Metallicities in long gamma-ray burst host galaxies atz < 0.5 calculated by the detailed modelling of optical and infrared line ratios

Abstract

We revisited the line spectra emitted from long GRB (LGRB) host galaxies at z<0.5 in order to calculate by the detailed modelling of the line ratios the physical conditions and relative abundances in LGRB hosts in this redshift range. We have found lower metallicities than in LGRB hosts at higher z. New results about metallicities and physical conditions in the different regions throughout the LGRB 980425 host at z=0.0085 are presented. In particular, we have found that the effective starburst temperature in the supernova (SN) region is the highest throughout the host galaxy. The low ionization parameter reveals that the radiation source is far or somehow prevented from reaching the emitting gas in the SN region. The models constrained by a few oxygen, nitrogen and sulphur line ratios to Hb in LGRB 980425 host satisfactorily reproduce the HeII/Hb and [ArIII]/Hb line ratios. The modelling of the observed [SIV]10.51\mu/[SIII]18.71\mum and [NeIII]10.6\mum/[NeII]12.81\mum line ratios from LGRB 031203 host galaxy at z=0.105 shows that the mid-IR lines are emitted from geometrically thin shock dominated filaments which are not reached by the photoionizing flux, while the optical lines are emitted from the radiation dominated outflowing clouds.