Numerical simulations of the Warm-Hot Intergalacti

arXiv:0801.1039v1[astro-ph]7Jan2008SSRvmanuscriptNo.(willbeinsertedbytheeditor)NumericalsimulationsoftheWarm-HotIntergalacticMediumSerenaBertone·JoopSchaye·KlausDolagReceived:20October2007;Accepted:22November2007AbstractInthispaperwereviewthecurrentpredictionsofnumericalsimulationsfortheoriginandobservabilityofthewarmhotintergalacticmedium(WHIM),thediffusegasthatcontainsupto50percentofthebaryonsatz∼0.Duringstructureformation,gravitationalaccretionshocksemergingfromcollapsingregionsgraduallyheattheintergalacticmedium(IGM)totemperaturesintherangeT∼105−107K.TheWHIMispredictedtoradiatemostofitsenergyintheultraviolet(UV)andX-raybandsandtocontributeasignificantfractionofthesoftX-raybackgroundemission.WhileOviandCivabsorptionsystemsarisinginthecoolerfractionoftheWHIMwithT∼105−105.5KareseeninFUSEandHubbleSpaceTelescopeobservations,modelsagreethatcurrentX-raytelescopessuchasChandraandXMM-NewtondonothaveenoughsensitivitytodetectthehotterWHIM.However,futuremissionssuchasConstellation-XandXEUSmightbeabletodetectbothemissionlinesandabsorptionsystemsfromhighlyionisedatomssuchasOvii,OviiiandFexvii.Keywordslarge-scalestructureoftheuniverse,intergalacticmedium,diffuseradiation,X-rays:diffusebackground,quasars:absorptionlines1IntroductionThecosmicbaryonabundancewasfirstinferredbyapplyingthetheoryofprimordialnucleosynthesistoobservationsoflightelementabundances(e.g.(Oliveetal.2000))S.BertoneAstronomyCentre,UniversityofSussex,Falmer,BrightonBN19QH,UnitedKingdomPresentaddress:SantaCruzInstituteforParticlePhysics,UniversityofCaliforniaatSantaCruz,1156HighStreet,SantaCruzCA95064,USAE-mail:[e-mail],LeidenUniversity,P.O.Box9513,2300RALeiden,TheNetherlandsE-mail:[e-mail]-Planck-Institutf¨urAstrophysik,P.O.Box1317,D-85741Garching,GermanyE-mail:[e-mail]2andhasbeenconfirmedbyobservationsofthecosmicmicrowavebackgroundradia-tion(CMB).ArecentmeasurementfromtheWilkinsonMicrowaveAnisotropyProberevealsthatΩbh2=0.0223+0.0007−0.0009,wheretheHubbleparameterish=0.73+0.03−0.04(Spergeletal.2007).Atz2mostofthebaryonsarethoughttoresideinthediffuse,photoionisedintergalacticmedium(IGM)withT∼104−105K,tracedbytheLyαforest1(e.g.Rauchetal1997;Weinbergetal.1997;Schaye2001).ThegastracedbytheLyαforest,togetherwiththemassingalaxies,fullyaccountsforthecosmicbaryonabundanceatz∼2andconfirmsthepredictionofnucleosynthesistheoriesandtheCMBmeasurements.However,whenthemassofstars,interstellargasandplasmainclustersofgalax-iesatredshiftz∼0isconsidered,onlyasmallfractionofthemassbudgetcanbeaccountedfor(e.g.Persic&Salucci1992;Fukugitaetal.1998;Fukugita&Peebles2004).WhilethegastracedbyLyαforestmayaccountforaboutathirdofthelow-zbaryons(e.g.Danforth&Shull2007),themajorityofthebaryonsremaininvisible.Unlessthebaryonbudgetathighredshifthasbeenoverestimatedbytwodifferentmeasurements,alargeamountofthelowredshiftbaryonsmustbe“missing”.Oneofthebasicpredictionsofcosmological,gas-dynamicalsimulationsisthedis-tributionofbaryonsintheuniverse.Howmanybaryonsarelockedinstars?Howmanyresideinclustersandfilaments?Whatisthestateofthediffusegas?Simulationsofstructureformationpredictthatthemissingbaryonsproblemismeasureofourtech-nologicallimitationsratherthanarealproblem.Inotherwords,thebaryonsareoutthere,butwecannotseethembecausetheyresideingasthatistoodilutetobedetectedinemissionandtoohottobetracedbyLyαabsorption.TheideaofgravitationalheatingoftheintergalacticgaswasfirstsuggestedbySunyaev&Zel’dovich(1972)andsubsequentlydeveloped(e.g.Nath&Silk2001;Furlanetto&Loeb2004;Rasera&Teyssier2006).Whilethelargeststructuresintheuniverseform,theIGMisheatedbygravitationalshocksthatefficientlypropagatefromthecollapsingre-gionstothesurroundingmedium.Simulationspredictthatgascompressedandheatedbyshockscanreachtemperaturesof108Kinrichclustersofgalaxies,whilefilamentsandmildlyoverdenseregionsareheatedtotemperaturesintherange105to107K(e.g.Cen&Ostriker1999;Dav´eetal.2001;Cen&Ostriker2006).ThelatterarecommonlyknownastheWarm-HotIntergalacticMedium(WHIM),whichrepresentsabouthalfthetotalbaryonicmassintheuniverseatz∼0.AtT∼105−107K,theIGMiscollisionallyionisedandbecomestransparenttoLyαradiation.Asaconsequence,theHiLyαforestdoesnottracethebulkofthegasmassatlowredshift.Althoughtheshock-heatedIGMemitsradiationintheUVandinthesoftX-raybands,thetotalenergyradiatedawaybymildlyoverdensegasisordersofmagnitudetoosmalltobedetectedbycurrentinstruments.Similarly,theabsorptionfromsuchagasalongthelineofsighttoabrightX-raysourceistooweaktoberesolvedbycurrentspectrographs(Richteretal.2008-Chapter3,thisvolume).InthisPaper,wereviewthepredictionsofnumericalsimulationsfortheproper-tiesandtheobservabilityoftheWHIM.InSect.2webrieflymentionthenumericaltechniquesusedtosimulatetheIGMandwediscussthemechanismsthatproducetheWHIM,namelyheatingbygravitationalaccretionshocksandother,non-gravitational,heatingprocesses.Amoredetaileddescriptionofthenumericaltechniquesthemselves1Sincethediffuseintergalacticmediumishighlyionised,thebaryondensityinferredfromLyαforestobservationsisinversely