Published in "The Hubble Deep Field", eds. M. Livio, S.M. Fall and P. Madau 1998
Abstract. The Lyman-break technique has opened the very high-redshift universe (e.g., z > 2) to the empirical investigation. The efficiency of the technique in detecting very distant star-forming galaxies has allowed the compilation of large samples at redshifts as high as z ~ 4.5 with well controlled selection criteria and homogeneous characteristics. Such data sets are essential to explore the spectrum of properties of early galaxies, including spectra, luminosity function, morphology, star-formation history, clustering and mass, that are necessary to constrain the theories of galaxy formation. Developed for a ground-based survey, the Lyman-break technique has been successfully transported to HST observations during the HDF. Although the HDF sample of Lyman-break galaxies is very small compared to the ground-based survey, it is considerably deeper and covers a larger redshift range. This has allowed us to study the luminosity distribution of these galaxies on a large dynamic range and probe their evolution with redshift. Here we discuss the salient features of this combined exploration, from the ground and from space, of star-forming galaxies during the first 15% of the age of the universe, emphasizing how the two surveys have complemented each other to help us unveil the fundamental traits of this population. More empirical information will be necessary (at high as well as intermediate redshifts) before we can map the Lyman-break galaxies onto systems at lower redshifts along an evolutionary sequence with some confidence. However, there is a good deal of fruitful work that can be started now by comparing the predictions of cosmological theories to the properties of the galaxies that the Lyman-break technique, both in its ground-based and space-borne incarnation, has allowed us to explore.
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