Galactic chemical evolution is the proportional buildup of helium and heavy elements or metals, i.e., elements other than hydrogen and helium, within a galaxy over time as a result of the continuous manufacture and expulsion of these elements by resident stars. The topic concerns itself not only with global or pangalactic changes but also with regional ones within spatially resolved galaxies.
The essence of chemical evolution can be illustrated by imagining a closed box containing interstellar gas of primordial composition situated at an arbitrary location within a galaxy. As portions of the gas collapse, fusion processes within the stars that are formed convert hydrogen into heavier elements, and this chemically enriched material is subsequently expelled into the interstellar medium through stellar winds, planetary nebula formation, or supernova eruptions. As this cast-off matter mixes with the surrounding medium, the composition of the latter changes such that the abundances of helium and the heavy elements increase relative to hydrogen. As a second generation of stars forms from this enriched material, the new stars possess a greater fraction of heavy elements than their predecessors. Thus, the enrichment cycle continues until enough material has been locked up in stellar remnants that the star formation process, which depends upon the availability of interstellar gas, is finally damped. A galaxy, then, can be thought of as an ensemble of these boxes.
The real picture is more complicated, of course. The boxes have no walls and, as such, are open to matter exchange with their surroundings in all directions. Nevertheless, the simple model does suggest some of the crucial processes which must be understood if we are to have a comprehensive understanding. For example, we must know the details of star formation and evolution, stellar nucleosynthesis and the rate of heavy-element production, the details of stellar death and matter ejection, and the efficiency with which ejected material is mixed into the interstellar medium.
Studies of galactic chemical evolution involve an interplay between (1) global and/or spatially resolved abundances, sometimes as a function of time, for one or more galaxies, and (2) models based upon a set of input parameters determined by the physics being tested. Observed abundances provide two basic sorts of information. First, ratios of heavy elements relative to hydrogen, such as O/H or Fe/H, serve as gauges of how far chemical evolution has progressed in a system, because they measure the extent to which hydrogen has been converted to heavier elements. As such, these ratios are particularly sensitive to the rate at which gas is cycled through stars, i.e., the star formation rate, and how that rate may have changed with time. Second, ratios of two heavy elements, such as N/O or O/Fe, provide information about differential elemental production by stars. That is, at what rate, say, is nitrogen produced relative to oxygen, or oxygen relative to iron? The answer here is tied to the production rates of individual elements as a function of stellar mass weighted by the relative number of stars at each mass (the initial mass function, or IMF) as well as to the history of star formation. Also, there is an element of time involved in all of this. For example, abundances measured in a star reveal enrichment levels at the time the star formed. In summary, chemical evolution can be traced indirectly by associating abundance patterns within a galaxy with local conditions, where the latter ultimately depend on time, and directly by observing abundances in stars of different ages or in galaxies of different look-back times.
The primary goal of this review is to describe the state of affairs concerning observed abundance patterns in galaxies. Because of author expertise, emphasis is placed on abundance patterns in spiral disks and elliptical galaxies as derived from emission-line analyses and photometric indices, respectively. However, for completeness and continuity, we also describe and compare the complementary results provided by stellar abundance work in the Milky Way and nearby galaxies. Our elemental scope is confined to those elements between carbon and iron on the periodic table (6 Z 26), i.e., those elements which are the most readily observed and for which there is the most information. Discussions of helium and the light elements are better taken up in the context of big bang nucleosynthesis, and for this the reader is urged to consult chapter 4 of Pagel (1997) and references therein for recent discussions of this topic. Likewise, elements beyond iron have been studied in part by Edvardsson et al. (1993), Wheeler, Sneden, & Truran (1989), and McWilliam (1997).
Numerous reviews of galactic chemical evolution and abundance patterns are available in the literature. An excellent, approachable introduction to the subject of chemical evolution is given in the comprehensive review by Tinsley (1980). The textbook by Pagel (1997) treats numerous topics related to galactic chemical evolution and the synthesis of elements. Additional material on observations and abundance studies in galaxies can be found in several recent conference proceedings, in particular, Friedli et al. (1998) and Walsh & Rosa (1999). Other useful works specifically treating element synthesis include books by Clayton (1983), Rolfs & Rodney (1988), and Cowley (1995), the review by Trimble (1991), and the conference proceedings by Edmunds & Terlevich (1992) and Prantzos, Vangioni-Flam, & Cassé (1993). Finally, QSO absorption-line systems are enabling chemical evolution studies to be carried out through the study of abundances as a function of look-back time. While these systems are beyond our scope, interested readers are urged to consult Lauroesch et al. (1996), Lu, Sargent, & Barlow (1996), and Pettini et al. (1999).
We begin with a discussion of abundances derived from emission lines in spiral galaxies, including the Milky Way, in Section 2. In Section 3 we turn to stars both inside and outside of the Milky Way, while abundances in elliptical galaxies from photometric integrated light are treated in Section 4. A summary is given in Section 5. Appendices explain techniques used to derive abundances from emission lines (Appendix A), stellar absorption lines (Appendix B), and the integrated starlight of composite systems (Appendix C). Unless otherwise stated, elemental abundances and ratios referred to in this review are by number, not mass.