Population III star clusters in the reionized Universe

In reionized regions of the Universe, gas can only collapse to form stars in dark matter (DM) haloes which grow to be sufficiently massive. If star formation is prevented in the minihalo progenitors of such DM haloes at redshifts z greater than or similar to 20, then these haloes will not be self-enriched with metals and so may host Population (Pop) III star formation. We estimate an upper limit for the abundance of Pop III star clusters which thus form in the reionized Universe, as a function of redshift. Depending on the minimum DM halo mass for star formation, between of the order of 1 and of the order of 1000, Pop III star clusters per square degree may be observable at 2 less than or similar to z less than or similar to 7. Thus, there may be a sufficient number density of Pop III star clusters for detection in surveys such as the Deep-Wide Survey (DWS) to be conducted by the James Webb Space Telescope. We predict that Pop III clusters formed after reionization are most likely to be found at z greater than or similar to 3 and within similar to 40 arcsec (similar to 1 Mpc comoving) of DM haloes with masses of similar to 1011 M-circle dot, the descendants of the haloes at z similar to 20 which host the first galaxies that begin reionization. However, if star formation is inefficient in the haloes hosting Pop III clusters due to the photoionizing background radiation, these clusters may not be bright enough for detection by the Near-Infrared Camera which will conduct the DWS. None the less, if the stellar initial mass function (IMF) is top-heavy the clusters may have sufficiently high luminosities in both Ly alpha and He ii lambda 1640 to be detected and for constraints to be placed on the Pop III IMF. While a small fraction of DM haloes with masses as high as similar to 109 M-circle dot at redshifts z less than or similar to 4 are not enriched due to star formation in their progenitors, external metal enrichment due to galactic winds is likely to preclude Pop III star formation in a large fraction of otherwise unenriched haloes, perhaps even preventing star formation in pristine haloes altogether after reionization is complete at z similar to 6.