colors and other quantities. The full data set is a vailable for retrieval at a Web site, which allows users to run specific mo dels with nonstandard parameters as well. We also make the source code avai lable to the community.