Evolution of galaxies and its significance for cosmology

Tinsley, Beatrice M.
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Evolution of the content and light of galaxies are studied to determine their significance for cosmology. A numerical computation of evolution starts from gas with Population I composition; then stars are formed at all times, at rates which are functions of stellar mass and 9 mass of gas in the galaxy. Discrete time steps of 10⁹ years are used, and 13 stellar masses. The stars are placed on the H-R diagram according to their masses and ages; each star finally becomes a white dwarf, while its excess mass enriches the interstellar gas. Different evolutionary sequences are constructed by adjusting four parameters of a stellar birthrate function. Then 'galaxies' resulting from each sequence of 10-12 x 10⁹ years are compared with observed local galaxies, with respect to colors in Johnson's 8-color (UBVRIJKL) system, mass/light ratio, relative mass of gas, and types of stars contributing to the light. 'Galaxies' closely resembling all normal types, Im to E, can be formed with a stellar birthrate proportional to the inverse square of stellar mass, and to the mass of gas in the galaxy; the types differ in initial rate of gas consumption and in the birthrate of very low mass stars. These types can all have the same age, and do not form an evolutionary sequence. Theoretical magnitude-redshift relations (m-z) are constructed, for a variety of cosmological models, using the computed past history of giant elliptical systems. Three values of Hubble's constant are considered, since effects of evolution are greater if this is small. The color-redshift relation is also discussed, but effects of evolution are small in the range observed. Inclusion of galactic evolution greatly reduces the differences in m-z relations between models. This is because those models with the greatest luminosity distance at a given redshift also have the greatest light travel-time, so the galaxies would be seen earlier when they were brighter. Many cosmological models are found to be consistent with the observed m [subscript V]-z relation, including models also consistent with empirical limits on the mean density of matter, with the minimum age of the universe inferred from evolution of star clusters, and with a steep number-flux density relation for radio sources.