摘要：Context.Dust is known to be produced in the envelopes of asymptotic giant branch (AGB) stars, the expanded shells of supernova (SN) remnants, and in situ grain growth within the interstellar medium (ISM), although the corresponding efficiency of each of these dust formation mechanisms at different redshifts remains a topic of debate. During the first Gyr after the Big Bang, it is widely believed that there was not enough time to form AGB stars in high numbers, hence the dust at this epoch is expected to be purely from SNe or subsequent grain growth in the ISM. The time period corresponding toz~ 5−6is thus expected to display the transition from SN-only dust to a mixture of both formation channels as is generally recognized at present.Aims.Here we aim to use afterglow observations of gamma-ray bursts (GRBs) at redshifts larger thanz> 4to derive host galaxy dust column densities along their line of sight and to test if a SN-type dust extinction curve is required for some of the bursts.Methods.We performed GRB afterglow observations with the seven-channel Gamma-Ray Optical and Near-infrared Detector (GROND) at the 2.2 m MPI telescope in La Silla, Chile (ESO), and we combined these observations with quasi-simultaneous data gathered with the XRT telescope on board theSwiftsatellite.Results.We increase the number of measuredAVvalues for GRBs atz> 4by a factor of~2–3 and find that, in contrast to samples at mostly lower redshift, all of the GRB afterglows have a visual extinction ofAV< 0.5mag. Analysis of the GROND detection thresholds and results from a Monte Carlo simulation show that although we partly suffer from an observational bias against highly extinguished sight-lines, GRB host galaxies at4
