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4.1. Lyman-Break Galaxies

In recent years our understanding of the earliest stages of galaxy formation and evolution has been revolutionized. With the commissioning of the Keck telescopes atop Mauna Kea and the availability of deep ground- and space-based imaging, the photometric technique outlined in Steidel & Hamilton (1992) has been used to routinely select high-redshift, star-forming systems. Dubbed the "Lyman-break" method, objects are identified on the basis of the redshifted continuum spectral discontinuity at (1 + z) × 912 Å and relatively flat (in fnu) continua longward of the redshifted Lyalpha [at (1 + z) × 1216 Å], as we expect from hot O and B stars. At the largest redshifts (z gtapprox 4), absorption due to the Lyalpha and Lybeta forests plays a role of equal or greater importance in attenuating the deep UV continua. These hydrogen absorptions from the Lyman limit and the Lyman forests cause high-redshift objects to effectively disappear in the bluer passbands. The technique is therefore sometimes referred to as the "dropout" technique (see Figs. 4 and 5). U-band dropouts were initially used to chart the distant universe at z ~ 3 (Steidel et al. 1996a, 1996b). This technique has now been pushed to higher redshifts: first to z gtapprox 4 (Dickinson 1998; Dey et al. 1998; Steidel et al. 1999), and now finally above z = 5 (Weymann 1998; Spinrad et al. 1998).

Figure 4

Figure 4. Detail of the Keck/LRIS BRI and HST/NICMOS F160W (~ H) images of the 0140+326 field. The images shown are 27".5 on a side; north is up and east is to the left. Objects at high redshift can be systematically and robustly selected on the basis of redshifted Lyman line and continuum absorptions of hydrogen. The galaxies targeted as B-band "dropouts" are labeled BD3 and BD12 and lie at z = 4.02 and z = 3.89, respectively. The serendipitously discovered z = 5.34 galaxy is labeled RD1 (R-band dropout; Dey et al. 1998) in this figure. 6C 0140+326 (Rawlings et al. 1996) was the most distant radio galaxy known at the time of the deep imaging and lies at z = 4.41. It is also a B-band dropout. Hydrogen absorption from the redshifted Lyman break and Lyalpha forest causes high-redshift objects to effectively disappear in the bluer passbands.

Figure 5

Figure 5. Spectrum of HDF 4-473.0. The total exposure time is 4 hr. It was selected from the deep HST optical and near-infrared images of the HDF (Williams et al. 1996; Thompson et al. 1999) as a V606 dropout (V606 > 30; I814 = 27.1 AB). The spectrogram shows a single emission line at 8029 Å. The asymmetric line profile and broadband colors are most consistent with this being Lyalpha at z = 5.60. Figure courtesy Weymann et al. (1998).

The hydrogen absorptions are ubiquitous: they are present in the spectra of O and B stars which will dominate the rest-frame UV continua of young, star-forming galaxies with conventional mass functions. They are also present for any system with a substantial hydrogen column present along our line of sight. In particular, bound-free absorption of hydrogen at the Lyman limit [at (1 + z) × 912 Å] will severely truncate a background spectrum if the total neutral hydrogen column at the relevant wavelength exceeds nH approx 3 × 1017 cm-2. Thus, Lyman absorption is expected even in systems whose spectral energy distributions are not dominated by hot, young stars; similar techniques have routinely been used to identify high-redshift quasars from deep multiband imaging such as the Palomar Observatory Sky Surveys (e.g., Kennefick, Djorgovski, & de Calvalho 1995; Stern et al. 1999c) and the Sloan Digital Sky Survey (Fan et al. 1999).

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