On the Contribution of AGN to the Cosmic Background Radiation

0.5 MB·English

by  D. R. Ballantyne

ACCEPTEDTOTHEAPJ
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
ONTHECONTRIBUTIONOFAGNTOTHECOSMICBACKGROUNDRADIATION
D.R.BALLANTYNE1ANDC.PAPOVICH2
AcceptedtotheApJ
ABSTRACT
WepresenttheresultsofcalculationsofthecosmicAGNbackgroundspectrumfrom3keV(4×10−4µm)
to 1000µm. These computations make use of the measured X-ray luminosity function and its evolution, as
wellasfitsfromsynthesismodelsofthecosmicX-raybackground(CXB)topredicttheAGNcontributionto
thecosmicinfraredbackground(CIRB)fordifferentmodelsofthelocationanddistributionoftheabsorbing
material. By comparing our results to observational constraints we conclude that the current deep Spitzer
7
surveys can account for the entire AGN contribution to the CIRB at 24µm, but these AGN are almost all
0
0 Compton-thin. In fact, the calculations show that Compton-thickAGN are a small fraction of the CIRB for
2 λ < 100µm. For this reason, the most efficient method of identifying the Compton-thick AGN population
is through hard X-ray imaging at energies & 40 keV. Selection of AGN based on their rest-frame near-IR
n
colors will miss low luminosity type 2 AGN due to contamination from the host galaxy. Finally, the AGN
a
J thatdominatethe CXBmusthavestar formationrates< 100M⊙ yr−1, consistentwiththem havingsimilar
properties as the sources which dominate the CIRB at z ∼ 1. Including the estimated re-radiated emission
6
fromstar formation,AGN andtheir hostgalaxiesmay contribute∼ 30%of the CIRB at 70µm, droppingto
2
∼10%at24µmand∼1%at1–10µm.

1 Subject headings: diffuse radiation — galaxies: active — galaxies: evolution — galaxies: formation — in-
v frared:galaxies—X-rays:galaxies
5
7
7 1. INTRODUCTION Silvaetal. (2004) madeuse of the latestX-ray luminosity
1 function(LFs)bycollectingobservedIRspectralenergydis-
The extragalactic cosmic background radiation encodes
0 tributions(SEDs)forAGNwithagivenX-rayluminosityand
within it the history of the formation and growth of galax-
7 columndensity.TheywerethenabletopredicttheAGNcon-
iesandstars,andthuselucidatingitsoriginisavitalstepto-
0 tributiontotheCIRBbyintegratingovertheX-rayLFandob-
ward understandingthese processes. In recent years signifi-
/
h cantprogresshasbeenmadeinthestudyoftheX-rayandin- servedlocalNH distribution. Thestrength ofthis procedure
p fraredbackgrounds(hereafter,CXBandCIRB,respectively). isthatitisbasedonobservationsofrealAGN,butbecauseof
- this,Silvaetal.(2004)werelimitedtosmallnumbersofob-
TheCXBisnowunderstoodtoarisefrompredominatelyab-
o
jectsforeachofthetheirluminosityandN binsandthusare
r sorbed, or type 2, active galactic nuclei (AGN) over a large H
t range of luminosity, redshift and absorbing column density possiblysubjecttothesignificantobject-to-objectvariability
s observedinAGNintheIR(e.g.,Weedmanetal.2005).Inad-
a (Setti&Woltjer 1989). The CIRB (Hauser&Dwek 2001)
dition,duetothelackofhighangularresolutionobservations
: is dominated by the thermal dust emission of star-forming
v in the mid-IR, Silvaetal. (2004) were forced to extrapolate
galaxiesovera wide range of redshift(Lagacheetal. 2005).
Xi Despitetheirdisparateoriginsandenergyranges,thereisan the observed SEDs beyond 20µm. An alternative approach
waspresentedbyTreisteretal.(2004,2006)whousedtheo-
r intimate connection between the CXB and CIRB since the
a X-rays absorbed by the obscured AGN which dominate the retical IR dust emission models to compute the expected IR
SED for a given X-ray absorption column and luminosity.
CXB will be thermalized and re-radiated in the IR. Thus, a
These models included a detailed radiative transfer calcula-
potentiallysignificantfractionoftheCIRBcouldbemadeup
tionandwillbeaccurateinthemid-IR,butdonotincludegas
bythisreprocessedaccretionenergy. Ifmostoftheaccretion
andthereforecannotbeeasilyconnectedtotheX-rayabsorp-
onto black holes over the history of the Universe has been
tionpropertiesofAGN.
obscured(e.g.,Fabian&Iwasawa1999) thena precisemea-
Here, wepresentthefirst predictionofthe AGNcontribu-
surement of the AGN contribution to the CIRB is required
tion to the entire extragalacticbackgroundlight from 3 keV
to understand the growth of the supermassive black holes.
to 1000 µm. This AGN background spectrum is based on
Similarly,astheCIRBisdominatedbystar-forminggalaxies
fits to the CXB and therefore automatically gives the AGN
with the longerwavelengthemission resultingfromgalaxies
fraction to the CIRB over its entire wavelength range. The
athighredshifts(Lagacheetal.2005),adeterminationofthe
calculationprocedureisdescribedinthenextsection,andwe
fractionofemissionfromAGNasafunctionofwavelengthis
present the results and discuss their implications in Sect. 3.
requiredtoaccuratelymeasurethehistoryofstarandgalaxy
Thestandardfirst-yearWMAPΛ-dominatedcosmologyisas-
formationovercosmictime.Givenaproceduretoconnectthe
sumedthroughoutthis paper,i.e., H = 70 kms−1 Mpc−1,
X-rayabsorptiontotheexpectedIRemissiononecanusethe 0
Ω =0.7,andΩ =0.3(Spergeletal.2003).
informationintheCXBtopredicttheexpectedAGNcontri- Λ m
butiontotheCIRB.
2. CALCULATIONS
1DepartmentofPhysics,TheUniversityofArizona,1118East4thStreet, The calculationsof AGN SEDs were performedusing the
Tucson,AZ85721;[email protected] photoionizationcodeCloudyv.05.07.06(Ferlandetal.1998)
2SpitzerFellow,StewardObservatory,TheUniversityofArizona,933N. andaredescribedindetailbyBallantyneetal.(2006b). This
CherryAvenue,Tucson,AZ85721;[email protected] technique has the advantage that it self-consistently treats
2 Ballantyne&Papovich
the atomic gas physics along with the detailed dust radia- the values of f observed by Bargeretal. (2005). This red-
2
tion physics (including PAHs and emission from very small shiftevolutionoff agreeswith therecentmeasurementsof
2
grains). Inaddition,physicalpropertiesofthemediumatten- Treister&Urry (2006), as well as the luminosity evolution
uatingtheAGN,suchasitsdistancefromthecentralengine, measuredby Gillietal. (2006). However, because the AGN
can be varied allowing for possible constraints to be placed backgroundspectrumiscalculatedbyintegratingtheindivid-
on important parameters. Although the IR radiative trans- ualN -averagedspectraoverallL andz,thespecificevo-
H X
ferusedinthesecalculationsismuchlesssophisticatedthan lutionmodeloff makeslittle impactto thefinalresult. In-
2
thoseusedbyTreisteretal.(2006),Ballantyneetal.(2006b) deed, there is negligible difference between the background
showedthattheSEDs,whenaveragedoveraN distribution, spectrum calculated with the above evolution of f and one
H 2
haveverysimilarpropertiestotheensembleofAGNfoundin wheref remainsconstantwithredshift.
2
thedeepsurveysofChandra,XMM-NewtonandSpitzer. Fur- AGNSEDswerecomputedwithCloudyforthefollowing
thermore,toconstructtheAGNbackgroundspectrumthein- column densities: logN = 20,20.5,...,24.0,24.5. Any
H
dividualN -averagedSEDsareintegratedoverallluminosi- objectobservedthroughacolumnlogN ≥22isconsidered
H H
tiesandredshifts. Thisprocedurefurtherreducestheimpact a type2 AGN. Thedistributionof columndensitiesthusde-
ofthesimplifiedradiativetransfertreatment. pendsonluminosityandredshift. We considertwo different
To calculate the observedSED of an AGN with X-ray lu- methodsofdeterminingtheN distribution. Inthefirstsce-
H
minosity L attenuated by a column density N , we first nario,anytype1or2AGNhasanequalprobabilitypofbeing
X H
define a modelAGN continuumextendingfrom 100 keV to assignedanycolumnwithintheirclassification:
> 1000µmusingthe‘agn’commandinCloudy. Thisspec-
trum has a X-ray photon index of Γ = 1.9 that falls off as N = 20.0,...,21.5 p=(1−f2)/4.0 (1)
ν−3 at energies≥ 100keV, a big bluebump temperatureof H 22.0,...,24.5 p=f2/6.0
(cid:26)
1.4×105 K (i.e., a 107 M⊙ black hole accreting at 10% of In the second method, the type 2 objects are distributed as
itsEddingtonrate),andα = −1.4. AlthoughthisSEDisa
ox observedinlocalSeyfert2galaxieswithahigherfractionof
goodphenomenologicaldescriptionofanAGNspectrum,itis
Compton-thicksources(Risalitietal.1999):
missingtheComptonreflectioncomponentinthehardX-ray
band(e.g.,Nandra&Pounds1994)andhasalowercutoffen- 20.0,...,21.5 p=(1−f )/4.0
2
ergythanwhatistypicallyobservedinSeyfertgalaxies(Matt N = 22.0,...,23.5 p=f /8.0 (2)
H 2
2001). Thereflectioncomponentandahighenergycutoffare ( 24.0,24.5 p=f /4.0
2
bothnecessarytoproducegoodfitstothepeakoftheCXBat
∼ 30keV(Comastrietal.1995;Gillietal.2006)andso the TheAGNbackgroundspectruminνIν isthengivenby
backgroundspectrumcomputedwiththisSEDwillunderpre-
νI (λ)= c
dict the peak intensity of the CXB. However, as described ν H0
below,theAGNbackgroundspectrumiscalculatedusingthe
phoarpduXla-tiroanylpuamraimnoestietrysf(uin.ec.t,ioenvo,NluHtiodnisotrfibtyuptieon2)/ttyhpaetw1erraetiaoll, zzmmianx llooggLLmXmXianx ddΦl(oLgXL,Xz)(1+z)2(SΩλm(L(1X+,zz))3d+2l ΩΛ)1/2dlogLX(3d)z,
usedinfitstothepeakoftheCXB(Ballantyneetal.2006a). R R
where dΦ(L ,z)/dlogL is the Uedaetal. (2003) X-ray
X X
Inaddition,theomissionofthereflectioncomponentwillnot luminosity function for AGN (in Mpc−3), S (L ,z) is the
λ X
affectthe dust heating in the obscuring medium. Therefore,
observed-frameAGN N -weighted SED (in νf units) cal-
H ν
thelowenergybackground,includingtheCIRB, willstillbe
culatedwithluminosityL atredshiftz, andd isthelumi-
X l
self-consistentlypredictedusingthepropertiesofaknownfit
nositydistancetoredshiftz (wherez =0andz =5).
min max
totheCXB.
AGNbackgroundspectrawerecalculatedforabsorberradiiof
ThisAGN spectrumilluminatesa constantdensity (nH = r = 1and10pcandwithboththesimpleandRisalitietal.
104 cm−3) cloudof gas and dustwith an inner radiuseither (1in999)N distribution.
H
r =1or10pcfromthecontinuumsource.Theradialextent
in
of the irradiated cloud is determined by requiring the X-ray
3. RESULTSANDDISCUSSION
spectrum to be absorbed by a column N . The transmitted
H
3.1. TheAGNBackgroundSpectrum
incident spectrum, outwardly directed diffuse emission and
reflectedradiationarecombinedundertheassumptionofthe Figure 1 plots the predicted AGN background spectrum
unified model to produce a ’unified’ SED for each LX and from 3 keV to 1000µm along with both recent observa-
NH (Ballantyneetal.2006b). Finally,ateveryLX andz,the tional constraints on the CXB and CIRB and the models of
unified SEDs undergo a weighted average over N to pro- Silvaetal. (2004). Between 1 and 3 keV the background
H
duce the N -averaged SEDs that are then used to calculate model has the correct shape, but falls at the lower edge of
H
theAGNbackgroundspectrum.Theweightsaregivenbythe the range of measured CXB intensities. This is also found
distribution of column densities predicted by the AGN type in the mostrecentsynthesismodels(Gillietal. 2006) andis
2/type1ratioatthegivenLX andz. likelyaresultofcontributionsfromnon-AGNsourcessuchas
WeemployamodelofanevolvingAGNtype2/type1ratio, clustersandstar-forminggalaxies.
suchthatthefractionoftype2swith41.5≤logLX ≤48and Thedottedanddashedlinesdenotethecontributionstothe
0 ≤ z ≤ 1isdeterminedbyf2 = K(1+z)0.3(logLX)−4.8, totalAGNbackgroundspectrumfromtype1(logNH < 22)
where K is determined by requiring the low-LX, z = 0 andtype 2 AGN (logNH ≥ 22), respectively. In agreement
type 2/type 1 ratio to be 4 (Maiolino&Rieke 1995). Be- with previous work (Silvaetal. 2004; Treisteretal. 2004;
tween z = 1 and 5 the value of f remains at its z = 1 Ballantyneetal.2006a), thetype2AGNdominatetheAGN
2
valuefortheappropriateLX. Ballantyneetal.(2006a)found backgroundlight at hard X-rays and in the mid- and far-IR.
thatthisevolutioncanself-consistentlyaccountforthespec- Type1 AGNonlydominatethe AGNbackgroundwherethe
trum of the CXB, the observed X-ray number counts and type 2 sources suffer the greatest extinction. The predicted
AGNandtheCosmicBackgroundRadiation 3
FIG.1.—(Top)TheheavysolidlineplotsthepredictedtotalAGNbackgroundspectrumfromhardX-raystothefar-IRundertheassumptionsofrin=1pc
andthesimpleNHdistribution.Thedottedanddashedspectraarethecontributionsfromtype1(logNH<22)andtype2AGN(logNH≥22),respectively.In
theX-raybandthedifferentcoloredregionsdenotetheCXBintensitiesmeasuredfrommultiplemissions(blue,ASCAGIS,Kushinoetal.2002;red,BeppoSAX,
Vecchietal.1999;yellow,ASCASIS,Gendreauetal.1995;cyan,XMM-Newton,DeLuca&Molendi2004).Theopenstarplotstherecentmeasurementofthe
intensityoftheCXBat1keVbyHickox&Markevitch(2006).IntheIR,thedot-dashedanddashedmagentalinesplotthepredictedcontributiontotheCIRB
fromAGNandAGN+hostgalaxies,respectively, calculatedbySilvaetal.(2004). ThegreenpointsandredsquaresareSpitzermeasurementsoftheAGN
contributiontotheCIRBpresentedbyTreisteretal.(2006)andBarmbyetal.(2006),respectively.Theblueandcyanlinesat15µmdenotetherangecoveredby
type1andtype2AGNasdeterminedbydeepISOsurveys(Matuteetal.2006).TheblackcrossisanestimatefromFaddaetal.(2002)oftheupper-limittothe
AGNcontributiontotheCIRBat15µm. TheblackcirclesareSpitzermeasurementsofthetotalCIRBat24µm(Papovichetal.2004)and70µm(Frayeretal.
2006). Thestacking analysis ofDoleetal.(2006)provides alower-limit at160µm. Finally, theyellow areashowstheallowed rangeofthetotalCIRBas
compiledbyHauser&Dwek(2001).(Bottom)Asinthetoppanel,butnowplottingamodelwhererin=10pc.
4 Ballantyne&Papovich
type1backgroundspectrumpassesthroughtherangeofval- of the sources found in the Chandra 2 Ms deep fields. The
uesmeasuredbyISOfromtype1AGNat15µm(Matuteetal. shallowerEGSsurveyresulted in Spitzer detectingonly68–
2006). ThisresultimpliesthatthedeepISOsurveyshaveob- 80% of the X-ray sources and therefore underestimates the
served the vast majority of the type 1 AGN contribution to AGNcontributiontotheCIRB.
the CIRB at 15µm, although difficulties in identification re-
3.2. TheImportanceofCompton-ThickAGN
sulted in underestimating the fraction due to type 2 objects
(Matuteetal.2006). An important problem in the study of both the CXB and
The two panels of Fig. 1 show the predicted AGN back- CIRB is the identification of Compton-thick AGN, objects
ground spectra for rin = 1 pc (top) and rin = 10 pc (top). with obscuring columns logNH ≥ 24. Such heavily buried
It is interesting to compare the shapes of the spectra in the AGN are all but invisible in the 2–10 keV X-ray band
IR band with those computed by Silvaetal. (2004) based (Mattetal. 1999), buta notinsignificantpopulationof them
on observations of nearby AGN. As our models do not in- arerequiredinordertofitthepeakoftheCXBat30–40keV
cludeanyemissionfromthehostgalaxy,theappropriatecom- (Gillietal.2006). Ithasbeenhopedthatmid-IRobservations
parison is with the dot-dashed magenta line which plots the will aid in the identification of Compton-thick AGN (e.g.,
results of Silvaetal. (2004) for AGN with little to no host Donleyetal. 2005). Figure 2 re-plotsthe r = 10pc AGN
in
galaxycontamination. Itisclearthatthe r = 10pc model backgroundspectrumfromFig.1,butnowshowsthecontri-
in
providesa closer match to the Silvaetal. (2004) results (for butionfromCompton-thin(logN <24)andCompton-thick
H
λ<40µm)withthemid-IRpeakoccurringatnearlyidentical AGN.We seethatCompton-thinAGNdominatetheproduc-
wavelengths.ThecomputedCIRBforthismodelpredictssig- tion of the AGN background at all wavelengths . 100µm.
nificantlymoreemissionbeyond&40µmthantheSilvaetal. In our models, the heavily obscured Compton-thick objects
(2004) spectrum, but this is because these authors extrapo- predominantly produce emission from cooler dust as the
latedtheobservedSEDstorest-framewavelengths> 20µm. warminteriorisshieldedfromview(Ballantyneetal.2006b).
Inrecentwork,Ballantyneetal.(2006b)foundthat,basedon Therefore,thepredictionsusingtheRisalitietal.(1999)N
H
comparisonswiththeobservedSpitzermid-IRnumbercounts distribution, in which half of the type 2 AGN are Compton
and LFs, averageAGN foundin the X-ray and mid-IRdeep thick (gray lines in Fig. 2), find a slightly weaker CXB at
surveyswerebetterdescribedbyamodelwheretheobscuring 1–3 keV as that emission is converted into enhanced far-IR
mediumaroundtheAGNis∼ 10pcfromthecentralengine radiation. This X-ray deficit would have to be filled in with
asopposedto∼1pc.Fig.1indicatesthatthelocalAGNthat additionalemissionfromothersources,orthrougharevision
Silvaetal.(2004)usedtoproducetheirCIRB predictionare inthehardX-rayLF.
alsobetterdescribedbyamodelwheretheobscuringmedium At 24µm the spectral intensity from Compton-thin AGN
aroundthe AGN is ∼ 10 pc from the centralengine. These is ∼ 5× greater than the Compton-thick sources. Assum-
independentmethodstogetherstronglyimplythata∼ 10pc ing the Risalitietal. (1999) N distribution, this factor re-
H
scaleobscuringmediumismorelikelythansmallerscalesin duces to about 3, although Ballantyneetal. (2006a) found
AGNbetweenz =0and1. thattheRisalitietal.(1999)distributioncannotholdoverall
Focusing in on the rin = 10 pc AGN background spec- LX and z as it underpredicts the observed 2–8 keV num-
trum, we see that the model is close to the observedSpitzer ber counts. Thus the true factor is likely between these two
estimates for the AGN contribution to the CIRB at 24µm values. The estimated contribution of AGN to the CIRB at
(Treisteretal. 2006; Barmbyetal. 2006). Comparisons to 24µm from the EGS surveylies below the modelprediction
dataatshorterwavelengthsarecomplicatedbytheimportant for the Compton-thinAGN contribution in either N distri-
H
host galaxy contamination that Spitzer will be be unable to bution,implyingthatthissurveyismissingCompton-thinob-
avoidatfaintIRfluxes(i.e.,high-zobjects).Thiswillresultin jects. Incontrast,themeasurementderivedfromtheGOODS
anoverestimationofthecontributiontotheCIRBduesolely surveyisconsistentwithourmodelfortheCompton-thinob-
fromreprocessedlight fromthe AGN. The host galaxycon- jectsat24µm. Thus,weconcludethatinadeepsurveysuch
tamination becomes increasingly important at shorter wave- asGOODS,theCompton-thinAGNarebasicallycompletely
lengths as a greater contribution from the Raleigh-Jeans tail identified and these AGN constitute the vast majority of the
of the old stellar population within the galaxy enters into AGN background at 24µm. Interestingly, our models show
the observations. This effect can be clearly seen at 3.6, 4.5, that Compton-thick AGN comprise only a small fraction of
5.7 and 8µm in Fig. 1. This effect will undoubtedly cause theAGNbackgroundatthiswavelength(Fig.2).Recentanal-
methods of AGN identification through near-IR properties ysis of the CXB has shown that the deep X-ray surveys are
(Alonso-Herreroetal. 2006) to miss type 2 AGN. That is, missing∼50%ofthehigh-L obscuredAGN(Worsleyetal.
X
near-IR selection of AGN only works when the re-radiated 2005;Ballantyneetal.2006a),manyofwhichareexpectedto
light from the AGN significantly outshines the host galaxy beCompton-thick(Gillietal.2006). Fig.2suggeststhatus-
intherest-framenear-IR.Longerwavelengths,however,will ingthemid-IRtoidentifysuchhighlyobscuredobjectswillbe
be immuneto this problemso that a simple comparisoncan extremelydifficultastheyconstituteonlyatinyfractionofthe
bemadebetweentheSpitzermeasurementsat24µmandthe AGNbackgroundatthesewavelengths.Compton-thickAGN
model. Thus,wefindthattheestimatedcontributionofAGN dodominatethetotalAGNemissioninthefar-IR(&100µm),
to the CIRB at 24µm from the GOODS survey (green dot; butthe CIRB reachesits peakinthis regiondueto emission
Treisteretal.2006)isconsistentwiththebackgroundmodel from star formation over a wide range of z (Lagacheetal.
and is unsurprisingly dominated by type 2 AGN. In con- 2005), and the AGN contribution is therefore a very small
trast,themeasurementfromtheExtendedGrothStrip(EGS; fractionof the total radiation emitted by star-forminggalax-
Barmbyetal.2006)liesbelowthebackgroundpredictionby ies. Thus,weconcludethat,irrespectiveoftheexactfraction
a factor of 1.7, and is therefore likely missing type 2 AGN. ofComptonthickAGN,theycontributeonlyasmallfraction
This difference is due to the different survey depths. In the tothetotalAGNextragalacticbackgroundlightexceptforin
GOODS survey, deep Spitzer observations detected > 90% the far-IR and the very hard X-ray band (Gillietal. 2006).
AGNandtheCosmicBackgroundRadiation 5
FIG.2.—AsinthelowerpanelofFigure1,butnowdottedanddashedlinesplotthecontributionsofCompton-thin(logNH<24)andCompton-thickAGN
(logNH ≥ 24),respectively,tothetotalAGNbackgroundspectrum. TheblacklinesshowtheresultsassumingthesimpleNHdistribution(eq.1). Thegray
linesplottheresultswhentheRisalitietal.(1999)NHdistribution(eq.2)isassumed.
Therefore, the best strategy to find Compton-thick AGN is by the Silvaetal. (2004) result is only 1 M⊙ yr−1. The
throughX-rayimagingatenergies&40keV. Silvaetal.(2004)curvepeaksatlongerwavelengthsthanour
model,butthisisjustaresultofthehighdusttemperature(for
3.3. AccountingforStar-Formation thisSFR)usedinourestimate.Thematchinamplitudeisstill
robust.Thus,ifthisisgenerallycorrect,thetypicalSFRinan
repArolocnegsswedithliegmhtisfsrioomnfrsotamr-ofoldrmstianrgsirnetghieonAsGeNmhitotesdtgianlatxhye, average AGN over all luminosities and z is ∼ 1 M⊙ yr−1.
However, only low-z AGN were used by Silvaetal. (2004)
mid- and far-IR can be an important contributor to the IR
to construct their SEDs. Indeed, the average luminosity of
spectra of AGN. Yun&Carilli (2002) derived an empirical
formula relating the instantaneousstar-formation rate (SFR) their AGN host galaxy sample is LIR = 1010 L⊙, which is
atthethresholdofstarbursts,andifentirelyattributedtostar
agnaldaxthiees.obFsiegr.ve3dshmoiwd-stoth-fearreIsRultcsonotfiniunuclmudiinngsttahri-sfosrimmipnlge formationcorrespondstoaSFRof∼ 1M⊙ yr−1. TheCXB,
in contrast, is mostly produced at z ∼ 1, where the cosmic
constanttemperature(T = 58 K; Yun&Carilli 2002) esti-
d SFRdensityis∼ 10×greater(Hopkins2004). Theseeming
matefortheeffectsofstar-formationontheAGNbackground
increaseintheAGNtype2fractionwithz alsoindicatesthat
spectrum assuming an instantaneous SFR of 1 (solid black
the SFR may be higher in these objects (Bargeretal. 2005;
line), 5,20 and 100 M⊙ yr−1 (sold gray lines). This star- Ballantyneetal. 2006a;Treister&Urry2006). At suchred-
formationspectrumwasaddedtoeachAGNSEDbetween3
shifts, the CIRB is dominated by the luminous IR galaxies
ainntdeg1r5a0l0(eµqm. 3(r)e.sTt-hfreamgael)apxrieiosrfitot bpyerYfournm&ingCathriellbia(c2k0g0r2o)uanldl withtypicalSFRs&20M⊙yr−1(Lagacheetal.2005).
The percentage of the CIRB produced by AGN and star-
haveSFRs> 20M⊙ yr−1,sothedusttemperaturesforthe1 formationintheirhostgalaxieswilldependbothontheSFR
and5M⊙yr−1modelswilllikelybeoverestimates.However, withinthehostgalaxyandthewavelengthofinterest. Fig.4
thiswouldonlymovethepeaktotherightontheplotandnot
plotsthepercentageofthe CIRBdueto AGN+ reprocessed
greatlyaffecttheamplitude.
star-formationlight between 1 and 100µm. The hatched re-
The dashed magenta line in Fig. 3 plots the Silvaetal.
gionshowstheallowedpercentageswhenthemodelsofFig.3
(2004) model of the AGN contribution to the CIRB includ-
aredividedbytheyellowregionindicatingthecompilationof
ingemissionfromhostgalaxies.ThemaximumSFRallowed
6 Ballantyne&Papovich
FIG.3.—AsinthebottompanelofFig.1,butnowincludinganestimatefortheIRemissionbetween3and1500µm(rest-frame)fromreprocessedlightdue
toaninstantaneousstar-formationrateofof1(blacksolidline),5,20and100M⊙yr−1(graysolidlines). ThesecalculationsusedtheYun&Carilli(2002)
estimatefortheIRcontinuuminstar-forminggalaxies,i.e.,aconstantdusttemperatureofTd=58K.TheseresultsalsoassumedthesimpleNHdistribution.
measurementsbyHauser&Dwek(2001). Similarly,thedata ing remnants of the higher redshift quasars. However, the
pointsat24and70µmwerecalculatedbydividingthemod- large population of LIRGs at z ∼ 1 indicates that there is
elsbytheSpitzermeasurementsofPapovichetal.(2004)and significantgalaxygrowthatthisepochwhichinvolvessignif-
Frayeretal.(2006). IfAGNhostgalaxiesdohavesignificant icantgasbeingcondensedinthenucleiofgalaxies. Thecor-
SFRsonaverage,then∼30%oftheCIRBat70µmcouldbe relationbetweenblackholeandbulgemassseeninspheroidal
duetoAGN+hostgalaxyemission(Fig.4).Atshorterwave- galaxies(Tremaineetal.2002)showsthatthestarformation
lengths the percentof the CIRB producedby AGN depends and black hole growth are connected during massive galaxy
onlyslightlyonthehostgalaxySFR,andis∼10%at24µm, formation at high z (DiMatteoetal. 2005). Therefore, it is
droppingto∼1%between1and10µm. highlyplausiblethattheymaybeconnectedwithreducedin-
Finally, Fig. 3 shows that AGN which produce the CXB tensity at z ∼ 1. Future measurements of the AGN contri-
cannot, on average, have SFRs & 100 M⊙ yr−1. Such very bution to the CIRB should be able to constrain the SFR in
high SFRs are the hallmark of the ultra-luminousIR galaxy AGNatthepeakofobscuredAGNactivity.Clearly,adetailed
population which are observed to be associated with major studyoftheCXBandCIRBprobestheintertwinedprocesses
mergerevents(Sandersetal.1988a,b)andwhoseevolutionin ofblackholegrowth,galaxyevolutionandstarformation.
spacedensityisverysimilartoquasars(Kim&Sanders1998;
Bargeretal. 2000); thatis, theypeakin density atz ∼ 2–3.
This result may imply that the moderate-luminosityAGN at The authors thank the anonymous referee for very help-
z ∼1thatgeneratetheCXBhavedifferentevolutionarypaths fulcomments,andR.GilliforkindlysendingtheCXBdata.
than the high-luminosity quasars and be fueled by different DRB is supported by the University of Arizona Theoretical
mechanisms(minormergers, interactions, etc.). Rather than Astrophysics Program Prize Postdoctoral Fellowship. Sup-
signifyingtherapidbirthofalargeellipticalgalaxy,thesein- portforthisworkwasprovidedbyNASAthroughtheSpitzer
termediate AGN are the signs of a more leisurely mode of Space Telescope Fellowship Program, througha contractis-
galaxyformation.Alternatively,theseAGNcouldbethefad- suedbytheJPL,CaltechunderacontractwithNASA.
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