If the reheating and ionization were due to OB stars, it is straightforward to calculate how many supernovae would have gone off, in each comoving volume, as a direct consequence of this output of UV, also how many supernovae would be implicated in producing the heavy elements detected in quasar absorption lines: there would be one, or maybe several, per year in each square arc minute of sky (Miralda-Escudé and Rees 1997). The precise number depends partly on the redshift and the cosmological model, but also on the uncertainties about the UV background, and about the actual high-z abundance of heavy elements.
These high-z supernovae would be primarily of Type 2. The typical observed light curve has a flat maximum lasting 80 days. One would therefore (taking the time dilation into account) expect each supernova to be near its maximum for nearly a year. It is possible that the explosions proceed differently when the stellar envelope is essentially metal-free, yielding different light curves, so any estimates of detectability are tentative. However, taking a standard Type 2 light curve (which may of course be pessimistic), one calculates that these objects should be approximately 27th magnitude in J and K bands even out beyond z = 5. The detection of such objects would be an easy task with the NGST (Stockman 1998). With existing facilities it is marginal. The best hope would be that observations of clusters of galaxies might serendipitously reveal a magnified gravitationally-lensed image from far behind the cluster.
The first supernovae may be important for another reason: they may generate the first cosmic magnetic fields. Mass loss (via winds or supernovae permeated by magnetic flux) would disperse magnetic flux along with the heavy elements. The ubiquity of heavy elements in the Lyman alpha forest indicates that there has been widespread diffusion from the sites of these early supernovae, and the magnetic flux could have diffused in the same way. This flux, stretched and sheared by bulk motions, can be the `seed' for the later amplification processes that generate the larger-scale fields pervading disc galaxies.