Early Chemical Evolution of Galaxies

Abstract

Initial conditions are set by Big bang nucleosynthesis from which we know that 90 per cent of baryons are dark and have essentially unknown chemical composition. In our own Galaxy, there are many clues from individual stars in different populations whereas in elliptical galaxies the data largely come from integrated spectra, but these raise problems enough like the Mg/Fe and G-dwarf problems. Irregular and blue compact galaxies display the primary--secondary transition in N/O; this in turn may be relevant to element ratios observed in damped Lyman-alpha systems at high red-shift, which offer rather little evidence for for pure SNII synthesis such as is found in the Galactic halo stars. A recent estimate of past star formation rates as a function of red-shift is presented and the appropriateness of the conventional conversion factor of 42 from SFR to metal production is discussed. For any reasonable value of this conversion factor, it is clear that most of the metals existing at z=2.5 have yet to be detected.