Metal enrichment and reionization constraints on early star formation

The epoch of reionization and formation of first stars are int er-linked topics that are of considerable interest. We use a simplified approach for studyin g formation of stars in collapsed haloes, and the resulting ionization of the inter-galactic medium (IGM). We consider a set of �CDM 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 f orming gas, and some variations of the initial mass function of stars. We find that a “normal” i nitial mass function (IMF) may satisfy these two constraints with a raised efficiency of sta r formation as compared to that seen in the local universe. Observations require a significa nt 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. Ultra-high mass stars or AGNs may not simplify models of reionization in that these may produce more ionizing photons but these do not contribute to production of metals and hence these help in reducing only the escape fraction for ionizing photons. However, suppression of very low mass stars is helpful in that it increases the produc tion of metals as well as ionizing photons and hence leads to a reduction in both escape fractions. Such a change is also warranted by observations of metal poor halo stars in the Galaxy. We also discuss correlations in parameters like the efficiency of star formation and the two e scape fractions with cosmological parameters.