Discovery of strong Iron Kα emitting Compton thick quasars at z=2.5 and 2.9

0.34 MB·English

by  C. Feruglio

TOAPPEARINAPJLETTERS
PreprinttypesetusingLATEXstyleemulateapjv.11/10/09
DISCOVERYOFSTRONGIRONKαEMITTINGCOMPTONTHICKQUASARSATZ=2.5AND2.9
C.FERUGLIO1,E.DADDI1,F.FIORE2,D.M.ALEXANDER3,E.PICONCELLI2,C.MALACARIA4
ToappearinApJLetters
ABSTRACT
Wereportthedetectionofthe6.4keVIronKαemissionlineintwoinfrared-luminous,massive,star-forming
BzK galaxiesatz=2.578andz=2.90in the CDF-S. TheChandra4 Msspectraof BzK4892andBzK8608
1
showareflectiondominatedcontinuumwithstrongIronlines,withrest-frameequivalentwidthsEW∼2.3keV
1
and 1.2 keV, respectively, demonstrating Compton thick obscuration of the central AGN. For BzK8608 the
0
lineidentificationcloselymatchestheexistingphotometricredshiftderivedfromthestellaremission. Weuse
2
theobservedluminositiesofthe IronKα line, of therest-framemid-IRcontinuumandofthe UV rest-frame
n narrowemissionlinestoinferintrinsicL2- 10keV&1044ergs- 1,about1.0–2.5dexlargerthantheobservedones,
a henceconfirmingthepresenceofanabsorberwithN >1024cm- 2. ThetwoBzK galaxieshavestellarmasses
J H
8 of 5×1010 M⊙ and, based on VLA 1.4GHz and submm870µmobservations,they appearto hostvigorous
starburstactivitywithSFR∼300-700M yr- 1thatisalsoopticallythick.WeestimatethattheAGNmightalso
1 ⊙
conceivablyaccountforanimportantfractionofthebolometricfar-IRemissionofthegalaxies. Theimplied

] volume density of Compton thick (CT) AGN with L2- 10keV >1044 erg s- 1 is in agreementwith predictions
O fromX-raybackgroundsynthesismodels. Thesesourcesprovideoneofthefirstclearcutobservationsofthe
C long-soughtphaseofsimultaneous,heavilyobscuredquasarandstar formationactivity,predictedbymodels
ofmassivegalaxyevolutionathighredshifts.
.
h Subjectheadings:galaxies:general—galaxies: active
p
-
ro 1. INTRODUCTION inferred (L2- 10keV >1044erg s- 1; Braito et al. 2004, Picon-
t celli et al. 2010). Their X-ray spectra are characterized by
s Highlyobscured,deeplyembeddedAGNmightcontribute
the presence of a strong Iron Kα 6.4 keV fluorescent emis-
a significantlytothetotalaccretionpowerintheUniverse(Mar-
[ conietal. 2004)andarerequiredtoaccountforthespectrum sionline, with largeequivalentwidthEW &1 keV, on a flat
reflection-dominatedcontinuum(Mattetal.2000).Notmuch
oftheX-raybackground(XRB, Gillietal. 2007). Inpartic-
1 evidence currently exists in the distant Universe of AGNs
ular, AGN obscured in the X-ray band by column densities
8v largerthanNH =1024 cm- 2 (Comptonthick AGN) represent shhyopwerilnugmpirnoomusingeanlta,xhyigahtEzW=0I.r9o3n(KIwαalsianwe:aaentIaRl.A2S0s0e5le)catnedd
7 20–25%oftheAGNdetectedbyIntegralandSWIFT/BATin
two possible detections in the CDFS reportedby Norman et
4 thelocaluniverse(Maliziaetal. 2009;Burlonetal. 2010).
al.(2002;atype2QSOatz=3.7)andbyTozzietal.(2006;a
3 The identification of highly obscured AGN is particularly
galaxyatz=1.53);bothconfirmedbyComastrietal. (2011).
. challengingsince, because of the high columndensities, the
1 In the following we present the significant Chandra de-
vastmajorityoftheirX-rayemissionbelowrestframe10keV
0 tection of the Iron Kα emission line in two massive BzK
isabsorbed,sothattheyarelargelymissedeveninthedeep-
1 galaxiesatredshiftz=2.578andz=2.90±0.10inGOODS-
est hard X-ray surveys available today. At high redshift the
1 South, using the recently acquired 4 Ms dataset. We used
v: qbuyessetlfeocrtisnugchgaolbajxeicetsswisimthahiniglyhpruatrisousedofbmyiindd-iinrefrcatreevdidtoenocpe-, for this purpose the entire set of 52 observations for a to-
Xi tical, or mid-infrared to X-ray fluxes, and by deriving aver- tAalrcehxivpeo(shutrtep:t/i/mcxec.ohfar4vaMrds.e,dauv/cadilaa/bCleonitnribth/CeDCFhSa.nhdtmral)D.Watea
ageX-raypropertiesbystackingtechniques(e.g.,Daddietal.
r adoptaΛCDMcosmology(H =70kms- 1 Mpc- 1;Ω =0.3;
a 2al0.0270b0;5D;0270h0e8r;eTafrteeirs;teFrioerteale.ta2l0.1200,0G8e,o2r0g0a9n;toApoleuxlaonsdeetraelt. ΩΛ=0.7)andaChabrierstella0rinitialmassfunction(IMMF).
2009,Eckartetal. 2010,Donleyetal. 2010). Thesestudies
2. X-RAYPHOTOMETRICANDSPECTRALANALYSIS
havesuggestedthatasubstantialfractionofmassivegalaxies
atz>1hosthighlyobscured,intrinsicallyluminousAGN. We havebeeninvestigatingthe propertiesof BzK-selected
Onlya fewtensofsecureComptonthickAGNareknown (Daddi et al. 2004), massive galaxies at redshift 1<z<3
sofarinthelocalUniverse(Comastrieta. 2004,DallaCeca withfaintornoX-rayemission,withtheaimofconstraining
etal. 2008andreferencestherein),andonlyforahandfulof their obscured AGN activity. We have focused in particular
them a quasar-like intrinsic hard X-ray luminosity has been on objects showing also a power-law emission in the near-
IRrestframe,anindependentindicationforthepresenceofa
luminous,obscuredAGN.Alargestatisticalsampleof750X-
[email protected]
1Laboratoire AIM, CEA/DSM - CNRS - Université Paris Diderot, rayundetectedgalaxieshasbeenassembledintheCOSMOS
Irfu/Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 field,andtheirX-raypropertiesinvestigatedthroughstacking.
Gif-sur-YvetteCedex,France Asacontrolsample,wealsostudiedindetailasmallersam-
2INAF -Osservatorio Astronomico di Roma, via Frascati 33, 00040
ple of 26 similarly selected galaxiesin GOODS-South, with
MonteporzioCatone,Italy
3Department ofPhysics, DurhamUniversity, SouthRoad, DH13LE, less than 200 net countsin the 0.5-7keV band in the CDFS
UK 4Msdatathatwouldremainthusundetectedintheshallower
4Universita’LaSapienza,PiazzaleAldoMoro5,00185Roma,Italy COSMOS Chandra observations. While the general results
2 Feruglioetal.
BzK4892 z=2.578 BzK4892 z=2.578
0−6
1
nts s keV−1−1 10−5 m s keV)−2−1−1 10−7
normalized cou 0−6 keV (Photons c2 10−8
1
0.5 1 2 5 1 2 5
Energy (keV) Energy (keV)
BzK8608 z=2.9 BzK8608 z=2.9
0−6
1
nts s keV−1−1 10−5 m s keV)−2−1−1 10−7
normalized cou 10−6 keV (Photons c2 10−8
1 2 5 1 2 5
Energy (keV) Energy (keV)
FIG.1.—ThespectraofBzK4892(upperpanels)andBzK8608(lowerpanels),fittedwithapurereflectionmodelplusanIronKα6.4keVline. Thespectra
areshownbothinobservedcountsunits(leftpanels)andinphysicalunits(keV2;rightpanels). Eachbincorrespondstoa2σmeasurement. Thereddottedline
representstheaveragebackgroundextractedfromthewholefield,excludingsources(Fioreetal.2011,inpreparation).
of these studies will be reported elsewhere (Feruglio et al. (Luo et al. 2010), likely because of the presence of another
2011, in preparation), we concentrate here on two remark- nearbyX-ray source(#193). In the 4 Ms data BzK8608has
ablesourcesinourGOODS-Scontrolsample, forwhichthe 92countsinthetotalband.Itisnotdetectedthe0.3-1.5band,
available Chandra data allow the unambiguous detection of anditshardnessratioisHR>0.54(3σ).
theIronKαemission. We extracted the Chandra spectra using a circular region
Both of the BzK galaxies under exam were selected from of 1.5′′ and 3′′ radius for BzK4892 and BzK8608 (off-axis
theD07sample.BzK4892isasourcewithspectroscopicred- ∼7.4′),respectively,estimatingthebackgroundinanannulus
shiftz=2.578(Szokolyetal.2004,Vanzellaetal. 2006;based aroundthe source position. We experimentedwith different
onmultiplenarrowemissionlines),detectedintheX-raysal- extractionradii, centering,andbackgroundestimates, taking
ready in the 1 Ms Chandra data (Giacconi et al. 2001) as careinallcasesofexcludingnearbydetectedsources(partic-
well as in the 2 Ms data (Luo et al. 2010). In the 4 Ms ularlyimportantforBzK8608),findingthatthefeaturesfound
dataset it has 75 net countsin the 0.5-7keV band and hard- inthespectrawererobusttothedetailextractionmethod.The
nessratioof0.58(thehardnessratioisdefinedasHR=hard- spectra for both galaxies show a flat continuum and promi-
soft/hard+soft,whereSandHarethesoftandhardbandnet nentemissionlinespeakingat∼1.78keV forBzK4892and
counts detected by Chandra). BzK8608 has a photometric ∼1.64keVforBzK8608(Fig. 1).
redshift z =2.94 (from D07; consistent with z =2.88 WeconsidertwospectralmodelstofitthedatainXSPEC:
phot phot
fromSantinietal. 2009).VLT+VIMOSspectroscopicobser- i) a reflection-dominated model, pexrav + zgauss, and ii)
vations(Popessoetal. 2009)showthatthereisnocontinuum a transmission (power-law) model, wabs*(pow+zgauss). A
emissionintheblue,consistentwithz∼3,butfailedtoyield narrow gaussian component is used to model the emission
areliableredshiftestimate. WetentativelydetectedintheVI- lines. Given the limited photon statistic, the slope of the
MOSspectrumalowsignaltonoisefeatureat6027Å(are- primarycontinuumhasbeenfixedtoΓ=1.8inbothscenar-
gionclearfromOHlines)that, ifidentifiedwithCIV,would ios. Based onC-statistics, the emissionline isdetectedwith
give z= 2.88. We notice that the photometric redshift for a significance of &4σ (∆C =17.5) for BzK4892 and &3σ
BzK8608isreliableasthegalaxyfulfillstheU-banddropout (∆C=7.5) for BzK8608 (see also Fig. 2) for the reflection-
criteria, corresponding to a Lyman continuum break (being dominatedmodel.Inthecaseofatransmissionmodelthesig-
undetectedinthedeepU-bandimagingofNoninoetal.2010; nificanceoftheline remainsabove4σ forBzK4892andjust
Fig.3). ThissourceisnotlistedintheChandra2Mscatalog below 3σ for BzK8608. To further evaluate the significance
Ironemittingquasarsatz=2.5- 3 3
FIG.2.—Upperpanels: ChandraimagesofBzK4892andBzK8608inthreeenergybands. ThemiddleimagesarecenteredattheenergyoftheIronKline.
Thecirclesshowtheextractionregions. Lowerpanels: confidencelevels(1,2and3σ)obtainedfromthefitoftheIronline. Theverticalbluelinesshowthe
opticalspectroscopic/photometricredshiftconstraints.
of the observed lines we also performed 105 simulations of 3.1. Intrinsicluminositiesandobscuration
thesourcecontinuumandbackgroundforbothsources. The
The observed (obscured) hard X-ray luminosities are
parreob3a×bi1li0t-y5thaantdth2e×l1in0e- s4,mreigshptecbteivdeulye.toTchheanspceecfltruacltufiatstioanres L2- 10keV = 1042.5 and 1042.8 erg s- 1 for BzK4892 and
BzK8608, respectively. Given the large N implied by the
summarizedinTable1. H
detection of high EW Iron Kα lines, the X-ray emission is
Based on the spectroscopic and photometric redshifts, we
optically thick and we cannot directly estimate the intrinsic
identifythe lines as Iron Kα for both sources. In particular,
we derivea spectroscopicredshiftfor BzK8608from the X- L2- 10keV using the X-ray data (e.g., La Massa et al. 2011).
InordertoestimatetheintrinsicluminositiesoftheAGNwe
ray spectrum, obtainingz=2.88±0.10. Figure 2 shows the
usedavarietyofindependentmethods.
1-2-3σconfidencelevelcontours,showingtheredshiftfitand
Following Iwasawa et al. (2005), a lower limit to the X-
thesignificanceoftheIronlinedetection.
ray luminosity can be obtained in the expectation that in
The rest frame equivalent widths of the Iron lines are
typical conditions the luminosity of the Iron Kα line is at
EW=2.3+- 10..06 keV for BzK4892 and EW=1.2±0.4 keV for most 3% of the 2-10 keV luminosity. For BzK4892 LKα =
BzK8608(for a reflection model, see Table 1), implyingab- 1.25×1042 erg s- 1 would imply a intrinsic 2-10 keV lumi-
sraeodsrdpbioitninogdnactloollotuhwme-nsesingeenrxigficyeceaodnficnIergoen1x0cKe24βsscaamtt-z12.=.82kB.e9zVK, i8tfh6ra0et8awl.sohTuolhwdescdoaern-- ne1no0ss4o4it.n8yeeotrfga4ls.×- (112.004A036ne)ragimltsep-r1lni.easTtivhaneerheesilgtaihmtieoarntleuLmiKsiαno/obLst2ai-tiy1n0,ekLde2Vf- r1oo0fmkeLVethv∼e-
tectionofIronKαat6.4keVexcludesthatthehighhardness
6.0µm luminosity. The 24µm flux of 591µJy implies
ratios in these sources might be due to high mass X-ray bi-
naries, as suggestedby Donleyetal. (2008),dueto spectral L6.0µm =1045.5 erg s- 1(the mid-IR SED of Mrk 231 is used
forthe small K-correction). While 24µmat z=2.578might
incompatibility(Persic&Raphaeli2002).
be contaminated by PAH features, we find similar results if
ThefitwiththetransmissionmodelgivesN =7±2×1023
H we use the 16µm flux of 209µJy (Teplitz et al. 2010) for
cm- 2 for BzK4892 and 7+3×1023 cm- 2 for BzK8608, and
- 2 whichnocontaminationisexpected(4.5µmrest-frame).This
imsosdtaetli.stiHcaolwlyevinedr,istthiensgeuiNshabvlaeluferosmarteheinficotnwsiitshteantrewfliethctitohne wouldsuggestL2- 10keV=1045±0.5ergs- 1usingtheLutzetal.
measured EW&1 keV. IndHeed, for N ∼ 6×1023 cm- 2, a (2004) correlation and a lower L2- 10keV =1044.4±0.5 erg s- 1
EW∼300 eV is predicted(Ikeda et al.H2009)and confirmed usingLanzuisietal. (2009). Athirdestimatecanbederived
from the luminosity of the UV-optical emission lines. The
byobservations(Guainazzietal.2005,Fukazawaetal.2010).
VLT/FORS1-2 optical spectrum of BzK4892 shows promi-
EW&1keVarethehallmarkofcolumndensitieslargerthan
1024cm-- .Therefore,inthefollowing,weadoptasbestfitthe nentLyα,CIV,HeIIandCIII]narrowemissionlines(Szokoly
etal. 2004,Vanzellaetal. 2006). Usingtheobservedratios
resultsfromthereflectiondominatedmodelforbothsources
between the line and the intrinsic 2-10 keV emission from
(TaFbig.u2r)e. 2 shows the Chandra images of BzK4892 and Mulchaeyetal. (1994),weinferL2- 10keV=1044.5±0.5ergs- 1.
UsingtheNetzeretal. (2006)conversionforUV-opticalline
BzK8608 in three energy bands, including one centered on luminosities would give larger X-ray luminosities by ∼0.5
the Iron lines. We note that BzK8608 is not detected in the
softband. dex, or L2- 10keV = 1045±0.5 erg s- 1. All in all, it appears
that the 2-10 keV luminosity of BzK4892 is in the QSO
range, well in excess of 1044 erg s- 1, and more likely of
3. DISCUSSION
4 Feruglioetal.
TABLE1
SPECTRALFITPARAMETERS
SourceID Model FeKαflux N EW Cstat/d.o.f.
H
10- 17erg/cm2s 1023cm- 2 keV
BzK4892 Abs 1.6±0.5 7.0+- 22..00 2.1±0.5 307/441
BzK4892 Refl 2.0±0.6 - 2.3+- 10..06 297/442
BzK8608 Abs 3.6±0.5 7.0+- 32..00 1.0±0.3 411/440
BzK8608 Refl 3.4±0.8 - 1.2±0.4 418/442
1044.5- 45ergs- 1. Theimpliedcolumndensityisatthelevelof larluminositiesrequirestarformationrates(SFR)inthehost
N ∼1024.5- 25cm- 2. galaxies at the level of 100-1000 M yr- 1. It is interesting
H ⊙
ForBzK8608,wemeasureL(Kα)=2.5×1042ergs- 1. This thus to compare these estimates to the inferred SFR for the
convertstoL2- 10keV∼1043.9ergs- 1(Iwasawaetal.2005),or twoBzK galaxies,inordertocomparetherelativeAGNand
∼1045 ergs- 1 adoptingLevensonetal. (2006). Thisgalaxy SFRcontributions.
isabout10timesfainterinthemid-IRthanBzK4892,which FromthestellarSED(UVtothenear-IRrestframe;Fig.3),
implies a lower L6.0µm. Using the Lutz et al. (2004) rela- we estimate SFR∼70 M⊙ yr- 1 for BzK4892, corrected for
tion,thisinturnsuggestsL2- 10keV=1044 ergs- 1. Wecannot dustreddeningbasedontheobservedUVslope(seeD07for
obtainanindependentestimatefromtheUV-opticalemission moredetails). Itis quitepossiblethoughthatalso thestellar
lines given that no lines were detected in the VIMOS spec- UVemissionmightbeopticallythick.Ifweinterpretitsmid-
trum.Theintrinsic2-10keVluminosityofBzK8608islikely IRemission(591µJy at24µm)asduetostar formationthis
≈1044 erg s- 1. The implied column density is likely in the wouldimplyawhoppingSFR∼104 M⊙ yr- 1,demonstrating
rangeN ∼1–5×1024cm- 2,hencethisobjectmightbeabor- that the mid-IR is likely completely dominated by the AGN
H
(thisobjectisamongthemostextrememid-IRexcessgalax-
derlineCTAGN.Wenoticethatbothsourceshaveestimated
ies in the sample of D07). The SED fit with a star forming
intrinsic2-10keVluminositiesintheQSOregime.
galaxy template (Fig. 3) shows an excess emission already
at 5.8 µm (observed frame) probably due to the AGN con-
tribution. The SED in the IRAC bands is indeed showing a
3.2. Bolometricluminositiesandstarformationrates steeppowerlaw, extendingallthewayto 24µm. Duetoits
The ratio of the bolometric to the 2-10 keV luminosity, faintopticalcounterpart(i∼25AB),BzK4892hashighmid-
Lbol/L2- 10keV, is typically of the order 30-50 for quasars infraredtoopticalfluxratio,F(24)/F(R)∼2000andtherefore
(Marconiet al. 2004). Assuming such a bolometric correc- isalsoclassifiedasadust-obscuredgalaxy(DOG,Deyetal.
tion,weestimateL ∼1046±0.5 ergs- 1 forBzK4892and∼ 2008). Itis an extremeobjectalso in the Fioreet al. (2008)
bol
1045.5±0.5ergs- 1forBzK8608.Theseveryhighluminosities, sample. Alonso-Herreroet al. (2006)reportsa 70 µm flux
ifexpressedinsolarunits,correspondtoL ∼1012.4±0.5L of3.3±1.8mJyforthisgalaxy,whichisalsolikelyaffected
bol ⊙
and∼1011.9±0.5L . Whenpoweredbystarformation,simi- bytheAGNemission. Ontheotherhand,thegalaxyisseen
⊙ at1.4GHzwithafluxof80µJyintheVLAdataofMilleret
al. (2008). Ifduetostarformation,usingtheradio-IRcorre-
TABLE2 lation,thiswouldimplySFR∼1100M yr- 1. Inspectingthe
GALAXYPROPERTIES ⊙
publiclyavailableApex+LABOCA870µm mapofGOODS-
BzK4892 BzK8608 S (Weiss et al. 2009), we find a 3σ signal at the position of
Ra 3:32:35.71 3:32:20.95 BzK4892of3.3mJy,whichwouldalsoconvertintoasimilar
Dec -27:49:16.1 -27:55:46.3 SFR,∼500M yr- 1.
⊙
Redshift 2.578 2.90±0.10 Even for the most luminous quasars it is generally found
HR 0.58 >0.54(3σ) that the far-IR emission in the sub-mm bands is due to star
SFRUV[M⊙yr- 1] 70 70 formation. If this is the case also for BzK4892, our results
SFRRadio[M⊙yr- 1] 1100 300 suggestthat the galaxyis witnessing also very powerfulstar
L(Kα)[ergs- 1] 1042.1 1042.4 formationactivityatthelevelof500-1000M⊙ yr- 1. Only5-
L2- 10keV,abs[ergs- 1] 1042.5 1042.8 10%ofthisisseendirectlyintheUV,implyingthatalsothe
L2- 10keV,Kα[ergs- 1] 1043.6- 44.8 1043.9- 45 UV-emission from stars is optically thick, similarly to local
LL22-- 1100kkeeVV,,U6µVmlin[eesrg[esrg- 1s]- 1] 11004444-.44-54.55 10-44 UaevnLedIr,RtowGwesnh(oaett.igics.e,eGtxhopaltedctahtedederratodetiboaeal.fnod2u08n07d20i;µndmmadaCejourirnvhmeadeerSgtFeaRrls.s.w2H0oo1uw0ld-)
i[ABmag] 24.75 24.72 formally imply L ∼1046- 46.5 erg s- 1, comparable to what
IRAC3.6µm[ABmag] 20.81 21.29 bol
inferredfortheobscuredAGN.Hence,theAGNislikelycon-
IRAC4.5µm[ABmag] 20.54 21.16
tributinganimportantfractionofthetotalL .
bol
IRAC5.8µm[ABmag] 20.1 20.84 ThemorphologyofthegalaxyfromHST+ACSimagingis
IRAC8.0µm[ABmag] 19.64 20.65 suggestive of a merger, showing two distinct clumps in the
MIPS24µm [mJy] 0.591±0.07 0.045±0.003 UV in the rest frame. When smoothing, after excludingthe
MIPS70µm [mJy] 3.3±1.8 - two bright knots, we detect significant faint low-level emis-
850µm [mJy] 3.3±1.1 - sion in the summed i+z band, extended over a diameter of
1.4GHz[µJy] 80±14 17.5±6.5 about0.75′′(6kpc).Overall,thisisconsistentwiththesizeof
Ironemittingquasarsatz=2.5- 3 5
BzK4892 z=2.578 BzK8608 z=2.9
FIG.3.—Opticaltomid-infraredSEDsofBzK4892(leftpanel,blacksymbols)andBzK8608(rightpanel). Thebluecontinuouslineshowsthefittedmodel
SEDofapurestarforminggalaxy.ThephotometryfromtheGOODSdatasetsistakenfromD07.HST+ACScolorimagesarealsoshownasthumbnails.
amassivegalaxyatz∼2,andwecannotexcludethatthetwo ComptonthickAGNwithL2- 10keV >1044 ergs- 1. However,
UVknotsmightbejustluminousHIIregionsinsideabigdisk wenoticethatoursamplingofL2- 10keV >1044 ergs- 1 AGN
galaxy. withComptonthickabsorptionat1.4<z<3 mightbesub-
The photometric information is of lower quality for stantially incomplete due to selection effects, and the lumi-
BzK8608duetoitsoverallfaintness. TheSFRinferredfrom nosityofatleastoneoftheBzK galaxiesinthispapermight
the UV is also 70 M⊙/yr. This object is also a mid-IR ex- besubstantiallyhigherthantheadopted1044ergs- 1limit.
cess source in D07 sample (by a less extreme factor of 6), We emphasize that the sources presented here have X-ray
with S24 =45µJy ( ∼1/10ofBzK4892), althoughit is nota spectrathatarethehigh-luminosityanalogousofthoseoflo-
DOG. The SED (Fig. 3) might also be consistent with pure calprototypeComptonthickAGN.Thisdiscoveryrepresents
stellaremission,butadeviationfromthestarformationtem- one of the first clear-cutevidencefor this class of objects at
plateisobservedatthe8µmband.Thisgalaxyisnotdetected highredshift. Ourresultsconfirmthatindeedthereisapop-
at 70 µm and 870µm, suggestinga lower AGN luminosity ulationofComptonthickQSOamongmassive,star-forming,
and/orSFR. Inspectingthe VLA 1.4 GHz data (Miller et al. dustobscuredgalaxies,assuggestedbyseveralstudies(D07,
2008) we detect a faint 3σ source at its position with a flux Fioreetal. 2008,2009,Treisteretal. 2010,Alexanderetal.
of17.5±6.5µJy. Thiscorrespondstoaluminosity3×1012 2008, Lanzuisi et al. 2009), and as predicted by models of
L and SFR∼300 M yr- 1. If the radio signal is real and merger-drivenAGN/galaxyco-evolution(Silk & Rees 1998,
⊙ ⊙
notduetoAGN,alsointhiscasethestellaremissionappears Fabian 1999, Di Matteo et al. 2005). Indeed, these lumi-
to be heavilyobscured, i.e. opticallythick in the UV, as ex- nousComptonthickquasarsappeartobehostedbygalaxies
pectedforveryactivestarburstsandmergers.TheHST+ACS with opticallythickUV emission, whichare a relativesmall
imagingofthisgalaxy(Fig.3)isnotparticularlytelling. minority among BzKs (Daddi et al 2007a), and imply the
Forbothgalaxieswederivesimilarestimatesofthestellar presence of a dense starburst usually connected with merg-
masses, atthelevelof5×1010M . Ourresultsthussupport ers. This seems to agree with evolutionaryscenarios, where
⊙
the picture that the luminous, Compton thick AGN that we mergersinducebothrapidaccretionontoSMBHandintense
discoveredarehostedbyfairlymassivegalaxiesatz∼2.5-3, star-formationactivity.
thatatthesametimealsohostvigorousstarformationactiv- The two sources presented here have high intrinsic X-ray
ityheavilyobscuredbydust.ThisisrelevantintheAGN-host andbolometricluminosity,andbrightIronKαline,allowing
galaxyco-evolutionscenario,inwhichaphaseofrapid,heav- their identification. They might represent just the tip of the
ily obscured BH growth accompanied by intense obscured icebergofamuchlargerpopulationofhighlyobscuredAGN
star-formationis predicted (Silk & Rees 1998, Fabian 1999, with somewhat lower intrinsic luminosity, whose Iron lines
Granatoetal. 2004). arestillremaininghardlydetectableeveninthedeepesthard
X-ray surveys. For example, the Gilli et al. model predicts
a∼10×higherspacedensityofComptonthicksourceswith
3.3. ImplicationsforComptonthicknuclearactivityathigh
redshift L2- 10keV>1043ergs- 1. DetectingIronlinesingalaxieswith
suchluminositiesmightbechallengingwithcurrentfacilities,
WederiveacrudevalueofthevolumedensityofCompton
eveniftheyhavesimilarEWasinBzK4892andBzK8608.
thickAGN withhighEWIronKαemission, based onour2
detections. We conservativelyuse the fullredshiftrangeex-
ploredbytheBzK selection(z=1.4-3),findingaspacedensity We thank the anonymous referee for useful comments.
of3×10- 6 Mpc- 3. Thisisconsistentwiththepredictionsof Thisresearchwas supportedbythe ERC-StG grantUPGAL
theGillietal. (2007)X-raybackgroundsynthesismodel,for 240039 and by the French ANR under contract ANR-08-
6 Feruglioetal.
JCJC-0008. EP acknowledges financial support from ASI (grantI/088/06/0). DMAthankstheScienceandTechnology
ResearchCouncilforsupport.
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