1. INTROUCTION

The measurement of element abundances in galaxies other than our own has a roughly forty-year history, beginning with early attempts to measure helium abundances in giant H II regions in the Magellanic Clouds and M33 (Aller & Faulkner 1962, Mathis 1962) and pioneering studies of heavy element abundances from forbidden lines in extragalactic H II regions (e.g. Peimbert & Spinrad 1970, Searle 1971, Searle & Sargent 1972). Since then this field has grown tremendously, with high quality oxygen abundance data in some 40 nearby spiral galaxies and more than 100 irregular and compact dwarf galaxies. The amount of data for other elements (C, N, Ne, S, and Ar) has also improved tremendously, thanks largely to improvements in visible-wavelength detectors and the launching of spacecraft observatories, such as IUE, HST, and ISO, which have opened up the UV and IR spectral regions for spectroscopy.

The direct importance of determining the distribution of metallicity and element abundance ratios in galaxies is the contribution these measurements make to chemical evolution, and by consequence the evolution of galaxies. The elements heavier than H and He in stars and the interstellar medium (ISM) are the accumulated product of previous generations of star formation. The overall metallicity (usually represented by O/H in H II regions/ISM, and by Fe/H in stars) is determined by the total amount of previous star formation. Element abundance ratios, particularly C/O, N/O, or s-process/Fe, track the relative contributions of low-mass stars and high-mass stars, incorporating information on the stellar initial mass function (IMF). The abundances can be affected by gas flows (infall, outflow, or internal flows). It is possible, with modeling, to infer important clues to the evolution of galaxies from abundance measurements.

Beyond galaxy evolution, abundance measurements provide important ancillary information relevant to other very important astrophysical problems, including:

• The dependence of the I(CO) / N(H₂) conversion on environment and metallicity, which is critical for determining the amount of molecular gas in galaxies.
• The metallicity dependence of the Cepheid period-luminosity relation, currently under debate with respect to the determination of the Hubble constant and the extragalactic distance scale.
• Understanding the color evolution of galaxies. Colors of composite stellar populations depend on both age and metallicity; metallicity measurements thus reduce degeneracies in the interpretation of colors.
• Stellar mass loss rates, particularly the radiatively-accelerated winds of O and Wolf-Rayet stars, likely depend on the metallicity of the individual stars.
• The cooling function of interstellar gas. Cooling of interstellar gas is generally dominated by metals (ions of metals in X-ray and ionized gas, singly-ionized metals in neutral gas, and molecules other than H₂ in molecular clouds), so the thermal balance in the ISM is a function of metallicity, with obvious implications for the formation of stars and galaxies.

In any field of investigation, a few key questions arise which form a framework for specific studies. I formulate a few of them below.

1. How do metallicity and element abundance ratios evolve within galaxies, and how do variations relate to the evolution of the gas content and stellar light?
2. What galaxy properties determine the observed compositions of galaxies? How is metallicity affected by galaxy dynamics (interactions, gas flows, angular momentum evolution)?
3. How did heavy elements get into the intergalactic medium (IGM)? Were they ejected from galaxies by supernova-driven winds, ejected in tidal streams during galaxy interactions and mergers, or did they come from the first, possibly pre-galactic, stars?
4. How well do simulations of galaxy formation and evolution reproduce the observed metallicities and distribution of abundances in galaxies?

The purpose of these lectures is to review the results of a variety of element abundance studies in galaxies other than our own in the nearby universe. I will not try to be all-inclusive, as the field is vast. For example, I will not attempt to discuss abundances in elliptical galaxies in detail, as better experts have already written extensive reviews on the subject (e.g. Worthey 1998, Henry & Worthey 1999), nor will I say much about luminous IR starbursts or low surface brightness galaxies. Much of the methodology behind abundance measurements in stars and ionized gas will be covered in great detail in the lectures by Lambert, Langer, and Stasinska; I will not spend much time on these subjects, but will highlight points of contention or uncertainty where appropriate. Likewise, Matteucci will discuss chemical evolution modeling in detail, so I will use the observational results to highlight areas where the data shed light on physical evolution of galaxies.