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Metal Enrichment and Reionization Constraints on Early Star Formation

The epoch of reionization and formation of first stars are inter-linked topics that are of considerable interest. We use a simplified approach for studying formation of stars in collapsed haloes, and the resulting ionization of the inter-galactic medium (IGM). We consider a set of LCDM models allowed by observations of CMB temperature and polarization anisotropies for this study. We constrain parameters related to star formation with the help of observations. We constrain subsets of these parameters independently by using the observed metallicity of the inter-galactic medium at z ~ 5 and the requirement that the Thomson scattering optical depth due to an ionized IGM as determined for the model from CMB observations be reproduced. We consider a range of initial metalicities for star forming gas, and some variations of the initial mass function of stars. We find that a "normal" initial mass function (IMF) may satisfy these two constraints with a raised efficiency of star formation as compared to that seen in the local universe. Observations require a significant fraction of metals to escape from haloes to the IGM. We can also place constraints on the ratio of escape fraction for metals and ionizing photons, we find that this ratio is of order unity for most models. This highlights the importance of using the constraints arising from enrichment of the inter-galactic medium. (Abridged)