The study of distant galaxies is empirically demanding - not surprisingly, as these galaxies are very faint.
Of course there are a variety of motivations to observe and perhaps understand distant "units of the Universe". We would like to detail the present-day "lumpiness" of the Cosmos and its evolution from a very smooth "sea" at decoupling. At the nominal redshift of the Cosmic Microwave Background the key fluctuations on reasonable scales are only of order 10-5. Of course at z ~ 0 we have a very inhomogenous distribution of baryons we call galaxies and the Intergalactic Medium (IGM hereafter).
Noting the obvious, studying distant galaxies is synonymous with traveling far back in cosmic time towards the birth of massive sub-structures and large galaxies. Can we now see directly the development of single galaxies of Milky Way dimensions?
We now believe that most galaxies form and accumulate either (1) by the infall of gas (and dark matter) as "monolithic" entities, self-gravitating by the time we can observe them, or (2) by a series of major and/or minor mergers. This is the now-popular "bottom-up" scenario. Here it is presumably difficult to catch the small and immature systems in the act of merging, depending perhaps on the appropriate dynamical time scales. Thus for scenario (2) we would anticipate young galaxies to illustrate complex morphologies, quite different from those of the mature galactic systems we study readily here and now, at zero redshift. There is indeed some evidence for "recent" mergers from the fine images of distant galaxies observed with the Hubble Space Telescope (HST) - see Stern & Spinrad (1999) for some plausible early merger examples (Fig. 1). And we'd like to push these examples back in cosmic time to even "younger" galaxy growth - but the first problem is, quite naturally, the location of small and dismally faint candidates for galaxies in formation.
Figure 1. HST images of five spectroscopically confirmed galaxies located in the HDF(N). Note the distortions, small tails, and multiple centeral components - presumably due to mergers. Overall the galaxies are obviously quite small at this stage of their evolution. From Stern & Spinrad (1999)
Another important contemporary research area emerging is the study of intergalactic (gaseous) matter usually seen in silhouette against a bright background source like a QSO or an unusually bright and distant galaxy. And now, new observational techniques are beginning to tell us about the interaction history of galaxies and the IGM (cf. Adelberger et al., 2003).
One of this paper's topics, directly or indirectly stated, is just how early in cosmic epoch (parameterized by redshift) we can study individual galaxies or their "pre-galactic" fragments. There is only a short time interval between the early epochs beyond z = 3 (see Figure 2). How can the galaxies evolve so quickly?
Figure 2. A plot of look back time (in Gyrs) versus redshift for three cosmological models. Most might now prefer the short-dashed curve. Note that at high z (z 3) the time intervals become quite short. Figure by Curtis Manning.
The historical view of our empirical and theoretical march outward toward higher redshift has shown a fairly rapid expansion. By 1976 a few radio galaxies had been located and studied at z > 0.5. The z = 1.0 threshold (for galaxies) was crossed in 1981. Of course Quasars and QSOs had been actively observed and known earlier at large distances - redshifts in the 1960s and 1970s taking us to z = 2.01 (Schmidt 1965) and then 2.88, and then to z = 3.5 (OQ 172; Baldwin et al., 1974). Finally, z = 4 for QSOs was surpassed by the Palomar two-color-based searches (Schneider, Schmidt & Gunn 1991), and searches for Ly on low-resolution grism spectra (Osmer 1999) were equally successful. Almost all the recent stages of the "QSO-z race" have emphasized red-IR photometry and unusual colors, since the z ~ 5 QSOs are heavily depressed by the Ly forest of the IGM (see Fan et al. 2001). The largest published QSO redshift to date is z = 6.28 (Fan et al., 2002; Pentericci et al., 2002a).
Now we are witness to the era of a friendly race toward higher and record-breaking galaxy redshifts. The current limit for galaxies, which we shall detail later in this publication, is near z = 6.5. Is this redshift close to the end of the "dark ages", where re-ionization by massive stars and/or early QSOs play as vital sources of ionizing radiation? We return to this topic, with empirical evidence, toward the conclusion of this review.