In astronomy, all chemical elements heavier than helium are collectively termed ‘metals’. The mass fraction of a baryonic component (e.g. hot gas, cold gas, stars) in metals is then referred to as its metallicity. As we will see in §3.4, the nuclear reactions during the first three minutes of the Universe (the epoch of primordial nucleosynthesis) produced primarily hydrogen (∼ 75%) and helium (∼ 25%), with a very small admixture of metals dominated by lithium. All other metals in the Universe were formed at later times as a consequence of nuclear reactions in stars. When stars expel mass in stellar winds, or in supernova explosions, they enrich the interstellar medium (ISM) with newly synthesized metals.
Evolution of the chemical composition of the gas and stars in galaxies is important for several reasons. First of all, the luminosity and color of a stellar population depend not only on its age and IMF, but also on the metallicity of the stars . Secondly, the cooling efficiency of gas depends strongly on its metallicity, in the sense that more metal-enriched gas cools faster.
Thirdly, small particles of heavy elements known as dust grains, which are mixed with the interstellar gas in galaxies, can absorb significant amounts of the starlight and reradiate it in infrared wavelengths. Depending on the amount of the dust in the ISM, which scales roughly linearly with its metallicity, this interstellar extinction can significantly reduce the brightness of a galaxy.
... the mass and detailed chemical composition of the material ejected by a stellar population as it evolves depend both on the IMF and on its initial metallicity. In principle, observations of the metallicity and abundance ratios of a galaxy can therefore be used to constrain its star-formation history and IMF. In practice, however, the interpretation of the observations is complicated by the fact that galaxies can accrete new material of different metallicity, that feedback processes can blow out gas, perhaps preferentially metals, and that mergers can mix the chemical compositions of different systems.
