Simulating high-z Gamma-ray Burst host galaxies
  We investigate the nature of high-z host galaxies of long Gamma-Ray Bursts
(LGRBs) by means of state-of-the-art numerical simulations of cosmic structure
formation and evolution of galaxies. We combine results from different runs
with various box sizes and resolutions. By assigning to each simulated galaxy
the probability to host a LGRB, assumed to be proportional to the mass of young
stars, we provide a full description of the physical properties of high-z LGRB
host galaxy population. We find that LGRBs at z>6 are hosted in galaxies with
typical star formation rates SFR \sim 0.03-0.3 Msun yr^{-1}, stellar masses M
\sim 10^{6-8} Msun, and metallicities Z \sim 0.01-0.1 Zsun. Furthermore, the
ratio between their doubling time and the corresponding cosmic time seems to be
universally equal to ~0.1-0.3, independently from the redshift. The
distribution of their UV luminosity places LGRB hosts in the faint-end of the
galaxy luminosity function, well below the current capabilities of space- or
ground-based optical facilities. This is in line with recent reports of
non-detection of LGRB hosts using extremely deep HST and VLT observations. In
conclusion, high-z LGRBs are found to trace the position of those faint
galaxies that are thought to be the major actors in the re-ionization of the
Universe.
