In the first part we also give a more technical overview of the methods, definitions and techniques adopted to measure the metallicity and chemical enrichment in stellar populations and in the interstellar medium, also discussing the strengths and weaknesses of the various methods. We will generally discuss the overall statistical properties of galaxies rather then focusing on individual galaxies, although we will unavoidably open some themes by quickly discussing the Milky Way (MW) or some nearby well-studied targets. In this review we primarily provide an overview of the chemical properties of galaxies in the local universe and at high-redshift from an observational perspective, but by also discussing how such properties give important constraints on the galaxy evolutionary processes, including extensive comparisons with theoretical models and numerical simulations. Major advances of several observational techniques and the advent of new major observing facilities have recently enabled astronomers to probe the chemical evolution in the early cosmic epochs, by directly probing the early enrichment process of galaxies and of the intergalactic/circumgalactic medium, hence setting tight constraints on the models of early galaxy formation. ![]() The analysis of the metallicity and chemical abundances on spatially resolved scales (gradients) gives additional information on the processes that have regulated the growth and assembly of galaxies (e.g., inside-out or outside-in formation and/or quenching), as well as information on other internal phenomena such as galactic fountains, stellar migration and radial gas inflows. In addition, different chemical elements are enriched on different timescales by different populations of stars therefore, the relative abundance of elements enables us to obtain unique constraints on the star formation history and on the late stages of the evolutions of single and multiple stars, stages which dominate the production of heavy elements. Therefore, the investigation of the metal content and of the metallicity in galaxies provides truly crucial information on the key mechanisms involved in the evolution of galaxies. ![]() The metallicity, i.e., the content of metals relative to hydrogen and helium, is also sensitive to dilution resulting from inflow of pristine gas. Indeed, the content of metals gives a measure of not only the integrated star formation in galaxies, but also on the fraction of metals lost through outflows and stripping. The investigation of the evolution of the content of chemical elements provides tight constraints on such models. The evolution of the baryonic component and how this results into the formation of stars and in the properties of galaxies as we see them in the local universe and across the cosmic epochs, has been subject to numerous models and cosmological simulations, which use different prescriptions and assumptions. ![]() However, the evolution of the baryonic component is much more complex as baryons interact with radiation and are subject to dissipative processes. Theory and cosmological simulations give a relatively simple scenario on how dark matter evolve from the primeval perturbations, forming dark matter halos and large scale structures that accrete within their gravitational potential (e.g., Springel et al. The evolution of the chemical properties of stellar populations and of the interstellar and intergalactic medium across the cosmic epochs provides unique information on the evolutionary processes driving the formation and evolution of galaxies. Finally, we briefly discuss the open problems and the prospects for major progress in this field in the nearby future. We discuss how the various observational findings compare with the predictions from theoretical models and numerical cosmological simulations. After an overview of the methods used to constrain the chemical enrichment in galaxies and their environment, we discuss the observed scaling relations between metallicity and galaxy properties, the observed relative chemical abundances, how the chemical elements are distributed within galaxies, and how these properties evolve across the cosmic epochs. In this review we provide an overview of these various areas. In recent years major progress has been made in constraining the chemical evolution of galaxies and inferring key information relevant to our understanding of the main mechanisms involved in galaxy evolution. The evolution of the content of heavy elements in galaxies, the relative chemical abundances, their spatial distribution, and how these scale with various galactic properties, provide unique information on the galactic evolutionary processes across the cosmic epochs.
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