The impact of star formation and gamma-ray burst rates at high redshift on cosmic chemical evolution and reionization

Abstract

Recent observations in the total luminosity density have led to significant\nprogress in establishing the star formation rate (SFR) at high redshift.\nConcurrently observed gamma-ray burst rates have also been used to extract the\nSFR at high redshift. The SFR in turn can be used to make a host of predictions\nconcerning the ionization history of the Universe, the chemical abundances, and\nsupernova rates. We compare the predictions made using a hierarchical model of\ncosmic chemical evolution based on three recently proposed SFRs: two based on\nextracting the SFR from the observed gamma-ray burst rate at high redshift, and\none based on the observed galaxy luminosity function at high redshift. Using\nthe WMAP/Planck data on the optical depth and epoch of reionization, we find\nthat only the SFR inferred from gamma-ray burst data at high redshift suffices\nto allow a single mode (in the initial mass function) of star formation which\nextends from z = 0 to redshifts > 10. For the case of the more conservative SFR\nbased on the observed galaxy luminosity function, the reionization history of\nthe Universe requires a bimodal IMF which includes at least a coeval high (or\nintermediate) mass mode of star formation at high redshift (z> 10). Therefore,\nwe also consider here a more general bimodal case which includes an\nearly-forming high mass mode as a fourth model to test the chemical history of\nthe Universe. We compute the abundances of several trace elements, as well as\nthe expected supernova rates, the stellar mass density and the specific SFR,\nsSFR, as a function of redshift\n for each of the four models considered. We conclude that observational\nconstraints on the global metallicity and optical depth at high redshift favor\nunseen faint but active star forming galaxies as pointed out in many recent\nstudies.\n