The largest X-ray-selected sample of z > 3 AGNs

19 Pages·2014·1.35 MB·English

MNRAS445,1430–1448(2014) doi:10.1093/mnras/stu1745
>
The largest X-ray-selected sample of z 3 AGNs: C-COSMOS
and ChaMP
E. Kalfountzou,1,2‹ F. Civano,1,3 M. Elvis,1 M. Trichas4 and P. Green1
1Harvard–SmithsonianCenterforAstrophysics,60GardenSt,Cambridge,MA02138,USA
2CentreforAstrophysics,Science&TechnologyResearchInstitute,UniversityofHertfordshire,Hatfield,HertsAL109AB,UK
3YaleCenterforAstronomyandAstrophysics,260WhitneyAve,NewHaven,CT06520-8121,USA
4AirbusDefence&Space,GunnelsWoodRoad,Stevenage,HertfordshireSG12AS,UK
Accepted2014August25.Received2014August22;inoriginalform2014May29 D
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ABSTRACT ed
Wepresentresultsfromananalysisofthelargesthigh-redshift(z>3)X-ray-selectedactive fro
m
galactic nucleus (AGN) sample to date, combining the Chandra Cosmological Evolution h
Survey and Chandra Multi-wavelength Project surveys and doubling the previous samples. ttp
s
Thesamplecomprises209X-ray-detectedAGNs,overawiderangeofrest-frame2–10keV ://a
luminositieslogLX =43.3–46.0ergs−1.X-rayhardnessratesshowthat∼39percentofthe cad
sourcesarehighlyobscured,N >1022cm−2,inagreementwiththe∼37percentoftype-2 em
AGNsfoundinoursamplebasHedontheiropticalclassification.For∼26percentofobjects ic.o
u
havemismatchedopticalandX-rayclassifications.Utilizingthe1/Vmax method,weconfirm p.c
o
that the comoving space density of all luminosity ranges of AGNs decreases with redshift m
above z > 3 and up to z ∼ 7. With a significant sample of AGNs (N = 27) at z > 4, it is /m
n
foundthatbothsourcenumbercountsinthe0.5–2keVbandandcomovingspacedensityare ras
/a
consistent with the expectation of a luminosity-dependent density evolution (LDDE) model rtic
atallredshifts,whiletheyexcludetheluminosityanddensityevolution(LADE)model.The le
/4
measuredcomovingspacedensityoftype-1andtype-2AGNsshowsaconstantratiobetween 4
5
thetwotypesatz>3.OurresultsforbothAGNtypesattheseredshiftsareconsistentwith /2/1
4
theexpectationsofLDDEmodel. 3
0
/1
Keywords: surveys–galaxies:active–X-rays:galaxies. 39
2
7
9
3
b
y
bothathighredshiftsandforlowluminosities.Thisrequireslarge g
1 INTRODUCTION u
samplesofAGNsspanningwiderangesofproperties.Whilemany e
s
Activegalacticnucleus(AGN)evolutionathighredshifts,before opticalsurveyshaveinvestigatedthespacedensityofhigh-redshift t o
n
theirdensitypeak,illuminatestheroleofAGNintheformationand AGNs (e.g. Richards et al. 2006; Jiang et al. 2009; Willott et al. 1
0
co-evolutionofgalaxiesandtheircentralsupermassiveblackholes 2010;Glikmanetal.2011;Ikedaetal.2011;Rossetal.2013),the M
(SMBHs) during the time of rapid SMBH growth. The so-called resultsarestillcontroversialduetotheirinevitableincompleteness, arc
downsizingevolutionhasbeenrevealedforbothAGN(e.g.Ueda especiallyatthefaintluminosityendduetothehostcontamination, h 2
et al. 2003; Hasinger, Miyaji & Schmidt 2005; Aird et al. 2010) and the bias against obscured sources. As compared with optical 02
3
andgalaxies(e.g.Cowieetal.1996;Kodamaetal.2004;Draper surveys,X-rayobservationsarelesscontaminatedbythehostgalaxy
etal.2009).Supportingthisidea,X-raysurveyshaveshownthat emissionandincludeAGNpopulationswithawiderangeofneutral
thenumberdensityofluminousAGNpeaksathigherredshiftsthan hydrogencolumndensity.
lessluminousones(e.g.Uedaetal.2003;Airdetal.2010).This For the investigation of absorption evolution (e.g. Ueda et al.
sortofcosmologicalco-evolutionscenarioisinferredfromthetight 2003;Hasinger2008;Draper&Ballantyne2010),X-rayselected
correlationexistslocallybetweenSMBHmassandgalacticbulge samplesincludealltypesofAGN(e.g.type-1/unobscuredandtype-
properties (e.g. Magorrian et al. 1998; Ferrarese & Merritt 2000; 2/obscured) and provide reduced obscuration bias in comparison
Gebhardtetal.2000;McConnell&Ma2013). with optically selected AGN. Although X-ray surveys have in-
Toelucidatetheco-evolutionofSMBHandgalaxies(e.g.Granato ferred the existence of an anticorrelation between the obscured
etal.2001,2004;Crotonetal.2006;Hopkinsetal.2006;Menci AGN fraction and the luminosity, several of these studies have
et al. 2008; Trichas et al. 2009, 2010; Kalfountzou et al. 2011, suggested that this fraction increases toward higher redshift from
2012,2014),theaccretionactivityintheUniversehastobestudied z=0toz∼2withlimitedsamplesatz>3(e.g.LaFrancaetal.
2005;Ballantyne,Everett&Murray2006;Treister&Urry2006;
(cid:2)E-mail:[email protected] Ballantyne2008;Hiroietal.2012).
(cid:3)C 2014TheAuthors
PublishedbyOxfordUniversityPressonbehalfoftheRoyalAstronomicalSociety
ThelargestX-ray-selectedsampleofz >3AGNs 1431
However, the evolution of AGN is still rife with uncertainty.
OnthebasisofhardX-raysurveys,manystudiesagreedthatthe
X-rayLuminosityFunction(XLF)ofAGNisbestdescribedbya
luminosity-dependentdensityevolution(LDDE)model(e.g.Ueda
etal.2003;Gilli,Comastri&Hasinger2007;Silvermanetal.2008;
Uedaetal.2014).Airdetal.(2010)preferredinsteadaluminosity
and density evolution model (LADE). In LADE, the shift in the
redshiftpeakoftheAGNspacedensityversusX-rayluminosityis
muchweakerthaninLDDEmodels,yetgivesasimilarlygoodfit
to their data. While the z < 2 downsizing behaviour is common
tobothmodels,quitedifferentnumbersofAGNsarepredictedat
higherredshifts(z≥3).
X-ray surveys (2–10 keV) are now sensitive enough to sample
the bulk of the z > 3 AGN population. Two studies have been
D
performedonhigh-redshiftAGNexploitingthedeepX-raysurveys ow
intheCosmologicalEvolutionSurvey(COSMOS)fieldcarriedout nlo
(wNithXM=M81–;NCewivtaonno(eNtAaGl.N2=0114)0,;lBimruitseadettoa2l–.12000k9e)ValnudmCinhoasnitdireas Figure1. SkyareaversusX-rayfluxsensitivitycurvesfortheC-COSMOS aded
L2A−G10NkeV >1044.2 and 1043.5ergs−1, respectively. A more recent (abreluae(bsolalicdkldinaes)headndlinCeh)a.MThPe/SvDeSrtSica(rlebdluseoldidaslhineed)lsinamepinledsicaantedstthheetflotuaxl from
studybasedonthe4MsChandraDeepFieldSouth(CDF-S;Xue correspondingto10percentofthetotalC-COSMOSarea(seeSection4). h
etal.2011)wasabletoinvestigatetheevolutionofz>3AGNdown TheverticalreddashedlineindicatestheChaMPX-rayfluxlimitwith>75 ttps
toLX∼1043ergs−1(NAGN=34;Vitoetal.2013).Theseresultsare percentcompletenessfromSDSS/UKIDSS/WISE(seeSection2.2).The ://a
consistentwithadeclineoftheAGNspacedensityatz>3,butthe totalarea,aftertheappliedcuts,usedforthisworkisrepresentedbythe ca
d
shapeofthisdeclineremainshighlyuncertainatz>4.Toovercome shadowedgreyarea. e
m
theselimitations,inthisworkwecombinedthetwolargestsamples ic
.o
ofz >3X-ray-selected AGNs withspectroscopicredshifts,both u
2.1 TheC-COSMOSsample p
derivedfromChandraX-rayObservatory(Weisskopfetal.2002) .c
o
surveys: the wide but shallow ChaMP survey (Kim et al. 2007; The Chandra-COSMOS survey (C-COSMOS; Elvis et al. 2009; m
Greenetal.2009),andthedeeperbutnarrowerC-COSMOSsurvey Civanoetal.2012)coversthecentral0.9deg2oftheCOSMOSfield /m
n
(Elvis et al. 2009). This combination results in the largest X-ray uptoadepthof200ksintheinner0.5deg2,withtheACIS-ICCD ra
s
AGNsamplewithN =211atz>3andN =27atz>4. imager(Garmireetal.2003)onboardChandra.TheC-COSMOS /a
At the same time, bAyGcNombining two surveysAwGiNth different flux X-ray source catalogue comprises 1761 point-like X-ray sources rtic
le
limits,weareabletodeterminethedensityevolutionofbothlow- detecteddowntoamaximumlikelihoodthresholddetml=10.8in /4
4
luminosity (L < 1044ergs−1) and high-luminosity AGNs. Our atleastoneband.Thislikelihoodthresholdcorrespondstoaproba- 5
sample includXes both obscured and unobscured AGNs, and their bilityof∼5×10−5thatacataloguesourceisinsteadabackground /2/1
4
separateevolutionhasbeendetermined. fluctuation (Puccetti et al. 2009). Given this likelihood threshold, 3
0
Thepaperisstructuredasfollows.InSection2,wediscussthe thefluxlimitreachedinthesurveyis5.7×10−16ergcm−2s−1 in /1
3
data sets used in this work and the selection of the high-z sam- the full band (0.5–10 keV), 1.9 × 10−16ergcm−2s−1 in the soft 92
ple. In Section 3, we present the optical and X-ray properties of band (0.5–2 keV) and 7.3 × 10−16ergcm−2s−1 in the hard band 79
3
the selected high-z AGN sample, and we explain the AGN type (2–10keV). b
y
classificationusingX-rayoropticaldata.InSections4and5,the The z > 3 C-COSMOS sample, as presented by Civano et al. g
u
numbercountsandspacedensityofthesamplearecomparedwith (2011),comprises107X-ray-detectedsourceswithavailablespec- es
modelpredictions.Section6summarizestheconclusions.Acos- troscopic(32)andphotometric(45)redshiftsplus30sourceswith t o
n
mologicalmodelwith(cid:3) =0.3,λ =0.7,andaHubbleconstant aformalz <3butwithabroadphotometricredshiftprobability 1
o o phot 0
of70kms−1Mpc−1isusedthroughout(Spergeletal.2003).Errors distribution,suchthatz +1σ >3.Allofthespectroscopic M
phot phot a
arequotedatthe1σ level. C-COSMOS sources have a quality flag 3 (two sources) or four rc
h
corresponding,respectively,toasecureredshiftwithtwoormore 2
0
emissionorabsorptionlinesandasecureredshiftwithtwoormore 2
2 SAMPLE SELECTION 3
emissionorabsorptionlineswithagood-quality,highS/Nspectrum
The high-redshift AGN sample used in this work has been se- (seeLillyetal.2007,2009forthoroughexplanationofqualityflags).
lectedfromtheC-COSMOSX-raycatalogue,combiningthespec- TunedphotometricredshiftsfortheC-COSMOSsourceshavebeen
troscopicandphotometricinformationavailablefromtheidentifi- computedandpresentedinSalvatoetal.(2011).Duetothelarge
cationcatalogueofX-rayC-COSMOSsources(Civanoetal.2011, numberofphotometricbandsandthesizeablespectroscopictrain-
2012)andtheChaMP(ChandraMulti-wavelengthProject)X-ray ing sample spanning a large range in redshift and luminosity the
catalogueusingonlythe323ChaMPobsidsoverlappingwithSloan estimatedphotometricredshiftsareexpectedtobequiterobustat
DigitalSkySurvey(SDSS;Richardsetal.2006)DR5imaging.In z>2.5evenatthefaintermagnitudes(i >22.5).TheCOSMOS
AB
Fig.1,weshowtheskycoverage(theareaofasurveythatissen- photometricredshiftsforX-ray-selectedsourceshaveanaccuracy
sitivetosourcesaboveagivenX-rayflux)usingtheobservedsoft of σ(cid:6)z/(1+zspec)=0.015 with a small fraction of outliers (<6 per
band(0.5–2keV)sourcedetectionsforthetwosurveys,andtheir cent),consideringthesampleasawholeati<22.5.Atfaintermag-
sum.Thiscorrespondsto2–8keVrestframeforz>3. nitudes,thedispersionincreasestoσ(cid:6)z/(1+zspec)=0.035with∼15
A schematic diagram of the sample selection with the detailed percentoutliers,stillremarkablygoodforanAGNsample.Forthe
numberofsourcesforeachstepispresentedinFig.2. z>3C-COSMOSsample,anaccuracyofσ(cid:6)z/(1+zspec)=0.014is
MNRAS445,1430–1448(2014)
1432 E.Kalfountzouetal.
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Figure2. Schematicflowdiagramofthehigh-zsampleselection.
achieved with only three catastrophic outliers (<9 per cent). The the small number of bands in which these objects are detected,
spectralenergydistributions(SEDs)ofthesourceswithphotomet- no photometric redshift is available for them. In X-ray-selected
ricredshiftlargerthan3havebeenvisuallyinspectedtogetherwith samples,non-detectionintheopticalbandhasbeenoftenassumed
the photometric fitting and the probability distribution of all the to be a proxy for high redshift (e.g. Koekemoer et al. 2004), or
possiblesolutions. forhighobscuration,oracombinationofboth.4ofthe15sources
Thereare91sourcesselectedinthe0.5–2keVband,14inthe have no detection in the soft band suggesting high obscuration,
2–10keV,and4inthe0.5–10keVbands.Thereare15C-COSMOS possiblycombinedwithhighredshift.Moredetailsaboutthesample
sourceswithoutacounterpartintheopticalbands,butwithaK-band selectioncanbefoundinCivanoetal.(2011)andarealsopresented
andIRAC (7),only IRAC(6)orno infrareddetection (2). Given inFig.2.
MNRAS445,1430–1448(2014)
ThelargestX-ray-selectedsampleofz >3AGNs 1433
2.2 TheChaMPsample
TheChaMPisawide-areanon-continuousX-raysurveybasedon
archivalX-rayimagesofthehighGalacticlatitude(|b|>20deg)sky
observedwithACISonChandra.Thefluxlevels(inergcm−2s−1)
reached in the survey are 9.4 × 10−16–5.9 × 10−11 in the full
(0.5–8keV),3.7×10−16–2.5×10−11(0.5–2keV)inthesoftand
1.7×10−15–6.7×10−11(2–8keV)inthehardband,respectively.
TheChaMPsurveyincludesatotalof392fields,omittingpointings
fromdedicatedserendipitoussurveyslikeC-COSMOS,theChan-
draDeepFields,aswellasfieldswithextended(>3arcmin)bright
opticalorX-raysources.ThelistofChandrapointingsavoidsany
overlapping observations by eliminating the observation with the
shorterexposuretime.Thesurveyhasdetectedatotalof>19000
X-ray sources (Kim et al. 2007; Green et al. 2009) over 33deg2 D
o
with∼15350X-raysourcespositionallymatchedtoSDSSoptical wn
counterparts(Greenetal.2009). Figure3. Opticaland/orinfraredChaMPsurveycompletenessasafunction loa
of0.5–2keVX-rayflux.ThereddottedlineindicatestheChaMPX-ray d
ThestudyoftheX-ray-detectedAGNpropertiesrequiresaccurate e
estimationofredshifts,luminositiesandsourceclassificationthus, fluxcutwith>75percentcompletenessfromSDSS/UKIDSS/WISE. d fro
goodqualityspectraor,whennotavailable,multibandphotometry. m
h
Hence for our X-ray analysis we chose only the 323 fields over- spatialresolutions,6.0arcsec(WISE;W1)and1–2arcsec(SDSS), ttp
lappingwithSDSSDR5imagingforwhichthesensitivitycurveis s
givenbyGreenetal.(2009),todetermineaccuratenumbercounts. weuse6arcsecasthematchingradiusforWISEcounterparts(Wu ://a
etal.2012). ca
Optical spectroscopy of ChaMP X-ray sources was described by d
Similarly, we searched the UKIDSS Large Area Survey (LAS; e
Trichas et al. (2012), where redshifts and classifications for a to- m
Lawrence et al. 2007) Data Release 10 for NIR counterparts to ic
tal of 1569 Chandra sources are presented. Since the ChaMP is .o
ChaMP X-ray sources. The photometric system is described in u
a Chandra archival survey, most ChaMP fields contain targeted p
Hewett et al. (2006), and the calibration is described in Hodgkin .c
sources selected by the target’s PI, and those targets are likely to o
etal.(2009).WeusedtheLASYJHKsourcetable,whichcontains m
bebiasedtowardsspecialX-raypopulationssuchasbrightAGN. /m
onlyfieldswithcoverageineveryfilterandmergesthedatafrom
Ofthetargetedsources∼90percenthaveasecurespectroscopic multipledetectionsofthesameobject.TheX-raysourcecatalogues nra
redshiftwith33ofthemhavingatz>3and29atz>4(seeTrichas werethenmatchedwithin3arcsecoftheX-raypositionseparately s/a
etal.2012).Thehighrateofhigh-redshift-detectedsourcesclearly to each UKIDSS band: Y (0.97–1.07µm), J (1.17–1.33µm), H rtic
showsthestrongselectionbiasesthatcouldaffectouranalysisif le
(1.49–1.78µm), and K (2.03–2.37µm) recovering also the areas /4
weincludedthetargetedsources.Therefore,weexcludealltargeted 4
withcoverageinasingleUKIDSSband.Theindividualbandlists 5
sources(153)toreducebiasinsamplepropertiesandsourcenumber /2
werethencombined.ForobjectsnotdetectedinaUKIDSSband, /1
counts. 4
we use the 5σ detection limits provided in Dye et al. (2006) of 3
ForSDSSpointsourceswithi<21andwithoutavailablespec- Y=20.23,J=19.52,H=18.73,andK=18.06.Matchingthe 0/1
troscopy,efficientphotometricselectionofquasarsispossibleusing ChaMPcataloguetoWISEandUKIDSS,wefind1103additional 392
anonparametricBayesianclassificationbasedonkerneldensityes- 7
WISE and/or UKIDSS counterparts which do not have a SDSS 9
timation as described in Richards et al. (2009). To select high-z 3
counterpart(thedetailednumbersarereportedinFig.2). b
candidateswithoutavailablespectroscopicorphotometricredshift, In summary, ∼70 per cent of the total ChaMP X-ray sample y g
SDSSdetectionisrequiredinatleasttheiandzbands,todetect u
have SDSS and UKIDSS/WISE photometry (9727 SDSS and/or es
Lymandropouts(e.g.Steideletal.1996). WISEand/orUKIDSSand1103WISEand/orUKIDSS).Thelimited t o
SearchingtheChaMPcatalogueforX-raysourceswithin4arcsec fractionofopticalmatchesshowshowopticalcounterpartsoffaint n 1
oftheopticalSDSSquasarcoordinate(95percentofthematched 0
X-raysourcesarefainterthantheSDSSmagnitudelimit(i=21.0). M
samplehasanX-ray/opticalpositiondifferenceoflessthan3arcsec; a
seeGreenetal.2009),yields9727uniquematches(∼63percentof SUDVS-eSxcqeusassacroslowuerrse,widiethntaifineedxtteonis<ion19fo.1rzfo>rs3peqcutarosasrcsoptoyib=yt2h0e.i2r rch 2
thetotalChaMPX-ray-selectedsample).Weadditionallysearched 0
usingugricolourcriteria(Richardsetal.2002). 2
forcross-matchesintheWide-fieldInfraredSurveyExplorer(WISE; 3
Based on the X-ray limits, the identification completeness of
Wrightetal.2010)andUKIRT(UKInfraredTelescope)Infrared
ChaMPX-raysourcesfallsrapidlyforobjectswithfainteroptical
DeepSkySurvey(UKIDSS;Warren,Hewett&Foltz2000;Hewett
counterparts.Fig.3showstheoptical(SDSSi-bandcounterparts)
etal.2006;Maddoxetal.2008).1
andinfrared(WISEandUKIDSScounterparts)completenessofthe
ForasourcetobeincludedintheWISEAllSkySourceCatalog
X-ray-selectedsampleasafunctionofthesoft(0.5–2keV)X-ray
(Wrightetal.2010),anSNR>5detectionwasrequiredforone
flux. This incompleteness can severely bias determination of the
of the four photometric bands, W1, W2, W3, or W4, with central
numbercountsandspacedensity,particularlyathighredshifts(e.g.
wavelengthsofroughly3.4,4.6,12,and22µm,andangularres-
Barger&Cowie2005).
olutionsof6.1,6.4,6.5,and12.0arcsec.Becauseofthedifferent
To address this issue, we set a relatively high X-ray flux limit
inChaMP,wherespectroscopiccompletenessishigher,andphoto-
1TheUKIDSSprojectisdefinedinLawrenceetal.(2007).UKIDSSusesthe metriccoverageallowsgoodphotometricredshifts.Weuseasoft
UKIRTWideFieldCamera(WFCAM;Casalietal.2007)andaphotometric flux limit for ChaMP at S0.5–2keV > 3 × 10−15ergs−1cm−2 as at
systemdescribedinHewettetal.(2006).Thepipelineprocessingandscience thesebrighterfluxesthecompletenessishigherthan≥75percent
archivearedescribedinHamblyetal.(2008). (see Fig. 3). The completeness fraction as a function of flux has
MNRAS445,1430–1448(2014)
1434 E.Kalfountzouetal.
been taken into account for the estimation of the number counts Table1. PhotometricredshiftreliabilitydefinedbyWuetal.
andcomovingspacedensity(seeSections4and5).Forsourcesnot (2012)andnumberofsourcesforChaMPsourceswithoutspec-
detected in the soft band, the 0.5–2 keV flux has been computed troscopicorSDSSphotometricredshifts.
byconvertingthe2–10keVfluxusing(cid:7) =1.8(seeSection3.2).
Oneofthemainadvantagesofourcompilationisthatwedonot Surveysa Reliability Nobjb Nobj−limc
(percent)
missthe fainthigh-redshift population, since this is recovered by
theC-COSMOSsurvey.Inthisway,theChaMPsampleisusedfor
W 955 359
the determination of the bright end of the luminosity function at U 27 19
highredshifts. U+W 67.4 19 16
S 70.4 637 367
S+W 77.2 733 526
2.2.1 Spectroscopicredshifts S+U 84.8 71 44
S+U+W 87.0 238 177
We compiled secure spectroscopic redshifts for a total of 1547 Total 2680 1508
sources. We have used 1056 sources (excluding target sources) D
aS=SDSS;W=WISE;U=UKIDSS. ow
from existing ChaMP spectroscopy (Trichas et al. 2012) for the bNumberofpoint-likeobjectsineachcombinationofsurveys. n
selectedChaMPfields.Additionalspectroscopicredshiftsaregiven cNumberofpoint-likeobjectsineachcombinationofsurveys loa
intheSDSS-III(N=91;Noterdaemeetal.2012)andSDSS-DR10 withS0.5–2keV>3×10−15ergs−1cm−2. ded
quasarcatalogues(N=145;Paˆrisetal.2014).Wealsosearched fro
theliteraturebycross-correlatingopticalpositionswiththeNASA betweensomeminimumandmaximumvalue,whichiscrucialfor m
h
ExtragalacticDatabase(NED),usinga2arcsecmatchradiuswhere dealingwithcatastrophicfailures.Theredshiftprobabilitydistribu- ttp
weTfhoeunhdig2h5-5remdsohrieftsospuerccetrsowsciothpiscpescatmropslceocpoicnsriesdtsshoifft.44 sources tniuomnbfoerrecaocuhntssoaunrdcecoismtaokveinnginsptoacaeccdoeunnstitfyo(rsteheeSeescttiimonatsio4naonfdt5h)e. s://ac
a
withz>3.AllofthesesourceshaveasoftbandX-raydetection,and AsforC-COSMOSselectionofhigh-zsources,wealsoincluded d
e
onlythreesourceslackahard-banddetection.Amongthem,there 13sourceshavingz +1σ >3andz <3.Thisadds m
aresevensourceswithz>4andonesourcewithz=6.016±0.005 another10objectstophtohtoemainzspahmotople,allofthpehmotodetectedinboth ic.o
u
(Jiangetal.2007).AllbutsixofthemhaveSDSSopticalspectra softandhardbands. p.c
withmeanS/N>4.5(noneofthemhasS/N<2.0)withatleasttwo o
m
broademissionlines(LyαandCIV)significantlydetected.Forfive /m
oftheremainingsources,redshiftshavebeenobtainedbyTrichas 2.2.3 High-zcandidateselectionandphotometric nra
et al. (2012) while for the source with the highest spectroscopic redshiftestimation s/a
r(e2d0s0h7i)f.tF(zor=306.s0o1u6rc)ewseofhtahveeCuhseadMtPhespeesctitmroastceopfriocmsamJiapnleg,ethtearle. For the remaining 7759 without a spectroscopic or photometric rticle
SDSSredshift,weselectedthehigh-redshiftAGNcandidatesusing /4
areavailablephotometricredshiftsderivedbyRichardsetal.(2009, 4
5
seeSection2.2.2)withanaccuracyofσ(cid:6)z/(1+zspec)=0.013andonly t(h∼e7ir0oppetriccaelnatn),d/doerspthiteeirbieninfrgariendclcuodleodurisn. SMDoSstSoDfRth6esceatsaolougrcuees, /2/1
onecatastrophicoutlier. 4
were rejected from Richards et al. (2009) selection criteria. The 30
remainingsourcescomefromlaterSDSSdatareleases. /13
9
2.2.2 SDSSphotometricredshifts Following the same morphological criteria as Richards et al. 27
(2009), a candidate is required to be unresolved in images taken 93
For the sources without spectroscopic redshifts, we derived pho- throughthetworedderfilters(e.g.gandrforz∼3selection).This by
tometric redshifts. The criteria used in SDSS DR6 have now minimizes contamination from low-z galaxies since even type-2 gu
been refined to include objects redder than (u − g) = 1.0 which AGNatz>3appearpointlike.However,weavoidusinganyfaint es
may well be high-z quasars. The resulting catalogue of ∼1 mil- fluxcutinordertoensurethatwedonotmissfainthigh-zcandidates t on
lionphotometricallyidentifiedquasarsandtheirphotometricred- sincenon-detectioncanimplyhigh-zdropouts.Werejectsources 10
shiftsfromSDSSDataRelease6(DR6)isdescribedinRichards withflagsindicatingthattheirphotometrymaybeproblematic(e.g. M
et al. (2009). Only point sources (type = 6) with i-band mag- blending of close pairs of objects, objects too close to the edge arc
h
nitudes between 14.5 and (de-reddened) 21.3 (psfmag i >14.5 of the frame, objects affected by a cosmic ray hit). Overall, we 2
andpsfmag i−extinction i <21.3; where psfmagarethepoint- reject5079non-pointlikesourcesorwithproblematicphotometry. 023
spread-functionmagnitudes).Theyestimatetheoverallefficiency Thisnumber(∼65percent)isingoodagreementwiththerejected
ofthecataloguetobebetterthan72percent,withsubsamples(e.g. numberofsourcesbyRichardsetal.(2009)usingthesamecriteria
X-ray-detected objects) being as efficient as 97 per cent. At the whichexplainthelackofaphotometricredshiftforthesesources.
faintlimitofthecataloguesomeadditionalgalaxycontamination Photometric redshift criteria must strike a quantifiable balance
isexpected. betweencompletenessandefficiency,i.e.aprobabilitycanbeas-
Thereare1611sourceswithSDSShigh-qualityphotometricred- signedbothtotheclassificationandtheredshift.UsingtheSDSS,
shifts and no spectroscopic redshifts in ChaMP (i.e. those with UKIDSS,andWISE2photometricdatacanhelpustoselectquasar
good ≥ 0.0, where good is the quality flag; 6 = most robust; candidatesmoreefficientlythanusingeachsurveyindividually(see
−6 = least robust; Richards et al. 2009). Among them there are Table1).Thephotometricredshiftreliability,definedbyWuetal.
14sourceswithz >3andonewithz >4,abovetheadopted
phot phot
ChaMPfluxlimit.Allofthesesourcesaredetectedinbothsoftand
hardband.TheSDSSphoto-zcodealsogivesaprobabilityofan 2WeusethecoloursrelatedtoWISEW3andW4magnitudesonlyforsources
objectbeinginagivenredshiftrange.Inthisway,wehavenotonly lackingSDSSand/orUKIDSSdetectionsbecauseWISEuncertaintiesare
themostlikelyredshiftbutalsotheprobabilitythattheredshiftis substantiallylarger(Wu,Zhang&Zhou2004).
MNRAS445,1430–1448(2014)
ThelargestX-ray-selectedsampleofz >3AGNs 1435
(2012)asthefractionofthesourceswiththedifferencebetweenthe suggesting that even in the case of type-2 AGNs the photometric
photometricandspectroscopicredshiftssmallerthan0.2isgivenin redshiftsarereliablyestimated.Overall,wefoundeightsourceswith
Table1.ThehighestreliabilitycanbereachedonlyintheUKIDSS z>3atgreaterthan1σ significance,foursourceswithz>3but
surveyedarea,whichismuchsmaller(4000deg2)thantheskycov- lowerthan1σsignificance,andtwosourceswithz +1σ >3.
phot phot
erageofbothSDSSandWISEsurveys.
Richardsetal.(2002)useda3Dmulticolourspacetoselecthigh-
2.2.4 TheChaMPhigh-zsample
redshift QSO candidates in SDSS: griz (g − r, r − i, i − z) for
candidates with z > 3.0. Following the SDSS group, we search The total z > 3 ChaMP sample includes 87 sources with z > 3.
forhigh-zcandidatesinthreeredshiftintervals(z(cid:5)3.0–3.5,z(cid:5) Among them there are 44 sources with secure spectroscopic red-
3.5–4.5, z (cid:5) 4.5). The details of the selection criteria are given shift,15sourceswithSDSSz >3and13sourceswithSDSS
phot
intheappendix.Ourselectioncriteriarequirethatoursourceslie z + 1σ > 3 available from Richards et al. (2009), and 15
phot phot
outsideofa2σ regionsurroundingthestellarlocus.Westillexpect sources with estimated photometric redshifts based on optical/
thesampletobecontaminatedbystarsandlow-zgalaxies.Forthis infrared colour–redshift relations (13 with z > 3 and 2 with
phot
reason,weusesomeadditionalcriteriadescribedbyRichardsetal. zphot+1σphot>3). D
(2002) to exclude objects in colour regions containing predomi- ow
nantly white dwarfs, A stars and unresolved red–blue star pairs. 3 THE C-COSMOS AND CHAMP nloa
Duringthecolourselectionprocess,nospecificlineisdrawnbe- z > 3 AGN SAMPLE de
tween optically selected quasars (type-1 AGN) and type-2 AGN. d
Taking into account that both type-1 and type-2 AGNs are unre- Insummary,wehaveassembledasampleofX-ray-selectedAGNs fro
m
solved in optical images at z > 3 and type-2 AGNs should lie at z > 3 in the C-COSMOS and ChaMP on the basis of both h
outsidethestellarlocusduetotheirredopticalcolours,weexpect spectroscopicandphotometricredshifts.Thetotalsampleincludes ttp
s
that the above criteria efficiently select both high-redshift AGN 209sourceswithz>3.Ofthese,45areselectedtobeatz>3from ://a
populations.Wefound53SDSS-detectedhigh-zcandidates. theirbroadP(z).Therearealso15C-COSMOSsourcesconsidered ca
d
To increase the reliability of the photometric estimation, we to be at z > 3 on the basis of their optical non-detection these e
m
also combine the SDSS selection with the redder baselines from are included only in the derivation of the upper boundary of the ic
.o
UKIDSS and WISE, where the contamination of the stellar locus logN–logScurve.Thepropertiesofthesamplemembersaregiven u
p
and low-redshift galaxies is lower. We used the combination of in Table A1 (Appendix A)and the detailed numbers are given in .c
o
UKIDSS and SDSS colours in the Y − K versus g − z colour– Fig.2.Fig.4showstheopticalandnear-infrared(i,K,and3.6µm) m
/m
colordiagramsuggestedbyWu&Jia(2010)toefficientlyseparate observedmagnitudedistributionsforthetotalhigh-zpopulationand
n
quasarswithredshiftz<4fromstars.Similarly,Wuetal.(2012) forsourceswithspectroscopicandphotometricredshifts,separately. ra
s
suggestedthatz−W1andg−zcolourscouldbeusedtoseparate Sourcesselectedasi-dropoutsarealsopresented. /a
stars from quasars. Based on these criteria, we have rejected 10 Thehard(2–10keVrest-frame)X-rayluminosityversusplaneis rtic
le
sources associated with stars based on both SDSS–UKIDSS and showninFig.5togetherwiththefluxlimitoftheC-COSMOSand /4
4
SDSS–WISEcolour–colordiagrams.Forsourcesdetectedonlyby ChaMPsurveys(dashedline)andtheappliedfluxcutforChaMP 5
/2
UKIDSS, we used the i = 21.3 upper limit and a Y − K versus (dottedline).Luminositieswerecomputedfromineverycaseas- /1
4
i − Y colour–color diagram to separate stars and low-z galaxies suminganintrinsic(cid:7)=1.8. 3
0
from high-z candidates. We found four high-z candidates. In the The C-COSMOS and ChaMP high-z sample is a factor of 4– /1
3
case of sources detected only by WISE, there is no efficient way 5 larger than all the previous individual X-ray-selected samples 92
7
detailedintheliteraturetoseparatehigh-zquasarsfromstars. at z > 3 (e.g. Brusa et al. 2009; Hiroi et al. 2012; Vito et al. 9
3
Photometricredshiftshavebeenestimatedforthehigh-zcandi- 2013).Mostimportantly,thisisthefirsttimethatasignificantsam- b
y
datesbycomparingtheobservedcolourswiththeoreticalcolour– ple of 29 X-ray-selected AGNs at z > 4 is assembled. At these g
u
redshift relations derived from samples with known redshifts redshifts previous studies had a maximum of nine sources. The es
(Richards et al. 2002; Wu & Jia 2010; Wu et al. 2012). A stan- z > 3 X-ray-selected AGN sample also covers more than a fac- t o
n
dardχ2minimizationmethodisusedtoestimatethemostprobable tor of 2 of soft (2–10 keV rest-frame) X-ray luminosity, and in- 1
0
photometric redshifts. Here, the χ2 is defined as (see Wu et al. cludesasignificantnumberofbothbroad-lineandnon-broad-line M
a
2004) AGNs. rc
h
χ2=(cid:2) [(mi,cz−mj,cz)−(mi,ob−mj,ob)]2, (1) ourTosadmispcluestsothbee colbasscsuifireeddAasGoNbsfcruarcetdionorrueqnuoibrsecsuereadch. Tohbejreectairne 2023
σ2 +σ2 twocommonlyadoptedmethodsforclassification:oneisbasedon
ij mi,ob mj,ob
the optical emission line widths (‘optical type’) or, if a spectrum
wherethesumisobtainedforallfourSDSScoloursand/orWISE isnotavailable,bythetypeoftemplatethatbestfitstheoptical–
and/orUKIDSScolours,m −m isthecolourinthecolour– infrared SEDs of the sources. The other is based on the column
i,cz j,cz
redshiftrelations,m −m istheobservedcolourofaquasar, densities, N , in the X-ray spectra (‘X-ray type’) or, if an X-ray
i,ob j,ob H
andσ andσ aretheuncertaintiesofobservedmagnitudes spectrumisunavailable,bythehardnessratio(HR;e.g.Hasinger
mi,ob mj,ob
intwobands.Theuncertaintyinthemeasurementwasobtainedby etal.2001).X-rayabsorptionshouldtypicallycorrelatewithoptical
mappingthe(cid:6)χ2 error.Sincetheabovestudiesaredominatedby AGN type. In the unified scheme (e.g. Lawrence & Elvis 1982;
optically selected quasars, we would expect that the photometric Antonucci1993;Urry&Padovani1995)asthenarrowemissionline
redshiftsuncertaintiesintype-1AGNsaresmaller.However,since AGNsareviewedthroughthedustytorus,andhencehavehigher
the Lyα break enters the g band at z ∼ 3.5, the g − r colours absorption column densities than broad emission line AGNs. In
quicklyreddenwithredshiftforbothpopulations.Alexandroffetal. fact,evidencehasbeenmountingovertheyearsthattheoptical-and
(2013)foundthatg−rcoloursareindistinguishableatan84per X-ray-basedclassificationsoftengivecontrastingresults(Lawrence
centconfidencelevelbetweentype-1andtype-2quasarsatz>2 &Elvis2010;Lanzuisietal.2013;Merlonietal.2014).
MNRAS445,1430–1448(2014)
1436 E.Kalfountzouetal.
D
o
w
n
lo
a
d
e
Fpliagnuerefo5r.thTehoebhjeacrtdsXin-roauyrlsuammipnloes.itBylu(ecosmqupaurteesd=wiCth-C(cid:7)O=SM1O.8S)sreadmsphlieft. d fro
m
Redcircles=ChaMPsample.Filled=spectroscopicredshift.Open=pho- h
tometric redshift. The dashed lines represent the 2–10 keV luminosity ttp
s
limit of the surveys computed from the 0.5–2 keV limiting flux. The ://a
dotted red line represents the completeness flux cut we have adopted at c
3×10−15ergcm−2s−1.Thedottedblacklinescorrespondtothefluxlimits ad
e
weimposedforthecomputationofthespacedensityandtheirassociated m
ic
areas,purple(43.4<logLX<44.0),green(44.0<logLX<44.7),orange .o
(logLX>44.7). up
.c
o
m
numbers of broad-line and non-broad-line AGNs are found. The
/m
classification for the 75 AGNs in C-COSMOS with photometric n
redshifts is obtained by the Salvato et al. (2011) photometric fit- ras
t(iAnrgnmouettsho&dIfilbtteinrtg2t0h1e1S)E.3DMvoiareχd2emtaiinlsimonizathtieonfitwtiintghccoandebLeEfPoHuAnRdE /article
in Salvato et al. (2011). Briefly, two libraries of templates were /44
5
used,dependingonmorphology,opticalvariability,andX-rayflux /2
ofthesource.Thefirstlibrary(definedinSalvatoetal.2009,table /1
4
2)consistsofAGNtemplates,hybrid(host+AGN)templates,and 30
/1
a few normal galaxies and was used for all the point-like optical 3
9
sources and for the extended sources with an X-ray flux brighter 2
7
than8×10−15ergcm−2s−1.Thesecondlibrary(asdefinedinIl- 93
bertetal.2009)includesonlynormalgalaxytemplatesanditwas by
usedfortheremainingsources(i.e.extendedandwithX-rayflux gu
<8×10−15ergcm−2s−1).Theflowchartinfig.6ofSalvatoetal. es
t o
(2011)summarizestheprocedure.Civanoetal.(2012),according n
Fanigdu3r.e64µ.mObbasnerdve(fdroAmBtmopagtnoitbuodtetodmis)trhibiguhti-oznoobfjeaclltst.heBlia-bckansdo,lKid-,bbanlude, tuonothbisscfiurtteidngA,GdiNvi.deAtbhoeusto4u0rcpeesrincteontob(2sc8urseodurAceGsN),ogfatlhaexipeshoatnod- 10 M
dot–dashed,reddashed,andgreensolidlinesrepresentthetotal,spectro- a
metric sample is best fitted with an unobscured quasar template, rc
scopic,photometricredshift,andi-dropoutsamples,respectively.Thei-band h
dropoutsarenotincludedinthei-bandhistogram. and 47 sources with an obscured quasar template. For 29 AGNs 20
withspectroscopicidentification,thephotometricandspectroscopic 23
typesmatch.Giventhemismatchrateof∼9percent,weestimate
3.1 Opticaltypes that∼7outofthe75AGNscouldhavebeenassignedthewrong
SEDclassification.
The optical type of the sources has been determined by the IntheChaMPz>3spectroscopicsample,asexpectedatthese
measured full width at half-maximum (FWHM) of the permitted fluxes(e.g.Brusaetal.2009),andduetothepredominantlySDSS
emissionlines.ThoseobjectswithemissionlineshavingFWHM spectroscopictargetselection,only2/44sourcesarenon-BLAGN.
>1000kms−1 (e.g.Stern&Laor2012)areclassifiedas‘optical The characterization of these sources based on their SED fittings
broad-line’ (BLAGN), and all others as ‘optical non-broad-line’ hasbeenobtainedbyTrichasetal.(2012).Inordertobeinagree-
(non-BLAGN),i.e.theyshownarrowemissionlinesorabsorption mentwiththespectroscopicChaMPsample,wefollowedthesame
linesonly,followingCivanoetal.(2011,2012). SEDfittingmethodforthecharacterizationofthe43sourceswith-
IntheC-COSMOSspectroscopicz>3sample,21of32sources out a spectroscopic classification. According to this fitting, 11 of
are classified as BLAGN. These are mainly associated with the
brighter optical sources (i ∼ 22–23) of the spectroscopic sam-
AB
ple (see Fig. 7). At fainter optical magnitudes (iAB > 23), equal 3http://www.cfht.hawaii.edu/arnouts/LEPHARE/lephare.html
MNRAS445,1430–1448(2014)
ThelargestX-ray-selectedsampleofz >3AGNs 1437
43sourcesarebestfittedwithanobscuredquasartemplate(non-
BLAGN).MoredetailsonthefittingcanbefoundinTrichasetal.
(2012)andRuizetal.(2010).Briefly,atotalof16templateshas
been used including QSO, Seyfert-2 galaxies, starburst galaxies,
absorptionlinegalaxiesandcompositetemplatesthatareknownto
harbourbothanAGNandastarburst.TheRuizetal.(2010)model
has been adopted, which fits all SEDs using a χ2 minimization
techniquewithinthefittingtoolSHERPA(Freeman,Doe&Siemigi-
nowska2001).Thefittingallowsfortwoadditivecomponents,one
associated with the AGN emission and the other associated with
thestarburstemission.Thefitwiththelowestreducedχ2hasbeen
chosenasthebest-fittingmodel.
Ageneralproblemofrelyingontheopticaltypeisthattheclassi-
ficationmaydependonthequalityoftheavailableopticalspectra,
D
sincegoodsignal-to-noiseratioisrequiredtodetectless-luminous ow
broad-emissionlinesabovethestellarcontinuumemissionofhost nlo
a
galaxies.Also,atz>3,theHα emissionlinemovesintothein- d
e
fAraGrNedtaynpdesso(1,.u8n,t1il.9r;ecOesnttelryb,rwocaks&difKficouslktit1o9o7b6s)erravpe.idIlnytelromseedthiaetier Fcliegsu=reC6h.aMHRPvsaemrsupsler.eFdislhleidft.=Bslupeecstqruoascreosp=icCre-dCsOhiSftM.OOpSensa=mpphleo.tRomedectriirc- d from
broadHβ emission,andwithoutHαthesemaybemisclassifiedas redshift.Sourceswithnohardbandorsoftbanddetectionareshownwith h
type-2.Nevertheless,sucheffectsarenotexpectedtobesignificant arrows.FourcurvesofconstantNH(1020,1022,5×1022,and1023cm−2) ttps
inoursample,astconsistspredominantlyofluminousAGNsfor arereportedfor(cid:7)=1.8(dashedlines)and(cid:7)=1.4(solidlines). ://a
whichcontaminationfromthehostgalaxiesisnegligible. ca
d
tobemade.UsingtheCIAO4spectralanalysispackage,SHERPA,5we em
have simulated X-ray spectra for AGN populations at 3 < z < 7 ic
.o
in order to quantify the evolution of X-ray spectral slopes due u
3.2 X-raytypes p
to the k-correction of the observed AGN spectra towards high-z. .c
o
Mostsourcesinoursamplehavealownumberofdetectedcounts Based on these simulations, we find that the HR distribution for m
(median∼25inthe0.5–8.0keVfullband).Inthiscountregime, theChaMPsamplepeaksat(cid:7)∼1.8–2.0whiletheHRdistribution /mn
spectralfitresultsarenotreliable,especiallyifmorethanonefree forC-COSMOSsamplepeaksat(cid:7)∼1.4–1.9.Hereafter,tobetter ras
pthaerapmaeratemreitserfisttaerde;leavrgeen.iFfotrhethfiestecroenavseorngse,swtheeuusenctehretaBinatyieessioann cwointhstrparienvitohuescsotuludmiens,dwenesfiitxyedantdhefoprhtohteonpuinrdpeoxsetoof(cid:7)co=m1p.a8riasnodn /article
EstimationofHardnessRatios(BEHR)method(Parketal.2006) convertedallsourcefluxes. /4
4
toderiveX-rayspectraltype.Hardnesscountratios(HR),defined Inthiswork,weadoptNH=1022cm−2asthedividingcriterion; 5/2
asHR=(CHB −CSB)/(CHB +CSB),whereCSB andCHB arethe AGNswithNH<1022cm−2and>1022cm−2areclassifiedasX-ray /14
countsinthesoftbandandhardband,respectively. unobscuredandobscured,respectively.Thiscriterionisadoptedby 30
BEHRisparticularlypowerfulinthelow-countPoissonregime, manyauthors,andisknowntobegenerallyingoodagreementwith /13
becauseitcomputesarealisticuncertaintyfortheHR,regardlessof theopticaltype(seee.g.Uedaetal.2003;Hiroietal.2012). 92
7
whethertheX-raysourceisdetectedinbothenergybands.Sources 9
3
with unconstrained upper or lower limits due to non-detections b
3.3 X-ray/opticalfluxratio y
(14 hard-only and 49 soft-only detections) have been computed g
u
byconvertingthe3σ fluxupperlimitintheundetectedbandinto TheX-ray/optical(X/O)fluxratioisaredshiftdependentquantity es
counts. for obscured AGN, given that the k-correction is negative in the t o
n
Toestimatethecolumndensity,curvesofconstantN asafunc- opticalbandandpositivefortheX-rays(Comastrietal.2003;Fiore 1
H 0
tion of redshift have been derived for two spectral slope values, etal.2003;Brusaetal.2010).Asaresult,obscuredsourceshave M
(cid:7) = 1.4 and (cid:7) = 1.8. The flatter spectral slope has been chosen higherX/Oathighredshift.Ontheotherhand,unobscuredsources arc
to be consistent with the assumptions adopted in producing the havesimilark-correctionsinthetwobands,andthedistributionin h 2
original X-ray catalogues (Kim et al. 2007; Puccetti et al. 2009). X/Oisnotcorrelatedwiththeredshift(Civanoetal.2012).Usually, 02
3
Thesteepervalueismorerepresentativeoftheintrinsicvalueifthe ther-ori-bandfluxisused(e.g.Brandt&Hasinger2005)while
spectrumisnotaffectedbyobscuration(Nandra&Pounds1994). a soft X-ray flux was used originally used for this relation with
TherelationshipbetweenHRandredshiftofourC-COSMOSand themajorityofluminousspectroscopicallyidentifiedAGNsinthe
ChaMPAGNsamplesisshowninFig.6.CurvesofN =1020,1022, EinsteinandASCAsurveys characterized byX/O=0±1(e.g.
H
5×1022,and1023cm−2arereportedfor(cid:7)=1.8(dashedlines)and Stockeetal.1991;Schmidtetal.1998;Lehmannetal.2001).The
(cid:7)=1.4(solidlines).WeobservethatC-COSMOSsampletendto samerelationhasbeenusedalsointhehardband,withoutreally
bemoreobscuredasexpectedduetothefainterX-raysensitivity accountingfortheX-raybandusedorthechangeinspectralslope
limit,thantheChaMPsample(Lawrence&Elvis1982;Uedaetal. (e.g. Alexander et al. 2001; Brusa et al. 2003; Fiore et al. 2003;
2003;Hasinger2008;Brusaetal.2010;Burlonetal.2011). Civano,Comastri&Brusa2005;Lairdetal.2009;Xueetal.2011).
Though the two samples (C-COSMOS and ChaMP) of z > 3 Fig.7showsthedistributionofX-raysoft(left)andhard(right)
AGNsshowdifferenttrendsregardingtheirobscuration,thelarge flux versus optical magnitude to illustrate the parameter space
HR errors and the similarity in this redshift range of the curves
withwidelydifferentNHvaluesforthesamespectralslope,donot 4http://cxc.harvard.edu/ciao/
allowanaccurateestimateofthecolumndensityforeachsource 5http://cxc.harvard.edu/sherpa/
MNRAS445,1430–1448(2014)
1438 E.Kalfountzouetal.
D
o
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lo
a
Figure7. X-rayflux(soft-left,hard-right)versusthei-bandmagnitudeforalltheX-raysourceswithani-bandcounterpart.Thegreyshadedregionrepresents d
e
tphheotloomcuestroicfrAedGsNhisftabloynogpethnesycmorbreollast.ioOnraXng/eOc=ircl0es±an1d.bSloaucrkcseqsuwariethsrseepcruerseenstpnecotnr-oBscLoApGicNreadnsdhiBftLsAaGreNr,erpersepseecnttievdelbyy.Gfirleleednusypmpebrollismaitnsdresporuersceenstwi-bitahnda d fro
m
dropoutsandblackleftpointingarrowsrepresentsoftandhardX-rayfluxupperlimitsforundetectedsourcesineachband.TheC-COSMOSsampleis h
representedbytheopenbigbluecircles. ttp
s
://a
spannedbythebroad-lineandnon-broad-linepopulations.TheX/O scheme, we have separated our total sample into broad-line and c
a
d
ratio(Maccacaroetal.1988)isdefinedas non-broad-lineAGNs(asdescribedinSection3.1). e
m
X/O=log10(fX/fopt)=log10(fX)+C+mopt/2.5, (2) (NIn=th1e02c2acsme−o2f)tghieveBsL28A/1G2N4sX,-trhaeyXob-rsacyurceldassosiufirccaetsiofonrc(cid:7)ri=ter1io.8n. ic.o
H u
p
wherefXistheX-rayfluxinagivenenergyrange,moptisthemag- Halfofthesesourceshaveaspectroscopicredshiftandallbutthree .co
nitudeatthechosenopticalwavelength,andCisaconstantwhich come from C-COSMOS sample. If we also take into account the m
dependsonthespecificfilterusedintheopticalobservations.For HRerrors,thenforthelowerHRlimits,11BLAGNsareclassified /m
n
both X-ray bands, the X/O = ±1 locus (grey area) has been de- as X-ray obscured sources and 41 are classified X-ray obscured ra
s
fined using as C(i) = 5.91 (Civano et al. 2012), which was com- sources for the upper HR limits. In the non-BLAGN subset, the /a
putedtakingintoaccountthewidthofthei-bandfiltersinSubaru, above criterion gives 49/71 X-ray obscured sources (detected in rtic
CFHT (Canada–France–Hawaii Telescope), or for bright sources bothsoftandhardbands)for(cid:7)=1.8.The27softbandsourcesin le/4
4
SDSS.Inthehardband,thelocusisplottedtakingintoaccountthe non-BLAGNsamplewithnodetectioninthehardband(reportedas 5
/2
bandwidthandthespectralslopeusedtocomputetheX-rayfluxes downwardarrowsinFig.6)haveveryhighupperlimitsontheHR, /1
((cid:7) =1.8).ThemajorityofBLAGNswithasecurespectroscopic duetotheconservativefluxupperlimitcomputedbyPuccettietal. 43
0
redshift,followthetrendof−1<log10(fX/fi)<1.However,given (2009),butmostofthemdonottherebysatisfytheNH>1022cm−2 /13
thevariationinαOXwithluminosity(e.g.Vignali,Brandt&Schnei- criterion. 92
der2003;Young,Elvis&Risaliti2010;Trichasetal.2013),there Forthetotalsample,wefindagreementbetweentheopticaland 79
3
canbesomeshiftinthelocationsofQSOswithluminositywithin X-rayclassificationfor∼74percent:∼77percentfortheBLAGNs b
theso-calledBLAGNregion.ThisshiftisconsistentwiththeX/O and∼69percentforthenon-BLAGNs.Theseratesareconsistent y g
u
relationbeingoriginallycalibratedonsoft-X-ray-selectedsources, withrecentstudies(e.g.Lanzuisietal.2013;Merlonietal.2014). es
brightintheopticalandalsointheX-rays.Thismightexplainthe Possibleexplanationcanbeamisclassificationoffainttype-1with t o
n
mildshiftbetweentheChaMPandC-COSMOSBLAGNs. strongoptical/IRcontaminationfromhostgalaxylight. 1
0
Apart from the AGN population found in the BLAGN region, To improve the statistics and gain information on the average M
there is also a significant population that lie at log10(fX/fi) > 1 propertiesofthetwosubclasses,wecomparedtheirmeanHRval- arc
suggestingobscurednuclei.Themaincharacteristicsofthissample ues as a function of redshift (Fig. 8). Despite the ∼30 per cent h 2
areasfollows:(1)lackofspectroscopicredshifts(opensymbols), misclassificationfortheindividualsources(seeTable2),themean 02
3
(2)non-BLAGNopticalclassification(greensymbols),and(3)low propertiesoftheBLAGNsandnon-BLAGNsseemtoagreewith
X-ray luminosities (1043ergs−1 < L2–10keV < 1044ergs−1) with theNH ∼1022cm−2 division.Theseresultsdoesnotchangeeven
N > 1022cm−2 for ∼65 per cent of them, which is consistent ifweuseonlysourceswithspectroscopicredshifts.Theupperand
H
withpreviousstudiesfindingthatmildobscurationiscommonat lowerlimitsdetectedonlyinthesoftorthehardbandwereused
theseluminosities(e.g.Silvermanetal.2010).Furthermore,nearly tocomputetheupperandlowerboundaryoftheshadedarea.We
75 per cent of all the sources with X/O > 1 are obscured, thus discusstheresultsinSection5.
confirmingthatselectionsbasedonhighX/Oratioareefficientin
findingsamplesofobscuredAGNs.
4 THE logN−logS OF THE z > 3 AGN
We derived the soft band number counts of the z > 3 and z > 4
3.4 ComparisonofopticalandX-raytypes
samplesbyfoldingtheobservedfluxdistributionthroughthesky
X-rayabsorptionisanalternativegoodindicatorofAGNtype.In coverageareaversusfluxcurveoftheC-COSMOSsurvey(Puccetti
ordertocompareouropticalclassificationtotheexpectedobscu- et al. 2009) and the ChaMP’s 323 fields (Green et al. 2009).
rationofBLAGNsandnon-broad-lineAGNsbasedontheunified Additionally, we have corrected the number counts for ChaMP
MNRAS445,1430–1448(2014)
ThelargestX-ray-selectedsampleofz >3AGNs 1439
D
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Figure8. ThemeanHRasafunctionofredshiftforBLAGN(blacksquares) m
andnon-BLAGN(orangecircles)z>3AGNsubsamples.Theerrorbars http
representthe68percentdispersion.Onlysourceswithbothsoftandhard s
banddetectionsaretakenintoaccountfortheestimationofthemeanHR ://a
c
ineachbin.Undetectedsourcesinoneofthesebandsareincludedonlyfor a
d
estimationoftheupperandlowerlimits(dashedareas).Thezphot =6.88 em
isnouorrcdeerwtiothbaeninucplpuedreldiminitthHeRfiugpu=re0..74hasbeenshifteddowntoHR=0.2 ic.ou
p
.c
o
Table2. ComparisonofopticalandX-raytypes.The m
upperandlowerlimitshavebeenestimatedtakinginto /m
n
accountonlytheerrorrangesinHR. ra
s
/a
Number Unobscured Obscured rtic
ofsources NH<1022cm−2 NH≥1022cm−2 le/4
4
NoBnL-BALGANGN 9262+−+−1119643 2489+−+−1191463 5/2/143
0
/1
3
9
incompleteness in the spectroscopic/photometric coverage as a 2
7
9
functionofX-rayflux(i.e.Fig.3). 3
b
To minimize the error associated with the most uncertain part y
g
ofthesensitivitycurve,wetruncatetheC-COSMOSsampleatthe u
e
flux corresponding to 10 per cent of the total area (blue dashed s
t o
line in Fig. 1). All the sources with a 0.5–2 keV flux above n
3 × 10−16ergcm−2s−1 have been considered (73 objects out of Figure9. Top:thebinnedlogN–logSrelation(withassociatederrors)of 10
thez>3(orangecircles)andz>4(bluesquares)QSOspopulation.The M
the81softbanddetected).Thefluxlimitappliedtothesampleis
consistentwiththesignal-to-noiseratiothresholdschosenbyPuc- grey shaded area represents the maximum and minimum number counts arc
undertheassumptionsdescribedinSection4.Theblueandorangecurves h
cetti et al. (2009), on the basis of extensive simulations, to avoid correspondtothepredictionbasedontheLDDE+exp(thicksolid),LDDE 20
theEddingtonbiasinthecomputationofthenumbercountsofthe (dotted),LADE(dashed),andCTAGNs(dashcross)modelsforeachred- 23
entireC-COSMOSsample.Thus,byapplyingafluxlimitcut,we shiftrange,respectively.Thesmallopencirclesrepresentsthenumbercounts
alsoreduce the Eddington bias affecting our sample. For ChaMP estimatedbyBrusaetal.(2009),thesmallopensquaresarefromCivano
thiswouldbeatS0.5−2keV >2×10−15ergcm−2s−1,belowtheflux etal.(2011),andsmallfilledtrianglesfromVitoetal.(2013).Middle:the
limitalreadyapplied. ratiooftheobservednumbercountsforz>3relativetotheLDDE+exp
ThebinnedlogN–logSrelationsfortworedshiftranges(z>3; model (thick solid line at the top panel). The thick solid line represents
orange points and z > 4; blue points, with associated errors) are theLDDE+expmodelN/Nexp =1,thedottedlinerepresentstheLDDE
relativetotheLDDE+expmodel,thedashedlinestheLADErelativetothe
plottedinFig.9(toppanel).Inintegralform,thecumulativesource
LDDE+expmodel,andthedashcrosslinestheCTAGNsmodelrelative
distributionisrepresentedby totheLDDE+expmodel.Bottom:theratiooftheobservednumbercounts
N(>S)=(cid:2)NS 1 , (3) mfoerdziu>mp4arneella.tiTvheetroattihoeoLfDthDeEL+DeDxEprmeloadtievle.tSoytmheboLlDsDarEe+siemxpilamrotdoetlhies
i=1 (cid:3)i N/Nexp>8.0andisnotpresented.
where N(>S) is the number of sources with a flux greater than S
and(cid:3),isthelimitingskycoverageassociatedwiththeithsource.
i
MNRAS445,1430–1448(2014)