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 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) = 21.80+/-0.15, H_2
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, OII, OIII and NII, as well as a large
variety of metal absorption lines. We find a GRB host galaxy that is highly
star-forming (SFR ~ 40 Msolar/yr), with a dust-corrected metallicity along the
line of sight of Zn/Hcorr = -0.5+/-0.2 (O/H ~ -0.3 from emission lines),
and a depletion factor of refractory elements of Zn/Fe = 0.85+/-0.04. The
molecular gas is separated by 400 km/s from the gas that is excited by the GRB
(implying a fairly massive host, in agreement with the derived stellar mass of
log(Mstellar/Msolar) = 9.9+/-0.2). Including emission line analysis, we isolate
and characterise three different gas-phases within the star-forming host
galaxy. Our main result is that the metallicity determinations from both
absorption and emission lines are consistent, which is encouraging for the
comparison of GRB host metallicities at different redshifts.
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