IFU observations of the GRB 980425/SN 1998bw host galaxy: emission line ratios in GRB regions

Context. The collapsar model predicts that the progenitors of Gamma-ray Bursts (GRBs) are metal poor in Fe group elements. The existence of low metallicity stellar populations could manifest itself in the characteristics of the GRB site immediate environment in the host galaxy. Aims. We analyse the strong emission lines from the sub-luminous host galaxy of GRB 980425, which showed the first connection with a supernova explosion (SN 1998bw). The host is of sufficient size to allow detailed resolved spectroscopy of individual H II regions and to search for regions with peculiar properties close to the the GRB site. Methods. Using integral field spectroscopy with VIMOS we study most of the high surface brightness part of the host including the H II region where the supernova and GRB occurred. Results. The star formation rate, reddening, equivalent width and stellar mass in the GRB region is similar to other H II regions in the host. Extreme values arise in the only region that shows emission lines from Wolf-Rayet stars, a region that is located 800 pc in projection from the GRB site. Strong emission line diagnostics of all H II regions imply oxygen abundances between 0.3 and 0.8 solar with the lowest values arising in the WR and GRB regions. Including uncertainties from the metallicity diagnostics, all metallicities are similar to within 3 sigma. We demonstrate that there is a good agreement between the luminosity weighted and mass weighted specific star formation rates (SSFR) in individual young H II regions. While the global average of the SSFR is similar to high redshift GRB hosts, there are significant variations between individual resolved H II regions. Comparing the measured emission line ratios of low redshift GRB hosts to theoretical models and observations of field galaxies, we find that GRBs are present in different environment metallicities while the regions of their origin are consistently very young. Similar line ratios of GRB hosts compared with those of the WR region can arise in spatially unresolved galaxies with bright H II regions close to the GRB location.