The nature of long-GRB host galaxies from chemical abundances

Abstract

Context. Gamma-ray bursts (GRBs) were the most energetic events after the Big Bang and they have been observed up to very high redshift. Measurements of the chemical abundances are now available for the galaxies hosting such events, which are assumed to originate from the explosion of very powerful supernovae (Type Ib/c), and provide the opportunity to study the nature of these host galaxies. Aims. We identify the hosts of long GRBs (LGRBs) observed at both low and high redshift to determine whether the hosts are galaxies of the same type at different cosmic epochs. Methods. We adopt detailed chemical evolution models for galaxies of different morphological type (ellipticals, spirals, irregulars) that follow the time evolution of the abundances of several chemical elements (H, He, α-elements, Fe), and compare the results with the observed abundances and abundance ratios in galaxies hosting LGRBs. Results. We find that the abundances and abundance ratios predicted by models devised for typical irregular galaxies can reproduce the abundances of the hosts at both high and low redshift. We also find that the predicted Type Ib/c supernova rate for irregulars is in good agreement with observations. Models for spirals and particularly ellipticals fit neither the high-redshift hosts of LGRBs (DLA systems) nor the low redshift hosts: in particular, ellipticals cannot possibly be the hosts of gamma-ray bursts at low redshift since they exhibit little star formation, hence no supernovae Ib/c. Conclusions. We conclude that the observed abundance and abundance ratios in LGRBs hosts suggest that these hosts are irregular galaxies at both high and low redshift, thus demonstrating that the host galaxies are the same type of galaxies observed at different ages.© 2010 ESO.