arXiv:1101.1503v2 [astro-ph.CO] 8 Jun 2011 aaae l 98 jrhe l 03 tnke l 2003; extremely have al. ( et 2006), afterglows Stanek simple al. spectrally 2003; et and Campana bright al. 2004; et 1993; Hjorth al. (Woosley et Malesani 1998; massive al. of (GRBs), et deaths Galama bursts a the gamma-ray years Long-duration with billion associated Bang. a dif Big very than the to less ter group existed Currentl local that the galaxies. environments in and ferent properties stars dust of appear extrapolate generations es- the we and first and are forms the -formation times dust of of how models ance these of to at understanding also but our dust evolves, to of only not quantity sential and properties The dacsi hssbetwr nbe yteddctdGRB dedicated the 2006a; by enabled tremendous al. recent were et satellite, The Watson subject 2011). this al. in et advances Zafar (e.g. 2010; al. galaxies opportunity et Schady distant an provide from e so tinctions only and not obtain 2002), to Sari & Granot 1998; inadfs epnigXryadUV and X-ray responding fast and tion ( etn h fego ihnol e eso eod after seconds of tens few a only the within afterglow the tecting -a tdi at (NIR) X-ray near- from spectra and al. afterglow et 2009). Greiner al. 2006; et Salvaterra al. 2009; et Kawai al. galax- et (see known Tanvir 2009; distant (QSO) most quasars at the and GRBs to ies of comparable als discovery or has the It than enabled greater optical. time, the first in the completeness for high dete a X-ray with in and complete tions, virtually afterglows of sample large ( z z K [email protected] UK; est odLietr E R,U;nt,[email protected] nrt3, UK; 7RH, LE1 Leicester, Road versity ae,JlaeMre e 0 K20 oehgnØ Denmark Ø, Copenhagen DK-2100 30, Vej [email protected] darach, Maries tayyaba, Juliane hagen, rpittpstuigL using typeset Preprint D utfraini h al nvrei ol eae topic debated hotly a is early the in formation Dust nti ae ecletfrtefis iealteavailable the all time first the for collect we paper this In ∼ = atversion raft γ 3 2 1 rytigr h aafrom data The trigger. -ray eateto hsc,Uiest fWrik oetyCV4 Coventry Warwick, of University Physics, of Department eateto hsc n srnm,Uiest fLeicest of University Astronomy, and Physics of Department akCsooyCnr,NesBh nttt,Uiest of University Institute, Bohr Niels Centre, Cosmology Dark 8 . 6 ) ic bevtoso hs ihzGB r be- are GRBs high-z these of observations Since 2). . T 9) R 893( 080913 GRB 295), rprisi h al nvretruhteaayi fthe of di analysis the the through in Universe regions early forming the star in study properties to probes excellent them 893ad000,at 050904, and 080913 R 893 ecmaetehg esitGB oasml flo of sample a ( to events GRBs extinguished possib moderately even with high of 090423, the lack GRB compare and We 050904 power GRB 080913. dust-attenuated GRB a in with extinction epochs dust all at for SEDs spect the construct fit to then We data X-ray and spectroscopy photometry, ubrsaitc,ti euthnsa eraei utco dust in decrease headings: a Subject at hints result this statistics, number Swift ayyaba ff h rgteso am-a us GB fegosadthei and afterglows (GRB) burst gamma-ray of brightness The rn pcsfralGB at GRBs all for epochs erent O Ghese l 04,wt t rcs localiza- precise its with 2004), al. et (Gehrels ctober Z afar A T E ff tl mltajv 11 v. emulateapj style X , 1. 5 2018 15, cierdeig u loaslt ex- absolute also but reddening, ective 1 D INTRODUCTION arach XLRN UTETNTO TTEEG FREIONIZATION OF EDGE THE AT EXTINCTION DUST EXPLORING akae,rinzto,fis tr ut xicin–gal – extinction dust, – stars first , ages, dark us:idvda GB002,001,ad050904) and 080913, 090423, (GRB individual burst: z .W J. z = Swift = atson 8 6 / . 10 . 2 / 9 n R 090423 GRB and 69) pia eecps de- , optical , / aepoie very a provided have , 09 1 F 6 z N ν . 9 n 6 and 69, > ial ∝ :GB050904 GRB 6: ν .T R. − β Sr tal. et (Sari ) c.uk anvir rf eso coe 5 2018 15, October version Draft . 9,rsetvl.W ahralaalbeotcladnear and optical available all gather We respectively. 295, A r Uni- er, Copen- , 2 V 7AL, J ABSTRACT ohan ∼ to c- f- o y - - . ; 0 . )above 3) .U F U. P. tn nsa-omn niomnsa ihredshifts. high at environments star-forming in ntent yb ta.20;Ptle l 00.Tesetu hw a shows spectrum The 2010). at al. break et sharp Patel 2009; 2009; al. al. et et (Greiner Fynbo (FORS2) 2 and Spectrograph at Reducer dispersion FOcal secured low the was opti- using 080913 (VLT) GRB an Large of 13, Very afterglow the September the 2008 of On spectrum 2009). cal al. et di (Greiner in bands 080913 GRB of al field Subaru the observed the and Observatories, Gemini Southern (ESO), European Observatory the at Telescopes 080913. GRB observing rsne nGenre l 20) ecretdtedt for data of the (MW) corrected Way We Milky the (2009). in al. extinction et Greiner in presented eali rvoswr seZfre l 2010). al. et Zafar (see work previous a in detail throughout. oercdt nthe in data tometric opinat sorption ain fGB002.Teatrlwo R 943was in non-detection 090423 with GRB bands, of NIR and afterglow optical The in observed 090423. GRB of vations ln h ieo ih oti us Shee ta.1998). al. et (Schlegel burst this to sight of line the along u eut,in results, our § ec ee.W eie3 derive We level. dence I group. local the outside curves of § sam extinction sample spectroscopic absolute spectroscopic first of large ple the first curves, the extinction to GRB-derived compare and shift edsrb ut-aeeghosrain fteafterglo the di of at observations multi- describe we eeto sls hn2 than less is detection oNRdt odtrietepoete fds at dust of X-ray properties the the fit determine jointly to and data GRBs SEDs NIR three the to all extract for We relativel epochs dust. multiple principle of at in amounts are small they even UV, to frame sensitive rest the in made ing and I pcrmwsotie ihteVTIfae Spectrom- Infrared VLT the with obtained was spectrum NIR H fegoso l known all of afterglows epeetorrslsfo h E tig ae on Based fitting. SED the from results our present we 3 epoieorconclusions. our provide we 5 a nrydsrbtos(Es tmlil epochs. multiple at (SEDs) distributions energy ral 0 n20 etme 3a 64:36UT in 06:48:33.6 analysed at 13 was September 050904 2008 GRB On of SED NIR to X-ray The t0:63 To 09Arl23, April 2009 on UT 07:56:32 At o l aaeesw ut netite tte6%confi- 68% the at uncertainties quote we parameters all For ynbo tn nvre ehr lcdt utextinction dust elucidate here We Universe. stant = Y ff z lwo rknpwrlw efidn evidence no find We power-law. broken or -law 2k s km 72 rn pcscridotwt di with out carried epochs erent bn u otehg esit(avre l 09.A 2009). al. et (Tanvir redshift high the to due -band & , 1 curnei on,bu aaismake galaxies blue young, in occurrence r eeiec o o ee fetnto in extinction of level low a for evidence le .I pt ftebae eeto n small and selection biased the of spite In 4. R haana z e esitGBetntosadfida find and extinctions GRB redshift wer = − 1 ∼ § 6 Mpc 2. edsusteoii fds thg red- high at dust of origin the discuss we 4 . .C S C. L. 9 eueti pcrmi diint pho- to addition in spectrum this use We 69. 40Åspotn h eeto fLy of detection the supporting Å 9400 xe:hg-esit–gamma-ray – high-redshift axies: MULTI-WAVELENGTH DATA − J 1 , , tarling Ω H σ z Λ σ and > infiat omlg where cosmology A significant. = pe iisfrtecsswhere cases the for limits upper Rs R 090423, GRB GRBs: 6 , 0 2 K . and 3and 73 ad rmtelightcurves the from bands A ndrew Swift ff E ff Ω rn ntuet.In instruments. erent rn pia n NIR and optical erent ( m B XTbgnobser- began -XRT − = .L J. -infrared Swift V 0 . evan ) 7i assumed is 27 = XTbegan -XRT z 3 0 > . 4 mag 043 .In 6. α griz ab- ws § 2 n y - ′ l 2

Table 1 Best fit parameters of the afterglow SEDs at different epochs with Small Magellanic Cloud (SMC) type extinction. The provided upper limits are 3σ.

GRB Epoch AV β1 β2 log νbreak (mag) Hz 050904 0.47d < 0.17 1.23 ± 0.08 ...... 1.25d < 0.08 1.17 ± 0.51 ...... 3.4d < 0.16 1.24 ± 0.07 ...... 080913 10.5m 0.14 ± 0.06 0.78 ± 0.02 ...... 26.9m 0.22 ± 0.11 0.65 ± 0.04 ...... 1.88hr 0.12 ± 0.03 0.79 ± 0.03 ...... 4.78d < 0.40 0.94 ± 0.04 ...... 090423 16.7hr < 0.10 0.45 ± 0.11 0.95 ± 0.11 15.1 ± 0.7 1.65d < 0.09 0.68 ± 0.13 1.18 ± 0.13 16.5 ± 0.9

Figure 1. NIR to X-ray SED of the afterglow of GRB 080913 at 10.5 eter And Array Camera (ISAAC) starting about 17.5 hours minutes (open red triangles), 26.85 minutes (blue circles), 1.88 hours (green squares) and 4.78 days (black triangles). Yellow stars correspond to after the burst trigger. A redshift of ∼ 8.2 was estimated XMM-Newton data at 4.4 days after the burst. The grey curve is the optical from a break observed around 1.14 µm due to Lyα absorp- spectrum of the afterglow taken at t0 + 1.88 hours. The solid lines represent tion by the neutral intergalactic medium (IGM). Another spec- the best-fit to the data; dashed lines illustrate the unextinguished power-law trum of the afterglow was obtained 40 hours after the burst from the best fit. Being close in time, the X-ray spectra for the first two epochs are superimposed. with the VLT’s Spectrograph using INtegral Field Observa- tions in the Near Infrared (SINFONI) and confirms the anal- avoid absorption caused by the neutral IGM. The absorption ysis of the ISAAC spectrum (Tanvir et al. 2009). The finding lines arising from ionic species were removed from the spec- was also confirmed with a spectrum obtained with Telesco- trum of GRB050904. Uncertainty in the Galactic extinction pio Nazionale Galileo (TNG) at ∼ 14 hours after the burst does not affect our results. The Dutra et al. (2003) redden- (Salvaterra et al. 2009). We use the J, H and K band pho- ing maps confirm the Schlegel et al. (1998) Galactic extinc- tometry of Tanvir et al. (2009). The data were corrected for tion maps up to E(B − V) = 0.25mag. Assuming an uncer- Galactic extinction of E(B − V) = 0.029mag (Schlegel et al. tainty of 15% (Schlafly et al. 2010) even for the case with the 1998). largest Galactic extinction (z′-band, GRB050904), this cor- For all three GRB afterglows the Swift XRT data were avail- responds to an uncertainty in the Galactic reddening value of able and downloaded from the Swift data archive and reduced . 0.015 mag. This indicates that the uncertainty due to the using HEAsoft (version 6.10). Spectra and lightcurves were Galactic extinction correction in the NIR is always smaller extracted in the 0.3–10.0 keV energy range. The X-ray spec- than our statistical uncertainties. tra at different epochs were analyzed using the latest associ- ated calibration files. All X-ray spectra were obtained near 3. RESULTS the time of optical/NIR spectra or relevant available photom- etry. To get an approximate X-ray flux level at each epoch, SEDs of the three GRB afterglows were obtained at multi- the afterglow lightcurves were obtained from the Swift XRT ple epochs. The data were fitted with dust-attenuated power- GRB curve repository at the UK Swift Science Data laws or broken power-laws. In addition to our standard ex- Centre1, created as described in Evanset al. (2007, 2010) tinction model (SMC type, see Schady et al. 2010; Zafar et al. and fitted by assuming a smoothly decay broken power-law 2011), we also fitted the data with the extinction curve in- (Beuermann et al. 1999). Considering the photon weighted ferred by Maiolino et al. (2004) to explain the spectrum of a mean time, the X-ray spectra were then normalized to the rel- QSO at z = 6.2 (although see Gallerani et al. 2010). evant SED time by using the lightcurve fit. The XMM-Newton observations of the afterglows of 3.1. Multi epoch SED of GRB050904 GRB080913 and GRB090423 were performed at ∼ 4.4 and The SED of this burst was examined in detail in Zafar et al. ∼ 2.2 days respectively. The data were collected from the (2010), where the SED of the afterglow of GRB050904 is XMM-Newton science archive (XSA)2. The data were re- constructed at 0.47, 1.25 and 3.4 days after the burst. We duced in 0.3–10.0 keV band using Science Analysis System use the results from the analysis here. The results of the (SAS, version 10.0.0). The spectrum of GRB080913 was SMC-extinction fits are reported in Table. 1 for complete- fitted using a single power-law with a best fit photon index ness. The fits using supernova (SN)-origin extinction from of Γ = 2.4 ± 0.4 and frozen for Galactic X-ray absorption Maiolino et al. (2004) suggest that there is no evidence in this of 3.2 × 1020 cm−2 (using the nH FTOOL; Kalberla et al. burst for this type of extinction. The SED of GRB050904has 2005). The best fit photon index for GRB090423 is Γ = been fitted by Haislip et al. (2006); Tagliaferri et al. (2005); 2.4 ± 0.3 with the fixed Galactic absorption of 2.9 × 1020 cm−2 Kann et al. (2010) finding no dust extinction. (Kalberla et al. 2005). Parts of spectra with a restframe wavelength shortward of 3.2. Multi epoch SED of GRB080913 Lyα (i.e. λ< 1216Å) are not included in the SED analysis to The SED of the afterglow of GRB080913 is extracted at four epochs i.e. 10.5 and 26.9 minutes, 1.88 hours and 1 www.swift.ac.uk/xrt_curves 4.78 days after the burst (see Fig. 1). To get precise photome- 2 http://xmm.esac.esa.int/xsa/ try in the z-band which is affected by strong Lyα absorption, 3

Figure 2. NIR and optical SED of the afterglow of GRB 080913 at 1.88 Figure 3. The NIR to X-ray SED of the afterglow of GRB 090423 at 16.7 hours after the trigger. The grey curve represents the optical spectrum. hours (black triangles) and 1.645 days (blue squares). The corresponding The solid black curve corresponds to the best fit SMC-dust–attenuated lines show the best fits to the data. The ISAAC and SINFONI spectra are power-law to the full NIR–X-ray SED, while the red line shows the best shown with the grey and cyan curves respectively. Red stars represent the fit power-law to the NIR-optical data only with no extinction. Inset: Zoom XMM-Newton data taken at 2.21 days after the trigger. of the optical spectrum and fits. The spectrum has been median filtered for display purposes. 3.3. Multi epoch SED of GRB090423 The SED of the afterglow of GRB090423 was constructed at 16.7 hours and 1.654 days (see Fig. 3). The VLT/ISAAC we performed spectro-photometric analysis at 1.88 hours us- and SINFONI spectra, NIR photometry and X-ray spectra ing the effective transmission function of the FORS2 z-Gunn were fitted at both epochs. The data from both epochs were filter. We constructed SEDs of the stars in the field using well fit with no dust using a broken power-law. The estimated the Sloan Digital Sky Survey (SDSS; Fukugita et al. 1996) 3σ upper limit on the AV with SMC extinction is 0.10 and and Two Micron All Sky Survey (2MASS; Skrutskie et al. 0.09mag at 16.7 hours and 1.654 epochs respectively. 2006) catalogues. We derived the count-to-flux conversion factor and estimated the afterglow band-integrated flux from 4. DISCUSSION = its measured counts. We computed the flux density at λrest Two apparently conflicting themes appear in studies of dust 1250Å using the procedure described in Zafar et al. (2010). at high redshift. The first is the expected decline in extinc- At 1.88 hour the flux density at 1250Å was 2.66 ± 0.63 µJy. tion as we get to early in the Universe, where galaxies are The NIR to X-ray SED at all epochs is well fitted with a sin- younger and dust has less time to form. In the local Uni- gle power-law and SMC type extinction (χ2/dof = 47/81). verse low and intermediate mass asymptotic giant branch The combined fit for dust extinction at all epochs resulted in (AGB) stars are believed to be the principal sources of in- marginal dust reddening of AV = 0.12 ± 0.03. Fitting each terstellar dust, requiring & 1 Gyr to evolve off the main se- epoch individually, we find dust extinction significant at the quence to produce dust (Gehrz 1989; Morgan & Edmunds 2σ level at 10.5 and 26.9 min, at 4σ at 1.88 hours and 1σ 2003; Marchenko 2006; Dwek et al. 2007). Because of the at 4.78 days (see Table 1). We also fitted the data using the time required to evolve to the giant branch, it has been posited Maiolino et al. (2004) extinction curve, resulting in a signif- that AGB stars should not contribute significantly to dust at icantly worse fit (χ2/dof = 69/81) and zero extinction. We z & 6 (Dwek et al. 2007). Spectroscopic (Douglas et al. 2010) also fitted the SED with Milky Way and Large Magellanic and photometric (Verma et al. 2007) studies of z ∼ 5 lyman- Cloud extinction curves but found no statistically significant break galaxies seem to confirm the general trend of little dust improvement in the fit compared to the SMC, and we found at high redshifts, showing low extinctions, and the sample no evidence for a 2175Å bump. As the simpler model, we of z-band dropout galaxies (z ∼ 7) shows little or no dust used the SMC to parameterize the extinction here and to pa- (Gonz´alez et al. 2010). rameterize the upper limit in GRB090423 below. However, the second, at first paradoxical, theme is the It is worth noting that the X-ray lightcurve for the afterglow very large dust masses discovered in quasar host galaxies at of GRB080913 shows some evidence of flaring, possibly re- z ∼ 6 from mm and sub-mm observations (e.g. Bertoldi et al. sulting in harder spectra (see Evans et al. 2007; Greiner et al. 2003; Carilli et al. 2001; Beelen et al. 2006; Wang et al. 2008; 2009). If the SED of the afterglow of GRB080913 is domi- Michałowski et al. 2010; Gall et al. 2011b), and the mystery nated by two separate components in optical-NIR and X-ray surrounding the origin of such large quantities of dust within , as observed at early times for GRB050904, we a billionyears of the . It has beensuggested that dust should fit the optical-NIR data independent of the X-rays. Us- produced on short time scales could be produced in the ejecta ing this method, we obtain results consistent with a single of Type II SNe (Kozasa et al. 1991; Todini & Ferrara 2001; power-law and no dust extinction, with a spectral slope of Morgan& Edmunds 2003; Nozawa et al. 2003; Dwek et al. β = 1.2 ± 0.1 and an upper limit of AV < 0.09 (see Fig. 2). 2007; Marchenko 2006; Hirashita et al. 2005), although ob- Previously Greiner et al. (2009); Kann et al. (2010) fitted the servations of local SNe do not support this hypothesis (e.g. optical-NIR SED of GRB 080913 and found no dust extinc- Meikle et al. 2007; Smith et al. 2008). Valiante et al. (2009) tion. estimated that under certain conditions AGB stars could pro- 4 duce dust at earlier epochs, beginning to dominate between 150 and 500Myr. However Michałowski et al. (2010) found that given the stellar masses in high redshift QSO hosts, AGB stars could not be responsible for the large masses of dust observed in these objects at z > 5 (see Cherchneff & Dwek 2009; Pipino et al. 2011; Gall et al. 2011a), and suggested rapid dust growth in the ISM as an alternative (see also Draine 2009). The modeling of Lyman alpha emitters at z ∼ 6 sug- gests that the largest galaxies are the most dust enriched and that dust content decreases with increasing redshift (Lai et al. 2007; Finkelstein et al. 2009; Dayal et al. 2010). In spite of the emphasis in the literature on these QSO hosts, it may be that such very massive systems, being exceptional and rare, tell us little about typical star forming environments at very high redshift. And while, young star-forming galax- ies at high redshift with strong nebular emission appear to be somewhat dust-obscured in spite of their blue continuua (Schaerer & de Barros 2010; Watson et al. 2010), the quan- Figure 4. Extinction in GRB afterglow versus redshift. The three high redshift GRBs (z > 6) are from this work while the 42 low redshift GRBs are tities of dust are not large, consistent with the idea that in from a sample studied in Zafar et al. (2011). There seems to be a dearth of the early universe, most of the star-formation is likely to GRB afterglows at z & 4 with AV ∼ 0.3. be less obscured than at later times. GRBs, associated as they are with the deaths of massive stars, usually occur in young, blue and sub-luminous galaxies with a high specific give a spectroscopic redshift for GRB060522 based on the star formation rate (Le Floc’h et al. 2003; Courty et al. 2004; Lyα break, however no estimate of the extinction is available Christensen et al. 2004; Prochaska et al. 2004; Savaglio et al. so far in the literature. Ruiz-Velasco et al. (2007) obtained 2009; Castro Cer´on et al. 2010) and are therefore likely to be the spectroscopic redshift of the afterglow of GRB060927. good probes of star-forming regions, especially at high red- We have studied the SED of the afterglow of this burst in our shift. spectroscopic sample of Zafar et al. (2011) and it is consistent In this work, we have determined that all the identified with AV = 0 (see Fig. 4). Moderate extinction is claimed in z > 6 GRBs have low or negligible extinctions. By compar- the case of GRB071025 (Perley et al. 2010), with extinction ing these results with the first spectroscopic GRB extinction corresponding to AV ∼ 0.5 (converting A3000 Å to AV using sample of Zafar et al. (2011), we find that at high-redshifts the SMC extinction curve ratio). A Maiolino et al. (2004) ex- (z & 4), GRB sightlines appear significantly less extinguished tinction curve is required to fit the SED. With this extinction, (see Fig. 4): the majority of reddened low-redshift GRBs are GRB071025 has a high enough extinction to begin to fill in observed with AV ∼ 0.3mag. While the numbers are small, the gap at high redshift around AV ∼ 0.3, though it is perhaps the absence of any GRBs at z & 4 with extinctions in this notable that it has the lowest redshift of all the bursts exam- range is striking. The first explanation to examine is observa- ined here. It is more anomalous in the sense that it has an ap- tional bias, where higher-redshift GRBs will be observed far- parently unique extinction curve since reanalyses have shown ther into the restframe UV where the extinction is likely to be that no other QSO or GRB any longer require such a peculiar more severe, possibly preventing spectra from being obtained extinction (see Gallerani et al. 2008; Zafar et al. 2010). or the afterglow from being detected. For the z > 6 GRBs, a The issues of bias and low number statistics are clearly im- restframe AV = 0.3 corresponds to 1–1.5 magnitudes of ex- portant and we cannot resolve them with this sample. The tinction in the observed J-band and 0.5–0.8 magnitudes in the fact that we infer very little dust in the afterglows studied here observed K, for an SMC extinction curve. Considering the does not imply that dusty environments do not exist along photometric observations given in Ruiz-Velasco et al. (2007); sightlines to z > 6 GRBs, but it suggests that there is less dust Haislip et al. (2006); Tagliaferri et al. (2005); Greiner et al. in highly star-forming regions at z & 4. This result is also po- (2009); Tanvir et al. (2009), we found that even with AV = 0.3 tentially interesting in the context of dark bursts – suggesting the GRBs at z > 6 could have been detected with rela- that highly-extinguished bursts may not exist at high redshift, tive ease and a photometric redshift obtained. In the case and that bright, very high redshift GRBs will be detectable as of GRB050904, it seems likely that a spectroscopic redshift long as they are observed in the NIR. would have been obtained even with restframe AV = 0.3. Finally, the comparison to the column densities of met- This work therefore hints at a low extinction of most z > 4 als is striking. In the cases of both GRB050904 and 22 bursts, since it seems that our z > 6 bursts would have been GRB090423, column densities of NH,X ∼ 3 × 10 and 23 found even with AV = 0.3. Therefore we have looked at all 1 × 10 are claimed (assuming solar abundances and neu- known z > 5 GRB afterglows in the literature. There are four tral elements, Watson et al. 2006b; Campana et al. 2011; bursts with firm estimates of z > 5 known. Three of them Gendre et al. 2006; Tanvir et al. 2009; Starling et al. in prep. have estimates of their dust extinction available. The four 2011) from the soft X-ray absorption. The dust-to-metals ra- bursts are: GRB050814 (Jakobsson et al. 2006) at z ∼ 5.3, tios in both cases are at most a few percent of the value in the GRB060522 (Cenko et al. 2006) at z = 5.11, GRB060927 local group. Given that the metals are likely to be highly- (Ruiz-Velasco et al. 2007; Zafar et al. 2011) at z = 5.46 and ionized (Schady et al. 2010; Watson et al. 2007), the metal GRB071025 (Perley et al. 2010) at z ∼ 5. Jakobsson et al. column must be substantially higher than found using neu- (2006) obtained a photometric redshift of the afterglow of tral medium models. This could drive the dust-to-metals ratio GRB050814 and the SED is consistent with no dust redden- down below a percent of local group values. While it is estab- ing (P. Jakobsson, private comm. 2010). (Cenko et al. 2006) lished that GRB environments can have low dust-to-metals ra- 5 tios (El´ıasd´ottir et al. 2009; Watson et al. 2006a; Schady et al. Dayal, P., Ferrara, A., & Saro, A. 2010, MNRAS, 402, 1449 2010), these high-redshift objects are extreme. Such extreme Douglas, L. S., Bremer, M. N., Lehnert, M. D., Stanway, E. R., & ratios could be related to the moreeffective destruction of dust Milvang-Jensen, B. 2010, MNRAS, 409, 1155 Draine, B. T. 2009, in Astronomical Society of the Pacific Conference due to the high of these bursts (e.g. Fruchter et al. Series, Vol. 414, Cosmic Dust - Near and Far, ed. T. Henning, E. Gr¨un, & 2001); the effects of dust destruction may be discernible not J. Steinacker, 453 only in very early colour changes in the GRB prompt phase Dutra, C. M., Ahumada, A. V., Clari´a, J. J., Bica, E., & Barbuy, B. 2003, UV emission, but also in the extinction curve at later times A&A, 408, 287 (Perna & Lazzati 2002). A small dust-to-metals ratio could Dwek, E., Galliano, F., & Jones, A. 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