NGC 4993 and other short gamma-ray burst host galaxies

Abstract

We present the spectral detailed modelling of NGC 4993 – the host galaxy of GW 170817 – and other short gamma-ray burst (SGRB) host galaxies. In order to determine their physical conditions and the element abundances, we have gathered spectroscopic and photometric data from the literature. The observation data are sometimes missing, preventing us from fully constraining the model. However, for most of the SGRB hosts the [OIII]5007/Hβ and [NII]6548/Hα line ratios are reported. The analysis of NGC 4993 by a composite model (photoionization+shock) confirms that an active galactic nucleus (AGN), most probably a low-ionization nuclear emission-line region (LINER) or a low-luminosity AGN (LLAGN) is the gas photoionization source. Shock velocities and preshock densities are similar to those found in the narrow line region of AGN. O/H and N/H have solar values. For the other SGRB of the sample, we found that O/H ratios are nearly solar, while N/H covers a much larger range of values at redshifts close to 0.4. In NGC 4993, the relative contribution to the spectral energy distribution of an old stellar population, characterized by a black-body temperature of Tbb = 4000 K, with respect to bremsstrahlung is higher by a factor of >100 than in most of the local AGN and starburst (SB) galaxies. For the other SGRB that compose the sample, Tbb ranges between 2000 K for SGRB 100206A and 8000 K for SGRB 111117A.