Annu. Rev. Astron. Astrophys. 1998. 36: 189-231 Copyright © 1998 by Annual Reviews. All rights reserved

1. INTRODUCTION

One of the most recognizable features of galaxies along the Hubble sequence is the wide range in young stellar content and star formation activity. This variation in stellar content is part of the basis of the Hubble classification itself (Hubble 1926), and understanding its physical nature and origins is fundamental to understanding galaxy evolution in its broader context. This review deals with the global star formation properties of galaxies, the systematics of those properties along the Hubble sequence, and their implications for galactic evolution. I interpret "Hubble sequence" in this context very loosely, to encompass not only morphological type but other properties such as gas content, mass, bar structure, and dynamical environment, which can strongly influence the large-scale star formation rate (SFR).

Systematic investigations of the young stellar content of galaxies trace back to the early studies of resolved stellar populations by Hubble and Baade and analyses of galaxy colors and spectra by Stebbins, Whitford, Holmberg, Humason, Mayall, Sandage, Morgan, and de Vaucouleurs (see Whitford 1975 for a summary of the early work in this field). This piecemeal information was synthesized by Roberts (1963), in an article for the first volume of the Annual Review of Astronomy and Astrophysics. Despite the limited information that was available on the SFRs and gas contents of galaxies, Roberts' analysis established the basic elements of the contemporary picture of the Hubble sequence as a monotonic sequence in present-day SFRs and past star formation histories.

Quantifying this picture required the development of more precise diagnostics of global SFRs in galaxies. The first quantitative SFRs were derived from evolutionary synthesis models of galaxy colors (Tinsley 1968, 1972, Searle et al 1973). These studies confirmed the trends in SFRs and star formation histories along the Hubble sequence and led to the first predictions of the evolution of the SFR with cosmic lookback time. Subsequent modeling of blue galaxies by Bagnuolo (1976), Huchra (1977), Larson & Tinsley (1978) revealed the importance of star formation bursts in the evolution of low-mass galaxies and interacting systems. Over the next decade, the field matured fully, with the development of more precise direct SFR diagnostics, including integrated emission-line fluxes (Cohen 1976, Kennicutt 1983a), near-ultraviolet continuum fluxes (Donas & Deharveng 1984), and infrared (IR) continuum fluxes (Harper & Low 1973, Rieke & Lebofsky 1978, Telesco & Harper 1980). These provided absolute SFRs for large samples of nearby galaxies, and these were subsequently interpreted in terms of the evolutionary properties of galaxies by Kennicutt (1983a), Gallagher et al (1984), Sandage (1986).

Activity in this field has grown enormously in the past decade, stimulated in large part by two major revelations. The first was the discovery of a large population of ultraluminous IR starburst galaxies by the Infrared Astronomical Satellite (IRAS) in the mid-1980s. Starbursts had been identified (and coined) from groundbased studies (Rieke & Lebofsky 1979, Weedman et al 1981), but IRAS revealed the ubiquity of the phenomenon and the extreme nature of the most luminous objects. The latest surge of interest in the field has been stimulated by the detection of star-forming galaxies at high redshift, now exceeding z = 3 (Steidel et al 1996, Ellis 1997). This makes it possible to apply the locally calibrated SFR diagnostics to distant galaxies and to directly trace the evolution of the SFR density and the Hubble sequence with cosmological lookback time.

The focus of this review is on the broad patterns in the star formation properties of galaxies and their implications for the evolutionary properties of the Hubble sequence. It begins with a summary of the diagnostic methods used to measure SFRs in galaxies, followed by a summary of the systematics of SFRs along the Hubble sequence and the interpretation of those trends in terms of galaxy evolution. It concludes with a brief discussion of the physical regulation of the SFR in galaxies and future prospects in this field. Galaxies exhibit a huge dynamic range in SFRs, over six orders of magnitude even when normalized per unit area and galaxy mass, and the continuity of physical properties over this entire spectrum of activities is a central theme of this review.

With this broad approach in mind, I cannot begin to review the hundreds of important papers on the star formation properties of individual galaxies or the rich theoretical literature on this subject. Fortunately, there are several previous reviews in this series that provide thorough discussions of key aspects of this field. A broad review of the physical properties of galaxies along the Hubble sequence can be found in Roberts & Haynes (1994). The star formation and evolutionary properties of irregular galaxies are reviewed by Gallagher & Hunter (1984). The properties of IR-luminous starbursts are the subject of several reviews, most recently those by Soifer et al (1987), Telesco (1988), Sanders & Mirabel (1996). Finally, an excellent review of faint blue galaxies by Ellis (1997) describes many applications to high-redshift objects.