Spatially resolved properties of the GRB 060505 host: implications for the nature of the progenitor

GRB 060505 was the first well-observed nearby possible long-duration gamma-ray burst (GRB) that had no associated supernova. Here we present spatially resolved spectra of the host galaxy of GRB 060505, an Sbc spiral, at redshift z = 0.0889. The GRB occurred inside a star-forming region in the northern spiral arm at 6.5 kpc from the center. From the position of the emission lines, we determine a maximum rotational velocity for the galaxy of v ~ 212 km s-1, corresponding to a mass of 1.14 × 1011 M⊙ within 11 kpc from the center. By fitting single-age spectral synthesis models to the stellar continuum, we derive a very young age for the GRB site, confirmed by photometric and Ha line measurements, of around ~6 Myr, which corresponds to the lifetime of a 32 M⊙ star. The metallicity derived from several emission-line measurements varies throughout the galaxy and is lowest at the GRB site. Using the two degree field galaxy redshift survey we can locate the host galaxy in its large-scale (~Mpc) environment. The galaxy lies in the foreground of a filamentary overdensity, extending southwest from the galaxy cluster Abell 3837 at z = 0.0896. The properties of the GRB site are similar to those found for other long-duration GRB host galaxies with high specific star formation rate and low metallicity, which is an indication that GRB 060505 originated from a young, massive star that died without making a supernova.