The above discussion has focussed on identifying young galaxies at the highest accessible redshifts. The observed optical emission then necessarily samples the rest-frame UV. However, galaxy formation is an ongoing process; locally we see several galaxies with metallicities close to primordial (Kunth et al. 1994; Thuan & Izotov 1997). These nearby young systems sample a different segment of the galaxy luminosity function from the high-redshift systems discussed above - the expectation is that high-redshift galaxies are the progenitors of present-day massive galaxies (L* systems and larger) while the local low-metallicity systems are all very low-mass dwarf galaxies. Nevertheless, high signal-to-noise (S/N) UV observations of the local protogalaxy population provide a very useful laboratory for studying and understanding the high-redshift population.
As mentioned earlier, models predict strong (50-200 Å equivalent width) Ly emission from young, dust-free galaxies forming their first generation of stars (e.g., Charlot & Fall 1991). However, International Ultraviolet Explorer (IUE) observations of local star-forming galaxies revealed Ly strengths considerably weaker than predicted by case B recombination: the Ly / H intensity ratio was always found to be 10, as opposed to the theoretical value of 33. Furthermore, some galaxies showed Ly absorption rather than emission. Small amounts of dust intermixed with the extended neutral gas was the assumed culprit (e.g., Hartmann, Huchra, & Geller 1984). Hartmann et al. (1988) showed evidence for an anticorrelation of Ly strength with metallicity, which conformed to a simplistic scheme of chemical enhancement and motivated searches for strong Ly emission in distant galaxies with anticipated primordial abundances. More recently, HST observations of the two most metal-deficient galaxies known, I Zw 18 (Z = Z / 51; Kunth et al. 1994) and SBS 0335-052 (Z = Z / 40; Thuan & Izotov 1997) show Ly in absorption rather than emission, at odds with the results of Hartmann et al. (1988).
Kunth et al. (1998a) present HST UV spectra of eight H II galaxies covering a wide range of metallicity. The observations were designed to cover both the Ly region and the region around O I 1302 and Si II 1304. The former region allows study of the Ly emission and absorption properties and an estimate of the H I column, while the latter allows a crude estimate of the chemical composition of the gas and a measure of the velocity of the gas with respect to the systemic velocity of the system as measured from optical emission lines. Surprisingly, they find that the primary indicator of Ly strength is kinematics, not metallicity. The four systems with metallic lines static with respect to the ionized gas show damped Ly absorption, while the four systems with Ly emission show the metallic lines blueshifted by 200 km s-1 with respect to the ionized gas. The implications are that even nearly primordial clouds undergoing star formation have sufficient dust columns to suppress Ly emission provided the kinematics of the neutral gas allows resonant scattering of the Ly emission. In all cases reporting Ly emission in the Kunth et al. (1998a) sample, an asymmetric profile with a sharp blue cutoff is observed.
In addition to the local star-forming galaxies with (1) broad, damped Ly absorption centered at the wavelength corresponding to the redshift of the H II gas and (2) galaxies with Ly emission marked by blueshifted absorption features, Kunth et al. (1998b) notes a third morphology of Ly line that is occasionally observed in the local universe: (3) galaxies showing "pure" Ly emission, i.e., galaxies which show no Ly absorption whatsoever and have symmetric Ly emission profiles. Terlevich et al. (1993) presents IUE spectra of two examples: C0840+1201 and T1247-232, both of which are extremely low-metallicity H II galaxies. Thuan, Izotov, & Lipovetsky (1997) present a high S/N HST spectrum of the latter galaxy, noting that with Z = Z / 23, it is the lowest metallicity local star-forming galaxy showing Ly in emission. At higher S/N and higher dispersion than the IUE spectrum, the line shows multiple absorption features near the redshift of the emission, bringing into question the "pure" designation. Tenorio-Tagle et al. (1999) have proposed a scenario to explain the variety of Ly profiles based on the hydrodynamical evolution of superbubbles powered by massive starbursts.
This scenario and observations of local star-forming galaxies have two important implications for studies of high-redshift protogalaxies. First, they provide a natural explanation for asymmetric profiles which seem to characterize high-redshift Ly (e.g., see Fig. 9), but also imply that although the asymmetric profile may be a sufficient condition for identification of a strong line with Ly, it is not a necessary condition. This point is particularly important for judging the identification of serendipitous and narrowband survey emission sources whose spectra are dominated by a solitary, high equivalent width emission line. Second, if Ly emission is primarily a function of kinematics and perhaps evolutionary phase of a starburst, attempts to derive the comoving star formation rate at high redshifts from Ly emission will require substantial and uncertain assumptions regarding the relation of observed Ly properties to the intrinsic star formation rate; use of the UV continuum ( 1500 Å) may be preferred for measuring star formation rates.
Figure 9. Co-averaged spectrum of the serendipitously discovered galaxy 0140+326 RD1 at z = 5.34 (I = 26.1; Dey et al. 1998). The total exposure time is 36.2 ks. Note the strong Ly emission and the continuum discontinuity shortward of the line which confirms the redshift identification. The "features" observed in the continuum are largely due to residuals from the subtraction of strong telluric OH emission lines (e.g., 8344 Å). The inset shows the asymmetric profile of the Ly emission line. Figure courtesy Dey et al. (1998).