The nature of the dwarf starforming galaxy associated with GRB 060218/SN 2006aj

Context: We present high resolution VLT UVES and low resolution FORS optical spectroscopy of supernova 2006aj and its host galaxy, associated with the nearby (z = 0.03342) gamma-ray burst GRB 060218. This host galaxy is a unique case, as it is one of the few nearby GRB host galaxies known, and it is only the second time high resolution spectra have been taken of a nearby GRB host galaxy (after GRB 980425). Aims: The resolution, wavelength range and S/N of the UVES spectrum combined with low resolution FORS spectra allow a detailed analysis of the circumburst and host galaxy environments. Methods: We analyse the emission and absorption lines in the spectrum, combining the high resolution UVES spectrum with low resolution FORS spectra and find the metallicity and chemical abundances in the host. We probe the geometry of the host by studying the emission line profiles. Results: Our spectral analysis shows that the star forming region in the host is metal poor with 12 + log(O/H) = 7.54+0.17-0.10 (~0.07 Zo), placing it among the most metal deficient subset of emission-line galaxies. It is also the lowest metallicity found so far for a GRB host from an emission-line analysis. Given the stellar mass of the galaxy of ~107 Mo and the SFRHalpha = 0.065 ± 0.005 Mo yr-1, the high specific star formation rate indicates an age for the galaxy of less than ~200 Myr. The brightest emission lines are clearly asymmetric and are well fit by two Gaussian components separated by Eœ 22 km s-1. We detect two discrete Na I and Ca II absorption components at the same redshifts as the emission components. We tentatively interpret the two components as arising from two different starforming regions in the host, but high resolution imaging is necessary to confirm this. Based on observations obtained at the ESO VLT under ESO programme 076.A-0737(A).

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