Progress in the field of distant galaxies has been rapid. Young, star-forming galaxies have been located by several means, and studied from space and the ground to magnitudes as faint as V = 28 and redshifts as large as z = 5.7, perhaps to even z = 6.7 or higher. A population of faint, dusty galaxies has been detected in the sub-mm region; their redshift distribution remains uncertain for now.
Observations with HST have established the prevalence of small protogalaxy candidates at high redshift. How and when do they merge to form the familiar Hubble types observed locally?
The last review of the subject of primeval galaxies in this journal, only 5 years ago, was largely a census of nondetection limits and a discussion of theoretical expectations of the predicted widespread population of young galaxies at high redshift (Pritchet 1994). The field is expanding so fast now that this review will be largely outdated at the time of press. We conclude with a brief discussion of ripe avenues for the field of deep extragalactic studies. Several of the next steps will involve new instruments and satellites slated for commissioning in the coming months and years. We also suggest some ventures to longer wavelengths that those typically employed in contemporary early universe studies. It is, of course, the redshift that pushes us in that direction.
Currently, much of the early universe extragalactic research has focused on simply procuring redshifts, identifying objects at earlier and earlier cosmic epoch, and studying the spatial clustering of (the Lyman-break) population. Spectroscopy has the potential to provide considerably more information. Detailed, higher resolution, higher S/N observations of some of the brighter Lyman-break galaxies can probe the ages, kinematics, and abundances of young (proto-)galaxies. Spinrad et al. (1999) suggests, from a very limited sample, that Ly emission strength anticorrelates with the strength of the rest-frame UV interstellar absorption lines. More recently, Pettini et al. (2000) have reported on intermediate resolution, high S/N spectroscopy of the lensed galaxy MS 1512-cB58 at z = 2.727. This detailed study probes the stellar IMF at early cosmic epoch, finding no evidence for a flatter IMF (at the high-mass end) or an IMF deficient in high-mass stars. They measure a metallicity of 0.25 solar and bulk outward motions of 200 km s-1, which may be an important mechanism of distributing metals in the IGM. This pioneering study lays the groundwork for follow-up studies on larger samples of (unlensed) sources.
The new generation of near-infrared spectrographs on large-aperture, ground-based telescopes such as the Keck Near Infrared Spectrometer (NIRSPEC) and the Very Large Telescope (VLT) Infrared Spectrometer And Array Camera (ISAAC), opens the window on observing the early universe to unprecedented redshifts. The first near-infrared detection of Ly emission will be a technological feat. What search techniques will prove most efficient at identifying sources at z 10, assuming that sufficiently luminous sources have collapsed at that redshift? These cameras should be sensitive to line emission at flux densities 10-17 ergs cm-2 s-1 out to ~ 2.5 µm in the windows between telluric OH emission. An hour spectrum with a 10 m telescope at resolving power R ~ 2000 should yield a detection at an S/N ~ 5 for an unresolved emission line of that strength. A more realistic, slightly resolved (spatially and spectrally) emission line would require several hours to yield an S/N of a few per resolution element. Detection of the anticipated 0.5 µJy continuum longward of Ly will remain challenging: NIRSPEC, as an example, will require 6 hr on integration to reach S/N ~ 1 per resolution element; multiple binning will be requisite.
Several new telescopes, cameras, and satellite missions are expected to be commissioned shortly. At sub-mm wavelengths, more sensitive cameras with improved spatial resolution such as SCUBA+ and BOLOCAM will be having first light in the next few years. Satellites such as SIRTF, Chandra, and XMM will open up the mid-infrared and X-ray universe considerably.
With the prospect of NGST becoming more realistic, one can consider deep imaging and spectroscopy in the infrared, perhaps to ~ 4 µm. This would open up the "dark age" to extreme, unprecedented redshifts; at z = 25, Ly propagates to 3.16 µm and we are probing the universe at a time less than 100 h50-1 Myr after the big bang. Imaging above and below this wavelength might be an excellent diagnostic for the very early quasars, galaxies, and supernovae.
The millimeter region of the spectrum has been a good region for molecular studies of our Milky Way galaxy. In the future it may also be the domain of choice for redshifted coolant lines such as [C II] 158 µm. This transition is potentially strong in both H I and H II regions; some local starbursts emit between 10-2 and 10-3 of the infrared (dust) continuum in this line. Its application to large redshifts is still uncommon, but unless the C/H ratio in young galaxies is disastrously low, it seems worth attempting detection of high-redshift [C II] 158 µm with modern millimeter interferometers. At z = 7, for example, the line redshifts to 1.26 mm. The opportunity then would be substantial - even for some physical study of a very distant (gas-rich) stellar system.
We are indebted to our close collaborators Andrew Bunker and Arjun Dey for extensive conversations on the subject of distant galaxies and helping shape our conception of the deep universe. We also gratefully acknowledge conversations and communications with Kurt Adelberger, Len Cowie, Carlos De Breuck, Mark Dickinson, George Djorgovski, Esther Hu, Ken Lanzetta, Curtis Manning, Pat McCarthy, Ed Moran, Leonidas Moustakas, Adam Stanford, Chuck Steidel, Eduard Thommes, Wil van Breugel, and Rogier Windhorst. We thank Alberto Fernández-Soto for providing Figure 6, Ken Lanzetta for providing Figure 10, Trinh Thuan for sharing the HST/GHRS spectrum of T1214-277, and Dave Hollenbach for conversations regarding the [C II] 158 µm emission-line strength. We are also indebted to Sam Maxie for considerable typographical efforts on the early draft of this manuscript. We acknowledge NSF grant AST 95-28536 for supporting some of the extragalactic research presented herein. Research by D. S. has been supported by IGPP/LLNL grant 98-AP017.