Reconciling the gamma-ray burst rate and star formation histories

While there are numerous indications that gamma-ray bursts (GRBs) arise from the deaths of massive stars, the GRB rate does not follow the global cosmic star formation rate and, within their hosts, GRBs are more concentrated in regions of very high star formation. We explain both puzzles here. Using the publicly available VESPA database of the Sloan Digital Sky Survey (SDSS) Data Release 7 spectra, we explore a multi-parameter space in galaxy properties such as stellar mass, metallicity, and dust to find the subset of galaxies that reproduces the GRB rate recently obtained by Wanderman & Piran. We find that only galaxies with present stellar masses below <10¹⁰ M _⊙and low metallicity reproduce the observed GRB rate. This is consistent with direct observations of GRB hosts and provides an independent confirmation of the nature of GRB hosts. Because of the significantly larger sample of SDSS galaxies, we compute their correlation function and show that they are anti-biased with respect to dark matter: they are in filaments and voids. Using recent observations of massive stars in local dwarfs we show how the fact that GRB host galaxies are dwarfs can explain the observation that GRBs are more concentrated in regions of high star formation than are supernovae. Finally, we explain these results using new theoretical advances in the field of star formation.