Galaxies as seen through the most Energetic Explosions in the Universe
  A gamma-ray burst (GRB) is a strong and fast gamma-ray emission from the
explosion of stellar systems (massive stars or coalescing binary compact
stellar remnants), happening at any possible redshift, and detected by space
missions. Although GRBs are the most energetic events after the Big Bang,
systematic search (started after the first localization in 1997) led to only
374 spectroscopic redshift measurements. For less than half, the host galaxy is
detected and studied in some detail. Despite the small number of known hosts,
their impact on our understanding of galaxy formation and evolution is immense.
These galaxies offer the opportunity to explore regions which are
observationally hostile, due to the presence of gas and dust, or the large
distances reached. The typical long-duration GRB host galaxy at low redshift is
small, star-forming and metal poor, whereas, at intermediate redshift, many
hosts are massive, dusty and chemically evolved. Going even farther in the past
of the Universe, at z > 5, long-GRB hosts have never been identified, even with
the deepest NIR space observations, meaning that these galaxies are very small
(stellar mass < 10^7 M_sun). We considered the possibility that some high-z
GRBs occurred in primordial globular clusters, systems that evolved drastically
since the beginning, but would have back then the characteristics necessary to
host a GRB. At that time, the fraction of stellar mass contained in proto
globular clusters might have been orders of magnitude higher than today. Plus,
these objects contained in the past many massive fast rotating binary systems,
which are also regarded as a favorable situation for GRBs. The common factor
for all long GRBs at any redshift is the stellar progenitor: it is a very
massive rare/short-lived star, present in young regions, whose redshift
evolution is closely related to the star-formation history of the Universe.
