Gamma-Ray Bursts And Host Galaxies

Abstract

We review the relation between Gamma‐Ray Bursts (GRBs) and host galaxies as they have been observed so far. The rate of discovery of GRBs has more than doubled with the launch of Swift; and more importantly, this new satellite has given much more precise and fast (“swift”) localizations of the bursts, allowing other telescopes (optical, x‐ray, and radio) to search for afterglows and counterparts, i.e. find underlying host galaxies. The number of host galaxies observed for GRBs, however, has remained limited, as the search has been non‐uniform and non‐systematic, and the reporting on such investigations has been sporadic. Furthermore, various selection effects and built‐in biases have plagued everyone’s analysis and understanding of the correlation between GRBs (e.g. long/short, young/old) and the host galaxies found, and it is interesting to ask whether the analysis of the GRBs for which host galaxies have been observed shows the hosts to be statistically representative of hosts of all bursts or not. We address that in the second part of this paper. Hosts of GRBs have also emerged as an important population of objects, particularly those with high redshifts, for various reasons that we explain, including star formation and its evolution in the universe. Issues relating GRBs and their host galaxies have also lately started to be examined by researchers; some have focused on long GRBs and examined the properties of their hosts, others have focused on short bursts, and some have examined “dark bursts” and their hosts. Bias and coincidence issues have recently been the subject of useful investigations. And although the selection effects and other bias factors are difficult to assess, we do address them (indirectly) in our own analysis here, for example by comparing the physical parameters of sources detected by Swift (only) and those detected by other Swift‐era or pre‐Swift instruments. Since there are enough GRBs for which the redshift/distance has been determined (with some degree of precision), we are able to perform statistical analyses on burst data (fluence, redshift, burst duration, etc.) obtained or gathered by Swift; we have thus calculated means and standard deviations and performed Kolmogorov‐Smirnov tests, plotting histograms and cumulative distributions of the most interesting parameters for the group of GRBs that have observed host galaxies and the group of bursts that do not (so far) have identified hosts. The analysis quickly and easily shows that host galaxies tend to be observed more for bursts of low redshifts; for the other parameters, we find the isotropric luminosity is statistically different for the two groups (K‐S“statistic” = 0.55 and 0.46, respectively, giving a significance probability of about 4.6×10−7 and 5.5×10−5, respectively). Other galaxy host properties (e.g. extinction coefficient, metallicity, etc.) probably play a role too, but we have not examined them (yet).