Cosmology 29 JAN, 2010

No evidence for dust extinction in GRB 050904 at z ~ 6.3

No evidence for dust extinction in GRB 050904 at z ~ 6.3

Context: GRB afterglows are excellent probes of gas and dust in star-forming galaxies at all epochs. It has been posited that dust in the early Universe must be different from dust at lower z. To date two reports directly support this contention, one of which is based on the spectral shape of GRB 050904 at z = 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We address the claimed evidence for unusual (SN-origin) dust in its host galaxy by simultaneously examining the X-ray and optical/NIR spectrophotometric data. Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4 days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn photometric data, the spectroscopic and z’-band photometric data at ~3 days from the Subaru telescope, as well as the critical UKIRT Z-band photometry at 0.47 days, upon which the claim of dust detection largely relies. Results: We find no evidence of dust extinction in the SED. We compute flux densities at lambda_rest = 1250 AA directly from the observed counts at all epochs. In the earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3 +- 0.2 mag) than previously reported and statistically insignificant (<1.5 sigma). Furthermore we find that the photometry of this band is unstable and difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that there is no evidence for dust extinction – the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type extinction curve. We conclude that there is no evidence of any extinction in the afterglow of GRB 050904 and that the presence of SN-origin dust in the host of GRB 050904 must be viewed skeptically. [abridged]

💡 Research Summary

The authors present a thorough re‑examination of the afterglow of GRB 050904, a gamma‑ray burst at a redshift of z ≈ 6.295, to test the claim that its host galaxy contains a substantial amount of supernova‑origin dust that produces an unusual extinction signature. Their methodology involves a complete re‑reduction of the Swift X‑ray Telescope (XRT) data and a careful re‑analysis of the optical and near‑infrared observations obtained at three epochs: 0.47 days, 1.25 days, and 3.4 days after the burst. The data set includes the critical UKIRT Z‑band photometry at 0.47 days (the basis of the original dust detection), VLT/FORS2 z‑Gunn imaging at 1.25 days, and Subaru/FOCAS spectroscopy together with z′‑band imaging at ≈3 days.

For each epoch the authors convert the observed counts directly into rest‑frame flux densities at λ_rest = 1250 Å, thereby avoiding any model‑dependent interpolation. They then fit the combined X‑ray and optical/NIR spectral energy distribution (SED) with a simple power‑law (F_ν ∝ ν^−β) and, where appropriate, with an additional dust extinction component using the “SN‑type” extinction curve proposed by Maiolino et al. (2004).

The key findings are:

  1. At the earliest epoch (0.47 days) the Z‑band suppression is only 0.3 ± 0.2 mag, significantly smaller than the ≈0.5 mag reported previously. This level of suppression corresponds to <1.5σ significance and is therefore statistically insignificant. The authors also demonstrate that the Z‑band photometry is intrinsically unstable, suffering from calibration difficulties due to atmospheric variability and filter transmission uncertainties.

  2. The SED at 1.25 days is perfectly reproduced by an unextinguished power‑law with β ≈ 1.1, identical to the X‑ray spectral index. When a SN‑type extinction curve is introduced, the best‑fit visual extinction at 3000 Å is only A(3000 Å) = 0.12 mag, and a 95 % confidence upper limit of A(3000 Å) < 0.27 mag is obtained. This effectively rules out any substantial dust extinction at this epoch.

  3. The third epoch (3.4 days) yields consistent results: the X‑ray spectrum remains unchanged, and the optical/NIR points continue to follow the same power‑law without requiring any reddening.

Overall, the authors conclude that the afterglow SED of GRB 050904 shows no credible evidence for dust extinction at any of the examined times. The previously reported SN‑origin dust signature is likely an artefact of photometric uncertainties and the challenging calibration of the Z‑band at such high redshift, where the Lyman‑α forest heavily contaminates the filter. Consequently, the hypothesis that the host galaxy of GRB 050904 contains a significant amount of supernova‑produced dust must be regarded with skepticism. The paper underscores the importance of simultaneous multi‑wavelength analysis and rigorous error assessment when probing dust properties in the early Universe.