|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
The notion of a "stellar population" is now so commonly used that we generally do not challenge ourselves to provide a specific definition for what really constitutes a population of stars. In the IAU Symposium 164, a range of possible definitions was offered, but perhaps the simplest is that a stellar population in a galaxy comprises the stars formed during a major event in the life of a galaxy (King 1995, Mould 1995). It has long been tacitly assumed, especially for the earliest stages of star formation in galaxies, that there exist some populations that are coeval for all disk galaxies, or perhaps even for all galaxies. What evidence do we really have that there was a substantial early episode of star formation in all galaxies? Are there characteristics of a young galaxy that determine the subsequent star-formation history or do external factors play a substantial role? To begin to answer such questions we can study distant galaxies or protogalaxies (e.g. Koo 1986, Djorgovski 1992), the formation of spheroids and disks (e.g. Schechter & Dressler 1987), or the special population of faint blue galaxies seen in faint galaxy number counts (Kron 1980, Tyson 1988, Colless et al 1993). Local galaxies can provide important details to galaxy evolution models, which makes the star-formation histories of the nearest galaxies a source of interest for cosmological questions. We are able to combine stellar photometry with spectroscopy to determine the fundamental population characteristics: the stellar luminosity function, spatial distribution, age, chemical abundance, and kinematics. The only external galaxies for which we can currently hope to make such detailed observations are the Magellanic Clouds and the Local Group dwarf spheroidal galaxies. This review focuses on the Magellanic Clouds.
Many of the characteristics that we attribute to a stellar population are strongly influenced by the properties of the Milky Way. For instance, we use the words "halo," "metal-poor," "high velocity," and "old" interchangeably to describe a certain population in our Galaxy, yet these words in the context of other galaxies are not necessarily interchangeable. As we will see, the "old" stars in the Large Magellanic Cloud (LMC) are not "halo" stars, and the "metal-poor" stars in the Small Magellanic Cloud (SMC) are not necessarily "old." We hope to show the reader that the Magellanic Clouds, with their relatively low metal abundances and large numbers of star clusters of all ages, can provide an instructive contrast to the notions of the stellar populations in our Galaxy. In addition, by studying the Magellanic Clouds, we may begin to better understand the characteristics of the stellar populations in a vigorously evolving system: systems that may have at some earlier epochs been similar to our Galaxy, but have subsequently evolved very differently.
In this review we concentrate on a quite limited set of questions that relate to these issues in the context of the Magellanic Clouds. What old populations are there in the Magellanic Clouds? How old is the oldest population in each galaxy and how do these populations compare to the Milky Way? Are the oldest populations in the Magellanic Clouds simply scaled versions of the halo of the Milky Way? What are the age-metallicity relations in each galaxy? What is the history of star formation from the earliest populations to the population characterizing the Clouds about 1 Gyr ago? Does the history of star formation as seen in the clusters reflect the history of star formation as seen in the field stars of the Magellanic Clouds?
We show that the available evidence suggests that the LMC really did lie relatively dormant for a substantial fraction of a Hubble time, while the SMC seems to have experienced a more constant star-formation rate. We know of no compelling observation that will tell us why the LMC resumed making stars several billion years ago. Although it is tempting to use the current dynamical models of Cloud-Milky Way interactions as a vehicle for understanding the LMC and SMC star-formation histories, these models are just beginning to predict the times of major episodes of star formation in the intermediate and old populations.
We approach these questions by summarizing the recently published observations of the field stars and of the rich luminous star clusters of the Magellanic Clouds. We also point out observations currently being made and observations that could and should be made in the future. It is our feeling that we are on the verge of a deep understanding of the general evolution of the Clouds, based on expected new data from the Hubble Space Telescope (HST), the southern 6.5-8-m class telescopes, proper motion surveys, wide-field optical and infrared surveys, and multifiber spectroscopy.
To make this review tractable, we have chosen to take Westerlund's (1990) review as our starting point. Unless it is important to our argument, we assume that Westerlund (1990) and other reviews and symposia (Feast 1995;, Olszewski 1988, 1993, 1995;, Da Costa 1993; many papers in Baschek et al 1993; many papers in Haynes & Milne 1991, Suntzeff 1992a, b, Mateo 1992, Graham 1988) can lead the reader to the earlier literature on the Magellanic Clouds.