Galactic Evolution

Summary. A program is described for studying the evolution of galaxies. Some initial results are presented, suggesting future applications of the program. In the models, stars are formed from interstellar gas, and the evolution of each star is followed from the main sequence to death. Semi-empirical tracks based on old open clusters and old-disk moving groups are used for the later stages of low-mass stars. Properties of the models that can be calculated at times from 1 to 11 x years (in discrete time steps of t0 yr) are: the detailed stellar population at 566 points of the H-R diagram; the mass of gas and its helium and metal content; the bolometric magnitude; integrated broad-band colors and magnitudes on Johnson's 8-color system; the mass to light ratio; the UV flux at 1700 A; integrated intermediate-band colors on McClure and van den Bergh's system; narrow-band colors on Wood's system; and narrow-band scanner colors and indices on Spinrad and Taylor's system. The models studied to date are homogeneous and contain solar-composition stars only. Metal-poor and super-metal-rich stars can be added (as integrated globular clusters and SMR K giants) to a computed population before its photometric properties are found. Stellar birthrates used include a variety of initial luminosity functions and functions of time or gas mass. These functions provide a set of models with which to survey the potentialities of the method. Future studies will use birthrate functions based on theories of star formation in various types of galaxies. Models discussed here were not planned to match any galaxy precisely, but the types of birthrate required for various typical galactic populations, and their overall evolution, are clearly indicated. Some results of interest are: (1) Models evolve redwards along the observed galactic 2-color relation, so UB V colors cannot tell if a galaxy contains only, or just predominantly, young stars. (2) The observed far ultraviolet flux could arise from young main sequence stars, in galaxies with a finite present birthrate. (3) Super- metal-rich stars are probably formed in elliptical galaxies and the central regions of spirals. (4) The infrared light of galaxies is predicted to be dominated by late giants, even in systems with a high mass-to-light ratio, (5) The Spinrad-Taylor indices are far the most sensitive of the properties studied to the distribution of stellar temperatures, luminosities, and abundances in a population. Use of broad-band colors alone can be seriously misleading. Key words: galaxies - evolution of galaxies