Revisiting the relationship between 6 μm and 2–10 keV continuum luminosities of AGN

19 Pages·2015·0.91 MB·English

MNRAS449,1422–1440(2015) doi:10.1093/mnras/stv299
µ
Revisiting the relationship between 6 m and 2–10 keV continuum
luminosities of AGN
S. Mateos,1‹ F. J. Carrera,1 A. Alonso-Herrero,1† E. Rovilos,2 A. Herna´n-Caballero,1
X. Barcons,1 A. Blain,3 A. Caccianiga,4 R. Della Ceca4 and P. Severgnini4
1InstitutodeF´ısicadeCantabria(CSIC-UniversidaddeCantabria),E-39005Santander,Spain
2InstituteforAstronomy,Astrophysics,SpaceApplications&RemoteSensing,NationalObservatoryofAthens,15236PalaiaPenteli,Greece
3PhysicsandAstronomy,UniversityofLeicester,UniversityRoad,LeicesterLE17RH,UK
4INAF–OsservatorioAstronomicodiBrera,viaBrera28,I-20121Milano,Italy
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Accepted2015February11.Received2015February10;inoriginalform2014October2 ad
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ABSTRACT h
WehavedeterminedtherelationbetweentheAGNluminositiesatrest-frame6µmassociated ttp
s
withthedustytorusemissionandat2–10keVenergiesusingacomplete,X-ray-flux-limited ://a
c
sample of 232 AGN drawn from the Bright Ultra-hard XMM-Newton Survey. The objects ad
e
have intrinsic X-ray luminosities between 1042 and 1046ergs−1 and redshifts from 0.05 to m
ic
2.8.Therest-frame6µmluminositieswerecomputedusingdatafromtheWide-fieldInfrared .o
u
Survey Explorer and are based on a spectral energy distribution decomposition into AGN p.c
o
andgalaxyemission.Thebest-fittingrelationshipforthefullsampleisconsistentwithbeing m
lliunmeainr,oLsi6tyµmra∝tioL02i.−s991l±0ak0re.g0V3e,lywiitnhdienpterinndseicntscoaftttehre,(cid:2)linloeg-oLf-6siµgmht∼X0-.r3a5ydaebxs.oTrphteioLn6.µAms/sLu2m−i1n0gkeVa /mnras
/a
constantX-raybolometriccorrection,thefractionofAGNbolometricluminosityreprocessed rtic
inthemid-IRdecreasesweakly,ifatall,withtheAGNluminosity,afindingatoddswithsimple le
/4
recedingtorusmodels.Type2AGNhavereddermid-IRcontinuaatrest-framewavelengths 4
9
<12 µm and are overall ∼1.3–2 times fainter at 6 µm than type 1 AGN at a given X-ray /2/1
4
luminosity.Regardlessofwhethertype1andtype2AGNhavethesameordifferentnuclear 2
2
dustytoroidalstructures,ourresultsimplythattheAGNemissionatrest-frame6µmisnot /1
0
6
isotropicduetoself-absorptioninthedustytorus,aspredictedbyAGNtorusmodels.Thus, 8
2
AGNsurveysatrest-frame∼6µmaresubjecttomodestdustobscurationbiases. 93
b
y
Keywords: galaxies:active–quasars:supermassiveblackholes–infrared:galaxies–X-rays: g
u
e
galaxies. s
t o
n
2
5
F
conductedusingdatafromSpitzer’sInfraredArrayCamera(IRAC) e
1 INTRODUCTION haveidentifiedapopulationofhighlyobscuredAGNwhosenumber bru
a
Inthestandardunifiedmodelofactivegalacticnuclei(AGN),the densityiscomparabletothatobservedforunobscuredAGN.Many ry
2
observeddifferencesbetweenAGNsubclassesaresolelyexplained oftheseobjectsmightbehiddenatopticalandX-raywavelengths 0
2
astheresultofanorientationeffectwithrespecttoanopticallyand by Compton-thick material (line-of-sight rest-frame gas column 3
geometricallythickdistributionofmoleculargasanddustontens densitiesN >1.5×1024cm−2;Mart´ınez-Sansigreetal.2005).
H
ofparsecsscales,frequentlyassumedtohaveatoroidalgeometry Uncovering the mid-IR thermal continuum associated with the
(Antonucci1993;Urry&Padovani1995).Thismaterial,hereafter AGNdustytorusemissioncanbeadifficulttask,duetocontam-
referredto as the dusty torus,absorbs the AGN bolometric lumi- ination from the AGN host galaxies. High (subarcsecond) spatial
nosityoutputproducedatrest-frameUV/opticalandX-raywave- resolution observations can isolate the AGN emission from other
lengths in the accretion disc/corona and re-emits it mainly in the contaminants on nearby objects with favourable geometries. Al-
mid-infrared(mid-IR)regime(rest-framewavelengths∼5–30µm). ternatively,mid-IRspectraldecompositioncanprovidereliablees-
Thus,itisclearthatthemid-IRspectralrangeoffersauniqueop- timates of the AGN dusty torus contribution to the infrared light
portunity to study the obscured accretion phenomenon. Surveys (e.g. Ballo et al. 2014). Studies such as these have found a tight
correlation between the intrinsic luminosity, traced by hard (2–
10 keV) X-rays, and reprocessed mid-IR emission of AGN over
(cid:3)E-mail:[email protected] morethanthreeordersofmagnitudeinluminosity(Lutzetal.2004;
†AugustoG.LinaresSeniorResearchFellow. RamosAlmeidaetal.2007;Horstetal.2008;Gandhietal.2009;
(cid:4)C 2015TheAuthors
PublishedbyOxfordUniversityPressonbehalfoftheRoyalAstronomicalSociety
Mid-IRcontinuumofAGN 1423
Levenson et al. 2009; Asmus et al. 2011, 2014; Mason et al. 2 AGN SAMPLE DESCRIPTION
2012).TherelationshipbetweenhardX-rayandmid-IRluminosi-
OurAGNsamplewasdrawnfromthewide-angleBUXS(Mateos
tiesseemstobeidenticalforAGNopticallyclassifiedastype1and
etal.2012).BUXSisalarge,complete,flux-limitedsampleofX-ray
type2,evenforCompton-thickAGN.IfthehardX-rayemission
isagoodtracerofboththeintrinsicpowerofAGNandtheheat- bright (f4.5−10keV >6× 10−14 ergs−1cm−2) AGN detected with
theXMM–Newtonobservatoryat4.5–10keVenergies.Thesurvey
ing source of the dusty torus, then these studies provide strong
is based on 381 high Galactic latitude (|b| > 20◦) XMM–Newton
observational support for the mid-IR light being a reliable proxy
observationshavinggoodqualityforserendipitoussourcedetection
for the AGN intrinsic power, regardless of any obscuration. Un-
thatwereusedtoderiveextragalacticsourcecountdistributionsat
fortunately,theAGNsamplesinvolvedinthesestudiesarehighly
intermediate X-ray fluxes (Mateos et al. 2008). BUXS contains
incompleteandbiasedtowardsinfrared-brightobjects.Recentstud-
255 AGN, after removal of Galactic stars and known BL Lacs
ies of local AGN detected with the Swift Burst Alert Telescope
(<3percent),detectedoveratotalskyareaof44.43deg2.
and AKARI all-sky surveys have shown a tight correlation be-
tween the hard X-ray (>10 keV energies) and monochromatic
mid-IR (9 and 18 µm) luminosities and that unabsorbed and ab- D
2.1 UV/opticalspectroscopicfollow-upandAGNclassification o
sorbed AGN follow the same correlation (Ichikawa et al. 2012; w
n
Matsutaetal.2012). AllthesourcesinBUXSareintheareacoveredbytheSloanDigi- loa
The sensitive all-sky infrared survey conducted with the Wide- talSkySurvey(SDSS)imagingsurveyDataRelease7intheugriz de
d
fi4e.6ld, 1In2fraanrded2S2uµrvmeywEixlplldorraemra(WticIaSlEly; Wincrirgehatseetoaulr. 2A0G1N0)caetn3s.u4s, bwaenudsse(dAtbhaezlaijkiealniheotoadl.-r2a0ti0o9e).stTimoaidtoerntcirfoystsh-meoatpcthicinalgcaolugnotreitrhpmartosf, from
ogvreeartlaysoiugrniufincdaenrtstfarnacdtiinognooffththeeoabgsecuorfedthaeccUrentiivoenrspeheimnopmroevnionng Pbuintetawuoethaalv.e(2d0e1te1c)1tio(sneseiMnaStDeoSsSe.tFaol.r2t0h1e2rfeomradientianilgs)t.wAollssoouurrcceess, https
inluminoussystems.Ithasalreadybeendemonstratedthatmid-IR weidentifiedtheiropticalcounterpartsfromourownimagingcam- ://a
c
colour-basedselectiontechniqueswithWISE,targetingthecharac- paign. ad
teristicredpower-lawthermalcontinuumemissionfromtheAGN Opticalspectroscopicidentificationsandclassificationsarecur- em
dustytorus(fν∝να withα ≤−0.5;Alonso-Herreroetal.2006), rently available for all but five objects. Such high identification ic.o
are highly reliable and effective at uncovering both unobscured completeness(98percent)guaranteesthatourstudywillnotsuffer up
andobscuredluminousAGN(e.g.Assefetal.2010,2013;Mateos frombiasesassociatedwithlowopticalidentificationrates.Thedata .co
m
etal. 2012, 2013; Sternet al. 2012; Yan etal. 2013). As demon- havebeencollectedfromtheliterature(mainlyfromSDSS)andas /m
stratedinMateosetal.(2013),WISEcanpotentiallyuncovermany part of our extensive optical follow-up identification programme n
Compton-thick AGN missed at other wavelengths, at least up to (fulldetailswillbepresentedinaforthcomingpaper). ras
/a
zrev∼ea1l.edDwesipthiteWsIuScEhrpemroagirnespso,othrleypchoynssitcraailnperdo.pertiesoftheAGN sioTnhlrionuegvheoluotcitthyiswpiadpthesr,≥ob1j5e0c0tskwmitsh−1deatnedcteindteUrmVe/odpiatitceaSleeymfeisr-t rticle
Inthispaper,weinvestigatehowwellwecanisolatetheAGN types1–1.5areclassifiedastype1AGN(143)whileSeyferttypes /44
9
emissionassociatedwiththedustytorusinthemid-IRregimeus- 1.8, 1.9 and 2 and those objects with a galaxy spectrum with no /2
ingtheWISEbroad-bandphotometricdataand,moreimportantly, emissionlinesareclassifiedastype2AGN(107).Inthisway,we /14
2
whetherthemid-IRcontinuumluminositiesderivedinthiswayare have a more uniform optical spectroscopic classification over the 2
/1
areliableisotropicproxyfortheintrinsicluminosityofAGN.Todo z range sampled by BUXS. For example, Seyfert 1.9s would be 0
6
sowehaveconductedaUV-to-mid-IRspectralenergydistribution classified as type 2 AGN at z (cid:2) 0.2, when the Hα line at 6563 82
(SED)decompositionintoAGNandgalaxycomponents.Thenwe Åisredshiftedfromouropticalspectra.2Wehavecheckedthatthe 93
havestudiedthecorrespondencebetweentheAGNluminositiesat results of our study do not change ifwe instead classify Seyferts by
g
rest-frame6µm,correctedfortheaccretiondiscandhostgalaxy 1.8and1.9astype1AGN. u
e
emission, and at 2–10 keV energies over more than three orders st o
ofmagnitudeinAGNluminosity.Wehaveusedthefinaldatare- n
2.2 Broad-bandUV-to-mid-IRphotometricdata 2
leaseoftheWISEsurvey(AllWISE;Cutrietal.2013)andalarge 5
F
uniformcompletesampleof232X-ray-selectedAGNdrawnfrom Asdiscussedbelow,oneofthemaingoalsofourstudyistodeter- e
b
theBrightUltra-hardXMM-NewtonSurvey(BUXS).Theobjects minetherest-frame6µmmid-IRcontinuumemittedbytheAGN ru
a
have z in the range 0.05–2.8, and intrinsic (absorption-corrected) dustytorus.Todosowehavedecomposedtherest-frameUV-to- ry
2–10 keV X-ray luminosities between 1042 and 1046ergs−1. The mid-IRSEDsofourAGNintoAGNandhostgalaxycomponents. 20
2
X-rayluminosities,correctedforabsorption,werecomputedfrom TobuildtheSEDs,wehavecollectedalltheavailablephotometric 3
adetailedanalysisoftheXMM–NewtonX-rayspectroscopicdata data from SDSS, the Two Micron All Sky Survey (2MASS), the
availableforallobjects. UKIRTInfraredDeepSkySurvey(UKIDSS)andWISE.
Thispaperisstructuredasfollows.Section2describestheAGN WehaveusedSDSSMODELMAGmagnitudesastheyprovidere-
sample and the multiwavelength data used in this study. In Sec- liable estimatesofthefluxes ofextended objects while, forpoint
tion 3 we describe the SED fitting approach used to isolate the sources,theyareindistinguishablefromthestandardpointspread
AGN mid-IR emission, the adopted technique to compute rest- function (PSF) magnitudes. We converted SDSS magnitudes and
frame 6 µm luminosities and discuss the effects associated with errors into flux densities following the SDSS recommendations.3
contaminationfromtheAGNhostgalaxies.InSection4wepresent
therelationshipbetween6µmand2–10keVcontinuumluminosi-
ties.ThemainresultsarediscussedinSection5andsummarizedin 1Thecodeisavailablefromhttp://saada.u-strasbg.fr/docs/fxp/plugin/
Section6.Throughoutthispaper,errorsare68percentconfidence 2Theopticalspectrafromourfollow-upidentificationprogrammedonot
for a single parameter, and we assume (cid:6)M = 0.3, (cid:6)(cid:7)=0.7 and typicallyextendtowavelengthslongerthan∼8000Å.
H0=70kms−1Mpc−1. 3http://www.sdss.org/dr7/algorithms/fluxcal.html
MNRAS449,1422–1440(2015)
1424 S.Mateosetal.
A 2percent errorwas added in quadrature to the catalogued flux thefullsampleofBUXSAGNwillbepresentedinaforthcoming
errorstoaccountfortheuncertaintiesinthezero-points. paper.
Wehaveusedthecross-matchingalgorithmofPineauetal.(2011) We have good quality XMM–Newton X-ray spectra in the ob-
toidentifytheWISEmid-IRcounterpartsofourAGN(seeMateos servedenergyrangefrom0.25to10keVforalltheAGNinthis
etal.2012fordetails).Allsourcesbutsix(fivetype1AGNand study.Themedian(mean)background-subtractednumberofcounts
onetype2AGN)havedetectionswithSNR≥2attheWISEwave- inthe0.25–10keVEPICspectra(MOS+pn)is1443(3127),where
lengthsof3.4,4.6and12µminAllWISE.ThemeanX-ray-mid-IR thecountsrangefrom52to75000.TheX-rayspectroscopicanaly-
separationis(cid:3)2arcsec.Only10objectshaveanon-zeroAllWISE siswasconductedwiththeXSPECpackage(v12;Arnaud1996).For
photometricqualityflaginatleastoneoftheWISEbandsindicating eachobjectwefittedthedatawithacombinationofdifferentmod-
thatthefluxesmaybecontaminatedduetoproximitytoanimage els to determine the shape of the direct and scattered continuum
artefact(cc flags;Cutrietal.2013).Afteracarefulvisualcheck components (modelled with power laws) and rest-frame line-of-
of the WISE images and SEDs, we found no issues in the WISE sightX-rayabsorption(consideringbothfullcoverageandpartial
fluxes for any of the sources so we decided to keep them in our coverageofthecontinuumsource).AllmodelsincludetheGalac-
sample. tic absorption for each source using column densities taken from D
o
WecomputedfluxdensitiesintheWISEbandsusingeitherellip- Dickey&Lockman(1990).Wehavealsoaccountedforthe‘soft w
n
ticalaperturemagnitudesfortheobjectsflaggedasextended(six excess’,i.e.anyemissionatenergies(cid:3)1keVinexcessoftheex- loa
d
intotal)orprofilefittingphotometryforthesourcesnotspatially trapolationoftheprimarycontinuum,analmostubiquitousfeature e
d
resolved.Weusedthemagnitudezero-pointsontheVegasystem intheX-rayspectraofAGN(e.g.Turneretal.1997;Porquetetal. fro
F (iso)=309.124,171.641,30.988and8.346Jyfor3.4,4.6,12 2004;Guainazzi,Matt&Perola2005;Piconcellietal.2005;Ma- m
aanνsdpe2c2trµumm,fre∝spνeαctwiviethlyα(W=ri−gh1t.Weteala.d2d0e1d01)..5Tpheerycecnotrruenspceorntdainto- tuesoinsgetthael.rm20a1l0a;nSdcboltat,cSktbeowdayrtm&odMelast.eToos2a0c1ce2p;tWthinetedreteetcatilo.n20o1f2a) https
ties to the cνatalogued flux errors in all bands to account for the modelcomponent,weusedtheF-testwithasignificancethreshold ://a
c
a
overallsystematicuncertaintyfromtheVegaspectrumintheflux of95percent. d
e
zero-points.Toaccountfordiscrepanciesbetweentheredandblue IfX-rayabsorptionwasnotdetected,wecomputed1σupperlim- m
ic
calibratorsusedfortheconversionfrommagnitudestojanskys,we itsusingthebest-fittingmodelparameters.ThoughBUXSincludes .o
added an additional 10percent uncertainty to the 12 and 22 µm manyobjectswithX-rayabsorptionclosetobeingComptonthick, up
.c
cataloguedfluxes(Wrightetal.2010). whentakingintoaccounttheuncertaintiesinthebest-fittingrest- o
m
At the stage of the project when we searched for the near-IR framecolumndensities,allX-rayabsorptionvaluesareconsistent /m
photometric information available for all the AGN in BUXS, we withbeingintheCompton-thinregime(N <1.5×1024cm−2). n
H ra
hadalready accurate sourcepositionsfortheoptical counterparts Thus,basedontheexistingX-raydatawedonothaveanysecurely s
/a
ofalltheobjectsinthesample.Therefore,toidentifythenear-IR confirmed Compton-thick AGN. The X-ray luminosities used in rtic
counterparts of our AGN, we cross-correlated the coordinates of this paper have been determined from the best-fitting models in le
/4
theiropticalcounterpartswiththe2MASScataloguesofpointand therest-frame2–10keVenergybandandarecorrectedforX-ray 4
9
extended sources (Jarrett et al. 2000; Cutri et al. 2003) and the absorption.ExamplesofourX-rayspectralmodellingareshownin /2
/1
UKIDSSDataRelease10catalogues(Lawrenceetal.2007)using Fig.2. 4
2
a simple position matching with a search radius of 2 arcsec. In 2
/1
total166objects(∼71percent)havedetectionsinthenear-IR.The 0
6
mean optical–infrared separation is <1 arcsec. We have checked 2.4 AGNsampleselection 82
9
that increasing the search radius does not produce any additional 3
For the study presented here, we have selected the 232 AGN in b
matches.FromUKIDSSweusedPetrosianmagnitudesforextended y
BUXSwithabsorption-correctedX-rayluminosities≥1042ergs−1 g
sourcesandthedefaultaperturemagnitudesforpointsources.Asfor u
SDSS,weaddeda2percenterrorinquadraturetothecatalogued 4an.6daWndIS1E2dµemte)cwtioitnhsSiNnRth≥e t2h.rOeeutshoofrttheesstew,1av3e7leonbgjethctsbaanredsty(p3e.41, est o
fluxerrorstoaccountforuncertaintiesinthezero-points.2MASS n
AGN(122Seyfert1s,15Seyfert1.5s)and95type2AGN(3Seyfert 2
andUKIDSSmagnitudeswereconvertedintofluxesusingthezero- 5
1.8s,14Seyfert1.9sand78Seyfert2s).TheX-rayluminositylimit F
points F (iso) = 1594, 1024 and 666.7 Jy for the 2MASS JHK e
ν s hasbeenimposedtoreducetoaminimumtheeffectsofhostgalaxy b
(Cohen, Wheaton & Megeath 2003) and F (iso) = 2232, 2026, ru
ν contaminationinthemid-IRregimebyincreasingthecontrastof a
1530, 1019 and 631 Jy for the UKIDSS ZYJHK (Hewett et al. ry
theAGNcontribution. 2
2006). 0
The selected AGN have 2–10 keV X-ray luminosities between 2
WenotethatifweusetheWISEmagnitudezero-pointsintheVega 3
1042and1046ergs−1andzintherange0.05–2.8.TableA1liststhe
system corresponding to the spectral shapes assumed to convert
opticalspectroscopicclassificationandintrinsicX-rayluminosity
magnitudestofluxdensitiesinUKIDSS(∼ν0)and2MASS(∼ν−2),
foralltheAGNusedinourstudywhileTable1summarizestheir
thedifferencesinthederivedfluxdensitiesintheWISEbandswould
mainproperties.Thedistributionoftheirzand2–10keVluminosi-
be lower than 1percent at 3.4, 4.6 and 22 µm and 9percent at
tiesisshowninFig.1.
12 µm, all below the systematic uncertainties in the catalogued
fluxes.
3 AGN MID-IR CONTINUUM LUMINOSITIES
Ourstudyaimstoinvestigatehowwellwecanisolatethemid-IR
2.3 X-rayspectralproperties
emissionassociatedwiththeAGNdustytorususingWISEbroad-
Herewegiveabriefsummaryoftheanalysisconductedtodetermine bandphotometricdata,andwhetherthemid-IRAGNluminosities
theX-raypropertiesofourAGN.FulldetailsontheX-rayspectral determinedinthiswayareareliableisotropicproxyoftheAGNin-
extractionprocedure,analysisofthedataandX-raypropertiesof trinsicpower.Todosowehavestudiedthecorrespondencebetween
MNRAS449,1422–1440(2015)
Mid-IRcontinuumofAGN 1425
Table1. SummaryofthemainpropertiesoftheAGNusedinthisstudy.
Sample N (cid:2)logL2–10keV (cid:8)logL2–10keV(cid:9) (cid:2)z (cid:8)z(cid:9) Nabs
(ergs−1) (ergs−1)
(1) (2) (3) (4) (5) (6) (7)
All 232 42.12–46.00 43.94(43.94) 0.056–2.860 0.558(0.396) 122(52.6percent)
Type1 137 42.32–46.00 44.23(44.38) 0.061–2.860 0.713(0.646) 37(27.0percent)
Type2 95 42.12–45.19 43.52(43.41) 0.056–1.266 0.334(0.240) 85(89.5percent)
Notes.Column1:sample;Column2:numberofsources;Column3:2–10keVX-rayluminosityrange(in
logarithmicunits).Column4:median(mean)X-rayluminosity(inlogarithmicunits);Column5:redshiftrange;
Column6:median(mean)redshift;Column7:number(fraction)ofobjectswithdetectedX-rayabsorption.
toensureareliabledeterminationofthedustytorusemission.We
D
haveusedthefittingcodeSEdAnalysisusingBAyesianStatistics o
w
(SEABASS)4describedindetailinRovilosetal.(2014).Theprogram nlo
usesthemaximumlikelihoodmethodandaMarkovchainMonte a
d
Carlo(MCMC)samplingtechniquetodetermineboththeoptimum ed
combinationofSEDtemplatesandtheassociateduncertainties. fro
m
h
ttp
s
3.1.1 SEDcomponents ://a
c
WefittedtheSEDswiththreemodelcomponentstoreproducethe ad
e
emissionfromtheAGNaccretiondisc,thedustytorusandthestellar m
emissionofthehosts. ic.o
TodescribetheUV/opticalemissionfromtheaccretiondisc,we up
haveusedtheaveragetype1quasarSEDofRichardsetal.(2006) .co
atλ<0.7µmandapowerlawλfλ∝λ−1 atlongerwavelengths. m/m
Figure1. HardX-rayluminosityversuszfortheAGNinBUXSselected Toreddentheaccretiondisc,wehaveusedtheGordon&Clayton n
forthisstudy. (1998)SmallMagellanicCloudextinctionlawatλ<3300Å,as ras
/a
therest-frame6µmand2–10keVX-raycontinuumluminosities i2t0b0e4s)t,rwephrieleseanttsλth>e3n3u0c0leaÅrrwededuesneidngthoefGquaalascatrisc(eHxotipnkcitniosnetlaawl. rticle
forourcomplete,flux-limitedsampleofX-ray-selectedAGN. fromCardelli,Clayton&Mathis(1989).Inbothcasesweassume /44
Wehaveusedrest-frame6µmmonochromaticluminositiesas RV=3.1. 9/2
a tracer of the AGN dusty torus emission. As such emission can Wehavecharacterizedthemid-IRemissionfromtheAGNdusty /14
2
bewellapproximatedbyapowerlaw(seee.g.Neugebaueretal. toruswiththetemplatelibraryofSilva,Maiolino&Granato(2004). 2
/1
1979;Elvisetal.1994;Richardsetal.2006;Assefetal.2010),to Thelibraryincludesfourtemplatesdividedaccordingtotheamount 0
6
computethe6µmluminositieswehaveusedasimplelinearinterpo- ofX-rayabsorption.Thesetemplatesweregeneratedbycombining 82
lation/extrapolationinlog–logspacebetweentheWISE-catalogued thenuclearmid-IRobservationsofalargesampleofSeyfertgalaxies 93
fluxesat4.6and12µm.Section3.1describesindetailhowwecor- andinterpolationusingthesmooth-densitydustemissionmodelof by
rected these luminosities for both host galaxy contamination and Granato&Danese(1994).OfthesewehaveusedtheSeyfert1tem- gu
e
forWtheeheamveisnsiootnufsreodmthteheWAISGEN3a.4ccµremtioflnudxeisscb.ecause,asshownin pdleantseiatinedstihnethtweoCSoemyfpetrotn2-ttheimnprleagtiemsceo:r1r0e2s2po<ndNing<to1X0-2r3acymc−o2luamnnd st on
Sweecthiaovne3n.1o,tthueseydarteheseWveIrSeEly2c2onµtmamflinuaxteesdbbeycastuasreli,gahst.thMisorseuorvveery, 1si0fi2c3a<tioNnHor<be1s0t-2fi4tctmin−g2X.W-raeyhcaovleumnontduesnesditeieitshHetorcthheooospetiacmalocnlgass-t 25 Fe
b
ismuchshallowerthantheonesconductedatshorterwavelengths, torustemplates,i.e.wehaveconsideredallthreetorustemplatesto ru
a
manyofourobjectsarenotdetected.Indeed,outofthe232AGN fittheSEDsofourtype1andtype2AGN. ry
selected for this study just 168 (72.4percent) are detected with Finally,wehavereproducedtheemissionfromthestellarpopu- 20
SNR≥2at22µm. lationoftheAGNhostsatrest-frameoptical–near-IRwavelengths 23
withthestellarpopulationsynthesismodelsofBruzual&Charlot
(2003).Weusedalibraryof75stellartemplateswithsolarmetal-
3.1 SEDdecompositionintotheAGNandtheirhosts
licityandaChabrierinitialmassfunction(Chabrier2003).Wehave
EvenatthehighAGNluminositiesinvolvedinourstudy,the4.6and used10exponentiallydecayingstarformationhistorieswithchar-
12µmfluxes(rest-frame∼1.5–4µmatz∼2)mightbecontami- acteristic times τ = 0.1–30 Gyr and a model with constant star
natedbytheemissionfromtheAGNhostgalaxies.Toisolatethe formation, and a set of ages in the range 0.1–13 Gyr (see Lusso
AGNcomponentandderivethe6µmluminosityassociatedwith etal.2013forasimilarapproach).Wehavecheckedthatnoneof
thedustytorus,wehavedecomposedtherest-frameUV-to-mid-IR ourfitsrequiresuchlong(>10Gyr)andprobablyunphysicalages.
SEDsofourobjectsintoAGNandgalaxyemission.Weusedthe We reddened the templates using the Calzetti et al. (2000) dust
best-fitting results to correct the 6 µm luminosities for contami- extinctionlawandarangeofE(B−V)galfrom0to2.
nation from the AGN host galaxies and accretion disc emission.
WestressthatthemaingoaloftheSEDfittinganalysisistoprop-
erly model all the continuum components in the observed SEDs 4http://astro.dur.ac.uk/∼erovilos/SEABASs/
MNRAS449,1422–1440(2015)
1426 S.Mateosetal.
ToavoiddegeneraciesbetweentheAGNandgalaxytemplates, thenullhypothesis86.7percent).Themedianuncertaintyassoci-
wehaveignoredanypossiblecontaminationfromdustheatedby atedwiththecorrectionfortheaccretiondiscemissiondetermined
starformationintheAGNhosts.AsdemonstratedinSection3.3, fromtheSEDfitsis13percent.Suchuncertaintieswereaddedin
atthehighAGNluminositiesinvolvedinourstudyandrest-frame quadraturetoour6µmluminosityestimates.
wavelengthssampledwiththeWISE4.6and12µmsurveys,con-
tamination due to star formation has a negligible effect on our
results. Nevertheless, as the star formation emission rapidly in- 3.1.3 SEDfittingofAGNwithoutdetectedUV/opticalbroad
creases at rest-frame wavelengths (cid:2)8 µm, contamination of the emissionlines
WISE22µmfluxesmightnotbenegligibleatthelowestredshifts
Inthe78AGNwithoutdetectedbroademissionlinesintheirrest-
(andAGNluminosities).Tominimizetheimpactofsucheffect,we
havetreatedall22µmdetections(168intotal)asupperlimits. frameUV/opticalspectra,theaccretiondiscemissionishighlyob-
scuredandtheobservedfluxesatrest-frameUV-to-near-IRwave-
Inthefollowingsubsections,wediscussinmoredetailtheSED
lengthsaredominatedbytheAGNhostgalaxies.Nevertheless,for
fittingprocedureandsummarizethemainresults.IntheAGNwith
signatures of the broad-line region in their rest-frame UV/optical AGNwithnuclearreddeninglevelsAV ∼2–5mag,theaccretion D
spectrum (i.e. Seyfert types 1 to 1.9) the contamination from the discemissionatnear-IRwavelengths(∼1–2µm)couldstillberel- ow
n
accretion disc at UV/optical and near-IR wavelengths is not neg- evant. As for these objects we do not have a direct view of the loa
accretion disc, we cannot constrain both the extinction and nor- d
ligible. Therefore, we have carried out the SED fits following a e
dsiioffnerleinnetsap(ip.er.oawchhefroerwAeGhNavweiathredleatteivcetelydUunVo/bospctuicreadlbvrioeawdoefmtihse- mtoafilixzatthioennoofrmsualcihzaatiocnomopfothneenatc.cTrehteioanppdrioscacrhelwateivefotlolotwheedtowruass d from
BLRandaccretiondisc;Section3.1.2)andforAGNwithoutde- cUoVm/poopntiecnaltbtorothademlineedsi.aWnveatlhueenocbrteaaitneeddafloirbrthareyAoGfrNedwdietnheddeateccctreed- http
tected UV/optical broad emission lines (i.e. where the accretion s
discemissionishighlyobscured;Section3.1.3).Nevertheless,we tiondisctemplatesusingarangeofE(B−V)disc valuesbetween ://a
0.65 and 10 with a step (cid:2)E(B−V) = 0.15. These templates ca
remindthereaderthat,tohaveamoreuniformopticalspectroscopic disc d
classificationoverthez rangesampledbyBUXS,throughoutthe wereaddedtoeachofthethreetorustemplatesusedfortheSED em
fitting.Werepeatedthefittingprocess,fixingtheaccretiondiscnor- ic
paperwehaveclassifiedSeyfert1.8–1.9(17intotal)astype2AGN malizationrelativetothetoruscomponenttothemedian−1σ and .ou
(seeSection2.1). p
median+1σ values, and used the best-fitting results to determine .c
o
theuncertaintiesinthecorrectionofthe6µmluminositiesforthe m
/m
3.1.2 SEDfittingofAGNwithdetectedUV/opticalbroad accretiondiscemission. n
emissionlines Thebest-fittingE(B−V)disc valueshaveanearlyuniformdis- ras
For the type 1 AGN (137) and intermediate Seyfert types 1.8– torfibEut(iBon−bVet)wdiescen=th6e.2c2h(o5s.e9n5)l.imWietsswtreitshs,ahomweedviaenr,(thmaetatnh)evmaleuae- /article
1.9 (17), we have used a range of reddening values for the surednuclearextinctionsshouldberegardedaslessaccuratethan /4
a(cid:2)cEcr(eBtio−nVd)isdics,c=E0(B.01−.AVs)diniscd,icbaetetwdeinenCa0ccaianndig0a.e6t5alw.(i2th00a8),stthepe fTohreombjeedcitasnw(hmeereanw)ecohnatvriebuatiodnireocfttvhieewredodfetnheedaaccccrreettiioonnddiisscc. 49/2/1
sAeVpa∼ra2timonagb,eotwreEe(nBo−ptVic)a∼lt0y.p6e51asasnudmitnygpea2GAalGacNticcostrarensdpaordndcsonto- a(6t.r9es±t-fr3a.m3)ep6erµcmen,tr.elTahtievemtoedthiaenbeusntc-fiertttainingtytorausssomcoiadteeld,iws6it.h5+−th32..e96 422/10
version.TotakeintoaccountanyintrinsicdispersionintheSEDs 6
correction for the accretion disc emission from the SED fits is 8
ofourAGN,thenormalizationsofthediscandtoruscomponents 46percent.Althoughaspointedoutpreviously,theextinctionval- 293
were left free to vary. The SED fitting was conducted both with uesarelessaccurate,wefoundthat67percentofthesourceshave b
y
andwithoutthestellarcomponent.Weacceptedthedetectionofthe dust-to-gasratioslowerthanGalacticbyafactorthatvariesfrom gu
stellarcomponentiftheχ2 differencebetweenmodels,(cid:2)χ2,was ∼1.5 to 90, again in very good agreement with Maiolino et al. es
higherthan6.17(2σ confidencefortwoparameters).5 Otherwise, (2001). t on
wedidnotapplyanycorrectionforhostgalaxycontaminationto Examples of the SED fitting procedure for type 1 and type 2 25
theobservedWISEfluxes. AGNareshowninFig.2.ThecontaminationfromtheAGNhost F
We have found a median (mean) nuclear extinction E(B− eb
VE()Bdisc−=V)d0is.c05be(lo0w.110).,1.wCitohns6id8eprienrgceonntlyotfhothseessoouurcrceessshhoawviinngg IginnalSTaeaxcbiteliesona2ts,r3we.s2et-arfenrpadom3r.et3tnhaeneadrm-sIeuRdmiaamnndacrmiozniedtdr-iIibRnuFtwiioganvse3flreaonnmgdtt4hh,serieAssdpGiesNcctuihvsoessleytds. ruary 2
evidenceforbothX-rayabsorption(seeSection2.3)andUV/optical andthereddenedaccretiondiscatrest-frame6µmfortype1and 023
reddening(49intotal),wefoundthatthegreatmajority(88percent)
type2AGNatdifferentX-rayluminosities.
havedust-to-gasratiosE(B−V)/N lowerthanGalacticbyafac-
H
tor varying from 1.5 to more than 100 in a few cases, in good
agreement with Maiolino et al. (2001). In 42 objects (38 with 3.2 Stellarcontaminationintherest-framenear-IR
L2–10keV >1044ergs−1)addingthestellarcomponentdidnotim- andmid-IR
provethequalityofthefitabovetheselectedthreshold.Themedian
(mean)contributionoftheaccretiondiscatrest-frame6µm,relative We have corrected our rest-frame 6 µm luminosities for the
tothebest-fittingtorusmodel,is11.8+7.6(12.9±7.8)percentand Rayleigh–Jeanstailoftheemissionofthedifferentunderlyingstel-
−6.1 larpopulationsoftheAGNhostgalaxiesasfollows.First,wehave
hasnosignificantluminositydependence basedontheSpearman
ranktest(correlationcoefficientρ=−0.12andprobabilitytoreject computedtheobservedfluxesinthe4.6and12µmpassbandsasso-
ciatedwithstarlightemissionusingthebest-fittingstellartemplates.
To accurately reproduce the measurement process with the wide
5The relationship between (cid:2)χ2 and the log-likelihood difference is bandpassesoftheWISEfilters,wehaveconvolvedthebest-fitting
(cid:2)χ2=−2(cid:2)(lnL)(seeappendix inRovilosetal.2014fordetails). stellarSEDswiththeWISErelativespectralresponse(RSR)filter
MNRAS449,1422–1440(2015)
Mid-IRcontinuumofAGN 1427
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4
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Figure2. ExamplesofthemodellingoftheXMM–NewtonEPICX-rayspectroscopicdataandSEDdecompositionforAGNclassifiedastype1(leftplots) /2
andtype2(rightplots),respectively.Top:theX-rayspectrafor2XMMJ095908.3+024309arebestmodelledwithasimpleunabsorbedpower-lawmodel /14
(abnrdoaNdH-b=an4d0p.h4o+−t87o..01n,iwndheexre(cid:13)th=e1c.o8l8u±mn0.d0e2n;ssiotylidisliinneus)n.iFtsoorf2X10M22McJm2−221)81p3lu.9s+a0lo0w16-e2n4etrhgeybsecsat-ttfietrteindgpmowodeer-llcaownswisittshosfaamneasbpseocrbtreadlpinodweexr-alsawtha((cid:13)to=ft1h.e77d+−ir00e..33c97t 22/106
absorbedcomponent(dottedlines).Bottom:thefiguresclearlyillustratethat,attheshortestwavelengthbandsofWISE,theemissionfromtheAGNhost 82
9
galaxiescandominateoverthatofthedustytorusespeciallyintype2AGNwheretheextinctionoftheAGNemissionissystematicallyhigher. 3
b
y
curves.Then,wehavedeterminedtheluminosityat6µmassociated etal.2003).OnlytheWISEfluxesat12µmaredominatedbythe gu
e
withstarlightemissionthroughlinearinterpolation/extrapolationin AGNatallluminosities,atleastuptoz ∼2.Analternativeway st o
log–logspace.Themedianuncertaintyassociatedwiththecorrec- to visualize the dependence of the level of stellar contamination n
2
tionforstellaremissiondeterminedfromtheSEDfitsis19percent. to the WISE fluxes on the AGN luminosity and optical class is 5
Thesewereaddedinquadraturetoour6µmluminosityuncertain- representing the distribution of WISE colours for our sources. If Fe
b
ties. weadoptthemid-IRcolourdefinitionofMateosetal.(2012)(i.e. ru
a
meFaisgu.r3edshwoiwthsWthIeSrEelaattiv3e.4c,o4n.6triabnudtio1n2fµrommasstaarlfiugnhctttioonthoefflAuGxeNs mloogs(ft4c.6oµnmta/mf3.i4nµamte)dveorbsjuesctlsogs(hfo12uµldm/bfe4.6lµocma)tfleduxindetnhseitbyortatotimos-)r,igthhet ry 20
2
luminosity.InthecaseswheretheemissionfromtheAGNhostswas part of the colour–colour diagram. Indeed, as we see in fig. 6 in 3
notdetectedwithsufficientsignificance((cid:2)χ2<6.17),weassigned Mateosetal.(2012)thelessluminousobjects,inparticularthose
azerovaluetothefractions.ForAGNopticallyclassifiedastype classifiedastype2AGN,haveoverallthebluestlog(f4.6µm/f3.4µm)
2,thestarlightcontaminationtotheWISEfluxesat3.4and4.6µm mid-IRcolours.
isimportantacrossthefullluminosityrangesampledbyourstudy. Fig.4illustratesthecorrectionforstarlightemissiontoourrest-
This is expected as dust extinction of the AGN emission is more frame6µmluminositiescomputedasindicatedabove.Thecorre-
importantintype2objects.Inaddition,influx-limitedsurveys,such spondingvaluesarereportedinTable2.
asBUXS,themostluminousobjectsaredetectedathigherzand
the WISE passbands are sampling shorter rest-frame wavelengths
3.3 Contaminationfromstarformationintherest-frame
wherethestarlightemissionpeaks.AsdustextinctionoftheAGN
mid-IR
emissionislesssevereintype1objects,thestarlightcontamination
totheirmid-IRfluxesissignificantlysmallerthanthatfortype2 As indicated in Section 3.1.1, we have not corrected our 6 µm
AGNbutstillnon-negligible,especiallyat3.4µm(Kotilainenetal. luminositiesfortheemissionfromdustinstar-formingregionsinthe
1992; Alonso-Herrero, Ward & Kotilainen 1996; Alonso-Herrero AGNhosts.Forexample,polycyclicaromatichydrocarbon(PAH)
MNRAS449,1422–1440(2015)
1428 S.Mateosetal.
D
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Figure4. Hostgalaxycontributiontotherest-frame6µmluminositiesas fro
m
afunctionoftheAGNluminosity.Largefilledsymbolsandverticalerror h
barsindicatethemedianandthe16and84percentiles(68percentenclosed, ttp
einqduiicvaatleedntwtoith1σth)eohfothriezdoinsttarilbeurtrioornboafrsv.aIlnuetsheincdaisfefserwenhtelruemthienolosiwtyerinlitmervitaolsf s://ac
a
theerrorbarreachedzero,theupperlimitcorrespondstothe68percentile d
e
(i.e.smallestintervalthatenclosesthe68percentprobability). m
ic
.o
u
featurescanprobestarformationintheAGNhostsovertime-scales p
.c
oftensofmillionsofyears(e.g.Peeters,Spoon&Tielens2004). o
m
Nevertheless,itisexpectedthatanyPAHredshiftedintotheWISE- /m
observedbandssuperimposedonthestrongAGNcontinuumwould n
ra
besmoothedoutduetothewidebandpassesofthefilters,especially s
/a
fortheextremelywide12µmfilter.Todemonstratethatthisisthe rtic
case,wehavedeterminedupperlimitsonthelevelofcontamination le
fromstarformationtoour6µmluminosities.Todoso,wehave /44
9
usedthemain-sequenceandstarburstgalaxytemplatesfromElbaz /2
/1
et al. (2011). For each object in our sample, we have determined 4
2
theupperlimitinthenormalizationofthestar-formingtemplates 2
/1
assumingthatthehostsofourAGNareluminousinfraredgalaxies 0
6
((SLFIRRGs)s)ofw∼ith10L08M−1(cid:10)000yµrm−1≤ba6s×ed1o0n11thLe(cid:10)Kaenndnicstuatrt(f1o9rm98a)tiroenlartaiotens. 8293
b
The less than equal sign indicates the additional constraint that y
g
none of the fluxes determined from each template in the WISE u
e
bands (taking into account the filter RSRs) should be above the st o
observedvalues.Then,wecomputedthe6µmluminositiesfrom n
2
linearinterpolation/extrapolationofthe4.6and12µmfluxesinlog– 5
F
logspaceaswedidfortheoriginaldata.Followingthisapproach, e
b
wefoundthatthecontributionfromstarformationat6µmiswell ru
a
rbaenlogwe 11054p2–er1c0e4n3terfgors−813.a3npderfcoern>to9f9tpheerAceGnNt owfitthheL2s–o1u0rkceVesinwtihthe ry 20
2
L ≥1043ergs−1. 3
2–10keV
If we assume instead that the hosts of our AGN with
z > 0.5 are ultraluminous infrared galaxies (ULIRGs) with
L8−1000µm ≤ 6 × 1012L(cid:10) (very IR luminous galaxies become
morenumerousathighz)andSFRs∼1000M(cid:10)yr−1,wefindthat
Figure3. ContributionfromtheAGNhostgalaxiestothemid-IRfluxes the contamination from star formation at rest-frame 6 µm is less
measuredwithWISEat3.4,4.6and12µm(observedframe).Largefilled than15percentfor89.1percentofthesesources.Table2listsour
symbolsandverticalerrorbarsindicatethemedianand16and84percentiles (veryconservative)medianupperlimitsonthelevelofcontamina-
(68percent enclosed, equivalent to 1σ) of the distribution of values in tionfromstarformationatrest-frame6µmasafunctionofAGN
different luminosity intervals indicated with the horizontal error bars. In classandluminosity.
the cases where the lower limit of the error bar reached zero, the upper For comparison, the median upper limit on the star formation
limitcorrespondstothe68percentile(i.e.smallestintervalthatenclosesthe contaminationtotheWISE12µmpassbandis18percentatAGN
68percentprobability). luminosities1042<L <1043ergs−1,andlessthan4percent
2–10keV
athigherluminosities.Attherest-framewavelengthssampledwith
MNRAS449,1422–1440(2015)
Mid-IRcontinuumofAGN 1429
Table2. ContributionfromthereddenedaccretiondiscandtheAGNhoststothemid-IRatrest-frame6µm.
Sample N (cid:2)logL2–10keV f6stµm f6dµiscm f6dµiscm+st f6sbµm
(ergs−1) (percent) (percent) (percent) (percent)
(1) (2) (3) (4) (5) (6) (7)
All 36 42–43 20.76+−2125..8818 3.66+−12.14.757 28.13+−1171..9243 7.93+−84..9729
Type1 12 42–43 10.95+−18.61.397 15.23+−15.47.296 27.15+−1180..4265 7.52+−95..3328
Type2 24 42–43 26.02+−1194..9954 2.94+−22..3126 29.25+−1172..1337 9.53+−75..7620
All 86 43–44 7.81+−85..9130 7.39+−74..1087 17.96+−86..7336 2.54+−31..2809
Type1 39 43–44 5.09+−63..7989 10.79+−65..0293 18.32+−88..5554 1.65+−21..5168
Type2 47 43–44 11.57+−56..9112 5.94+−33..8138 17.41+−85..2663 3.19+−31..2921
All 88 44–45 2.12+−22..3102 10.82+−55..2857 13.84+−74..0384 0.37+−00..9235(3.72+−92..2496) Dow
Type1 65 44–45 1.40+−21..6470 11.40+−65..7242 14.18+−64..8688 0.32+−00..6221(3.25+−62..1174) nlo
Type2 23 44–45 4.40+−52..1888 8.73+−34..6101 13.17+−42..1901 0.63+−10..3434(6.27+−14.34.530) ade
d
Type1 21 45–46 0.23+−00..8233 11.33+−45..2534 12.45+−66..9240 0.01+−00..0051(0.11+−00..6056) fro
m
Notes.Column1:sample;Column2:numberofsources;Column3:2–10keVX-rayluminosityrange(in h
logarithmicunits).Columns4and5:mediancontributionfromtheAGNhostsstellaremissionandthereddened ttp
ahcocsrtestaionnddthisecraetdrdeestn-efrdamacecr6eµtiomn,dreisscpeacttirveeslty-f.rCamoleum6nµ6m:.mCeodliuamnonf7th:e(vseurmycoofnthseercvoantitvreib)umtieodnisafnroumpptehrelAimGiNts s://ac
forthelevelofcontaminationfromstarformationatrest-frame6µmassumingthattheAGNhostsareLIRGs ad
e
orULIRGs(valuesinbrackets;seeSection3.3fordetails).Theerrorbarsrepresentthe16and84percentiles. m
Inthecaseswherethelowerlimitoftheerrorbarreachedzero,theupperlimitcorrespondstothe68percentile ic.o
(i.e.smallestintervalthatenclosesthe68percentprobability).Pleasenotethattocomputethenumberswehave u
p
notincludedtheonlytype2AGNinthesamplewithL2–10keV>1045ergs−1. .co
m
theWISE3.4and4.6µmpassbands,thecontaminationfromstar 4 CORRELATION OF L6µm AND L2–10keV /mn
formationshouldbenegligible. LUMINOSITIES ras
samWpelenootfeotbhjaetcttos,cio.em.ipnuctleudthinegabthoovseenfuomrwbehrischwweehahvaeveusoendlythuepfpuelrl 4.1 Linearregressiontechnique /article
limitsontheir22µmfluxes.Fromthecomparisonofthe12µmflux /4
ldiimstirtisbufotironf2s2aµnmdi(ff2n2oµtm/dfe1t2eµctme)dfl),uwxedehnasviteyfroautinods(tuhsaitntgh1eσobujpepcetsr fFnroraosmmiteien2so–aw1t0orkewsentV-,frwXaem-raweyi6llluµrmemfienraosstoitLiteh6sµecmmorwirdeh-cIitRleedmLf2oo–nr10oXkce-hVrraoyamraeabttsihcoerlpurtemisotin-- 49/2/14
in BUXS that have escaped detection at 22 µm are ∼4–5 times 22
faavienrtaegreatlo1w2eµrm(∼th0a.8n–t0h.o9setimdeetsecstmedalaletr2)2mµimd-,IRbufltuthxedyeanlssiotyharvateioosn. (Ls2e–e10SkeeVc,tiwone2h.a3v).eTpoedrfeotremrmedineatlhineeraerlarteiognresshsiiponbeitnwleoegn–Llo6gµmspaancde /1068
usingtwodifferentapproaches:theleast-squaresχ2 minimization 2
TfohrimsiinmgprleiegsioenvseninltohweeorbljeevcetlssnooftcdoentetacmtedinwatiitohnWfrIoSmEdatu2st2iµnmst.ar- (pProrepsosseedtably.1K9e9l2ly)a(2n0d0t7h,ehBeraeyaefsteiarnKm07a)x.i6mFuomrthliekeKli0h7ootedcmhneitqhuoed, 93 by g
the data are fitted using the Metropolis–Hastings MCMC algo- ue
s
3.4 Reliabilityofrest-frame6µmluminositiesforz>1AGN rithmsampler.Wehaveadopteduniformpriordistributionsforthe t o
n
regressionparameters.Thebest-fittingparametersandtheircorre- 2
ItiswellknownthatthereissomecurvatureintheIRcontinuum spondinguncertaintieshavebeendeterminedbytakingthemedian 5 F
emission of AGN at rest-frame wavelengths close to the ∼1 µm andthe16and84percentiles(68percentenclosed,equivalentto eb
inflection point between the UV and near-IR bumps (Elvis et al. 1σ) from the posterior probability distributions of the model pa- rua
1994;Richardsetal.2006).Thismeansthatourlinearinterpola- rameters using 104 iterations from the MCMC sampler. The two ry 2
tion/extrapolation approach, based on the WISE fluxes at 4.6 and regressiontechniquesaccountformeasurementerrorsinboththe 0
2
12µm,mightbeoverestimatingthe6µmluminositiesforobjects independentanddependentvariables.However,sincetheK07ap- 3
at z > 1, when the WISE 4.6 µm band is sampling rest-frame proach determines simultaneously both the regression parameters
wavelengths(cid:3)2µm.Toinvestigatesucheffect,wehavecompared andtheintrinsicscatter,7itshouldprovidemorereliableestimates
ourderived6µmluminositieswiththoseobtainedusingtheWISE oftheparameters,theirassociateduncertaintiesandofthestrength
fluxesat12and22µm.WehaveusedforthiscomparisonallAGN ofthelinearcorrelation.Therefore,fromnowonwewillrefertothe
inBUXSwithz>1anddetectionwithSNR≥2at22µm(19outof resultsobtainedwiththeK07technique,butpresentthoseobtained
37objectswithz>1).Wehavefoundamedian(mean)difference fromtheleast-squarestechniqueforcomparisonwithresultsinthe
inthelogarithmofthe6µmluminositiesofjust−0.02(−0.02). literature.
Althoughthenumbersinvolvedinthecomparisonaresmall,these
resultsshowthatatuptoz∼2,rest-frame6µmAGNluminosi-
ties can be robustly determined using the WISE 4.6 and 12 µm 6http://idlastro.gsfc.nasa.gov/ftp/pro/math/linmix_err.pro
fluxesalone.Inaddition,weconfirmthatcontaminationfromdust 7Ifxandyaretheindependentanddependentvariables,respectively,the
emissioninstar-formingregionstotheWISE22µmfluxesisnot scatterinyatfixedxthatwouldhavebeenmeasurediftherewerenoerrors
important. iny.
MNRAS449,1422–1440(2015)
1430 S.Mateosetal.
Table3. Summaryoftheresultsofthelinearregressionanalysis.
Sample Method N ρ(p-value) τ(p-value) σobs σint α β γ
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
All LS 232 0.90(<10−5) 0.42(<10−5) – 0.44±0.01 1.01±0.01 –
All Bayesian 232 0.918+−00..001102 – 0.38 0.350+−00..002108 0.302+−00..002265 0.986+−00..003300 2.003+−00..112103
Type1 LS 137 0.88(<0.0013) 0.40(<10−5) – 0.52±0.01 0.89±0.01 –
Type1 Bayesian 137 0.904+−00..001270 – 0.40 0.353+−00..002282 0.377+−00..003334 0.941+−00..004421 2.372+−00..119775
Type2 LS 95 0.88(<10−5) 0.45(<10−5) – 0.24±0.01 1.00±0.01 –
Type2 Bayesian 95 0.889+−00..002237 – 0.40 0.325+−00..003217 0.180+−00..004487 0.926+−00..005589 1.513+−00..118508
Notes.Column1:sample;Column2:linearregressiontechnique.LS:leastsquares;Column3:numberofobjects;Column4:linear
correlationcoefficient.Forleastsquares,theSpearmanrankandtheassociatedprobabilitythatthecorrelationisentirelycausedby D
o
distancebasedonourMonteCarlosimulations;Column5:Kendall’spartialcorrelationcoefficientandprobabilitythatthecorrelation w
n
isentirelycausedbydistancebasedonourMonteCarlosimulations;Column6:observedscatterinL6µmabouttheregressionline; loa
Column7:intrinsicscatterabouttheregressionline.AtagivenX-rayluminosity,σint denotesthestandarddeviationinthemid-IR de
l(ueqmuiantoiosinty2;).CUolnucmerntasin8tiaensdar9e:1iσnt.erceptandslopeoftheregression,respectively(equation1);Column10:constantinlinearspace d fro
m
h
Thelinearregressiontechniquesusedhereassumethatthemea- Equation(1)canbewrittenas ttp
s
sthuereXm-eranytelrurmorisnaorseitGyamuessaisaunr.eHmoewntesvoerr,tthheisuinscneorttathinetcieasseasfosorceiiathteedr (cid:2)L6µm(cid:3)=γ ×(cid:2)L2−10keV(cid:3)β, (2) ://aca
with the correction of the mid-IR luminosities for the accretion 1044 1044 de
m
ddiastca apnodintstwariltihghlutmeminiosssiitoyn.LTaondovaesrscoocmiaetedthiusncperortbalienmtie,sfoLr−eaancdh mwhederieanwaenhdavtheeco1m6pauntded8γ4 (p∼er1c0eαn)tialensdoitfst1hσe upnrocbeartbaiilnittyiesdeansstihtye ic.oup
L+ (allvaluesinlogarithmicunits)wehavecomputedaGaussian distributions of γ. The latter were computed with a Monte Carlo .co
Lfun−ctLio−ntwoitLh+meLa+nLisaenqduadlistpoerthsieonenσclosusecdhtphraotbiatbsiilnittyeggriavlefnrobmy samplingoftheposteriorprobabilitydistributionsofα. m/mn
the fits (68.3percent for the X-ray luminosity uncertainties and ra
s
S9E5A.4BApSeSr)c.eWntefhoarvteheusmedidt-hIeR1lσumuinncoesrittayinutniecserdteariinvteiedsidnetrhivisedwafryoimn 4T.a2blTe3hesuLm6mµmar–iLze2s–1th0ekerVesruelltastioofnourregressionanalysis.Thebest- /article
ourregressionanalysis. /4
fittinglinearrelationsforthefullsampleandthetype1andtype2 4
TodeterminetheL6µm–L2–10keVrelationship,wehaveconducted AGN obtained with the K07 technique are illustrated in Fig. 5. 9/2
thelinearregressionanalysistreatingL2–10keV astheindependent TableA1liststheX-rayand6µmluminositiesofourAGN. /14
variableandL6µmasthedependentvariable.Ourdecisionisbased AlthoughtheK07techniquedeterminesthestrengthofthecor- 22
onthefactthathardX-rays,producedatdistancesmuchcloserto relation between L6µm and L2–10keV, we have also computed the /106
thecentralSMBH(subparsecscales),areamoredirecttracerofthe 8
Spearman rank-order correlation coefficient (ρ) for comparison. 2
AGN intrinsic power than the mid-IR emission, originating from 9
BoththeSpearmanrankandtheBayesiancorrelationcoefficients 3
thedustytorusattensofparsecscales.8Wehaveassumedalinear b
confirmthetightcorrelationbetweenmid-IRandX-rayluminosi- y
rela(cid:2)tionoft(cid:3)heform (cid:2) (cid:3) tiesovermorethanthreedecadesinAGNluminosity(from1042to gue
log L6µm =α+β×log L2−10keV , (1) 1046 ergs−1). st o
1044 1044 Toinvestigatetheroleofdistanceinourluminosity–luminosity n 2
correlation, we have also performed a partial Kendall τ correla- 5
where the luminosities are in units of ergs−1 and α and β are F
thecorrelationparameters.Theluminositieshavebeennormalized tLiontesttoankitnhgeidnitsotaanccceo.uTnottchoemcpoumtemtohnedseigpnenifidceanncceeooffLt6hµemmaenad- ebru
at L = 1044ergs−1, roughly the mean AGN power of our 2–10keV a
2–10keV sured Spearman ρ and Kendall τ correlation coefficients for our ry
sample. 2
sample,wehaveconducteda‘scramblingtest’(e.g.Merlonietal. 0
2
2006;Bianchietal.2009).Theideabehindthistestisasfollows: 3
8Fromastatisticalpointofviewwenotethat,inthepresenceofintrin- foreachsource,wekeepitsX-rayluminosityandz.Wethenassign
sicscatter,thebest-fittingslopesofforwardandinverseregressions(i.e. them 6 µm fluxes from the sample at random with replacement
switchingthedependentandindependentvariables)canbeverydifferent. (bootstrap) and they are converted to a mid-IR luminosity using
Tocircumventsuchproblem,bothvariablesareoftentreatedsymmetrically the z of the source. In this way, we are removing any intrinsic
usingthebisectorline(e.g.Isobeetal.1990).However,ifthedependent correlationbetweenL6µm andL2–10keV thatisnotassociatedwith
variableofthelinearregressionanalysisdependsontheparameterusedto distanceeffects.WehaverepeatedthisMonteCarlotest105times
selectthesample(althoughourAGNsampleisX-rayfluxlimited,westress
calculatingeachtimetheSpearmanρandKendallτcorrelationco-
thatweareonlyusingobjectsfromBUXSwithL2–10keV>1042 ergs−1), efficients.Thesimulationswereconductedforboththefullsample
thetruncationofthedatasetwillartificiallyflattenthemeasuredslopedue
andforthetype1andtype2AGNsamples.Theresultsarelisted
totheincreasedlossofobjectswithlowX-rayluminositiesatlowmid-IR
inTable3andareillustratedinFig.6forthefullsample(similar
luminosities(K07).Thus,theconventionalforwardfitrelation,i.e.fitting
L6µm atagivenL2–10keV,shouldprovidethemoreaccurateestimateof resultswereobtainedforthetype1andtype2AGN).Thevalues
theexistingrelationshipbetweenthemid-IRandX-rayluminositiesforour ofthecorrelationcoefficientsobtainedfortheoriginaldatasetare
AGN. wellabovethedistributionofthoseforthesimulatedsamples.Our
MNRAS449,1422–1440(2015)
Mid-IRcontinuumofAGN 1431
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Figure5. Rest-frame6µmmonochromaticluminositiesagainst2–10keVintrinsic(i.e.correctedforabsorption)X-rayluminosities.The6µmluminosities up
havebeencorrectedfortheaccretiondiscemissionandcontaminationfromtheAGNhostgalaxiesasindicatedinSection3.Thebest-fittinglinearrelations .c
o
inlog–logspacederivedwiththeK07Bayesianapproachforthefullsampleandthetype1andtype2AGNareindicatedwithsolid,dottedanddashedlines, m
/m
respectively.Wecompareourresultswiththerelationderivedfortype1AGNbyLutzetal.(2004,greyshading).
n
ra
s
/a
TheshadedareainFig.5showstherelation(and1σ dispersion) rtic
for type 1 AGN from Lutz et al. (2004). We have compared the le
/4
mid-IR/X-rayluminosityratiodistributionsofourAGNandthe42 4
9
aotb6jecµtmsinanLduetzsteimtaalt.e(s20o0f4th)ewiinthtriLn2s–i1c0kXeV-r>ay1l0u4m2ienrogssi−ty1.,Adectceocrtdioinngs /2/14
2
tothetwo-sampleKolmogorov–Smirnov(KS)test,theprobability 2
/1
ofrejectingthenullhypothesis(thetwosamplesaredrawnfromthe 0
6
sameparentpopulation)is61.3percent.Wecanthereforeconclude 82
9
thatthetwodistributionsareconsistentwitheachother.Wehave 3
b
alsocomparedtheL12µm/L2–10keVluminosityratiodistributionsof y g
ourAGNandthosefromGandhietal.(2009)withX-rayluminosi- u
e
ties>1042ergs−1(37AGN),whereL12µmarethemonochromatic st o
luminositiesinνL unitsatrest-frame12µm.Todothecomparison, n
ν 2
wecomputedL12µm(correctedfortheaccretiondiscemissionand 5 F
contaminationfromtheAGNhostgalaxies)foreachofourAGNas e
b
Figure6. ResultsoftheMonteCarlosimulationsconductedtodetermine indicatedinSection3.AccordingtotheKStest,thedistributionsof ru
thhiestocgorraremlartieopnrebseetnwtseethneLd6isµtrmibauntidonL2o–f10thkeeVK(esnodliadllhpisatrotigarlamcosr)r.eTlahtieonlacrgoe- L12µm/L2–10keV areconsistentwitheachother(47.3percentprob- ary 2
abilityofrejectingthenullhypothesis).Therefore,ourresultsare 0
efficient(wheredistanceeffectshavebeentakenintoaccount)whilethe 2
compatible with published mid-IR:X-ray relations for local AGN 3
insetplotillustratestheresultsfortheSpearmancorrelationcoefficient.The
verticaldashedlinesrepresentthecorrelationcoefficientsobtainedforthe (e.g. Krabbe, Bo¨ker & Maiolino 2001; Lutz et al. 2004; Gandhi
originalsample. et al. 2009; Levenson et al. 2009; Asmus et al. 2014). None the
less,theydonotsupporttheluminosity-dependentrelationshipde-
analysis shows that the probability that the measured correlation rivedforthetype1AGNintheChandraCOSMOS(C-COSMOS)
betweenL6µmandL2–10keVisentirelyduetotherangeofdistances andChandraDeepFieldSouth(CDFS)surveyssuggestedbyFiore
inoursampleisalmostalways<10−5(seeTable3). etal.(2009).
At a given X-ray luminosity, the standard deviation in 6 µm We have demonstrated that at AGN luminosities
luminositiesisverysimilarforbothtype1(∼0.353dex)andtype2 L > 1042ergs−1, the mid-IR emission detected with
2–10keV
(∼0.325dex)AGNandtheslopesofthelinearregressionrelations WISE at rest-frame wavelengths ∼6 µm is dominated by hot
arecompatible,withintheuncertainties.Despitethis,type1AGN dust heated by the AGN. Therefore, it is possible to indirectly
are overall ∼1.3–2 times brighter than type 2 AGN at 6 µm at a infertheintrinsic2–10keVX-rayluminositiesofAGNwithdata
given X-ray luminosity, albeit with a large overlap between both from WISE by inverting equations (1) and (2) after correcting
populations. the 6 µm luminosities for contamination from the AGN hosts
MNRAS449,1422–1440(2015)