Metallicity effects in long gamma-ray burst populations in a ΛCDM Universe

Abstract

The use of long γ-ray burst as star formation tracers is suspected to be affected by a possible dependence of the production or luminosity of these sources on the metallicity of their stellar progenitors. Selection effects are an alternative explanation. Our aim is to explore the nature of metallicity effects in long γ-ray burst populations using hydrodynamical cosmological simulations that include chemical evolution. We construct long γ-ray burst and host galaxy model populations using galaxy catalogues built from cosmological hydrodynamical simulations, making different assumptions on the nature of metallicity effects. We explore the ability of these models to reproduce an observational dataset that combines redshifts, prompt γ-ray emission observables from Swift and Fermi satellites, and HG properties from the largely unbiased BAT6 sample. Our results suggest that metallicity effects are more prompted to enhance the production rate of these sources at low metallicities, than to increase the burst luminosities. This is a statistically robust result based on the deviance information criterion. The metallicity threshold of these effects lies in the range [0.3 − 0.6]Z , but can not be constrained more precisely with present data and models. In the self-consistent star formation and metal enrichment scenario presented by our simulation, only models with a metallicity-dependent long γ-ray bust rate are successful at reproducing the γ-ray properties of these sources, their redshift distribution, and the masses and metallicities of their host galaxies, simultaneously. Models with a metallicity-dependent luminosity can reproduce observations, but are not statistically favoured in comparison to a metallicity-dependent production rate. Our simulations also predict that high metallicity hosts are possible even in the presence of a metallicity threshold for long γ-ray burst production. Our results support the view of long γ-ray bursts being produced by the collapse of low-metallicity, massive stars. This strongly suggests that they are biased tracers of the cosmic star formation at lower redshifts.

On the cosmological evolution of long gamma-ray burst properties

“…In addition, Juneau et al (2005) showed that star formation peaks at lower redshifts for lower-mass galaxies, qualitatively consistent (if lGRBs do indeed preferentially occur in lower mass galaxies) with our finding of a higher lGRB rate relative to the global star formation rate at low redshifts. This result is supported by Bignone et al (2018) who showed that lGRBs' preference for low metallicity environments increases the rate of lGRBs at low redshift.…”

The best place and time to live in the Milky Way

“…We link both LGRB and the TP formation rate to the metallicity evolution within the MW. We consider that LGRBs preferably form in evironments with Z ă 0.4Z d (Bignone et al 2017(Bignone et al , 2018. Assuming a different metallicity threshold value (Tab.…”

“…Hosts metallicity studies suggest that in most cases GRBs happen in galaxies with metallicity Z lower than a threshold value " 0.7 Z d (Vergani 2018;Palmerio et al 2019). Population studies (Bignone et al 2017(Bignone et al , 2018 suggest that this metallicity threshold should be in the range 0.3-0.6 Z d .…”

The best place and time to live in the Milky Way

“…Studies of the host metallicity have suggested that GRBs in most cases occur in galaxies whose metallicity Z is lower than a threshold value ∼0.7 Z (Vergani 2018;Palmerio et al 2019). Population studies (Bignone et al 2017(Bignone et al , 2018 suggest that this metallicity threshold lies in the range 0.3−0.6 Z . Assuming a threshold value Z c = 0.4 Z (Bertelli et al 1994;Virgili et al 2011), we therefore modeled the LGRB population under this A41, page 3 of 11 A&A 647, A41 (2021)…”

“…We linked the LGRB and the formation rate of TPs to the metallicity evolution within the MW. We considered that LGRBs preferably form in evironments with Z < 0.4 Z (Bignone et al 2017(Bignone et al , 2018. Assuming a different metallicity threshold (Table 4) mainly affects the rate of lethal LGRBs in the outer regions of the Galaxy, where the metallicity is indeed close to the threshold we did adopt.…”