[Paper Review] Are LGRBs biased tracers of star formation? Clues from the host galaxies of the Swift/BAT6 complete sample of bright LGRBs. II: star formation rates and metallicities at z < 1

J. Japelj, S. D. Vergani+22 more|arXiv (Cornell University)|Apr 4, 2016

Gamma-ray bursts and supernovae108 references36 citations

TL;DR

This study analyzes the host galaxies of 14 bright long gamma-ray bursts (LGRBs) at z < 1 from the Swift/BAT6 complete sample to assess their suitability as tracers of star formation. Using nebular emission lines, the authors measure dust extinction, star formation rates (SFR), and metallicities, finding that LGRB hosts are biased toward low-metallicity, low-mass galaxies with lower average SFRs than expected from star formation tracers, due to a strong aversion to high metallicities.

ABSTRACT

Long gamma-ray bursts (LGRBs) are associated with the deaths of massive stars and could thus be a potentially powerful tool to trace cosmic star formation. However, especially at low redshifts (z < 1.5) LGRBs seem to prefer particular types of environment. Our aim is to study the host galaxies of a complete sample of bright LGRBs to investigate the impact of the environment on GRB formation. We study host galaxy spectra of the Swift/BAT6 complete sample of 14 z < 1 bright LGRBs. We use the detected nebular emission lines to measure the dust extinction, star formation rate (SFR) and nebular metallicity (Z) of the hosts and supplement the data set with previously measured stellar masses M$_{\star}$. The distributions of the obtained properties and their interrelations (e.g. mass-metallicity and SFR-M$_{\star}$ relations) are compared to samples of field star-forming galaxies.We find that LGRB hosts at z < 1 have on average lower SFRs than if they were direct star-formation tracers. By directly comparing metallicity distributions of LGRB hosts and star-forming galaxies, we find a good match between the two populations up to Z $\sim 8.4-8.5$, after which the paucity of metal-rich LGRB hosts becomes apparent. The LGRB host galaxies of our complete sample are not inconsistent with the mass-metallicity relation at similar mean redshift and stellar masses. The cutoff against high metallicities (and high masses) can explain the low SFR values of LGRB hosts. We find a hint of increased incidence of starburst galaxies in the Swift/BAT6 z < 1 sample with respect to that of a field star-forming population. Given that the SFRs are low on average, the latter is ascribed to low stellar masses. Nevertheless the limits on the completeness and metallicity availability of current surveys, coupled with the limited number of LGRB host galaxies, prevent us from reaching more quantitative conclusions.

Motivation & Objective

• To determine whether long gamma-ray burst (LGRB) hosts at z < 1 are unbiased tracers of cosmic star formation.
• To investigate the role of host galaxy properties—particularly metallicity, stellar mass, and SFR—in LGRB formation efficiency.
• To compare the distributions of SFR, metallicity, and stellar mass in LGRB hosts with those of field star-forming galaxies.
• To assess whether the observed SFRs in LGRB hosts are consistent with a star-formation-driven origin, given their environmental biases.

Proposed method

• Spectroscopic analysis of nebular emission lines (e.g., Hα, [O II], [O III]) in host galaxies of 14 z < 1 LGRBs from the Swift/BAT6 complete sample.
• Use of emission line ratios to derive dust extinction, star formation rates (SFR), and nebular metallicities (12 + log(O/H)).
• Supplementation with stellar masses (M‹) from prior work (Paper I, Vergani et al. 2015) to enable comparison with mass-metallicity and SFR-stellar mass relations.
• Comparison of LGRB host properties with those of field star-forming galaxies from deep surveys (e.g., VVDS, VIMOS VLT Deep Survey) to assess biases.
• Statistical analysis of metallicity distributions and their relation to SFR and stellar mass, including testing for deviations from the mass-metallicity and SFR–mass relations.
• Investigation of correlations between high-energy GRB properties (Eγ,iso, Lγ,iso, Eγ,peak) and host metallicities to rule out selection effects.

Experimental results

Research questions

• RQ1 Are LGRB hosts at z < 1 representative of the general star-forming galaxy population in terms of SFR and metallicity?
• RQ2 Does the observed SFR in LGRB hosts reflect a true star-formation rate, or is it suppressed due to environmental biases?
• RQ3 To what extent do LGRB hosts deviate from the mass-metallicity and SFR–stellar mass relations observed in field galaxies?
• RQ4 Is the preference for low-metallicity environments in LGRB hosts responsible for their lower average SFRs?
• RQ5 Is there evidence for an enhanced incidence of starburst galaxies among LGRB hosts compared to field populations, and if so, is it consistent with SFR-driven models after accounting for metallicity bias?

Key findings

• LGRB hosts at z < 1 have on average lower star formation rates (SFRs) than expected if they were direct tracers of star formation, indicating a bias against high-SFR environments.
• The metallicity distribution of LGRB hosts shows a clear deficit of metal-rich galaxies (12 + log(O/H) > 8.4–8.5), with a sharp cutoff above this threshold.
• LGRB hosts are consistent with the mass-metallicity relation at similar redshift and stellar masses, suggesting that the metallicity bias is the primary driver of their observed properties.
• The low SFRs in LGRB hosts are likely a consequence of their strong aversion to high-metallicity and high-mass galaxies, rather than intrinsic SFR suppression.
• There is a hint of increased incidence of starburst galaxies (high specific SFR) among LGRB hosts compared to field populations, but this is likely due to selection in low-mass, low-metallicity systems rather than a true enhancement.
• No significant correlation is found between high-energy GRB properties (Eγ,iso, Lγ,iso, Eγ,peak) and host metallicity, indicating that sample selection is not biased by luminosity or energetics.