ABSTRACT
The third chapter discusses the critical subject of galactic chemical evolution. It is, in principle, possible to determine the chemical composition of a galaxy as a function of position and time by measuring chemical element abundances of stars with different birthplaces and ages. Besides, the growth of chemical element abundances in a galaxy provides a clock for galactic aging. For example, older stars contain less iron, on average, than younger stars. The chemical evolution models usually follow the evolution of the element’s mass because of stellar nucleosynthesis, mass ejection and the infall of gas of primordial chemical composition. The existence of an age–metallicity relation is an essential issue for developing chemical evolution models. Of particular interest are damped Lyman-alpha (DLA) systems, which are concentrations of neutral hydrogen gas detected in the spectra of quasars. DLA systems are interesting probes for objects at large redshifts. These objects are not luminous enough to be observed otherwise. DLA systems are predominantly neutral, making it easier to measure heavy elements abundance. DLA systems are observed in absorption rather than in emission of the stars. Therefore, they offer the opportunity to study the properties of the gas in the early galaxies directly.
