The two fundamental requirements of any PG search are a sound search strategy - covering a sufficient area of sky to a sufficient depth, and an ability to recognise a PG amongst the background of older and more normal galaxies. Therefore, it will be useful to have some idea of what the basic properties of such and object might be.
4.1 Surface Density
An estimate of the surface density of PGs on the sky can be made by extrapolating the volume density of local galaxies (~ 0.015h3 Mpc-3) to high redshift. Suppose that all galaxies form at a particular redshift by gravitationally collapsing over a period of 1 Gyr, rapidly forming their first generation of stars: then the resulting areal density of PGs as a function of their formation redshift is shown in Figure 2. PGs should be very abundant irrespective of the uncertainties in the cosmology, with at least 103 in each area of sky the size of the moon!
|Figure 2. Expected surface density of PGs as a function of their formation redshift, assuming a collapse time of 1 Gyr. The three curves represent the same cosmologies as in Figure 1.|
4.2 Angular Size
This is one of the most difficult properties to estimate. The angular size of a PG depends upon when the bright phase occurred relative to the collapse as well as the redshift and cosmology. If the star formation occurs soon after the onset of collapse then PGs could be low-surface brightness objects with angular diameters 10-30 arcsec (Partridge and Peebles 1967). At z 5 and with a surface density of 105 deg-2 (see Figure 2) PGs would be virtually overlapping on the sky and very difficult to detect as discrete sources using conventional techniques. Possibly the best way to to detect such objects is to investigate the integrated background light resulting from such a population of sources and experiments to do just this have been carried out. At the other extreme is the idea, stretching back a number of years, that the star formation rate (hereafter SFR) is closely coupled to the gas density and hence the bulk of the star formation occurs in the central regions of collapsed PGs (Eggen et al. 1962, Larson 1974). In this case we can expect the visible signature of PGs to have an angular diameter 0.1-0.5 arcsec, making them very difficult to resolve from the ground due to the limitation imposed by atmospheric ``seeing'' irregularities. Such objects would be excellent targets for optical satellite missions, such as the Hubble Space Telescope with its diffraction-limited 0.1 arcsec imaging capability.