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arXiv:astro-ph/0508177v26Oct2005Mon.Not.R.Astron.Soc.000,000–000(0000)Printed5February2008(MNLATEXstylefilev1.4)CosmologicalandastrophysicalparametersfromtheSDSSfluxpowerspectrumandhydrodynamicalsimulationsoftheLyman-αforestMatteoViel&MartinG.HaehneltInstituteofAstronomy,MadingleyRoad,CambridgeCB30HA5February2008ABSTRACTThefluxpowerspectrumoftheLyman-αforestinquasar(QSO)absorptionspectraissensitivetoawiderangeofcosmologicalandastrophysicalparame-tersandinstrumentaleffects.ModellingthefluxpowerspectruminthislargeparameterspacetoanaccuracycomparabletothestatisticaluncertaintyoflargesamplesofQSOspectraisverychallenging.WeusehereacoarsegridofhydrodynamicalsimulationsrunwithGADGET-2toobtaina“bestguess”modelaroundwhichwecalculateafinergridoffluxpowerspectrausingaTaylorexpansionofthefluxpowerspectrumtofirstorder.Inthisway,wein-vestigatehowtheinterplaybetweenastrophysicalandcosmologicalparametersaffectstheirmeasurementsusingtherecentlypublishedfluxpowerspectrumobtainedfrom3035SDSSQSOs(McDonaldetal.2004).WefindthattheSDSSfluxpowerspectrumaloneisabletoconstrainawiderangeofparam-etersincludingtheamplitudeofthematterpowerspectrumσ8,thematterdensityΩm,thespectralindexofprimordialdensityfluctuationsn,theeffec-tiveopticaldepthτeffanditsevolution.ThethermalhistoryoftheIntergalac-ticMedium(IGM)is,however,poorlyconstrainedandtheSDSSdatafavoureitheranunplausiblylargetemperatureoranunplausiblysteeptemperature-densityrelation.ByenforcingathermalhistoryoftheIGMconsistentwiththatinferredfromhigh-resolutionQSOspectra,wefindthefollowingvaluesforthebestfittingmodel(assumingaflatUniversewithacosmologicalconstantandzeroneutrinomass):Ωm=0.28±0.03,n=0.95±0.04,σ8=0.91±0.07(1σerrorbars).Thevaluesforσ8andnareconsistentwiththoseobtainedbyMcDonaldetal.withdifferentsimulationsforsimilarassumptions.Weargue,however,thatthemajoruncertaintiesinthismeasurementarestillsystematicratherthanstatistical.Keywords:Cosmology:intergalacticmedium,cosmologicalparameters–large-scalestructureofuniverse–quasars:absorptionlines1INTRODUCTIONThelastfewyearshavenseentheestablishmentoftheLyman-αforestasoneofthemajorobservationaltoolstoprobethematterpowerspectrum.Measure-mentsofthematterpowerfromtheLyman-αforestdataextendtosmallerscalesandprobeacomplemen-taryredshiftrangethanthoseusingtheCosmicMi-crowaveBackground(CMB),galaxysurveysorweakgravitationallensing.TheLyman-αforestisthuside-allysuitedtoconstraincosmologicalparameterswhichaffectthepowerspectrumonsmallscalesliketheneu-trinomassandthemassofDarkMatter(DM)particles(Seljaketal.2004;Vieletal.2005).Inacombinedanal-ysiswithCMBdatatheLyman-αforestresultshavealsobeenpivotalintighteningconstraintsonthepowerlawindexofthepowerspectrumofprimordialdensityfluctuationsn.Aconsistentpictureemergessuggestingthatthefluctuationamplitudeofthematterpowerspectrumσ8isratherhigh,thatthespectralindexofprimordialden-sityfluctuationsisconsistentwiththevaluen=1andthatthereisnoevidenceforarunningofthespectralin-c0000RAS2M.Viel&M.G.Haehneltdex,significantneutrinomassoradeviationfromacolddarkmatterspectrumatsmallscales(Croftetal.1998,McDonaldetal.2000,Huietal.2001,Croftetal.2002[C02],McDonald2003,Vieletal.2003,Viel,Haehnelt&Springel2004[VHS],Desjacques&Haehnelt2004,Viel,Weller&Haehnelt2004,McDonaldetal.2004[M04a],McDonaldetal.2004b[M04b],Vieletal.2005,Lidzetal.2005).MajoruncertaintiesaretherebytheassumedeffectiveopticaldepthandthermalhistoryoftheInter-galacticMedium(IGM)andthenumericallimitationsinobtainingaccuratetheoreticalpredictionsforthefluxpowerspectrumforalargeparameterspace(seeVHSandM04aforadetaileddiscussion).VHSandViel,Weller&Haehnelt(2004)recoveredthelineardarkmatterpowerspectrumamplitudeanditsslopefromanewsetof27highresolutionhighsignal-to-noiseQSOs(theLUQASsample,Kimetal.2004[K04])andreanalysedtheearlierresultsofC02.Vieletal.foundσ8=0.94±0.08,n=0.99±0.03(1σ)andnoevidencefora(large)runningspectralindexwhentheycombinedtheLyman-αforestdatawiththeWMAPdata.SimilarresultshavehavebeenobtainedsubsequentlybytheSDSScollaborationfromamuchlargersampleof3035low-resolutionlowS/Nspectrawithsignificantlywiderreshiftcoverage(M04b;Seljaketal.2004).ThesefindingscorrectedearlierclaimsofSpergeletal.(2003)whousedthedataofC02combinedwithCMBandgalaxysurveydatatoargueforasig-nificanttiltawayfromthecanonicalHarrison-Zeldovichn=1spectrumandpossiblyalsoforarunningofthespectralindex(seeSeljaketal.(2003)forthefirstsug-gestionthatthisresultmaybeduetotheassumptionofanunplausiblylargeeffectiveopticaldepth).LargesamplesofQSOabsorptionspectraoffertheopportunitytoobtainthefluxpowerspectrumforawiderangeofredshiftswithanaccuracyatthepercentlevel.Despitethegenerallygoodagreementbetweendif-ferentgroupsachievedinthelastcoupleofyearsthereare,however,stillmajoropenissuesofwhatandhowlargetheuncertaintiesinthecosmologicalandastro-physicalparametersinferredfromthefluxpowerspec-trumare(seeVHSandM04bfordiscussionswithdiffer-entviews).Itwillbeimportanttoresolvetheseissuesiffurtherprogressistobemade.ThetwodatasetsusedbyVHSandM04bandtheirtheoreticalmodellingwereverydifferent.TheSDSSQSOdatasetanalysedbyM04aconsistso
本文标题:Cosmological and astrophysical parameters from the
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