The Implications of Wilkinson Microwave Anisotropy Probe Observations for Population III Star Formation Processes

Abstract

In an earlier paper, we pointed out the strong likelihood for universal reionization to occur twice, giving rise to a much larger Thomson optical depth due to the intergalactic medium than that in the case of a single rapid reionization at z ~ 6. The latest Wilkinson Microwave Anisotropy Probe (WMAP) observations of Kogut et al. indicate that the universe indeed appears to have entered a significantly ionized state at a very high redshift. In light of this new development, we perform a more focused analysis of the Thomson optical depth in the context of the spatially flat, cosmological constant-dominated cold dark matter model (?CDM) constrained by WMAP observations. While the current uncertainties of the observed Thomson optical depth are still relatively large, with ?e = 0.17 ? 0.04 (68%), important implications for Population III (Pop III) star formation processes at high redshift can already be inferred. We are able to draw four conclusions: (1) In the absence of a top-heavy initial stellar mass function (IMF) for Pop III metal-free stars and without a dramatic upturn in the star formation efficiency and in the ionizing photon escape fraction at high redshift (z > 6), we find ?e ? 0.09. (2) With a top-heavy IMF for the Pop III metal-free stars, a plausible star formation efficiency, and an ionizing photon escape fraction ?20%, it is expected that ?e ? 0.12, which could be raised to ?e = 0.14, if the metal-enrichment efficiency of the intergalactic medium by Pop III stars is reduced by a factor of 5. (3) To reach ?e = 0.15-0.17 requires at least one of the following three conditions: the cosmological model power index n is positively tilted to n ? 1.03, Pop III star formation in minihalos with molecular hydrogen cooling has an efficiency c*(H2, III) > 0.01, or the ionizing photon escape fraction from Pop III galaxies is close to unity. The highest ?e that we have obtained is 0.24 for a WMAP-consistent ?CDM model with n = 1.03, a Pop III star formation efficiency of 1% for minihalos, and an ionizing photon escape fraction of 100% from Pop III galaxies. (4) For the WMAP-normalized running index ?CDM model, we obtain ?e = 0.12 with a Pop III star formation efficiency of 1% for minihalos and an ionizing photon escape fraction of 100% from Pop III galaxies. If the current observed value of the Thomson optical depth withstands future data, we will have strong observational evidence that Pop III stars are massive and that their formation efficiency may be much higher than current theoretical works suggest. Alternatively, there may be unknown, nonstellar ionizing sources at very high redshift.