FUV and X-ray absorption in the Warm-Hot Intergalactic Medium

0.83 MB·English

by  P. Richter

SSRvmanuscriptNo.
(willbeinsertedbytheeditor)
FUV and X-ray absorption in the Warm-Hot Intergalactic
Medium
P.Richter · F.B.S.Paerels · J.S.Kaastra
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Received:20September2007;Accepted:21September2007
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Abstract TheWarm-HotIntergalacticMedium(WHIM)arisesfromshock-heatedgascol-
7
lapsinginlarge-scalefilamentsandprobablyharboursasubstantialfractionofthebaryons

] inthelocal Universe. Absorption-line measurements intheultraviolet (UV)andintheX-
h raybandcurrentlyrepresentthebestmethodtostudytheWHIMatlowredshifts.Wehere
p
describethephysical properties of theWHIMand theconcepts behind WHIMabsorption
-
o linemeasurementsofHIandhighionssuchasOVI,OVII,andOVIIIinthefar-ultraviolet
r andX-rayband.WereviewresultsofrecentWHIMabsorptionlinestudiescarriedoutwith
t
s UVandX-raysatellitessuchasFUSE,HST,Chandra,andXMM-Newtonanddiscusstheir
a
implicationsforourknowledgeoftheWHIM.
[

Keywords galaxies: intergalactic medium · quasars: absorption lines · cosmology:
1
v large-scalestructureoftheUniverse
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9 1 Introduction
0
.
1 As recent cosmological simulations imply, the temperature of the intergalactic medium
0 (IGM)undergoesasignificantchangefromhightolowredshiftsparalleltotheproceeding
8
of large-scale structure formation in the Universe (e.g., Cen&Ostriker 1999; Dave´ etal.
0
2001). As a result, a substantial fraction of the baryonic matter in the local Universe is
:
v expectedtoresideintheso-calledWarm-Hot IntergalacticMedium(WHIM). TheWHIM
Xi represents a low-density (nH ∼10−6−10−4 cm−3), high-temperature (T ∼105−107 K)
plasmathatprimarilyismadeofprotons,electrons,HeII,andHeIII,togetherwithtracesof
r
a somehighly-ionised heavy elements. TheWHIMis believedtoemerge from intergalactic
gasthatisshock-heatedtohightemperaturesasthemediumiscollapsingundertheaction
P.Richter
Institutfu¨rPhysik,Universita¨tPotsdam,AmNeuenPalais10,D-14469Potsdam,Germany
E-mail:[email protected]
F.B.S.Paerels
Department ofAstronomy andColumbia Astrophysics Laboratory, Columbia University, 550West120th
Street,NewYork,NY10027,USA
J.S.Kaastra
SRON,Sorbonnelaan2,3584CAUtrecht,TheNetherlands
2
of gravity in large-scale filaments (e.g., Valageasetal. 2002). In this scenario, part of the
warm (photoionised) intergalactic medium that gives riseto theLya forest in the spectra
ofdistantquasars(QSO)isfallingintothepotentialwellsoftheincreasinglypronounced
filaments,gainsenergy(throughgravity),andisheatedtohightemperaturesbyshocksthat
runthroughtheplasma.
Becauseofthelowdensityandthehighdegreeofionisation,directobservationsofthe
shock-heatedandcollisionallyionisedWHIMarechallengingwithcurrentinstrumentation
(incontrast tothephotoionised IGM, whichiseasilyobservable through theLya forest).
DiffuseemissionfromtheWHIMplasmamusthaveaverylowsurfacebrightnessanditsde-
tectionawaitsUVandX-rayobservatoriesmoresensitivethancurrentlyavailable(see,e.g.,
Fangetal.2005;Kawaharaetal.2006).ThemostpromisingapproachtostudytheWHIM
withobservationsatlowredshiftistosearchforabsorptionfeaturesfromtheWHIMinFUV
and in the X-ray regime in the spectra of quasars, active galactic nuclei (AGN) and other
suitedextragalacticbackgroundsources.AstheWHIMrepresentsahighly-ionisedplasma,
themostimportantWHIMabsorptionlinesarethoseoriginatingfromtheelectronictransi-
tions ofhigh-ionisation stateions (hereafter referredtoas”high ions”)ofabundant heavy
elementssuchasoxygenandcarbon.Amongthese,five-timesionisedoxygen(OVI)isthe
most valuable high ion to trace the WHIM at temperatures of T ∼3×105 K in the FUV
regime.IntheX-rayband,theOVIIandOVIIItransitionsrepresentthekeyobservablesto
tracetheWHIMathighertemperaturesintherange3×105<T <107.Inadditiontothe
spectral signatures of high ions of heavy elements the search for broad and shallow Lya
absorptionfromthetinyfractionofneutralhydrogenintheWHIMrepresentsanotherpos-
sibilitytoidentifyandstudythemostmassiveWHIMfilamentsintheintergalacticmedium
with FUV absorption spectroscopy. Finally, for the interpretation of the observed WHIM
absorptionfeaturesinUVandX-rayspectrathecomparisonbetweenrealdataandartificial
spectrageneratedbynumericalsimulationsthatincluderealisticgasphysicsisofgreatim-
portancetoidentifypossiblepitfallsrelatedtotechnicalandphysicalissuessuchaslimited
signal-to-noiseratiosandspectralresolution,line-broadeningmechanisms,non-equilibrium
conditions,andothers.
Inthis chapter, wereview the physics and methodology of theUV and X-ray absorp-
tionmeasurementsofwarm-hot intergalacticgasatlowredshiftandsummarisetheresults
ofrecentobservationsobtainedwithspace-basedobservatories.Theoutlineofthischapter
is the following. The ionisation conditions of the WHIM and the most important absorp-
tion signatures of this gas in the UV and X-ray band are presented in Sect.2. Recent UV
absorption measurements oftheWHIM atlow redshift arediscussedinSect.3. Similarly,
measurementsoftheWHIMintheX-rayarepresentedinSect.4.InSect.5wecomparethe
resultsfromWHIMobservations withpredictionsfromnumericalsimulationsandgivean
overview of WHIM measurements at high redshift. Finally, some concluding remarks are
giveninSect.6.
2 PhysicalpropertiesoftheWHIM
2.1 WHIMionisationconditions
TheoccurrenceandcharacteristicsoftheWHIMabsorptionsignaturesintheFUVandX-
raybandaredeterminedtoahighdegreebytheionisationconditionsinthegas.Webriefly
discuss the WHIM ionisation properties, as this is crucial for interpretation of the WHIM
absorptionlinesinFUVandX-rayspectrathatariseinsuchwarm-hotgas.Generally,there
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are two processes that determine the ionisation state of warm-hot gas in the intergalactic
medium:collisionalionisationcausedbythehightemperatureofthegasincollapsedstruc-
turesandphotoionisationbythecosmicFUVbackground.
2.1.1 Hydrogen
Byfar most of themass of the WHIM is inthe form of ionised hydrogen. Therefore, un-
derstanding theprocesses that lead totheionisation ofhydrogen is essentialfor the inter-
pretation of WHIM absorption lines and for a reliable estimate of the baryon content of
warm-hotintergalacticgas.Theionisationpotentialofneutralhydrogenis13.6eVandthus
bothionisationbyparticlecollisionsandionisationbyhigh-energyphotonscontributetothe
ionisationofHIinwarm-hotgas.Westartwithcollisionalionisation,whichisbelievedto
dominatetheionisationofhydrogenattemperatures>105K.
Incollisional ionisation equilibrium (CIE)– the most simple approach tocharacterise
theionisationconditionsinlow-density,high-temperatureplasmas–theionisationfraction
dependsonlyonthegastemperature.Ifweignoreanycharge-exchangereactions(whichis
justifiedincaseofhydrogen), theneutral hydrogen fraction inCIEissimply theratiobe-
tweentherecombinationcoefficienta (T)andthecollisionalionisationcoefficientb (T):
H H
a (T)
fHI,coll= b H(T). (1)
H
Above gas temperatures of ∼1.5×104 K collisions by thermal electrons efficiently
ionisehydrogentoahighdegree,andalreadyatT ∼3×104 Ktheneutralhydrogenfrac-
tioninthegasislessthenonepercent. Forthetemperature rangethatischaracteristicfor
theWHIM, T =105−107 K,one canapproximate theionisation fraction inacollisional
ionisationequilibriumintheway
log fHI,coll≈13.9−5.4logT+0.33(logT)2. (2)
where T is inunits K (Richteretal. 2006a; Sutherland&Dopita 1993). Thus, for WHIM
gaswithT =106KtheneutralhydrogenfractioninthegasinCIEisonly∼2.4×10−7.
Next to particle collisions, photons with energies >13.6 eV contribute to the ionisa-
tionoftheWHIM,inparticularinthelow-temperature WHIMtailat∼105 Kandbelow.
Suchionisingphotonsinintergalacticspaceareindeedprovidedbythemetagalacticultra-
violet (UV) background, originating from the hard radiation emitted by QSOs and AGN.
Fig.1showsthespectralshapeoftheUVbackgroundatz=0(leftpanel)andtheredshift-
dependence of the hydrogen photoionisation rate from the UV background (right panel)
basedonthemodelsbyHaardt&Madau(1996).
Consideringphotoionisation, onegenerallycanwritefortheneutral-hydrogen fraction
inthegas:
n a (T)
e H
fHI,photo= G , (3)
HI
wherea (T)denotesthetemperature-dependent recombinationrateofhydrogen,n isthe
H e
electrondensity,andG isthephotoionisation rate.G dependsontheambientionising
HI HI
radiation field Jn (in units ergcm−2s−1Hz−1 sr−1) in the WHIM provided by the meta-
galacticUVbackground(seeFig.1):
¥
GHI=4p shnnJn dn ≈2.5×10−14J−23s−1. (4)
nZ
L
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Fig.1 Leftpanel:SpectralshapeofthemetagalacticUVbackgroundatz=0(fromHaardt&Madau1996).
Plottedisthefluxofphotons(Fn =4p Jn )againstthefrequencyn .Thehydrogenionisationedgeisindicated
withadashedline.Rightpanel:Redshift-dependence ofthehydrogenphotoionisationrateG fromtheUV
backgroundfortherangez=0toz=5.AdaptedfromHaardt&Madau(1996).
Here,n L isthefrequencyattheLymanlimitands n denotesthephotoionisationcrosssec-
tion of hydrogen, which scales with n −3 for frequencies larger that n (see Kaastraetal.
L
2008 -Chapter9,thisvolume). Wehaveintroduced thedimensionless scalingfactorJ
−23
which gives the metagalactic UV radiation intensity at the Lyman limit in units 10−23
ergcm−2s−1Hz−1 sr−1.Forz=0wehaveJ ∼1−2,whileforz=3thevalueforJ
−23 −23
is ∼80, thus significantly higher (Haardt&Madau 1996). Assuming n =n and insert-
e H
ingaproperfunctionfora (T),wefinallycanwriteforthelogarithmicneutralhydrogen
H
fractioninapurelyphotoionisedWHIMplasma
16n T−0.76
logfHI,photo≈log JH 4 , (5)
−23 !
wheren isthehydrogenvolumedensityinunitscm−3andT isthetemperatureinunits104
H 4
K.Thus,forpurelyphotoionisedintergalacticgasatz=0withn=5×10−6 andT =106
K we find that the neutral hydrogen fraction is fHI,photo ∼2.4×10−6. This is ten times
higherthanforCIE,indicatingthatcollisionsdominatetheionisationfractionofhydrogenin
intermediateandhigh-temperatureWHIMregions.However,notethatatlowertemperatures
nearT =105 Katthesamedensitywehave fHI,photo∼ fHI,coll.SincethisistheWHIM
temperatureregimepreferentiallydetectedbyUVabsorptionfeatures(e.g.,OVIandbroad
Lya ), photoionisation is important and needs to be accounted for when it comes to the
interpretationofWHIMabsorbersobservedintheFUV.FromaWHIMsimulationatz=0
including both collisional ionisation and photoionisation (Richteretal. 2006b; see Fig. 2)
findthefollowingempiricalrelationbetweenneutralhydrogenfractionandgastemperature
foraWHIMdensityrangebetweenlogn =−5.3and−5.6:
H
logfHI≈0.75−1.25logT. (6)
ThisequationmayserveasathumbruletoestimateionisationfractionsinWHIMab-
sorbersatz=0ifthegastemperatureisknown(e.g.,frommeasurementsofthelinewidths;
seeSect.2.2.1).
5
Fig. 2 The neutral hydrogen fraction, log fHI = log(nHI/nH), in a WHIM simulation
(photoionisation+collisional ionisation), is plotted as a function of the gas temperature, log T. The
lightgrayshadedindicatescellsinthedensityrangelognH=−5to−7.Thedarkgrayshadedarearefers
tocellsthathavelognH=−5.3to−5.6,thusadensityrangethatischaracteristic forWHIMabsorbers.
AdaptedfromRichteretal.(2006b).
Fig.3 CIEionfractionsofselectedhighionsofoxygen(OVI,OVII,OVIII;leftpanel)andneon(NeVIII,
NeIX; right panel) in the WHIM temperature range log (T/K)=4.5−7.0, based on calculations by
Sutherland&Dopita(1993).
2.1.2 Oxygenandothermetals
While hydrogen provides most of the mass in the WHIM, the most important diagnostic
linestostudythisgasphasearefromhighlyionisedmetalssuchasoxygen, neon, carbon,
magnesium,andothers.Therefore,theunderstandingoftheionisationpropertiesoftheob-
servedhighionsoftheseelementsisasimportant asforhydrogen. Asforhydrogen, both
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collisionalionisationandphotoionisationneedtobeconsidered.Withitssingleelectron,hy-
drogencanonlybeeitherneutralorfullyionised.Heavyelements,incontrast,haveseveral
electrons availableandare–evenatveryhightemperatures –usuallyonlypartlyionised.
Thus,electronictransitionsexistforsuchhighly-ionised metals(”highions”)inwarm-hot
gas.OfparticularimportanceforobservationsoftheWHIMarethehighionisationstatesof
oxygen,OVI,OVII,andOVIII,astheyhavestrongelectronictransitionsintheUV(OVI)
andatX-raywavelengths(OVII&OVIII)andoxygenisarelativelyabundantelement.An-
otherimportantmetalforobservingwarm-hotgasintheUVandX-raybandisneon(NeVII,
NeVIII,NeIX,NeX).Incollisionalionisationequilibrium, theionisationstateoftheseel-
ementsisdeterminedsolelybythetemperatureofthegas.Foreachelement,theionisation
fractions of the ionisation states (e.g., four-times vs. five-times ionised) then are charac-
terised by the respective ionisation potentials (IPs) of the individual ionisation levels. For
instance,atT ∼1−3×105K,asignificantfractionoftheoxygenisfive-timesionised(O+5
orOVI,IP=138eV).Six-timesionisedoxygen(O+6orOVII,IP=739eV)andseven-times
ionisedoxygen(O+7 orOVIII,IP=871eV)predominantlyexistathighertemperaturesin
therange3×105−3×106 Kand3×106−107K,respectively.Fig.3showstheionisation
fractionsofthemostimportanthighionsofoxygenandneon,basedontheCIEcalculations
ofSutherland&Dopita(1993);seealsoKaastraetal.2008-Chapter9,thisvolume.High
ionsofotherelementssuchascarbon,nitrogen, siliconandmagnesiumarelessimportant
for WHIM observations as their observable transitions trace lower temperature gas (e.g.,
CIV,SiIV)ortheabundanceoftheseelementsintheintergalacticmediumaretoolow.Itis
important tonoteatthispoint,thatthediscussedrelationbetweenionisationstate/fraction
and gas temperature explicitly assumes that the gas is in an ionisation equilibrium. This
may not be generally the casein the WHIM, however, as the densities are generally very
low. For instance, under particular non-equilibrium conditions the timescales for cooling,
recombination, andion/electronequilibrationmaydiffersignificantlyfromeachother(see
forinstanceBykovetal.2008-Chapter8,thisvolume).Insuchacase,thepresenceofhigh
ionssuchasOVIand/ormeasuredhigh-ionratioswouldnotserveasareliable”thermome-
ter”fortheWHIMgas.Inaddition,WHIMfilamentsmostlikelyneitherareisothermalnor
dotheyhaveaconstantparticledensity.Infact,asWHIMsimulationsdemonstrate,WHIM
absorbersseemtorepresentamixofcoolerphotoionisedandhottercollisionallyionisedgas
withasubstantialintrinsicdensityrange.Theabsorptionfeaturesfromhighionsarisingin
suchamulti-phasemediumthereforearegenerallydifficulttointerpretintermsofphysical
conditionsandbaryonbudget.
Inview of thehigh energies required to produce the high ions of oxygen and neon in
combinationwiththespectralshapeofthemetagalacticbackground radiation(seeFig.1),
photoionisationofhighmetalionsintheWHIMislessimportantthanforhydrogen.How-
ever,forOVIphotoionisationisimportantatlowredshiftsinWHIMregionswithverylow
densitiesorinsystemslocatedclosetoastronglocalradiationsource(e.g.,inOVIsystems
associated with the background QSO). Note that at high redshift, most of the intervening
OVIappearstobephotoionisedowingtothesignificantlyhigherintensityofthemetagalac-
ticbackgroundradiationintheearlyUniverse(seeSect.5.2).
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Table1 DataonOandNehighionshavingobservableabsorptionlines
Ion [X/H]1 Ionisation Absorption Band CIEtemperature2
potential[eV] lines[A˚] range[106K]
OVI −3.34 138 1031.926 FUV 0.2−0.5
1037.617
OVII −3.34 739 21.602 X-ray 0.3−3.0
OVIII −3.34 871 18.969 X-ray 1.0−10.0
NeVIII −4.16 239 770.409 EUV 0.5−1.3
780.324
NeIX −4.16 1196 13.447 X-ray 0.6−6.3
1[X/H]isthelogofthenumberdensityofelementXrelativetohydrogenforSolarabundances,takenhere
fromAsplundetal.(2004).
2CIEmodelsfromSutherland&Dopita(1993).
2.2 WHIMabsorptionsignaturesintheUVandX-rayband
2.2.1 UVabsorption
Asindicatedintheprevioussubsection,five-timesionisedoxygen(OVI)isbyfarthemost
important highiontotracetheWHIMattemperatures ofT ∼3×105 Kintheultraviolet
regime (assuming CIE, see above). Oxygen is a relatively abundant element and the two
lithium-like (1s22s)2S1/2 →(1s22p)2P1/2,3/2 electronic transitions of OVI located in the
FUVat1031.9and1037.6A˚ havelargeoscillatorstrengths(f =0.133,f =0.066).
1031 1037
NexttoOVI,NeVIIItracesWHIMgasnearT ∼7×105 K(incollisionalionisationequi-
librium) and thus is possibly suited to complement the OVI measurements of the WHIM
inahigher temperatureregime. ThetwoavailableNeVIIIlinesarelocatedintheextreme
ultraviolet (EUV) at 770.4 A˚ (f =0.103) and 780.3 A˚ (f =0.051), allowing us to
770 780
tracehigh-column density WHIMabsorbers at redshifts z>0.18 withcurrent FUV satel-
litessuchasFUSE.However,asthecosmicabundanceofNeVIIIisrelativelylow,NeVIII
isnotexpectedtobeaparticularlysensitivetraceroftheWHIMattheS/Nlevelsachievable
withcurrentUVspectrographs.ThesameargumentholdsforthehighionMgX,whichhas
two transitions in the EUV at even lower wavelengths (ll 609.8, 624.9 A˚). So far, only
OVIandinonecaseNeVIIIhasbeenobservedintheWHIMatlowredshift(seeSect.3.2).
NotethatWHIMabsorptionfeaturesbyOVI(andNeVIII)aremostlyunsaturatedandthe
lineprofilesarefullyornearlyresolvedbycurrentUVinstrumentssuchasFUSEandSTIS,
whichprovidespectralresolutionsofR=l /Dl ≈20,000and45,000,respectively.Table
1summarisesphysicalparametersofOandNehighionsandtheirobservabletransitionsin
theUVandX-raybands.
Four-timesionisednitrogen(NV;I.P.is98eV)alsoisbelievedtotracepredominantly
collisionallyionisedgasattemperaturesnearT∼2×105K,butitslowercosmicabundance
togetherwithitsdeficiencyinlowmetallicityenvironments duetonucleosynthesis effects
(e.g.,Pettinietal.2002)makesitverydifficulttodetectintheWHIM.Otheravailablestrong
high-ion transitionsintheUVfromcarbon (CIV ll 1548.2, 1550.8 A˚)andsilicon(SiIV
ll 1393.8,1402.8A˚)arebelievedtotracemainlyphotoionisedgasattemperaturesT<105
K,butnottheshock-heatedwarm-hotgasathighertemperatures.
Nexttohigh-ionabsorptionfromheavyelements,recentUVobservations(Richteretal.
2004;Sembachetal.2004;Lehneretal.2007)haveindicatedthatWHIMfilamentscanbe
detectedinLya absorptionofneutral hydrogen. Although thevastmajorityofthehydro-
gen in the WHIM is ionised (by collisional processes and UV radiation), a tiny fraction
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(f <10−5,typically)ofneutralhydrogenisexpectedtobepresent.Dependingontheto-
HI
talgascolumndensityofaWHIMabsorberanditstemperature,weakHILya absorption
atcolumndensities12.5≤logN(HI)≤14.0mayarisefromWHIMfilamentsandcouldbe
usedtotracetheionisedhydrogencomponent.TheLya absorptionfromWHIMfilaments
isverybroadduetothermallinebroadening,resultinginlargeDopplerparametersofb>40
kms−1.Suchlinesaregenerallydifficulttodetect,astheyarebroadandshallow.Highres-
olution, highS/NFUVspectraofQSOswithsmoothbackground continuaarerequired to
successfullysearchforbroadLya absorptioninthelow-redshiftWHIM.STISinstalledon
theHSTistheonlyinstrumentthathasprovidedsuchdata,butduetotheinstrumentallimi-
tationsofspace-basedobservatories,thenumberofQSOspectraadequateforsearchingfor
WHIMbroadLya absorption(inthefollowingabbreviatedas”BLA”)isverylimited.
ThebvaluesoftheBLAsareassumedtobecomposedofathermalcomponent,b ,and
th
anon-thermalcomponent,b ,inthewaythat
nt
b= b 2+b 2. (7)
th nt
Thenon-thermalcomponentmayincpludeprocesseslikemacroscopicturbulence,unre-
solvedvelocity-components,andothers(seeRichteretal.2006aforadetaileddiscussion).
Thecontributionofthethermalcomponenttobdependsonthegastemperature:
2kT T
b = ≈0.13 kms−1, (8)
th m A
r r
whereT isinK,kistheBoltzmannconstant,mistheparticlemass,andAistheatomic
weight.Fortheshock-heatedWHIMgaswithlogT ≥5onethusexpectsb ≥40kms−1.
th
Thenon-thermal broadening mechanismsareexpectedtocontributetosomedegreetothe
totalbvaluesinWHIMabsorbers(seeRichteretal.2006a),sothatthemeasuredbvalueof
aBLAprovidesonlyanupperlimitforthetemperatureofthegas.
2.2.2 X-rayabsorption
ThehighestionisationphaseoftheWHIMwillproduceandabsorblineradiationprimarily
intheHe-andH-likeionsofthelow-Z elements(C,N,O,Ne),andpossiblyintheL-shell
ionsofFe.Inpractice,muchoftheattentionisfocusedonoxygen,becauseofitsrelatively
high abundance, and because the strongest resonance lines in He- and H-like O are in a
relatively’clean’wavelengthband.Forreference,theLya transitionsofCVI,NVII,OVIII,
andNeXoccurat33.7360,24.7810,18.9689,and12.1339A˚,respectively(wavelengthsof
the1s−2p doubletweightedwithoscillatorstrength;Johnson&Soff(1985).TheHe-
1/2,3/2
likeionsCV,NVI,OVII,andNeIXhavetheirstrongesttransition,then=1−2resonance
line,at40.2674,28.7800,21.6015,and13.4473A˚ (Drake1988;seealsoTable1).Dataon
the higher order series members can be found in Verneretal. (1996). As far as the Fe L
shellionsareconcerned, themost likelytransitiontoshowupwouldbethestrongest line
inNe-likeFeXVII,n=2p−3dl 15.014A˚.Inaddition,alllowerionisationstagesofC,N,
O, and Ne(with theexception of neutral Ne ofcourse) canalso absorb by n=1−2; the
strongest ofthesetransitions would be 1s−2p inOVI at 22.019 A˚ (Schmidtetal.2004).
Likewise,thelowerionisationstagesofFecouldinprincipleproducen=2−3absorption.
Thethermalwidthsofallthesetransitionswillbeverysmall,requiringresolvingpowers
oforderR∼10000(C,N,O,Ne)forgastemperaturesoforder106Ktoberesolved;forFe,
therequirementisevenhigher,byafactor∼2.Aswewillsee,forpracticalreasons,these
requirementsexceedthecurrentcapabilitiesofastrophysicalX-rayspectroscopybyalarge
9
factor.DuetothesmallDopplerbroadening(ignoringturbulentvelocityfieldsfornow),the
lineswillrapidlysaturate.ForHe-andH-likeOresonancelineabsorption, saturationsets
inat an equivalent width of order 1 mA˚ (Kaastraetal.2008 -Chapter 9, this volume), or
columndensitiesoforderafewtimes1014 ionscm−2.Thechallenge,therefore,forX-ray
spectroscopypresentedbytheIGMistodetectsmallequivalentwidth,near-saturationlines
thatareunresolved.
2.3 ThebaryoncontentoftheWHIMasmeasuredbyUVandX-rayabsorbers
OneimportantresultfromabsorptionlinemeasurementsoftheWHIMintheUVistheob-
servednumberdensityofWHIMabsorbers,usuallyexpressedasdN/dz,thenumberofab-
sorbersperunitredshift.Forinstance,fromrecentmeasurementswithFUSEandHST/STIS
onefindsforOVIabsorbersandBroadLya absorbersatz≈0valuesofdN/dz(OVI)≈20
anddN/dz(BLA)≈30(seeSect.3.2).Currently,theWHIMabsorberdensityisonlymea-
surableintheUV,sinceintheX-raybandboththeobservednumberofWHIMabsorption
linesandtheavailableredshift pathforWHIMobservations istoosmalltoderivestatisti-
callysignificantvaluesofdN/dz(OVII)anddN/dz(OVIII).
Aparticularlyinterestingquestionnowis,howtheobservednumberdensityofhigh-ion
lines or BLAs translates into an estimate of the cosmological baryon mass density of the
WHIM,W (WHIM).ToobtainsuchanestimateofthebaryoncontentoftheWHIMfrom
b
UVandX-rayabsorptionmeasurementsonehastoconsidertwomainsteps.First,oneneeds
totransformtheobservedcolumndensitiesofthehighions(e.g.,OVI,OVII,OVIII)intoa
totalgascolumndensitybymodellingtheionisationconditionsinthegas.Inasecondstep,
one then integrates over the total gas column densities of all observed WHIM absorbers
along the given redshift path and from that derives W (WHIM) for a chosen cosmology.
b
Throughout thepaper wewill assume aL CDMcosmology withH =70 kms−1Mpc−1,
0
W L =0.7,W m=0.3,andW b=0.045.Forthefirststeptheuncertaintyliesintheestimate
of the ionisation fraction of hydrogen of the WHIM. For this, it is usually assumed that
theWHIMisincollisionalionisationequilibrium,butphotoionisationandnon-equilibrium
conditionsmayplayasignificantrole.InthecaseofusingmetalionssuchasOVItheun-
knownoxygenabundance(O/H)ofthegasintroducesanadditionaluncertainty(seebelow)
fortheestimateofW (WHIM).Forthesecondstep,itisimportanttohavealargeenough
b
sampleofWHIMabsorptionlinesandasufficienttotalredshiftpathalongdifferentdirec-
tionsinordertohandlestatisticalerrorsandtheproblemofcosmicvariance.Asmentioned
earlier,theserequirementscurrentlyarefulfilledonlyfortheUVabsorbers.
ThecosmologicalmassdensityW bofOVIabsorbers(and,similarly,forotherhighions)
intermsofthecurrentcriticaldensityr canbeestimatedby
c
W b(OVI)= m mr cHcH0 (cid:229)ij fOVI,Nij((OO/VHI))ijijD Xj. (9)
Inthisequation,m =1.3isthemeanmolecularweight,m =1.673×10−27 kgisthemass
H
per hydrogen atom, H is the adopted local Hubble constant, and r =3H 2/8p G is the
0 c 0
currentcriticaldensity.Theindexidenotesanindividualhigh-ionabsorptionsystemalong
alineofsight j.Eachmeasured high-ion absorption systemi ischaracterised by itsmea-
suredioncolumndensity(e.g.,N(OVI)ij),theionisationfractionofthemeasuredion(e.g.,
f ),andthelocalabundanceoftheelementmeasuredcomparedtohydrogen(e.g.,the
OVI,ij
localoxygen-to-hydrogenratio,bynumber).Eachlineofsight jhasacharacteristicredshift
10
rangeD zinwhichhigh-ionabsorptionmaybedetected.Thecorrespondingcomovingpath
lengthD X availableforthedetectionofWHIMhigh-ionabsorbersthenisgivenby:
D Xj=(1+z)2[W L +W m(1+z)3]−0.5D zj. (10)
Inanalogy,wecanwriteforthecosmologicalmassdensityoftheBLAs:
W (BLA)= m mHH0 (cid:229) N(HI)ij . (11)
b r cc ij fHI,ijD Xj
Ascanbeeasilyseen,theadvantageofusingBLAsforderivingtheWHIMmassdensity
isthatthemetallicityofthegasisunimportant forthedeterminationofW .Thedisadvan-
b
tageis,however,thattheionisationcorrectionsareverylargeanduncertain,sincetheyare
determinedindirectlyfromtheBLAlinewidths(seeSect.2.2.1).
3 UVmeasurementsoftheWHIM
3.1 PastandpresentUVinstruments
ThefirstandsecondgenerationsofspacebasedUVspectrographssuchasCopernicusand
the International Ultraviolet Explorer (IUE) did not have sufficient sensitivity to system-
atically study intervening absorption in the intergalacticmedium along alarge number of
sightlines.Theearlylow-andintermediateresolutionspectrographsinstalledontheHubble
SpaceTelescope(HST),namelytheFaintObjectSpectrograph(FOS)andtheGoddardHigh
ResolutionSpectrograph(GHRS),wereusedtostudythepropertiesofthelocalLya forest
andinterveningmetal-linesystems(e.g.,Stockeetal.1995;Shulletal.1998).Whileinter-
vening OVI absorption has been detected with these instruments (e.g., Trippetal. 1998),
the concept of a warm-hot intergalactic gas phase was not really established at that time.
Withtheimplementationofthehigh-resolutioncapabilitiesoftheSpaceTelescopeImaging
Spectrograph(STIS)installedonHSTthefirstsystematicanalysesofWHIMOVIabsorbers
assignificantlow-redshift baryonreservoirs cameoutin2000(seeTrippetal.2000),thus
relativelysoonaftertheimportanceofashock-heated intergalacticgasphasewasrealised
incosmologicalsimulationsforthefirsttime(e.g.,Cen&Ostriker1999;Dave´etal.2001).
TheSTIS echellespectrograph together withtheE140M gratingprovides ahighspectral-
resolutionofR≈45000,correspondingtoavelocityresolutionof∼7kms−1 intheSTIS
E140M wavelength band between 1150 and 1730 A˚ (e.g., Kimbleetal. 1998; Woodgate
1998). Anexample for a STIS quasar spectrum withintervening hydrogen and metal-line
absorptionisshowninFig.4.NotethatatthespectralresolutionoftheSTISE140Mgrat-
ing all intergalactic absorption lines (i.e.,hydrogen and metal lines) are fully resolved. In
1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) became available, covering the
wavelength range between 912 and 1187 A˚. Equipped with a Rowland-type spectrograph
providingamediumspectralresolutionofR≈20000(FWHM∼20kms−1)FUSEisable
toobserveextragalacticUVbackgroundsourcesbrighterthanV=16.5magwithacceptable
integrationtimeandsignal-to-noise(S/N)ratios(foradescriptionofFUSEseeMoosetal.
2000;Sahnowetal.2000).Withthisresolution, FUSEisabletoresolvethebroaderinter-
galacticabsorptionfromtheHILymanseries,whilemostofthenarrowmetal-lineabsorbers
remainjustunresolved.ThisisnotaproblemforOVIWHIMstudieswithFUSE,however,
sincethespectralresolutionisveryclosetotheactuallinewidthsandtheOVIabsorption
usually is not saturated. FUSE complements the STIS instruments at lower wavelengths