Abstract
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. The majority of our GRBs was 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, visual attenuations (A_V), oxygen abundances and
emission-line widths (sigma). 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.
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