Can We Use Grb to Probe High Redshift Star - Formation

— We present a detailed analysis of the selection effects that plague GRB observations. We find that these effects may partially explain the different redshift distributions between BeppoSax/HETE2 and Swift bursts. It is mandatory to consider these effects to determine the redshift evolution of GRBs. PACS 95.55.Ka – cosmology-observations. PACS 98.70.Rz – γ-ray sources; γ-ray bursts. Gamma Ray Bursts (GRBs) are one of the great wonders of Universe. They combine several of the hottest topics of 21 st century astrophysics. On one side they are privileged laboratories for fundamental physics, including relativistic physics, acceleration processes and radiation mechanisms. On the other side, being GRB associated to the death of massive stars, they can be used as a cosmological tool to investigate star-formation and metal enrichment at the epochs of galaxy birth, formation and growth. There are two main research areas to the latter purpose. The first one uses the GRB as a background beacon for spectroscopy of UV lines to characterize the physical and chemical status of the matter along the line of sight. The second includes statistical studies of the GRB redshift distributions [1] to infer information on the histories of the GRB-and star-formation rates and of the metal enrichment in the Universe (e.g. [2]). Although the techniques adopted, and therefore the reference communities, are somewhat different, these research areas are interconnected. As an example, on one hand spectroscopy of UV lines can be used to determine the metal content of the absorption systems. On the other hand, it is well known that galaxy scale properties like metalicity, star-formation rate and mass are correlated. Then, metalicity determinations obtained through GRB spectroscopy could be, at least in principle, plugged in the GRB population studies, to obtain a better constraint on the models. Unfortunately, both research areas are plagued by large uncertainties. (1) Absorption line spectroscopy is certainly a powerful tool. Spectroscopy of galaxies and QSOs is routinely used to study both the galaxy Inter-Stellar-Matter (ISM) and the Inter-Galactic-Matter (IGM) along the line of sight. However, star-formation regions and c Società Italiana di Fisica