Metal enrichment and reionization constraints on early star formation

Abstract

The epoch of reionization and formation of first stars are interlinked topics that are of considerable interest. We use a simplified approach for studying the formation of stars in collapsed haloes and the resulting ionization of the intergalactic medium (IGM). We consider a set of Λ cold dark matter models allowed by observations of cosmic microwave background 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 IGM 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 metallicities for star-forming gas, and some variations of the initial mass function (IMF) of stars. We find that a 'normal' 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, and find that this ratio is of order unity for most models. This highlights the importance of using the constraints arising from enrichment of the IGM. Ultrahigh mass stars or active galactic nuclei may not simplify models of reionization in that these may produce more ionizing photons but these do not contribute to the production of metals and hence help in reducing only the escape fraction for ionizing photons. However, suppression of very low mass stars is helpful in that it increases the production 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 escape fractions with cosmological parameters.