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

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: kwrsema@science.uva.nl
² Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁴ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁵ Department of Physics and Astronomy, University of Victoria, Elliott Building, 3800 Finnerty Rd., Victoria, BC, V8P 1A1, Canada
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁷ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁸ INAF, Astronomical Observatory of Trieste, 34131 Trieste, Italy

Received: 18 August 2006
Accepted: 19 December 2006

Abstract

Context.We present high resolution VLT UVES and low resolution FORS optical spectroscopy of supernova 2006aj and its host galaxy, associated with the nearby () 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) =  (~), 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  and the SFR  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 ∼22 km s^-1. We detect two discrete and 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.