GRB hosts through cosmic time - VLT/X-Shooter emission-line spectroscopy of 96 GRB-selected galaxies at 0.1 < z < 3.6

TL;DR

This study presents emission-line spectroscopy of 96 GRB host galaxies across a broad redshift range, revealing how their physical properties like star formation and metallicity evolve over cosmic time.

Contribution

It provides the largest sample of GRB host galaxy spectra to date, analyzing their properties and evolution from z=0.1 to 3.6, highlighting changes in star formation and metallicity with redshift.

Findings

Median SFR increases from 0.6 to 15 M_sun/yr from z~0.6 to 2.

Higher [OIII]/[OII] ratios at high z indicate evolving galaxy ionization conditions.

Less than 20% of GRB hosts at z<1 have super-solar metallicity, showing GRBs prefer low-metallicity environments.

Abstract

[Abridged] We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 GRB-selected galaxies at 0.1<z<3.6, the largest sample of GRB host spectroscopy available to date. Most of our GRBs were detected by Swift and 76% are at 0.5<z<2.5 with a median z~1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFRs), visual attenuations, oxygen abundances (12+log(O/H)), and emission-line widths. We find a strong change of the typical physical properties of GRB hosts with redshift. The median SFR, for example, increases from ~0.6 M_sun/yr at z~0.6 up to ~15 M_sun/yr at z~2. A higher ratio of [OIII]/[OII] at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the BPT diagram. Oxygen abundances of the galaxies are distributed between 12+log(O/H)=7.9 and 12+log(O/H)=9.0 with a median of 12+log(O/H)~8.5. The fraction of GRB-selected galaxies with super-solar metallicities is around 20% at z<1 in the adopted metallicity scale. This is significantly less than the fraction of star formation in similar galaxies, illustrating that GRBs are scarce in high-metallicity environments. At z~3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z ~< 0.5 Z_sun. Together with a high incidence of galaxies with similar metallicity in our sample at z~1.5, this indicates that the metallicity dependence observed at low redshift will not be dominant at z~3.