The extinction properties of long GRB host galaxies from H and He I recombination lines

0.67 MB·English

by  K. Wiersema

Mon.Not.R.Astron.Soc.000,1–10(2010) Printed6January2011 (MNLATEXstylefilev2.2)
The extinction properties of long GRB host galaxies from H and HeI
recombination lines⋆
1 K. Wiersema1 †
1
1UniversityofLeicester,UniversityRoad,LeicesterLE17RH
0
2

n
a
Accepted04January2011
J

4

ABSTRACT
]
O Inthispaperweshowhowaself-consistenttreatmentofhydrogenandheliumemissionline
fluxesofthehostsoflonggamma-rayburstscanresultinimprovedunderstandingofthedust
C
propertiesinthesegalaxies.Inparticular,wefindthatevenwithmodestsignaltonoisespec-
.
h troscopywecandifferentiatedifferentvaluesforRV,theratiooftotaltoselectiveextinction.
p TheinclusionofPaschenandBrackettlines,evenatlowsignaltonoise,greatlyincreasethe
- accuracy of the derived reddening. This method is often associated with strong systematic
o
errors,causedbythe needformultipleinstrumentsto coverthewide wavelengthrange,the
r
t requirementto separatestellar hydrogenabsorptionfromthenebularemission,andbecause
s
of the dependancy of the predicted line fluxes on the electron temperature. We show how
a
[ these three systematic errors can be negated, by using suitable instrumentation (in particu-

larX-shooterontheVeryLargeTelescope)andwidewavelengthcoverage.Wedemonstrate
1
thismethodusinganextensiveopticalandnear-infraredspectroscopiccampaignofthehost
v
galaxyofgamma-rayburst060218(SN2006aj),obtainedwithFORS1,UVESandISAACon
4
theVLT,coveringabroadwavelengthrangewithbothhighandlowspectralresolution.We
1
8 contrastourfindingsofthissourcewithX-shooterdataofastarformingregioninthehostof
0 GRB100316D,andshowtheimprovementoverexistingpublishedfluxesoflongGRBhosts.
.
1 Keywords: gamma-rays:bursts
0
1
1
:
v
1 INTRODUCTION main driver of stellar winds. Measuring the gas phase iron abun-
i
X dancefromemissionlinesofgalaxiesiscomplicated,astheseemis-
Aslonggamma-raybursts(hereafterreferredtosimplyasGRBs)
sionlinesarefaintandironiseasilyandrapidlydepletedintodust
r are produced by massive stars, as evidenced by their associated
a grains. Oxygen isagood alternative: itsabundance can be easily
supernovae (e.g. Galama et al. 1998; Hjorth et al 2003; Pian et
measured(thelinesarebright)eitherdirectly(usingoxygenlinesat
al. 2006; Starling et al. 2010; but see also Fynbo et al. 2006),
variousionisationstagesincombinationwithelectrontemperature
thereisaclearlinkbetweenhighmassstarformationandGRBs.
anddensitysensitivelines);orindirectlythroughthestrongforbid-
This link is apparent in the properties of their host galaxies: the
denlinescoupledwithradiationtransportsimulations.Inorderto
hostsshowbrightnebularemissionlinesassociatedwithactivestar
deriveaccurateabundances,oneneedstocorrectfortheextinction
formation, and show spectral energy distributions and morpholo-
experiencedbythehotgasthatemitsthedifferentemissionlines.
giesgenerallyconsistentwithblue,metal-poor,starformingdwarf
Besides deriving element abundances from emission lines,
galaxies(e.g.Savaglio,Glazebrook&LeBorgne2009).Fromhigh
therearefurtherreasonswhyweareinterestedintheextinctionin
resolutionimagingitisevidentthatthepositionsoflongGRBsin
thestarformingregion(s)inthehostgalaxies.Particularlyinterest-
their hosts are strongly correlated with thelocation of starforma-
ingisthecombinationofafterglowspectroscopyandphotometry,
tionwithinthesehosts(e.g.Fruchter etal2006). Itisthereforea
which probes the line of sight dust and gas properties within the
reasonableassumptiontomakethatthepropertiesofthestarform-
host,withhostgalaxyspectroscopyinemission.Thiscombination
ingregionsinthehostalsoleadtoinsightinthepropertiesofthe
mayconstrainthepossibledestructionofdustbytheGRB,itmay
progenitor star:theprogenitor only livesfor ashort time.A con-
showhowspecialtheregioninwhichtheGRBtookplaceiswith
strainingparameterontheevolutionoftheprogenitorobjectisits
respect to other star forming regions, and may shed light on the
metallicity,ormorespecificallytheabundanceofiron,asitisthe
(depletion)chemistryintheISMinGRBhostgalaxies.
In this paper we focus on the extinction in star forming re-
⋆ BasedonobservationsmadewithESOTelescopesattheParanalObser- gions withinGRB hosts (perhaps better described asthe attenua-
vatoryunderprogrammeID381.D-0723 tionofemissionlinesbydustwithinthehosts).Thehostgalaxyof
† E-mail:[email protected] GRB060218isaparticularlyattractivetargettodothis.Thesuper-
2 K. Wiersema
novaaccompanyingthisGRBiswellstudied,andthereforeacon-
nectionwithhostgalaxyproperties(spectroscopic,spectralenergy
distribution;seee.g.Savaglio,Glazebrook&LeBorgne2009)may
providetightconstraintsontheprogenitorevolution.Inaprevious
paper(Wiersemaetal.2007;hereafterW07)weperformedastudy
of the metal abundances in this host and the ISM velocity struc-
ture,bothobtainedthroughemissionandabsorptionlinesdetected
inaVLTUVESspectrumofthesupernova(2006aj).Weobtained
threemoredatasetsofthissource,whichwewillusetoprobethe
extinctionpropertiesofthishostinthispaper.Thesedataarepart Figure1.Thisplotshowsthepositioninrestframewavelengthofthemost
ofadedicatedVLTprogram(program381.D-0723,PIWiersema) usefulHandHeIlines.Thehydrogenlines(thehydrogenseriesiseasily
aimed at a thorough understanding of the stellar population(s) of recognisablebyeye)areindicatedbyaredsquare,withfluxesnormalised
thishostgalaxy,andconsistofopticalspectroscopyatmediumand toHβforcaseBrecombinationandTe =104Kandne =100cm−3.The
HeIlinesareshownwithbluetriangles,theirfluxesarenormalisedtoHeI
highresolution(usingtheFORS1andUVESinstruments)andlow
resolutionKbandspectroscopy(usingtheISAACinstrument). λ5876,forthesameTeandneashydrogen.Thenominalrangesofthethree
armsofX-shooter areindicated byhorizontal dotted lines. However, we
Thispaper isorganizedasfollows: inSection2wedescribe notethattheNIRarmefficiencydropsveryrapidlyredwardoftheKband.
thewayweuseHandHeIlinefluxratiostofittheextinctionprop- ItisclearfromthisplotthattheHeIlinesfillinimportantwavelengthgaps
erties; inSection3 wedescribe the observations and theway we left bythe hydrogen series –amajority oftheX-shooter spectral orders
measureemissionlinefluxesfromthespectra;inSection4wefit containsoneormoreemissionline.
thedatausingourmodelsandinSection5wediscussourresults.
Throughout the paper we adopt a cosmology with H = 71
0
kms−1Mpc−1,Ω =0.27,Ω =0.73.
m Λ
then proceed tomeasure theline fluxes and upper limitson non-
detectedlines.HydrogenlinefluxesarenormalizedtoHβ,whereas
fortheHeIlinesweuseHeIλ5876:theselinesaremostfrequently
2 METHOD detectedinlow-zhostsandarebright.Forthepredictedfluxratios
R weuseafixedelectrondensityn =102cm−3:thevastmajority
Weassumethattheforegroundabsorbingscreenofdustisuniform, p e
ofGRBhostsandstarformingregionswithinthesehostsshowflux
andexpresstheextinctionfunctioninA /A andtheobservedand
λ V ratiosofthe[OII]λ3727,3729doubletorthe[SII]λ6716,6731
predictedratiosoftwoemissionlines,R andR atwavelengthsλ
o p 1 doubletinthelowdensitylimit.Notealsothatatlowdensitiesthe
andλ :
2 emissivities are fairly weak functions of the density (Osterbrock
2.5log(R /R ) 1989).WeuseaseriesofdifferentelectrontemperaturesT ,from
A = o p . (1) e
V (A −A )/A 0.5 to 2.5× 104 K, and obtain the predicted line flux ratios for
λ2 λ1 V
hydrogen fromHummer&Storey(1987) fortheseT valuesand
e
We now use the Cardelli, Clayton & Mathis (1989) electrondensity.HeIemissivitiesarestillsubjecttosomedebate,
parametrizationoftheextinctioncurve, butgenerally thedifferencesinemissivitiesbetween different ap-
proaches isconsiderably smaller thanthemeasurement errorswe
A /A =a(λ)+b(λ)/R ,
λ V V canrealisticallyexpectfromeventhebrightestGRBhosts.Weuse
withtheusualnotationR =A /E(B−V),forarangeofR values. theemissivitiesandtheinterpolationoverelectrontemperatureas
V V V
NotethatwewillusethisonlowredshiftGRBhosts,soweexpecta determinedbyPorteretal.(2005)andPorter,Ferland&MacAdam
possible∼2200Åfeaturenottohaveaneffectontheanalysis.Any (2007;theirappendixA)tocomputeR forHeIusingthesameT
p e
lineratiowherethefluxratioscanbeaccuratelypredictedthrough gridandfixedn asforhydrogen.
e
atomicphysicscanbeused,butitismostcommontousehydrogen After the fluxes of the hydrogen and HeI lines in range are
(HI) or helium (HeI or HeII) lines because of the large number measured(detailedinSection3),wearriveforeachsourceatagrid
ofavailabletransitionsintheopticalwindow,theirbrightness,the ofvaluesofA /A (forarangeofR values)andthecorresponding
λ V V
lineemissivitiescanbeaccuratelymodelled,andbecausethereis 2.5log(R /R )values(forarangeofT ).Anexamplefortwodif-
o p e
nosensitivitytoabundance. ferentvaluesofR isshowninFigure7.Wenowfitastraightline
V
HeIIlinesaregenerallytoofaintinmostGRBhoststarform- tothepoints,foreachcombinationofR andT ,recordingtheχ2.
V e
ingregionstobeusefulinthiscontext,buttheHeIlinesdoprovide WedothisseperatelyforHandHeI.Theslopeofthebestfitting
meaningfulconstraintsonA andR .Inaddition,theHeIlinesfill straight line gives the attenuation (it is equal to −A , see Eq. 1).
V V V
gapsinthewavelengthcoverageoftheHseries.Aswewilldemon- ThebestdescriptionofR andT isnowgivenbythemodelwith
V e
stratefurtherbelow,thisisparticularusefulforX-shooterspectra: thelowestχ2.
mostordershaveaHorHeemissionlinein(atleastatthelowred- The use of hydrogen lines to derive R and A simultane-
V V
shiftsweconsider inthispaper).Figure1showsthisgraphically: ouslyiscommonlyusedingalacticemissionlineregions,seee.g.
we plot the wavelength and flux of the relevant H and HI lines, Greve(2010)andPetersen&Gammelgaard(1997)andreferences
wherethehydrogenfluxesarenormalisedtoHβandtheHeIfluxes therein.Theseauthorspointoutafurthersimplificationthatisuse-
toHeIλ5876,assumingcaseBrecombination, A = 0,T = 104 fulwhenonlylimitedlinesareavailable:thefluxratiosofemission
V e
Kandn =100cm−3(seebelowfordetails). linesfromthesamespeciesthat comefromthesameupper level
e
WenowcomputeA /A forthewavelengthsλofallusefulH arefairlyinsensitiveton andT .
λ V e e
andHeIlinesinthespectrausingarangeofR valuesfrom2.5to Wenowproceedtodemonstratethestrengthsandweaknesses
V
5.5(seealsoPetersen&Gammelgaard1997).ItisclearthatA /A ofthemethodoutlinedaboveforthecaseofstarformingregions
λ V
changes most strongly with changing R for the bluer lines. We inlongGRBhostgalaxies.
V
The extinctionpropertiesof GRBhostgalaxiesfromH andHeI 3
The datawere reduced using standard routines inIRAF,us-
ingroutinesintheccdprocandspecredpackages,usingcalibration
data(biasframes,flatfields,arclampframesandstandardstarex-
H α
posures)takenthesamenightsasthesciencedata.Fluxcalibration
ofthedataofSep4wasachievedusingexposuresofFtypespec-
trophotometricstandardstarLTT1788(Hamuyetal.1992,1994).
ThestandardstarforthesecondnightofdatawasDA4starHZ4
(Oke1990).Theextracted,fluxcalibrated,individualspectrafrom
thetwonightswerecombined throughaweightedaverage,using
the signal to noise per bin as weight. As expected, the signal to
noiseisdominatedbythesecondnightspectra,thathaveconsider-
ablybetterseeingconditions.Weappliedameanextinctioncurve
[O III] 5008 forParanal,anddereddenedthespectrafortheGalacticextinction
valueof E(B−V) = 0.142andR = 3.1,seeforadiscussion
Gal V
W07.Publishedphotometryofthehost(Sollermanetal.2006)was
usedtobringthespectrumtoanabsolutescale.Wemeasureacon-
tinuumsignaltonoiseof∼50perpixelandaarclineFWHM(as
ameasurefortheresolution)of5.0Å(at∼5000Å).
Amultitudeofemissionlinesaredetected,seeFigure2,where
-150 -100 -50 0 50 100 150 we have labelled lines commonly found in high signal to noise
Velocity (km/s) spectraofbluecompactgalaxiesandWRgalaxies(manylabelled
linesarenotdetected).Inthefollowingwewillonlydiscussthehy-
Figure4.ThetoppanelshowstheHαemissionlinefromtheUVESdata
drogenandHeIlines,themetallineswillbediscussedinafuture
describedinSection3.1.3;thelowerpanelshowsthe[OIII]λ5008emission
linefromW07.Inadditiontotheselines,severalotheremissionlinesare publication(Wiersemaetalinprep.).
resolved.
3.1.2 ISAACspectroscopy
3 OBSERVATIONS
WeacquireddeeplowresolutionISAACKbandspectroscopy,see
3.1 ThehostofGRB060218 Table 1. Data were reduced using the ESO ISAAC data reduc-
tionsuite,version 5.7.0, using calibrationdata (darkframes, flat-
ThedataofthehostgalaxyofGRB060218presentedinthispaper
fields,arclampframes)takenthesamenight.Reductionincluded
were taken with the aim of characterizing both the young stellar
skysubtraction,flatfielding,wavelengthcalibration,spectralimage
population that produced the GRB itself (e.g. the WR star popu-
registrationandco-addition.Thedataweretakenusing30arcsec-
lation) and apossible underlying old stellarpopulation (seeW07
ondnodthrows,anda10arcsecond jitterboxwidth.Wavelength
forthemotivation).Inthispaperwewillrestrictourselvespredom-
calibration from the arc line frames was refined using night sky
inantly tothe hydrogen and helium linesand the inferred attenu-
lines, and the host galaxy spectrum was extracted using tasks in
ation properties, further physical properties of the source will be
IRAF.ObservationsoftheB3VstarHip021575(HD29376)were
discussedinfuturepublications.
taken for telluricline calibration: telluriccorrection and flux cal-
GeneraldetailsofourobservationsarelistedinTable1.
ibration was achieved using the IDL / SpeXtool package (Cush-
ing,Vacca&Rayner2004),inparticularthextellcor generaltask,
whichusesahighqualityVegamodelempiricallyconvolvedwith
3.1.1 FORS1spectroscopy
knowntelluricabsorptionfeaturestomatchtheentiretelluricstan-
WetargetedthishostwithFORS1,asthisinstrumenthasbeenblue dardspectrumindetectorunits(therebyimplicitlysolvingforthe
optimised (the similar FORS2 instrument is red optimised). The instrument response). The resulting model is then applied to the
600Bgrismwasused.Whilethisgrismdoesnotgiveuscoverage sciencetargetspectrum,producingatelluricfeaturecorrectedand
attheredendofthevisualrange,itshigherresolutioncomparedto fluxcalibratedspectrum(exceptingslitlosses).Tobringthespec-
e.g.the300Vgrism(astudyofthishostwiththisgrismispresented trum onto an absolute flux scale, one generally uses photometry
inHanetal.2010)allowsmoreaccuratedeblendingofblueBalmer of the source. In the case of this source, Kocevski et al. (2007)
linesandmoreaccuratefitsforstellarabsorption.Inaddition,the giveK =18.73±0.34,uncorrectedforGalacticextinction.How-
s
higherresolution,thoughstilllow,allowsamoreaccurateviewof ever,theKocevskietal.measurementsweredonewithpoorspatial
theemissionlinesmaking uptheWRstar bumpclaimed byHan resolution. We therefore examine the reduced acquisition images
et al. (2010). We selected a 1.3 arcsecond slit. The width of the from the second epoch of ISAAC data, which provide excellent
slitwaschosenbasedonhighresolutionimagingfromtheHubble pixelscaleundergoodseeingconditions.Wecombinethethree10
SpaceTelescope(HST),seee.g.Starlingetal.(2010)foranHST second acquisitionexposures (K filter),andtietheastrometryto
s
imageofthehost,sothatwearemorelikelytogetallthegalaxy the FORS1 acquisition images. The host galaxy is detected, and
lightintheslit. isresolved,butdetectionsignificanceislow.Weperformaperture
Observationswereperformedattwoepochs,withpoorseeing photometrywithrespecttofour2MASSstarsinthefield.Wefind
conditionsinthefirstepoch.Acquisitionwasdoneusingdirecton- K = 18.3±0.3 for the host, but because of the poor detection
s
sourceacquisitiononthesourcephoto-center,asthehostisbright rateandthesmallnumberof2MASSstarsinthefield,weusethe
foraGRBhost(r =20.16±0.03;Sollermanetal.2006),thestar Kocevskietal.valuetobringthespectrumtoanabsolutescale.
forming region(s) (see Section 3.1.3 and W07) are located at or Theslitpositionanglewaschosensuchthatanearby2MASS
nearthecenterofthehostgalaxy. star fell into the slit, with the aim of providing a more accurate
4 K. Wiersema
Table1.VLTobservationsofthehostofGRB060218.Theseeingisasmeasuredfromtheacquisitionimage(s).
Inst.+grism+slitwidth(”) Date Exptime(s) Seeing(”) Airmass
FORS1+600B+1.3” 2008-09-04 2×1345 1.5 1.3
2008-10-01 2×1345+1×900 1.1 1.3
Inst.+dichroiccenterwl(filters)+slitwidth(”) Date Exptime(s) Seeing(”) Airmass
UVES+437and860nm(HER5+OG590)+1.2 2008-08-02 1400 0.9 1.4
Inst.+mode+centerwl(µm)+slitwidth(”) Date Exptime(s) Seeing(”) Airmass
ISAAC+SWS1-LR+2.2+1.0 2008-07-17 12×60 0.8 1.7
2008-09-10 108×60 0.4 1.5
fluxcalibrationincasetheacquisitiondatawerenotdeepenough 3.2 ThecaseforX-shooter:sourceAinthehostof
to detect the host, particularly since the value from Kocevski et GRB100316D
al.(2007)hasalargeuncertainty.However,thespectrumandacqui-
To demonstrate the advantages X-shooter offers compared to our
sition images show that this object, 2MASS 03213974+1651305
observationswithmultipleinstrumentsofthehostofGRB060218,
(K=12.55)isinfactablendoftwostars,separatedby0.9arcsec-
wemakeuseofthespectraweobtainedofastarformingregionin
onds,andwiththecomponentsnotfullycoveredbytheslit.Weare
thehostgalaxyofGRB100316D. Thisstarformingregionisnot
thereforeunabletofurtherfinetunetheabsolutefluxoffsetofour
theonehostingtheGRB,butsimplyservesasanillustrationofthe
spectrumusingthisobject.
methodsused.Theobservations,thedatareductionandcalibration
and the basic properties of this region are reported in Starling et
al.(2010).Moredetailedpropertiesofthishost,includingthestar
3.1.3 UVESspectroscopy formingregionwheretheGRBoccured,willbereportedinafuture
paper(Floresetal.inprep.).
We acquired a high resolution spectrum of this source with the
aimofverifyingthemultiplevelocitycomponentsobservedinboth
emissionandabsorption([OIII]andNaI+CaII,respectively)in
3.3 Linefluxmeasurements
UVES spectroscopy of SN2006aj, the supernova accompanying
GRB060218 (W07). We selected the central wavelengths of the Besides the narrow nebular emission lines, the spectra also show
spectra such that we would cover both the Hα emission line and absorption components. As demonstrated for Hδ in Figure 5, a
the [OII] doublet in the red and blue arms, lines that will aid in clearabsorptioncomponentispresentunderneaththeemissionline,
detecting changing physical parameters over the various velocity made by stellar atmosphere absorption. The strength (equivalent
components(seealsoSection5.1). width) of the absorption component of the line is a strong func-
Becauseofaspatialcoordinateoffseterroronlytheexposures tion of the age of the stellar population dominating the observed
taken on Aug 2 can be used here. We reduce the data using the luminosity, the starformation history (starburst, continuous) and
standardprescriptionswithintheESOUVESdatareductionpack- the metallicity (e.g. Gonzalez Delgado & Leitherer 1999). Most
age,version4.2.4.Acrudefluxcalibrationisachievedusingspec- ofthesepropertiesarepoorlyconstrainedorcompletelyunknown
trophotometricstandardstars(EG274;Hamuyetal.1992, 1994) for GRB hosts. Subtracting a constant equivalent widthof all in-
takenonAug5andAug8,nospectrophotometricstandardswere tegrated hydrogen line fluxes to account for absorption (as done
observedonthenightthedataweretaken.Becauseoftheshortex- inLevesqueetal.2010andforsomesourcesinSavaglio,Glaze-
posuretime,nocontinuumemissionisdetected,butwedodetecta brook & Le Borgne 2009) will lead to inaccurate line fluxes and
dozenemissionlines,includingthethreebrightestBalmerlinesand createinconsistencieswheninferringstellarpopulationproperties.
the[OII]lines.Inthispaperweonlyusethevelocityinformationin MoreaccurateisthemethodemployedbyHanetal.(2010),who
someofthelines,andfurthercalibrationofthelinefluxesthrough fit a linear combination of single stellar population models from
theFORS1dataandthedatainW07willbedoneinfuturepapers. the evolutionary population synthesis code of Bruzual & Charlot
TheUVESspectrumisconvertedintovacuumwavelengthsusing (2003).AsGRBstarformingregionsareoftenconsideredbursty,
theIRAFdisptranstaskandbroughtontoaheliocentricwavelength metalpoorandthedominantstellarpopulationislikelyveryyoung
scale,afterwhichwetransformthewavelengthsontothesolution (astheGRBsformfromthemassiveendoftheIMF),weprefernot
ofW07bymeasuringthewavelengthoffsetsofsixlinesdetected torelyonmodels.Weperformfreefitsonthecontinuum,emission
in both the current UVESdata and the UVES data inW07. This andabsorptionlines,acceptingthismaycomewithlargererrorson
allowsustodirectlycomparetheprofilesofthedetectedemission theemissionlinefluxes.Thechoiceofspectralresolutionandre-
lineswiththeonesintheW07UVESdataonafixedrelativeveloc- quiredsignaltonoiseforthe060218FORS1datawasmotivatedby
ityscale.InFigure4weshowacomparisonoftheHαemissionline thenecessityoffittingabsorptionlinecomponentsinthismanner.
fromthedatadescribedabovewiththe[OIII]λ5008lineasanal- The emission and absorption components are adequately fit
ysedinW07.Thecomplexlinemorphologyseeninbothemission by Gaussian profiles (except in the UVES data of the host of
andabsorptionislikelycausedbytwoormorestarformingregions GRB060218, seeSection5.1).Wefiteach line regionseparately
and/orabrightstarformingregionwithoutflow(s)andgalaxydisk- using the ngaussfit task in the stsdas package within IRAF. We
likeemission.TheHSTimagingdataarenotabletoresolvethis, startedthefittingwiththeHδline,whoseabsorptionandemission
butthedetectionofseveralresolvedemissionlinesmayenableus components are easy to separate and have good signal to noise.
toconstrainthesemodelssomewhat(Wiersemaetalinprep.). AsdemonstratedinFigure5wefitsimultaneouslythecontinuum
The extinctionpropertiesof GRBhostgalaxiesfromH andHeI 5
Figure2.TheVLTFORS1600Bspectrum(seeSection3.1.1)ofthehostofGRB060218,plottedinrestframewavelengthscale.Indicatedbyverticallinesare
strongandweak(nebular)emissionlines,aswellassomeabsorptionfeatures,commonlydetectedinstarforminggalaxies(notethatnotallthelabelledlines
aresignificantlydetected).LineslabelledinitalicfontindicateGalacticabsorptionlines.Clearabsorptioncomponentscanbeseenunderneaththehydrogen
andheliumemissionlines.Atthescaleofthisplot,theerrorspectrumisnotvisible(apartfromthelowestsignalarea,thedashedlinearound∼3200Å).
(asafirstorderpolynomial);theemissionlinecenterwavelength, thesefreefitsontheHδlineswefound theabsorption andemis-
Gaussianfullwidthathalfmaximum(FWHM)andamplitude;and sion line to be centered at the same wavelength to within errors.
absorption line center wavelength, Gaussian FWHM and ampli- Inthefitsoflineswithrelativelypoorsignaltonoisewetherefore
tude. Errors on these fitted parameters are calculated by resam- tietheemissionandabsorptionlinecenterstogetherinthefits.In
pling,whichweuseaschecksonfitquality.Fromtheseparameters severalcases,mostnotablytheBalmerlinesbluewardofHδ,theH
we compute the emission and absorption line fluxes and equiva- orHeIemissionlinesareblendedwithotheremissionlines,orthe
lentwidths,andtheir1σuncertainties.AscanbeseeninFigure5, underlyinghydrogenandheliumabsorptionlinesareblendedwith
theFWHMof theabsorption lineisconsiderably largerthanthat other,unrelated,absorptionlines.Inseveralcasesitwaspossibleto
oftheemissionline,allowinginmostcasesreasonablefits.From deblendbyfittinganadditionalGaussianfixedatthewavelengthof
6 K. Wiersema
Figure3.TheISAACspectrumtakenwiththeLRKgrism,plottedinrestframewavelengths.Thetopplotshowsthecombined2dimensionalspectrumbefore
telluriclinecorrectionandsensitivitycalibration.Thelowergraphshowstheextracted,telluriclinecorrectedandfluxcalibratedspectrum.Severalemission
andabsorptionlinesthatarecommonlydetectedin(ultracompact)HIIregions,starburstgalaxiesandWRstarsareindicated.Whereascontinuumemissionis
detectedovernearlytheentirespectralrange,mostofthelinesindicatedarenotsignificantlydetected.
theadditionalcomponent(forexampletheHǫtransitionisblended Ofthesourceswithfourdetectedlines,allbut020903havepoorfit
with[NeIII]),butinsomecasestheresolutionoftheFORS1data χ2,inmostcasesdominatedbyasingleemissionline(oftenHδ).
wasnotsufficienttoobtainaccurateemissionandabsorptionfluxes, Thefitsfor020903havegoodreducedχ2values,butnopref-
andwedonotusetheselinesinfurtheranalysis.Wewillreportthe erenceforaR orT isclearfromthisdata(thereducedχ2ranges
V e
absorption equivalent widthsand theirimplicationsforthestellar from0.93to1.05)becauseofthelowA (intherangeA =0.23to
V V
populationsinafuturepaper. 0.33forthewholeR ,T grid)andthesmallA /A range.Itisclear
V e λ V
Inthenear-infraredwefurtherexcludelinesthatareaffected fromthesefitsthatwerequiresignificantlymorelinesandA /A
λ V
bysharpresidualsfromtelluriclinecorrections,or,inthecaseof range to constrain these parameters. In addition we note that the
sourceAinthehostof GRB100316D, bybadpixels.Wefurther Hanetal.sampledealsmainlywithhost-integrated spectra(with
excludelinesfromtheanalysisthathaveverylargeerrors(insome the exception of GRB060505), and since each host may contain
casesthisisbecauseofproximitytotheedgesoftheechelleorders multiple bright star forming regions, the measured attenuation is
wheresignaltonoiseoccasionallydropssteeply,orwhentheline asuperposition of these,and not easilycoupled toGRBenviron-
isrightinanareawheretelluricabsorptionbringstheresponseto mentparameters(i.e.theinfluenceofWRstarsassociatedwiththe
nearzero). starformationepisodeproducingtheGRBprogenitor).Aswewill
showinSection5,thismayalsobeevidentinourdataonthehost
ofGRB060218throughourhighresolutionspectroscopy.
4 FITSOFTHEEXTINCTION
4.1 LiteratureBalmerfluxdata 4.2 ThehostofGRB060218
Several optical spectroscopic studieshavebeen performed onthe Itisclearfromthefitsinsection4.1thatalarger A /A baseline
λ V
nearby(long)GRBhostgalaxypopulation,generallywiththeaim isbeneficial.Thedataof thehostof GRB060218 arewellsuited
ofcomparingtheGRBhostpopulationwithothersamplesofgalax- todemonstratethepossibilities.Thishosthasbeenobservedwith
ies,seeforexampleHanetal.(2010),Levesqueetal.(2010).Han HST(e.g.Starlingetal.2010)showingittobebeabluecompact
etalfitstellarpopulationmodels,therebyaccountingforstellarab- galaxywhichappearstocontainonebrightmassivestarformation
sorption,andlistthefluxesoftheHα,β,γandδlinesforeighthost region(butseeSection5.1andWiersemaetalinprep.).
galaxies (for the host of GRB060505 both the values for the in- Figure7(top)showsanexampleoftwoofthefits,atR =3.1
V
tegratedhost spectrumand theregionwhere theGRBtook place and 5.0, and T = 104 K, for the hydrogen and helium lines de-
e
are given; Tho¨ne et al. 2008). Han et al. (2010) compute the ex- tectedintheFORS1and ISAACdatadescribed above. Notethat
tinctionbyassumingR =3.1andT =104 K,usingtheratiosof becauseoftherelativelylowresolutionoftheFORS1600Bgrism
V e
twolinesatatime(e.g.Hα/Hβ).Figure6showsourfitsonthese wearenotabletoderiveaccuratefluxesofalldetectedhydrogen
emissionlinefluxesfortwovaluesofR andT =104K.Asinour linesbecauseofblendingwithotheremissionlines(e.g.Hǫ)and/or
V e
method thefluxesarenormalisedtoHβ,thefitstoallsources go absorptionlines(linesbluewardofH9),thisiseasilyvisibleinFig-
roughlythroughthesamepoint,andtheslopeisequalto−A .Itis ure2.Thisisimportant:eventhoughthelinesbluewardofHδget
V
clearfromthisfigurethatmostGRBhostsintheHanetal.sample everclosertogetherinA /A space(seeFigure1),thispartofthe
λ V
havelow A .ThiscannotnecessarilybeextrapolatedtoallGRB A /A rangeismostsensitivetoR changes.Afurthercaveatisthe
V λ V V
hostsingeneral:thereisabiasagainstdustysightlinesinthede- fluxofthePaαline,whoseuncertaintyislargebecauseofthelarge
tectionof afterglows(seee.g.Greiner etal. 2011), whichmayin uncertaintyinthehostKbandmagnitude.
turntranslateinabiasagainstdustrichhostgalaxies. Usingthefluxesasdeterminedabovewefindacceptablefits
Ofthesourcesinthissample,onlyfourhavefourBalmerline for the hydrogen lines for most models on our T ,R grid, with
e V
detections(thehostsofGRBs990712,020903, 031203,060218), the lowest χ2 for the T = 1.5 × 104 K and R = 4.5 model
e V
makingitimpossibleinmostcasestodistinguishtheeffectsofR (χ2 = 1.3). The χ2 distribution shows that the models with high
V red
andT throughfitχ2 (wehavetwofreeparameterstofitforeach R valuesperformbetterthanmodelswithlowR (. 3.5)forall
e V V
pointinourT ,R grid:theslopeandoffsetof thestraightline). temperaturesconsidered.ForallmodelswithR &3.1,highertem-
e V V
The extinctionpropertiesof GRBhostgalaxiesfromH andHeI 7
Figure7. AnexampleoftwoofthefitsintheTe,RVgridonthedataofthehostofGRB060218(topfourpanels)andsourceAinthehostofGRB100316D
(lowerfourpanels).Theleftcolumnshowsafitusingthehydrogenlines,withTe=104K,displayedarethefitsforRV =3.1and5.Therighthandcolumn
showsafitoftheneutralheliumlines,usingthesameparameters.IndicatedarethecorrespondingbestfitvaluesforAV.Thesefitsarenottheoneswiththe
bestχ2foreitherofthesources,butservetodemonstratetheinfluenceofRV onthefit,andTe=104Kallowseasycomparisonwithliteraturevalues.Note
thatthefitsdisplayedfor100316DusealsothebluestHeIlineinthefit.
peratures(T >12500K)fitbetterthanlowerones.Broadlyspeak- 4.3 X-shooterdata
e
ingtheseresultsindicateapreferenceforahightemperature,high
HSTimagingofthehostgalaxyofGRB100316D showsthatthe
R attenuation, consistent withthefindings in W07and a“grey”
V
sourcewerefertoassourceA(seeStarlingetal.2010fordetails
extinctioncurve.Thebestfittingmodelcorrespondstoanattenua-
tionA =1.02±0.16magnitudes.WefittheHeIlinesseperately ofthissource)isasingle,largestarformingregionunrelatedtothe
V
GRB.Weusethedataofthissourceonlytoshowthepossibilities
andfind,asexpectedfromthesmallnumberoflinesdetectedand
X-shooteroffersinthestudyofdustinGRBhosts.
thelargeerrorbars,thatwecannotreliablydistinguishR andT
V e
preferences: reduced χ2 values range from 0.2 to 0.4. We do see WefitthefluxesdeterminedinSection3.3.Inthiscase,wedo
thattheA valuesfoundfromtheHeIlinefitsareconsistentwith detectHeIlinesatredwavelengths,andbecauseofthehigherres-
V
thehydrogen ones. Adopting thesame parametersasfor thebest olutionofX-shootercomparedtoFORS1600Bgrism,wecande-
hydrogenfitwegetA =1.3±0.7. blendseveralofthebluerBalmerlines(butnotH8).Unfortunately,
V
While these fits show the possibility to distinguish models theredshift of thishost galaxy(z = 0.0591, Starlinget al. 2010)
with different R ,T from a relatively small number of lines, we issomewhat unfortunate:several ofthebrighterlinesinthenear-
V e
note that the difference in χ2 between the best and worst fitting infrared fall on areas heavily affected by telluric absorption (e.g.
model(R =2.5,T =2×104)isonly1.7.Alargernumberofde- Paγ),onbadpixels(e.g.Brǫ)orareredshiftedoutofthewindow
V e
tectedlines,particularlyaround A /A ∼ 0.5andbluewardofHδ, (wavelengthsredwardsof∼2.26µmarestronglyaffectedbyther-
λ V
canimprovethisconsiderably.Takingthebestfittingmodelatface mal noise and left out of the analysis, this affects e.g. Brγ). We
value,thepreferenceforaR largerthanthemeanGalacticvalue furthernotethatnoinfraredimagingofsourceAwasavailableat
V
of3.1isinteresting.LargevaluesofR areindicativeofsightlines thetimeofwritingtoassessthequalityofthefluxcalibration.
V
dominated by relatively large dust grains. Dust is extremely sen- Thehydrogenlinesarewellfitbythemodels,seeFigure7for
sitivetoenvironmental properties,asevidenced bythewidevari- twoexampleswhichclearlyshowasignificantlylowerattenuation
ety of Galactic extinction law shapes (Valencic, Clayton & Gor- thaninthehostofGRB060218.Thislowattenuationmeansthere
don2004),andfairlygreyextinctioncurvesmaypointtoregions islittlesensitivityto R and T .The χ2 has abroad minimumat
V e
withmassivestars(and/ordenseenvironmentswithshockeffects) χ2 =0.86forT =7500KandR =3.5.ForallvaluesofR the
red e V V
and/or fairly incomplete dust mixing and processing through the χ2 increaseswithincreasingelectrontemperature(toamaximum
galaxy. of8.7at20000K,i.e.1.0abovethebestfit),allowingustocon-
WecautionthattheUVESspectroscopygivessometentative cludetheelectrontemperatureinsourceAislikely. 104 K.For
hints for more than one star forming region in this host (perhaps eachtemperaturethereisverylittlechangeinχ2withchangingRV,
two,iftheGaussiancomponentsmakingupthelinearetwodistinct theentirerangeRV = 2.5−5.5onlychangesχ2 by0.1,whichis
starformingregions),whichwasnotevidentinHSTimaging.This expectedwhenAV isverylow.TheAV foundforwhatisformally
mayimplythatthefitsweperformhereareofthesuperpositionof thebestfittingmodel(Te=7500K,RV =4.5)isAV =0.19±0.06.
thedustpropertiesofthesetworegions,withpotentiallydifferent TheHeIfluxesarealsowellfitwiththemodels(seeFigure7
R andA .Wethereforewillnotcomparethedustpropertiesfound foranexample).Wefitthedatasettwice,includingandexcluding
V V
abovefurtherwiththosefoundfromabsorptionspectroscopy. theHeIlineatrestframe3187 Å:thislineisobservedat3376Å
8 K. Wiersema
Figure6.AnexampleofoneofthefitsontheBalmerHα,Hβ,HγandHδ
emissionlinesfromaseriesoflongGRBhostsfromHanetal(2010).Both
plotsshowafitusingTe = 104 K,withinthetopplotRV = 3.1,andthe
lowerplotRV =5.TheAVvaluesthatcomefromthesefitsareshowninthe
panels.Thesefitsarenotnecessarilytheoneswiththebestχ2,butnicely
demonstratetheinfluenceofRVonthefit.
agreementwiththeHresults,butsimilartotheHlinestheχ2does
Figure 5. Fit of the Hδ lines of the X-shooter spectrum of source A in notchangemuch:fortheT =104 Kmodelstheχ2 changesfrom
e
the host of GRB100316D (top) and the FORS spectrum of the host of 3.2to3.9fromR = 2.5to5.5.Thebestfittingmodel(T = 104
V e
GRB060218 (bottom). The weak emission line blueward of Hδ in both
KandR =3.0)givesχ2 =0.8forA =0.43±0.31.
spectra is[SII],whichisincluded inthefit.Thelineisclearly madeup V red V
ofanemissioncomponent(formedbyhotgas)andanabsorptioncompo-
nent (from stellar atmosphere absorption). Both emissionand absorption
linesareadequatelyfitbyGaussianprofiles.Theresultingfitisshownin 5 DISCUSSION
red,thebluedashedlineindicates thecontinuumlevel. TheHδemission
5.1 Caveats,implicitassumptionsandcalibration
linefluxesforthesetwosourcesarevirtuallyidentical,whiletheabsorption
equivalentwidthofthehostofGRB060218issignificantlylower.Thisplot requirements
demonstratesthenecessityofadequatespectralresolution.
In the list below we briefly outline a few important caveats, im-
plicitassumptionsandcalibrationrequirementstoallowthefitsas
describedabovetobesuccessful.Weillustratethesethroughthefit
andthevalidityofthemeanParanalatmosphericextinctioncurve
resultsofthetwosourcesabove.
isuncertainattheseverybluewavelengths.Thefactthatthisline
is even detected is quite remarkable, and will prove valuable for • Signal to noise: As can be seen from the poor results de-
GRBhostsathigherredshifts,asitextendssensitivitybeyondthe scribedinSection4.1,werequiregoodsignaltonoisetodetectalso
Balmerlimit(seeFigure1).Wefindthatincludingthisuncertain theweakerblueBalmerlinesandtheweakerPaschenandBrackett
line flux in the fit increases the χ2 significantly (doubling it for lines.Theheliumlinesareevenfainterthanthehydrogenlines,but
mostmodels),butweshowafitincludingthislineinFigure7for formanimportantadditionparticularlywheninfraredspectroscopy
completeness.Excludingthisline,theHeIlinefluxesshowbroad isnotpossibleorwhentheredshiftisfairlyhigh(Figure1).
The extinctionpropertiesof GRBhostgalaxiesfromH andHeI 9
• Redshift:thesignaltonoiserequirementsnaturallytranslate 5.2 TheadvantagesofX-shooter
inaredshift requirement, but theredshift playsamoreimportant
The list of caveats and requirements above shows the necessity
roleinthepositionoftheemissionlines.Thefitsaboveshowthe
of covering a wide (restframe) wavelength range but also the in-
necessityofdetectionofoneormoreinfraredlines(e.g.Paα−δand
herent problems of using multipleinstruments todo this. TheX-
Brγ,δ),whichbecomeincreasinglyhardtodetectwithincreasing
shooterspectrographontheESOVLTsolvedmanyoftheseissues.
redshift.
The(fixed)mediumresolutionofX-shooterissufficienttodeblend
• Highresolutionimaging:theredshiftplaysaroleinthisre-
manyoftheblueBalmerlines,providegoodfitsontheabsorption
quirementtoo.Werequirehighresolution(i.e.spacebasedimag-
components and to give good telluric line corrections. The very
ing)toestablishexactlywhichregionsofthehostarecoveredby
largewavelengthrange(inFigure1weshowthez=0wavelength
theslit,andtomakesurewearenotaveragingmultiplestarforming
coverageofthethreeX-shooterarms)meansthatalargenumberof
regionstogether.Additionally,acomparisonwithdustpropertiesin
linesarewithinreach,and,becauseoftheveryhighefficiencyof
absorptionfromtheafterglowspectrawillrequireunderstandingof
theinstrument,wedetectalsothefainterlines.Thearchitectureof
thespatialpositionoftheGRBwithinthehost.
theinstrument issuchthat acquisitionisdone simultaneously for
• Spectralresolution:Asdiscussed inSection3.3andFigure
allthreearms,meaningthattheslitscovertheexactsameregions
5,ahighspectralresolutionandsignaltonoiseisrequiredtoeffec-
inthehost.Thesimultaneouscoverageofallthreearmsmeansthat
tivelyfittheabsorptioncomponentsundertheHandHeIlines,and
some consistency in flux calibration can be achieved through the
to deblend these lines from other emission and absorption lines,
continuumaswell.Incaseswherethereistroublebringingtherel-
This is particularly important for the higher Balmer lines, which
ativefluxcalibrationforeacharmontoanabsolutescale,thereare
arevitaltodiagnoseR .Thereisanadditionaluseofhighresolu-
V insome cases sufficient linesin each individual arm toallow fits
tionspectra,asdemonstratedinFigure4:theresolved[OIII]lines
foreacharmseparately. Usingothersources(stars)intheslitfor
(confirmedbytheHαdata)canbeexplainedbytwostarforming
additional flux calibration, as described in Section 3.1.2 will not
regions (W07). The detection of two velocity components in ab-
bepossibleduetotheshortslit.Thereisstillastrongrequirement
sorptioninthesupernovaspectraindicatesthatinthisscenariothe
for accurate pre-imaging, as the X-shooter acquisition CCD only
GRBislocatedintheredmostcomponent. However,thereareal-
operatesintheopticalwavelengthrange.Anadditionalbenefitof
ternativescenariostoexplainthemultiplecomponents(Wiersema
themethodsshownaboveisthatafitonthelinesintheUVBand
etalinprep).InthecaseofouranalysisofthehostofGRB060218
VISarmsprovidesstrongenoughconstraintsontheattenuationpa-
theUVESspectramayimplythatwearereallyseeingtheaverage
rametersthattheresultingfitcanbeusedtorefinefluxcalibration
propertiesoftwostarformingregionsintheanalysisinSection4.2.
intheNIRarm,providingawaytogetadditionalfluxcalibration
• Calibration: The wide wavelength range required for this
fromthesciencedataitself.
analysisoftenmeansacquiringdatawithmorethanoneinstrument
The data above show some of these advantages already,
(butsee5.2).Thisputsrequirementsontheabsolutefluxcalibra-
though theexposure timefor this dataset isshort, and conditions
tion:withinoneinstrument(orX-shooterarm)arelativefluxcali-
werenotideal,asthedataweretakeninToOmode(seeStarlinget
brationwillbesufficientasweconsiderfluxratios,butwhencon-
al.2010).Highersignaltonoiseobservationsarelikelytoimprove
sidering multiple instruments, an absolute calibration is required.
theresultsinSection4.3.
Eachwavelengthrangeoffersitsownproblems,inadditiontothe
commonproblemswithabsolutefluxcalibration(spectrophotome-
try),illustratedabove.Particularlyintheinfraredthiscanbevery
5.3 Reddening,extinctionandattenuationinGRBhosts
difficult, as the telluric transmission has to be fit for in addition
tothesensitivity,slitlossesandextinction,puttingadditionalcon- Longgamma-rayburstsareaccompaniedbybrightafterglowsand
straintsontheobservingconditionsandtelluricstandardstars(e.g. inmostcasesbybrightsupernovae.ThismakesGRBsidealprobes
theirspectraltype).Wenotethatahigherspectral resolutionalso of distant galaxies: spectra can be obtained of the afterglows to
greatlyimprovesthequalityofthetelluriccalibration. study the host ISM in absorption, and, when the afterglow has
Tofinalizethefluxcalibrationwerequiregoodphotometry,but fadedaway,spectraofthehostprobethehostgalaxyinemission.
formanyGRBhoststhereisalackofhighqualityphotometryat DustpropertiesinGRBhostsareofparticularinterest,becauseof
infraredwavelengths.ThisisillustratedinSection4.2.Therequire- thedegeneracyofdustreddeningwithotherparametersofinterest
ment forgood multiband imagingalsoplays animportant rolein (e.g.deviationsfromsynchrotronmodels,searchesforhighredshift
the acquisition process. GRB hosts are extended sources and the burststhroughLymanbreaksetc.),thepossiblemetallicitylimitson
starformingregionsofinterestareblue,meaningthatdirectacqui- GRBprogenitors(e.g.theamountofmetalslockedupindust),the
sitioncanbeproblematicwhen multipleinstrumentsfor different influenceofdustscatteringonafterglowpolarimetry,thenatureof
wavelengthregionsareinvolved,asthephotocenteroftheregions darkbursts,etcetc.
ofinterestiswavelengthdependent. Broadly speaking the dust properties of GRB hosts can be
• Aftergloworsupernovaspectra:Tomaximizethescientific studiedusingtheaftergloworusingthehostgalaxyinemission.In
returnitishelpfultohavehighresolutionspectroscopyoftheaf- thefirstcase,theamountofreddeningexperiencedbytheafterglow
terglow or SN in question. The detection of dust-tracing absorp- andtheshapeoftheextinctioncurve(orthegrainsizedistribution)
tionlines(suchasNaI,KIordiffuseinterstellarbands)willallow canbederivedthroughfitsontheafterglowspectralenergydistri-
comparisonofthelineofsightextinctionproperties;anydetected bution(e.g.Galama&Wijers2001;Starlingetal.2007;Schadyet
velocitystructureofabsorptionlineswillaidindeterminingthepo- al.2010andreferencestherein),throughstudiesoftherelativecol-
sitionoftheGRBwithrespecttothestarformingregion(seeW07 umndensitiesofgasphasemetals(e.g.Savaglio&Fall2004)and
andthediscussionoftheUVESdataabove);andknowledgeofthe the wavelength dependancy of afterglow polarisation (e.g. Klose
SNbehaviourwillhelpinestimatinganyresidualsupernovacon- et al. 2004; Wiersemaet al in prep). The distance of the gasand
tinuum or nebular phase contribution to the measured fluxes (see dust that is seen in absorption in afterglow spectra can be deter-
alsoKu¨pcu¨ Yoldas¸,Greiner&Perna2006). minedthroughvariableexcitedmetastablefinestructurelines(e.g.
10 K. Wiersema
Vreeswijketal.2007), excitedmolecular transitions(e.g.Sheffer Han,X.H.,Hammer,F.,Liang,Y.C.,Flores,H.,Rodrigues,M.,
etal.2009),ionisationstates(e.g.Prochaska,Chen&Bloom2006) Hou,J.L.,Wei,J.Y.2010,A&A514,24
andanysignsofdustdestructionbytheGRBphotonfield(i.e.re- HjorthJ.etal.,2003,Nature,423,847
lease of elements into the gas phase that were locked up in dust Hummer,D.G.&Storey,P.J.1987MNRAS,224,801
grainsleadstoobservablevariabilityofresonancelines). Klose,S.,Palazzi,E.,Masetti,N.,Stecklum,B.,Greiner,J.,Hart-
Thestudyofdustinhostgalaxiescanbeapproachedbystudy- mann,D.H.,Schmid,H.M2004,A&A420,899
ingdustinthestarformingregionshostingGRBs,whichmeasures Ku¨pcu¨ Yoldas¸,A.,Greiner,J.&Perna,R.2006,A&A457,115
theinfluenceofdustonemissionlineemittinghotgas(inthispa- Kocevski,D.etal.2007,ApJ663,1180
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6 CONCLUSIONS
Porter,R.L.,Ferland,G.J.&MacAdam,K.B.2007,ApJ,657,
We have presented a spectroscopic dataset of the host of 327
GRB060218,which,togetherwithalreadypublisheddataforastar Prochaska,J.X.,Chen,H.-W.&Bloom,J.S.2006,ApJ648,95
formingtegioninthehostofGRB100316D,weusetogetfurther Savaglio,S.,Glazebrook,K.,LeBorgne,D.2009,ApJ691182
insightintotheattenuationof theemissionlinesbydust.Specifi- Savaglio,S.&Fall,S.M.2004,ApJ614,293
callyweshowhowhydrogenandheliumfluxesofmanyrecombi- Schady,P.etal.2010,MNRAS401,2773
nationtransitionscanbefitsimultaneously togetinsight intothe Schlegel,D.J.,Finkbeiner,D.P.&Davis,M.1998,ApJ,500,525
electrontemperatureandtheR ofthedust.X-shooteristheideal Sheffer,Y.,Prochaska,J.X.,Draine,B.T.,Perley,D.A.,Bloom,
V
instrumenttodothisforalargesampleofGRBhosts,aslongas J.S.2009,ApJ701,L63
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glowspectralenergydistributionsandreddeningfitsonintegrated Curran,P.A.,Kouveliotou,C.,vanderHorst,A.J.,Heemskerk,
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ACKNOWLEDGMENTS
Vreeswijk,P.M.etal.2007,A&A468,83
We thank Sandra Savaglio, Johan Fynbo, Cedric Ledoux, Palle Wiersema,K.,etal.2007,A&A,464,529
Møller, Sara Ellison, Sung-Chul Yoon, Elena Pian, Rhaana Star-
ling,RalphWijers,NialTanvirandPaulVreeswijkforusefuldis-
cussion, and the anonymous referee for helpful comments. We
thank the ESO staff for obtaining the data discussed here. KW
acknowledges support from STFC. The financial support of the
British Council and Platform Beta Techniek through the Partner-
shipProgrammeinScience(PPSWS005)isgratefullyacknowl-
edged.
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