Since it first was presented (), the Lyman-break technique had stood out as one of the most powerful tools for identifing high-z galaxy candidates. As of this writing, more than 550 candidates have been spectroscopically confirmed to be at z ~ 3 over an area ~ 0.3 square degrees and about 50 at z ~ 4 over an area ~ 0.23 square degrees (). In number density, the bright ends of the z ~ 3 and z ~ 4 populations have similar values to the local L > L* galaxies, for a flat cosmology. Even if merging may have played a role in changing these values over time, just the number of stars contained in Lyman-break galaxies at z > 2 accounts for ~ 20-30% of all the stars known today. The basic fact is that the Lyman-break galaxy populations are a significant fraction of the total galaxy population today. Thus, understanding the nature of the Lyman-break galaxies remains a gateway to understanding the evolution of galaxies.
The identification of high-z candidates is based on the detection of the Lyman break at 912 Å, which is the strongest discontinuity in the stellar continuum of star-forming galaxies. A galaxy at, say, z = 3 will have the Lyman break redshifted to 3648 Å. If a pair of filters is chosen to straddle the break, the galaxy will appear extremely red in this color. In order to avoid as much as possible low-z interlopers, one or more filters are generally added longward of the Lyman break to select only candidates which are blue in this(these) additional color(s). With a careful selection of the color criteria, the Lyman-break technique is extremely successful at identifying high-z candidates; spectroscopic confirmations give a ~ 95% success rate for the z ~ 3 sample and a ~ 80% success rate for the z ~ 4 sample (, , ). The lower success rate at z ~ 4 is due to the incidence of low-z interlopers, namely elliptical galaxies at z ~ 0.5-1 whose 4,000 Å break falls inside the selection window of the filters.
While the determination of the intrinsic nature of the Lyman-break galaxies, whether they are massive systems or galaxy fragments, and what kind of progenitors they are, is still a source of heated debate (e.g., , , ), the identification of their observational low-z counterparts appears less controversial.
By selection, Lyman-break galaxies are UV-bright, actively star-forming systems, with a preferentially blue spectral energy distribution (SED). Observed star formation rates (SFRs) range from a few to 50 M yr-1, for a Salpeter Initial Mass Function (IMF) in the range 0.1-100 M (). This range of values is typical of what observed in Local, UV-bright starburst galaxies (e.g., ). The restframe UV and B-band half-light radii are around 0.2-0.3 arcsec, which correspond to spatial radii ~ 1-3 h50-1 kpc, depending on q0 (, ). The similarity of the half-light radii at both UV and B suggests that the UV is a reliable tracer of the full extent of the light-emitting body. Ground-based optical spectra, which correspond to the restframe 900-1800 Å range for a z ~ 3 galaxy, show a wealth of absorption features, and sometime P-Cygni profiles in the CIV(1550 Å) line (cf. the figures in ), typical of the predominance of young, massive stars in the UV spectrum. Currently limited ground-based near-IR spectroscopy (e.g. ) has revealed nebular line emission in these galaxies. Hybrid line equivalent widths constructed using the UV flux density f(UV) as denominator, namely, EW'(Å) = F(line)/f(UV), show that the observed values for the high-z galaxies fall in the loci observed for local starburst galaxies (). In summary, the observational properties of the Lyman-break galaxy population fully resemble, in the restframe UV-optical range, those of low-redshift, UV-bright starburst galaxies ().
The Lyman-break galaxies share another global characteristic with the Local starbursts. If we parametrize the observed UV stellar continuum with a power law, F() , Lyman-break galaxies cover a large range of values, roughly from -3 to 0.4, namely from very blue to moderately red (Figure 1, left panel). This range is not very different from that covered by the Local, UV-bright starbursts (Figure 1, right panel). Population synthesis models (e.g. ) indicate that a dust-free, young starburst or constant star-formation population have invariably values of < -2.0, for a vast range of metallicities. What does cause the UV stellar continuum of Lyman-break galaxies to be redder than expected for a young star-forming population?
Figure 1. (Left Panel) The distribution of the observed UV spectral indices of the z 3 galaxies (M. Dickinson 1998, private communication). Only a small fraction of all the galaxies are blue enough ( < -2) to be classifiable as dust-free, young star-forming populations. Different symbols corrsponds to different color selections. (Right Panel) The distribution of for a UV-bright sample of Local starburst galaxies (). Note that the range of values is similar to the high-z sample. The UV spectral slopes of the local starbursts correlate with Es(B - V), the color excess of the optical stellar continuum due to dust reddening (, ). Blue UV spectra correspond to small values of the color excess, red UV spectra to large values of Es(B - V). The best fit line through the data is shown.