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Returning to Figure 1, it must be remembered that everything we have learnt on the distant universe up to this point required `decoding' the information `encrypted' in the absorption spectra of QSOs. It is easy to appreciate, therefore, the strong incentive which motivated astronomers in the 1990s to detect high resdshift galaxies directly. After many years of fruitless searches, we have witnessed since 1995 a veritable explosion of data from the Hubble Deep Fields and ground-based surveys with large telescopes. The turning point was the realisation of the effectiveness of the Lyman break technique (Figure 19) in preselecting candidate z appeq 3 galaxies (Steidel et al. 1996). Although these galaxies constitute only ~ 3 - 4% of the thousands of faint objects, at all redshifts, revealed by a moderately deep CCD exposure at the prime focus of a 4m telescope (see Figure 20), they can be readily distinguished on the basis of their colours alone, when observed through appropriately selected filters (Figure 21).

Figure 19

Figure 19. (Courtesy of Kurt Adelberger). An illustration of the principles behind the Lyman break technique. Hot stars have flat far-UV continua, but emit fewer photons below 912Å, the limit of the Lyman series of hydrogen (top panel). These photons are also efficiently absorbed by any H I associated with the sites of star formation (middle panel) and have a short mean free path - typically only ~ 40Å - in the IGM at z = 3. Consequently, when observed from Earth (bottom panel), the spectrum of a star forming galaxy at z appeq 3 exhibits a marked `break' near 4000Å. With appropriately chosen broad-band filters, this spectral discontinuity gives rise to characteristic colours; objects at these redshifts appear blue in (G - curly R) and red in (Un - G). For this reason, such galaxies are sometimes referred to as U-dropouts. A more quantitative description of the Lyman break technique can be found in Steidel, Pettini, & Hamilton (1995).

Figure 20

Figure 20. A typical deep CCD image recorded at the prime focus of a 4m-class telescope. This particular image (of the field designated DSF2237b) was obtained with the COSMIC camera of the Palomar Hale telescope, by exposing for a total of two hours through a custom made curly R filter. In the 9 × 9arcmin field of view (corresponding to co-moving linear dimensions of 11.6 × 11.6 h-1 Mpc at z = 3) there are ~ 3300 galaxies brighter than curly R = 25.5; 140 of these (circled) show Lyman breaks which place them at redshifts between z = 2.6 and 3.4.

Figure 21

Figure 21. All the ~ 3300 galaxies from Figure 20 are included in this colour-colour plot. The shaded region shows how the 140 candidate Lyman break galaxies are selected for subsequent spectroscopic follow-up. The symbol size is proportional to the object magnitude; circles denote objects detected in all three bands, while triangles are lower limits in (Un - G) for Un dropouts.

The Lyman break technique has been very successful at finding high redshift galaxies thanks to the combination of the increasingly large and UV sensitive CCDs used to identify candidates on the one hand, and the multi-object spectroscopic capabilities of large telescopes required for follow-up and confirmation on the other. Thus, samples of spectroscopically confirmed z appeq 3 galaxies have grown from zero to more than one thousand in the space of only five years. As can be seen from Figure 22, the spectroscopic redshifts generally conform to expectations based on just two colours. Such large samples have made it possible to trace the star formation history of the universe over most of the Hubble time and to measure the large-scale properties of this population of galaxies, most notably their clustering and luminosity functions (see Steidel 2000 for a review).

Figure 22

Figure 22. The location of spectroscopically confirmed galaxies (from the surveys by Steidel and collaborators) on the (Un - G) vs. (G - curly R) plot. Triangles denote objects undetected in the Un band; stellar symbols are used for Galactic stars.

In parallel with this work on the Lyman break population as a whole, in the last few years we have also begun to study in more detail the physical properties of some of the brighter galaxies in the sample. The questions which we would like to address are:

  1. What are the stellar populations of the Lyman break galaxies?
  2. What are their ages and masses?
  3. What are their levels of metal enrichment?
  4. What are the effects of star formation at high z on the galaxies and the surrounding IGM?

Many of these questions link observational cosmology with stellar and interstellar astrophysics, and this will become evident as we now explore them in turn.

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