Implications of WMAP Observations On the Population III Star Formation Processes

Abstract

In an earlier paper (Cen 2003) we pointed out the strong likelihood for the universal reionization to occur twice, giving rise to a larger Thomson optical depth. Here we perform a more focused analysis of the Thomson optical depth in light of the WMAP observations. While the current uncertainties on the observed Thomson optical depth are still relatively large, with tau_e=0.17+-0.04 (68%) (Kogut et al 2003), important implications on 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 ionizing photon escape fraction at high redshift (z>6), we find tau_e =< 0.09; (2) with a top-heavy IMF for the Pop III metal-free stars and plausible star formation efficiency and ionizing photon escape fraction, it is expected that tau_e =< 0.12; (3) it is possible to reach tau_e = 0.15, if the metal enrichment efficiency of the intergalactic medium by Pop III stars is very low thus Pop III era is prolonged; (4) to reach tau_e >= 0.17 requires either of the following two 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_*(H_2,III)>0.01 (with ionizing photon escape fraction greater than 30%). If the current observed value of Thomson optical depth withstands future data, we will have strong observational evidence that Pop III stars are massive and their formation efficiency may be much higher than current theoretical works suggest. Alternatively, there may be unknown, non-stellar ionizing sources at very high redshift.