X-shooting GRBs at high redshift: probing dust production history*

Evolved asymptotic giant branch (AGB) stars and Type  Ia supernovae (SNe) are important contributors to the elements that form dust in the interstellar medium of galaxies, in particular, carbon and iron. However, they require at least a Gyr to start producing these elements, therefore, a change in dust quantity or properties may appear at high redshifts. In this work, we use extinction of γ-ray burst (GRB) afterglows as a tool to look for variations in dust properties at zz ≥ 3. We use a spectroscopically selected sample of GRB afterglows observed with the VLT/X-shooter instrument to determine extinction curves out to high redshifts. We present 10 new zz ≥ 3 X-shooter GRBs of which six are dusty. Combining these with individual extinction curves of three previously known zz ≥ 3 GRBs, we find an average extinction curve consistent with the SMC-Bar. A comparison with spectroscopically selected GRBs at all redshifts indicates a drop in visual extinction (AV) at zz > 3.5 with no moderate or high-extinction bursts. We check for observational bias using template spectra and find that GRBs up to zz ∼ 8 are detectable with X-shooter up to AV ∼ 0.3 mag. Although other biases are noted, a uniformly low dust content above zz > 3.5 indicates a real drop, suggesting a transition in dust properties and/or available dust building blocks. The remarkable increase in dust content at zz  3.5 dust drop could be the result of low stellar masses of GRB host galaxies.