Host galaxies of long gamma-ray bursts in the Millennium Simulation

In this work, we investigate the nature of the host galaxies of long gamma-ray bursts (LGRBs) using a galaxy catalogue constructed from the Millennium Simulation. We developed an LGRB synthetic model based on the hypothesis that these events originate at the end of the life of massive stars following the collapsar model, with the possibility of including a constraint on the metallicity of the progenitor star. A complete observability pipeline was designed to calculate a probability estimation for a galaxy to be observationally identified as a host for LGRBs detected by present observational facilities. This new tool allows us to build an observable host galaxy catalogue which is required to reproduce the current stellar mass distribution of observed hosts. This observability pipeline predicts that the minimum mass for the progenitor stars should be similar to 75M(circle dot) in order to be able to reproduce BATSE observations. Systems in our observable catalogue are able to reproduce the observed properties of host galaxies, namely stellar masses, colours, luminosity, star formation activity and metallicities as a function of redshift. At z > 2, our model predicts that the observable host galaxies would be very similar to the global galaxy population. We found that similar to 88 per cent of the observable host galaxies with mean gas metallicity lower than 0.6Z(circle dot) have stellar masses in the range 10(8.5)-10(10.3) M-circle dot, in excellent agreement with observations. Interestingly in our model, observable host galaxies remain mainly within this mass range regardless of redshift, since lower stellar mass systems would have a low probability of being observed while more massive ones would be too metal-rich. Observable host galaxies are predicted to preferentially inhabit dark matter haloes in the range 10(11)-10(11.5) M-circle dot, with a weak dependence on redshift. They are also found to preferentially map different density environments at different stages of evolution of the Universe. At high redshifts, the observable host galaxies are predicted to be located in similar environments as the global galaxy population but have a slightly higher probability to have a close companion.