Abstract
We present the first reported case of the simultaneous metallicity determination of a gamma-ray burst (GRB) host galaxy, from both afterglow absorption lines as well as strong emission-line diagnostics. Using spectroscopic and imaging observations of the afterglow and host of the long-duration Swift GRB121024A at z = 2.30, we give one of the most complete views of a GRB host/environment to date. We observe a strong damped Ly-alpha absorber (DLA) with a hydrogen column density of log N(HI) = 21.88 +/- 0.10, H2 absorption in the Lyman-Werner bands (molecular fraction of log(f)~ -1.4; fourth solid detection of molecular hydrogen in a GRB-DLA), the nebular emission lines H-alpha, H-beta, [O II], [O III] and [N II], as well as metal absorption lines. We find a GRB host galaxy that is highly star-forming (SFR ~ 40 solar masses/yr ), with a dust-corrected metallicity along the line of sight of [Zn/H]corr = -0.6 +/- 0.2 ([O/H] ~ -0.3 from emission lines), and a depletion factor [Zn/Fe] = 0.85 +/- 0.04. The molecular gas is separated by 400 km/s (and 1-3 kpc) from the gas that is photoexcited by the GRB. This implies a fairly massive host, in agreement with the derived stellar mass of log(M/M_solar ) = 9.9+/- 0.2. We dissect the host galaxy by characterising its molecular component, the excited gas, and the line-emitting star-forming regions. The extinction curve for the line of sight is found to be unusually flat (Rv ~15). We discuss the possibility of an anomalous grain size distributions. We furthermore discuss the different metallicity determinations from both absorption and emission lines, which gives consistent results for the line of sight to GRB 121024A.
Highlights
The study of gamma-ray burst (GRB) afterglows has proven to be a powerful tool for detailed studies of the interstellar medium (ISM) of star-forming galaxies, out to high redshifts (e.g. Vreeswijk et al 2004; Prochaska et al 2007; Ledoux et al 2009; Sparre et al 2014)
We present the first reported case of the simultaneous metallicity determination of a gamma-ray burst (GRB) host galaxy, from both afterglow absorption lines as well as strong emission-line diagnostics
The intrinsic spectrum of the afterglow is well fitted with simple power-law segments, so the imprints of the intergalactic medium (IGM) as well as the ISM surrounding the burst are relatively easy to distinguish from the afterglow in the observed spectrum
Summary
The study of gamma-ray burst (GRB) afterglows has proven to be a powerful tool for detailed studies of the interstellar medium (ISM) of star-forming galaxies, out to high redshifts (e.g. Vreeswijk et al 2004; Prochaska et al 2007; Ledoux et al 2009; Sparre et al 2014). For GRB damped Lyα absorbers (GRB-DLAs, N (H i)> 2×1020 cm−2 Wolfe et al 2005), a direct comparison of metallicity from the two methods is interesting because it can either provide a test of the strong-line methods or alternatively allow a measurement of a possible offset in abundances in H ii regions and in the ISM. We measure abundances of the GRB host galaxy in absorption and compare them with the metallicity determined by strong-line diagnostics using observed nebular lines from [O ii], [O iii], [N ii] and the Balmer emission lines.
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