Book Description

Long [gamma]-ray bursts (GRBs) are uniquely powerful explosions at cosmological distances. As they mark the deaths of massive stars, they act as beacons of star formation and point out faint galaxies in the distant universe. Thus, they allow us to probe the conditions and the evolution of galaxy formation and metal enrichment throughout the universe. However promising as these prospects are, they need rely on a firm foundation based on the understanding of how the formation of gamma-ray bursts depend on the galactic environments. That is, do GRBs trace all star formation, or are they biased to metal poor and low mass hosts? Here I will explore the host galaxies of these events in order to understand how they relate to the properties of their galaxy populations. Like gamma ray bursts, core-collapse supernovae (CCSN) are the "grand-finale" of the life of massive stars. Providing a census of all massive star formation, they are an ideal control group to compare GRB hosts with at low redshifts. I employ this method to compare restframe properties of the host populations, concluding that GRB hosts are in comparison to CCSN hosts drawn from a compact, low mass and irregular galaxy population. This suggests an inherent bias amongst GRB progenitors, and that they prefer low metallicity environments. Furthermore, the GRB locations on their hosts have higher surface luminosities than for CCSNe, suggesting that GRB progenitors are more massive and short lived than those of CCSNe. Although the low redshift sample only appear to trace star formation in sub-luminous irregular galaxies, I will also show that this need not be strictly true everywhere: I will study the luminosity-metallicity and mass-metallicity relations of GRB hosts up to z ~ 6, and show that at high redshift where the universal metallicity is lower than in the present day universe, GRB hosts appear to follow the metallicity relations of that era. While GRBs might be biased tracers of star formation in the local universe, this suggests that above z ~> 3, the universal metal enrichment is low enough that GRBs trace all star formation. Even at intermediate redshift, I will show that not all GRB hosts are blue and sub-luminous. The host of the dark burst GRB080207 is extremely red, massive and with high inferred dust and gas content. I will discuss how the difficulties of obtaining accurate positions for highly extinguished bursts may have adversely affected host samples and follow-up strategies, and show that the increasing number of well studied dark burst suggest that many of them are massive and dust rich. This implies that, even at lower redshifts, a complete census of all GRBs may trace a higher fraction of star formation then inferred by only optically bright bursts.