Galaxies as seen through the most energetic explosions in the universe

Gamma-ray bursts (GRBs) are explosions of stars (longer than 2
seconds) or merging compact objects (neutron stars / black holes,
shorter than 2 seconds). Although GRBs are the most energetic events
after the Big Bang, their detection and localization are still very
difficult. Systematic search during the past 17 years led to 330
spectroscopic redshifts; for about half of them, the host galaxy was
studied in some detail. Despite the small numbers, their impact on our
understanding of galaxies is large. GRB hosts offer the opportunity to
explore regions of the universe which are observationally hostile for
traditional means, due to gas and dust absorption, or to the large
distance. The typical long-GRB host galaxy at low redshift is small,
star-forming and metal poor, whereas explorations in recent years at
intermediate-redshift have revealed the presence of more massive,
dusty and chemically evolved galaxies. Finally, z > 5 long-GRB hosts
have given so far elusive results, indicating very small galaxies,
smaller than what is reachable today with NIR deep surveys. The common
factor is the stellar progenitor: in long GRBs these are very massive
rare/short-lived stars, present in young regions whose redshift
evolution is closely related to the star-formation history of the
universe.