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

Abstract

GRB 060505 was the first well-observed nearby possible long-duration 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~212km/s corresponding to a mass of 1.14x10^11 M_solar 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 Halpha line measurements, of around 6 Myr which corresponds to the lifetime of a 32 M_solar star. The metallicity derived from several emission line measurements is lowest at the GRB site with 1/5 Z_solar but roughly solar in the rest of the galaxy. Using the 2dF 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 south west 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.