Metallicity properties of the simulated host galaxies of long gamma-ray bursts and the fundamental metallicity relation

Abstract

By combining high-resolution N-body simulations with semi-analytical models of galaxy formation, we study the implications of the collapsar model for long-duration gamma-ray bursts (LGRBs) on the metallicity properties of the host galaxies. The cosmological model that we use reproduces the fundamental metallicity relation – the metallicity decreases with increasing star formation rate for galaxies of a given stellar mass. This was recently discovered for the Sloan Digital Sky Survey galaxies. We select host galaxies that house pockets of gas particles, that are young and that have different thresholds for their metallicities; these can be sites of LRGB events, according to the collapsar model. The simulated samples are compared with 18 observed LGRB hosts with the aim of discovering whether the metallicity is a primary parameter. We find that a threshold of metallicity for the LGRB progenitors, within the model galaxies, is not necessary to reproduce the observed distribution of host metallicities. The low metallicities of most LGRB hosts are consistent with the expectation that GRBs trace star formation. The star formation rate appears to be the primary parameter tracing the occurrence of a burst event. Finally, we show that only a few LGRBs are observed in massive, highly extinct galaxies, despite the fact that these galaxies are expected to produce many such events. We identify these missing events with the fraction of dark LGRBs.