4.1 Some Broadband Searches
The first PG searches were motivated by the models of Partridge and Peebles (1967), which suggested that PGs would undergo their first burst of star formation near the maximum expansion phase of a perturbation (z 10). Thus PGs were predicted to be large (~ 10"), red objects. Such sources were searched for by Partridge (1974); his 7 candidate PGs all appear to be normal galaxies at z 0.3 (Koo 1986). Davis and Wilkinson (1974) developed an innovative large aperture (~ 1000 arcsec2) photometer to search for fluctuations in the background that might have been due to a high surface density population of diffuse sources. No evidence was found for such a population, down to a flux level of 0.5% of the red ( 7500Å) night sky brightness for a 100 arcsec2 source. Comparable flux limits were achieved in a search for fluctuations in the infrared K band ( 2.2 µm) by Boughn et al. (1986); this survey is noteworthy insofar as it extended the search for PGs to much higher redshifts (possibly 10).
Further constraints on PG's and high redshift galaxies using broadband surveys may be found in Koo (1986), Tyson (1988), and Guhathakurta, Tyson, and Majewski (1990); see also Smith, Thompson, and Djorgovski (1993).
4.2 Quasars and PG Searches
Models of galaxy formation that included dissipation (Larson 1974) completely changed the search strategy for PGs. PGs were now predicted to be quite compact, and the question naturally arose as to whether there were any PGs lurking in surveys of quasars (e.g., review by Koo 1986; Koo and Kron 1988). The answer appears to be no: for example, none of the ~ 250 QSO candidates in the CFHT survey (Crampton, Cowley, and Hartwick 1990) have spectra which could be placed in the PG category (Hartwick, private communication).
The question nevertheless remains as to whether QSO's themselves could be PGs. This conjecture has a long history that has been reviewed by Koo (1986 and references therein; Djorgovski 1994). Recently Terlevich and collaborators have revived this idea (e.g., Terlevich and Boyle 1993); they propose that quasars could be the luminous, star forming cores of very young elliptical galaxies undergoing their first burst of star formation (but see Heckman 1991, Filippenko 1992). Perhaps a more plausible scenario is one in which the formation of a luminous AGN (powered by conventional means - e.g., infall onto a massive collapsed object) is both associated with and contemporaneous with galaxy formation (Djorgovski 1994): in this case some (though not necessarily all) quasars may be the beacons that signal galaxy formation.
4.3 Redshift Surveys
Deep redshift surveys are generically related to broadband searches for PGs, because the objects for which redshifts are obtained are first identified in broadband images. A number of such redshift surveys have been completed (e.g., Broadhurst, Ellis and Shanks 1988; Colless et al. 1990; Lilly et al. 1991; Colless et al. 1993; Lilly 1993; Tresse et al. 1993), and several others are nearing completion. The limiting magnitudes for these surveys are typically B 22.5 (Colless et al. 1990, 1993), or Ilim 22.5 (Lilly 1993) or 22.1 (Tresse et al. 1993). Many thousands of objects now have redshifts, and few if any have characteristics that could ambiguously lead to classification as a PG.
4.4 Searches for Redshifted 21 cm Radiation from
Gunn and Peterson (1965) first noted that a diffuse neutral intergalactic medium would produce an absorption trough at wavelengths shorter than Ly in the spectra of high-redshift quasars. The fact that no such trough is seen (e.g., Steidel and Sargent 1987) provides an extraordinarily sensitive limit to the cosmological mass density in a diffuse neutral IGM, H I < 10-5: either the IGM is ionized (e.g., Miralda-Escudé and Ostriker 1990), or it is strongly clumped.
In the latter case it is in principle possible to search for redshifted 21 cm radiation from the expected structures (Hogan and Rees 1979). This topic is nevertheless somewhat orthogonal to the rest of this review, for several reasons. First, a typical (~ mJy) sensitivity limit in redshifted 21 cm corresponds to a neutral hydrogen mass ~ 1014 M at z 3 - i.e., a protocluster rather than a protogalaxy. Second, objects observed in redshifted 21 cm are likely to be in an evolutionary stage preceeding the epoch at which stars start to form (in contrast to the other search strategies that we discuss). Finally, and perhaps most important, these searches implicitly assume as their starting point that massive objects have collapsed at early epochs - i.e., that structure forms in a ``top-down'' hierarchy, as proposed by, for example, Sunyaev and Zel'dovich (1975). In contrast, we have for the most part assumed in this review a ``bottom-up'' hierarchy of structure formation - a view that is supported by many lines of reasoning (e.g., Ostriker 1993). Nevertheless, a discussion of such objects is clearly relevant to any discussion of galaxy formation.
Searches for redshifted 21 cm radiation from H I in protoclusters have been reported by a number of groups (Davies, Pedlar, and Mirabel 1978; Bebbington 1986; Hardy and Noreau 1987; Noreau and Hardy 1988), at redshifts ranging from 3.3 to 8.4. Perhaps the most sensitive survey at the low end of this redshift range is that of Uson, Bagri and Cornwell (1991a), who reach a detection limit of around 1014 M in neutral hydrogen. The null results that these authors report are not surprising if structure forms in a bottom-up manner - i.e., as expected in Universes dominated by CDM (but see Uson et al. 1991b).
4.5 Searches for UV-Optical Emission Lines
A PG can be thought of as a giant H II region that has been photoionized by massive stars from the first burst of star formation. Hence it is expected that PGs should possess a rich emission line spectrum; even in the earliest (very low metallicity) phases of PG evolution one might expect strong Balmer and Lyman emission lines. One of the most intensively applied methods for detecting PGs is in fact searching for redshifted Ly 1216Å (Meier 1976; Hogan and Rees 1979), which we discuss extensively in this section of the paper. Other emission lines have been proposed for PG searches in the near-infrared (1-2.5µm); these include [O II] 3727Å (Thompson et al. 1994), and H (Pritchet and Hartwick 1994), which is less affected by dust absorption than Ly.
There are three principal search techniques for faint emission line objects, illustrated in Figure 6. The performance of these techniques is mainly distinguished by volume surveyed and by limiting brightness.
Figure 6. Search strategies for emission line objects. (a) Slitless spectroscopy; (b) long-slit spectroscopy; (c) narrow-band imaging. Although the volume surveyed by slitless spectroscopy is largest of the three techniques, the limiting flux is brighter because each object is superimposed on a night-sky background from all wavelengths passed by the optics.
(1) Slitless spectroscopy [e.g., Fig. 6(a)] allows the sampling of a very large volume of space, but results in a relatively bright limiting threshold, because the sky brightness from all wavelengths passed by the bandpass of the system is superimposed on any spectral feature. This technique is commonly used in QSO grism/grens surveys (e.g., Crampton et al. 1987, 1990), and two different PG searches using this technique have been reported by Koo and Kron (1980). These latter searches reach limiting fluxes ~ 10-14-10-15 erg cm-2 s-1 and cover solid angles ~ 1 deg2.
(2) Long-slit spectroscopy [Fig. 6(b)] generally results in the best limiting flux for detecting (unresolved) emission lines, because the spectral resolution, and hence sky background per resolution element, is lowest. However, the volume surveyed is usually much smaller than that achieved with other techniques. The principal long-slit surveys are those of Cowie and Hu (see Cowie 1988), Lowenthal et al. (1990), and Djorgovski and collaborators (Thompson, Djorgovski, and Trauger 1992; Djorgovski, Thompson, and Smith 1993). Typical limiting fluxes are 10-16-10-17 erg cm-2 s-1, and areas surveyed are 10-4 deg2. Anecdotal evidence suggests that many other groups involved in faint object spectroscopy have searched for serendipitous emission-line objects that happen to lie along their slits, without success. Pritchet and Hartwick (unpublished) have detected several such objects in long-slit spectroscopy with the CTIO 4 meter telescope and RC spectrograph, but they are almost certainly low luminosity [O II] 3727Å emitters - i.e. ``normal'' intermediate redshift galaxies.
(3) Narrow-band imaging [Fig. 6(c)] imposes a restriction on z, the range of redshifts that is surveyed, and so results in a smaller surveyed volume. Most of this work consists of optical searches for Ly at redshifts of order 2-6 using CCDs and narrowband interference filters (e.g., Cowie 1988; Pritchet and Hartwick 1987, 1990; Rhee, Webb, and Katgert 1989; Smith et al. 1989; Wolfe et al. 1992; Djorgovski and Thompson 1992; Djorgovski et al. 1993; De Propris et al. 1993; Møller and Warren 1993; Macchetto et al. 1993), or with a Fabry-Perot system (Djorgovski et al. 1993; Thompson et al. 1992). The limiting flux for these surveys is much improved over slitless spectroscopy - typical flux limits are between 10-16 and 10-17 erg cm-2 s-1, and typical CCD fields now approach 0.01-0.1 deg2 (or larger for CCD mosaics) on 4 meter class telescopes.
More recently, narrow-band imaging surveys have been extended to the infrared. Pritchet and Hartwick (1994) have completed a CCD survey for redshifted Ly at z 7 (~ 9100Å), and have achieved a remarkably faint limiting flux of ~ 10-17 erg cm-2 s-1 by employing narrow band filters designed to avoid the strongest OH emission lines (which are the dominant source of sky brightness in the region 7000Å-2.3µm). Using infrared arrays, Parkes, Collins, and Joseph (1994) have pushed the search for redshifted Ly out to z 9, using a similar ``low OH'' technique in the infrared J band; furthermore, Thompson et al. (1994) have searched for redshifted [O II] 3727Å emission from PGs associated with QSO's at z > 4. These latter two infrared array surveys reach limiting fluxes of order 10-15 erg cm-2 s-1, over relatively small solid angles of 10-4-10-3 deg2 (the small sizes of the surveys being dictated by the small areas of infrared arrays, a limitation that is expected to improve in the future).
The result of all of these surveys is identical: no emission line PGs have been found.
Figure 7 shows the constraints on surface density and limiting flux for observations of redshifted Ly. (Similar diagrams have been presented by a number of authors - e.g., Koo 1986, Pritchet and Hartwick 1990, Djorgovski and Thompson 1992.) We have assumed that the upper limit on surface density is one object per sampled solid angle. The plotted points should be interpreted as excluding the region to the upper left of each point (higher fluxes and higher surface densities). The stippled area roughly combines the limits from different groups; PGs should possess lower fluxes and/or surface densities than the shaded area, and the integral PG luminosity function should not pass through this shaded region. (However it should be noted that the survey limits plotted in Fig. 7 are independent, and hence the combined limit of all the surveys should be even stronger than the shaded area.)
Figure 7. Limits that have been placed on the surface density and apparent Ly flux of PG's, from various searches for redshifted Ly radiation. Each plotted number refers to a separate limit by a different group, and excludes the region to the upper left of the point (i.e., PG's can exist at fainter fluxes or lower surface densities). The shaded region combines the limits from different groups: the integral luminosity function of PG's should not pass through the shaded region. (In fact the limits imposed by combining all of these surveys are even more stringent, because the surveys are independent.) The solid line is a model with z = 3, 0 = 1, and other parameters taken from Fig. 5. This model should not be taken too seriously since the plotted limits refer to surveys at a range of redshifts (2-5). The meaning of the plotted numbers is as follows: 1, 2 - Koo and Kron (1980) photographic and CCD; 3 - Pritchet and Hartwick (1987); 4,5 - Cowie (1988) long slit and narrow band; 6 - Rhee et al. (1989); 7 - Smith et al. (1989); 8 - Pritchet and Hartwick (1990); 9 - Lowenthal (1990); 10 - Wolfe et al. (1992); 11,12,13 - Djorgovski and collaborators (Djorgovski and Thompson 1992, Djorgovski et al. 1993, Thompson et al. 1992, 1993) long slit, two Fabry-Perot surveys; 14 - De Propris et al. (1993); 15 - Macchetto et al. (1993); and 16 - Møller and Warren (1993). See text for further details.
The limits plotted in Fig. 7 are illustrative only. Different groups use different criteria for defining their limiting fluxes (in terms of both the number of noise for a real detection, and also the size of the image), and it is difficult to correct the published information to a uniform system.
The solid line in Fig. 7 represents a rough model computed as in Fig. 5. Insofar as the model cuts through the region excluded by the limits, this model appears to be in disagreement with the observations. However, this model has been computed for z = 3, z = 0.1, whereas the observations cover a range of redshifts (z 2-5) and redshift intervals. To rectify this problem, we convert the observed surface density and flux limits into corresponding volume density and absolute luminosity limits (Fig. 8). In this diagram the limits can be compared with one model independently of zGF; the model in this figure is that described in Section 3 and used (in observational form) in Fig. 5. The meaning of the reference numbers is the same as in Fig. 7.
Figure 8. Limits on PG comoving densities and absolute Ly luminosities. These limits were computed assuming h50 = 1 and 0 = 1. The model is as described in Section 3. The reference numbers for the limits, and the meaning of these limits and the shaded area, can be found in the caption to Fig. 7. See text for further details.
Again it can be seen that primeval galaxies as predicted by our simple dust-free model should have been easily detected if they were unresolved. The discrepancy between model and observations exceeds a factor of 100X in surface density, or 10X in luminosity, if galaxies formed in the redshift interval 2-5. The limits at higher redshift are not as strong at the present time.