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3.3 Detecting `Pregalaxies' at Very High Redshift

What is the chance of detecting the ancient `pregalaxies' that ionized the IGM at some redshift zi > 5? The detectability of these early-forming systems, of subgalactic mass, depends which of the two scenarios in 3.2 (above) is nearer the truth. If B were correct, the individual high-z sources would have magnitudes of 31, and would be so common that there would be about one per square arc second all over the sky; on the other hand, option A would imply a lower surface density of brighter (and more readily detectable) sources for the first UV (Miralda-Escudé and Rees (1998), Barkana and Loeb (1999)). There are already some constraints from the Hubble Deep Field, particularly on the number of `miniquasars' (Haiman, Madau and Loeb 1999). Objects down to 31st magnitude could be detected from the ground by looking at a field behind a cluster where there might be gravitational-lens magnification, but firm evidence is likely to await NGST.

Note that scenarios A and B would have interestingly different implications for the formation and dispersal of the first heavy elements. If B were correct, there would be a large number of locations whence heavy elements could spread into the surrounding medium; on the other hand, scenario A would lead to a smaller number of brighter and more widely-spaced sources.