Author: Daniel Perley
Abstract: Only a minute fraction of all massive stars explode as GRBs. Determining whether this fraction is constant factor in all environments or if it depends on external factors (such as metallicity) provides a strong observational constraint on the nature of the GRB progenitor and is essential to understand before the exciting promise of using GRBs as star-formation rate indicators at high redshifts can be realized. To this end, we compile a large sample of Swift GRB host galaxies based on a combination of results from recent large programs with Keck, VLT, HST, and Spitzer, showing unambiguously that the GRB host population at z<1.5 deviates strongly from what would be expected for an unbiased SFR-tracing population (even when the effects of dust extinction are fully accounted for). This effect corresponds to approximately an order-of-magnitude difference in the ratio between the GRB and star-formation rates between high-mass and low-mass galaxies, consistent with the notion of a strong dependence of the GRB rate on metallicity. Interestingly, however, this dependence on host mass almost completely vanishes at z∼2. Using new results from our ongoing legacy project with the Spitzer Space Telescope, we investigate possible causes of this rapid transition with redshift and examine whether or not GRBs may be able to serve as useful star-formation rate tracers at high redshifts despite their pronounced metal-aversion at lower redshifts.