The Compton-thick quasar at the heart of the high-redshift giant radio galaxy 6C 0905+39

0.22 MB·English

by  M. C. Erlund

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed2February2008 (MNLATEXstylefilev2.2)
The Compton-thick quasar at the heart of the high-redshift giant
radio galaxy 6C0905+39
M. C. Erlund,1⋆ A. C. Fabian1, Katherine M. Blundell2
and Carolin S. Crawford1.
1InstituteofAstronomy,MadingleyRoad,CambridgeCB30HA
8
2UniversityofOxford,Astrophysics,KebleRoad,OxfordOX13RH
0
0
2

n
2February2008
a
J

2
ABSTRACT
2 OurXMM-Newtonspectrumofthegiant,high-redshift(z = 1.88)radiogalaxy6C0905+39

showsthatitcontainsoneofthemostpowerful,high-redshift,Compton-thickquasarsknown.
]
h Itsspectrumisveryhardabove2keV.ThesteepXMM-Newtonspectrumbelowthatenergyis
p showntobeduetoextendedemissionfromtheradiobridgeusingChandradata.Thenucleus
- of6C0905+39hasacolumndensityof3.5+1.4×1024cm−2andabsorption-correctedX-ray
o −0.4
r luminosityof1.7+−00..91×1045ergs−1 inthe2−10keVband.Alowerredshiftactivegalaxy
t inthesamefield,SDSSJ090808.36+394313.6,mayalsobeCompton-thick.
s
a
[ 1 INTRODUCTION sorption along the line-of-sight istaken to be 1.91×1020cm−2
(Dickey&Lockman1990).
1
Thepowerful,classicaldoubleFRII(Fanaroff&Riley1974)radio
v
galaxy 6C0905+39 is unusual as it is one of the highest redshift
1
4 (z = 1.88) giant radio galaxies known, spanning 111arcsec on
2 DATAREDUCTION
3 thesky(Law-Greenetal.1995)correspondingtoaprojectedsize
3 of945kpcinthecosmologyassumedinthispaper.Radiogalax- XMM-Newton observed 6C0905+39 on 2006 October 30 for
1. ies have long been associated with obscured, (optically defined) 58.0ks with EPIC-pn and 61.9ks with each of the EPIC-MOS
0 Type 2 quasars, because they are considered to be the same ob- cameras.Thedatawerereduced usingthestandardpipeline, run-
8 jects as radio-loud quasars seen through an obscuring torus (e.g. ningtheSAStoolsEPCHAINandEMCHAINfortheEPIC-pnand
0 Barthel 1989 and Antonucci 1993). X-ray observations are par- bothEPIC-MOScamerasrespectively.Theresultingfileswerefil-
: ticularly efficient at detecting obscured AGN as they provide the teredtoremoveflares,leaving42.2ksofgood-timeforthePN,and
v
observer with several diagnostics, such as the photoelectric cut- 53.1ksforbothMOS1andMOS2whendead-timeintervalsare
i
X off, the iron line equivalent width (EW) and the X-ray to optical alsotakenintoaccount.
r or [O III] λ5007 ratio (e.g. Bassanietal. 1999 or, for a review, Spectraforthebackground(anareaofskyfreefromsources
a Comastri 2004). Sources with an obscuring column greater than near 6C0905+39 on the same chip), the central source (within a
1.5×1024cm−2,atwhichtheThomsonscatteringopticaldepth 15arcsec circle) and the extended X-ray emission which is dis-
reachesunity,areknownasCompton-thickAGN.Itisnon-trivial cussed elsewhere (Erlundetal. 2007) were extracted separately
to calculate the column density of such sources as there is little for each instrument and stacked and fittedusing XSPECv11.3.2.
(seeWilman&Fabian1999)directemissionbelow10keVmean- GiventhelownumberofcountsintheMOSspectra,onlythePN
ing that analysis relies on interpreting the reflected spectrum at spectrawereusedforanalysisofthecore.
lowerX-rayenergies.Suzakuisstartingtoprovideawindowinto
theX-rayregimeabove10keVenablingdirectdetectionofnuclear
emissioninthebrightestCompton-thicksourcessuchasNGC5728
(Comastrietal.2007).SuchsourcesareimportantsincetheX-ray 3 RESULTS
background gives strong evidence for a significant population of
TheX-rayemissionextendingalongtheradioaxisoverhundreds
Compton-thickAGN(e.g.Comastri2004,Worsleyetal.2005).
of kpc to the east and west, which was first detected by Chan-
In this paper, we present recent XMM-Newton observa- dra and presented in Blundelletal. (2006), is clearly visible in
tions of 6C0905+39 (which is located at RA 09h08m16.9s, Dec theXMM-Newtondata(seeFig.1).Intotalthereare2566counts
+39d43m26sinJ2000).Throughoutthispaper,allerrorsarequoted (1783 background) inthe source, 2125 (1624 background) inthe
at 1σ unless otherwise stated and the assumed cosmology is extended emission and 441 counts (159 background) in the core
H0 = 71kms−1Mpc−1, Ω0 = 1 and Λ0 = 0.73. One arc- inthe0.2−8keVbandaftersummingtheMOS1,MOS2andthe
second represents 8.518kpc on the plane of the sky at the red- PNdata.ThesmallnumberofcountsinthenucleusinourChandra
shift (z = 1.883 ±0.003) of 6C0905+39 and the Galactic ab- observation(Blundelletal.2006)precludedanydetailedanalysis.
(cid:13)c 0000RAS
2 M. C. Erlundet al.
Figure1.ThreecolourplotofXMM-Newtondata(188′′×116′′imagewith2arcsecpixels;theimagehasbeensmoothedbyaGaussianwithσof2pixels):
0.2−1keVband(red),1−3keVband(green)and3−10keV band(blue)allonasquare-rootscale. Thewhiteoverlaidcontours areVLAA-array
1.425-GHzradiocontours(0.4,1.7and7mJy/beam),displayingthecoreandtwolobesoftheradiosource.Thebluesourcetothewestof6C0905+39is
anobscuredAGN(seesection3.3).
Allspectraldatadescribedinthepresentpaperwerefittedoverthe
0.5−10keVband.
3.1 Theextendedemission 10-3
TheextendedemissionisdiscussedindetailinErlundetal.(2007);
however,webrieflymentionithereinthecontextofitseffectasa
potential contaminant of the core spectrum. Fig. 1 clearly shows -1V
ke
the extended emission from the bridge of 6C0905+39 and that -1c
ttehnedneudmembeirssoiofnXi-srnayotpjehtoetomnisssiinocnre(saisnecsettohwisawrdosutlhdebceovreer.yTnhaerreoxw- unts se 10-4
o
and sowouldnot beresolved byChandra), but inverse-Compton c
up-scatteringofthecosmicmicrowavebackgroundbyspentradio
plasmafromthelobes(Blundelletal.2006).
Thebridgespectra(excluding a15arcsec regionaround the
core) were extracted from the EPIC-pn, MOS1 and MOS2 data
partial covering model
andwerefittedwithaGalactic-absorbedpower-lawgivingapho- power law model
ton index of Γ = 1.75+0.27 and a normalisation of 2.5+0.3 × 10-5
−0.24 −0.3
10−6ctkeV−1cm−2s−1withareduced-chisquaredofχ2ν =0.9 0.5 1 2 5
with 90 degrees of freedom (d.o.f.). The absorption-corrected X- Energy [keV]
rayluminosityoftheextendedemissioninthe2−10keVbandis
LX =1.8+−00..33×1044ergs−1. Figure2.Thespectrumofthecoreof6C0905+39.Thesolidlineshowsthe
bestfitpartialcoveringmodelandthedottedlineshowsthecontinuationof
thedirectlyobservedpower-lawcomponent.
3.2 Thecore
OnlythePNdatahavebeenusedinthefollowingfitsbecauseeach
MOSspectrumcontainsroughlyaquarterofthecountsinthePN an excess in the hard X-rays which can, in principle, be repro-
spectrum but a similar amount of noise. Spectral analysis shows ducedbycoldreflection(XSPECmodelwabs*zwabs*[powerlaw
thattheMOSspectraareconsistentwiththePNspectrum. +pexrav]).Thismodelisnotagoodfithowevergivingχ2 =2.2
ν
The XMM-Newton spectrum of the core is strikingly hard. for7d.o.f.(χ2 = 15.4) withanimplausibly largeamount ofre-
A simple Galactic and intrinsic absorbed power-law is a poor flection(reflectionfraction∼33)andaphotonindexofΓ=1.6.
fit with χ2 = 2.7 for 8 d.o.f. (χ2 = 21.7). The best-fit pa- Apartial-coveringmodelwitharedshiftedironlineisagood
ν
rameters give a photon index of Γ = 0.93, a normalisation of fit(Fig.2).Includinganironline,themodelgivesacoveringfrac-
1.3×10−6ctkeV−1cm−2s−1 andnointrinsicabsorption.The tion of 0.97+0.02, intrinsic absorption of 2.6+1.0 × 1024cm−2,
−0.03 −0.7
spectrum (Fig. 2) appears to have a power-law component with photon index Γ = 2.3+0.2 and a normalisation of 5.0+0.7 ×
−0.3 −0.8
(cid:13)c 0000RAS,MNRAS000,000–000
3
10−5ctkeV−1cm−2s−1 for thepower-law. Theredshifted iron
line (modelled as a Gaussian) was fixed at 6.4keV in the rest-
frame of the source with σ = 0.1keV, giving an equivalent
width EW ∼ 0.9keV; the normalisation of this component is
2.2+1.5 ×10−5ctcm−2s−1. Thisgives χ2 = 0.9 with6d.o.f.
−1.4 ν
(χ2 = 5.4).Withoutanironline,thefitvaluesaresimilar:acov-
ering fraction of 0.95+0.01; an intrinsic absorption of 2.1+1.0 ×
−0.04 −0.6
1024cm−2; a photon index Γ = 2.1+0.2 and a normalisation of
−0.2
3.2+0.6×10−5ctkeV−1cm−2s−1,givingχ2 =1.1for7d.o.f.
−3.2 ν
(χ2 =7.8).Includinganironlineimprovesthefitby∆χ2 =2.4,
sothedatamarginallyfavouritsaddition.Theabsorption-corrected
luminosityinthe2−10keVbandforthepartialcoveringmodel
with an iron line is LX = 3.2+−00..32 × 1045ergs−1 and the ob-
served flux is FX = 3 × 10−14ergcm−2s−1 (the errors are
poorly constrained). The column density of 6C0905+39 is that
of a Compton-thick AGN and its luminosity implies that it is a
quasar.SuchCompton-thickquasarsarerare,Fig.3(adaptedfrom
Gandhietal.2006)illustratesthispoint.ItshowsaselectionofX- Figure3.ThisfigureisadaptedfromGandhietal.(2006)(theirFig.7,see
rayselectedtype-2quasarswhicharenotreflectiondominated.We theircaptionfordetailsofhowthisplotwasproducedandtherelevantref-
infer from this that 6C0905+39 isparticularly intrinsically lumi- erencesforeachsource).TheX-rayluminosityaxisshowstheabsorption-
nousandhighlyobscured. correctedluminosityinthe2−10keVbandofeachsource.B20902+39
The paucity of counts in the Chandra data (14 ± 4 in the (B20902,Fabianetal.2002),SWIREJ104406.30+583954.1andSWIRE
0.5−7keV band) means that no analysis was performed on the J104409.95+585224.8(SW104406,SW104409respectively;Pollettaetal.
2006)and6C0905+39havebeenadded.ThisselectionofX-rayselected
originaldetectionofthecore(Blundelletal.2006).TheChandra
type2quasarsfromtheliteratureexcludessourceswhichareclearlyreflec-
dataareconsistentwiththeXMM-Newtondatainasmuchasthey
tiondominated.Thisimpliesthat6C0905+39isoneofthemostintrinsi-
are incapable of constraining the spectral shape of the nucleus.
callyluminousandoneofthemostobscuredquasarsyetdetected.
However,inthe2arcsecregionextractedfortheChandraanalysis
therearesignificantlyfewersoftphotonsthanintheXMM-Newton
data(15arcsecregion).Toquantifythiswemeasuretheobserved
count-ratebelow1.5keV,whichis1.5±0.9×10−4counts−1,and
compareittothatpredictedfromamodeloftheXMM-Newtondata
inthesameband,whichgivesacount-rateof5×10−4counts−1.
This deficit is significant at ∼ 4σ level and implies that the soft
photonsarefromtheextendedemissionwhichislessofacontam-
inantinthesmallerChandraaperture.
Giventhat thereisaconsiderable amount ofextended emis-
sion from the lobes of the radio source (clearly seen in Fig. 1)
thatislikelytobecontaminatingtheXMM-Newtonnucleusspec-
trum,wenowtryahighlyobscuredAGNwithapower-lawcom-
ponentatlowenergiesfromtheextendedemission.Inordertotest
this, we fit a model consisting of Galactic absorption; a power-
law with a photon index fixed to the value found for the ex-
tended emission (Γ = 1.75, see Erlundetal. 2007); an intrinsi-
cally absorbed power-law (with a photon index fixed to Γ = 2
as the data are unable to constrain it); and a redshifted iron line
with it energy fixed at 6.4keV and σ = 0.1 (XSPEC model
wabs[powerlaw + zwabs(powerlaw + zgauss)]). The best-
fitting model is shown in Fig. 4. The intrinsic column density is
3.5+1.4 ×1024cm−2 andthenormalisationofthiscomponent is
−0.4
2.7+1.4×10−5ctkeV−1cm−2s−1.Theredshiftedironlinehas
−0.1
anormalisationof2.7+11.2×10−4ctcm−2s−1.Thismodelgives
−2.5
a good fit to the data with χ2ν = 1.1 for 7 d.o.f. (χ2 = 7.4); Figure4.ThebestfitobscuredAGNwithcontaminationisshowninblack.
statisticallythe partial covering model isa slightlybetter fit with ItconsistsofanabsorbedGalacticandintrinsicabsorbedpower-law(green
χ2 = 5.4 for 6 d.o.f. which gives ∆χ2 = 2. The observed line); anadditional power-law component fixedtohavethesamephoton
flux is FX = 2.8+−10..45 × 10−14ergcm−2s−1. The absorption- indexastheextendedemission(Γ = 1.75,redline)andaredshiftediron
correctedluminosityinthe2−10keVbandofthecentralsourceis line(blueline).
LX,core = 1.7+−00..91×1045ergs−1.Thenormalisationofthecon-
taminatingpower-lawis1.5+0.2×10−6ctkeV−1cm−2s−1and
−0.2
itsabsorption-correctedX-rayluminosityinthe2−10keVband
is LX,ext = 1.1+−00..12 ×1044ergs−1. The luminosity of the ex-
tendedX-rayemissionisLX = 1.9+−00..33×1044ergs−1 implying Compton scattering of the cosmic microwave background comes
that roughly a third of the power in the emission due to inverse- fromclosetothecore.
(cid:13)c 0000RAS,MNRAS000,000–000
4 M. C. Erlundet al.
Themeasuredobscuringcolumnandabsorptioncorrectedlu-
V minosity suggests that 6C0905+39 is a genuine Compton-thick
e
ec/k 10−4 quasar. (Wenote that the column density measured directly from
nts/s X-rayspectra depends onthegeometry oftheobscuring medium
ed cou 10−5 apsleshtoorwusngbeyomMeattrtyesteael.m1s9r9e9le.vHaonwt.)evTehre,rienathreevperreysefnetwcaCsoemapstiomn--
normaliz 10−6 athgicokoqduCasoamrspktonno-wthnicakthqiugahsarerdcsahnidfti.dNatoerminatnheetCahl.a(n2d0r0a2d)edeepteficteeldd
South with z > 3. Apart from such clear examples, very few
1
Compton-thickquasarsareknownespeciallyatredshiftz∼2,de-
χ 0 spitethefactthatobscuredAGNareexpectedtomakeup∼30per
1
− centofthehardX-raybackgroundand70percentshouldbefound
1 2 5 between 1 < z < 3, according to populations synthesis models
channel energy (keV)
(seee.g.Normanetal.2002andGillietal.2001).
Figure5.ThePNspectrumofthesecondobscuredAGNinthefield(the Theabsorption-correctedluminosityinthe2−10keVband
bluesourcetothewestof6C0905+39inFig.1).Thesolidlineisthebest
canbeconvertedtothebolometricluminosityusingresultsderived
fitabsorbedpower-lawmodelwitharedshiftedironline.
by Vasudevan&Fabian (2007) who found that low and high Ed-
dington ratio1 AGN have a bolometric correction of 19±4 and
55±13respectively.Inordertodeterminewhether6C0905+39isa
3.3 AnotherobscuredAGNinthefield?
low-orhigh-Eddingtonsource,theblackholemassisestimatedus-
ThehardX-raysourcetothewestof6C0905+39(thebluesource ingtheblack-hole-mass–bulge-massrelationship(Magorrianetal.
inFig.1locatedatRA09h08m08.3s, Dec+39d43m12s inJ2000 1998) and the stellar mass (measured by Seymouretal. 2007) as
coordinates)isanSDSSsourceandso,followingtheSDSSnam- a proxy for the bulge mass. This gives a black hole mass for
ingconvention,isSDSSJ090808.36+394313.6.Ithasaspectrum 6C0905+39 ofMBH ∼ 2×109M⊙.Thisisconsistentwiththe
consistentwithitbeinganobscuredAGN(seeFig.5)withcolumn value calculated using the method presented in Marconi&Hunt
density of NH ∼ 1.28+−00..4316 × 1024cm−2 when the power-law (2003), who make use of the black-hole-mass–bulge-luminosity
componentisfixedatΓ=2becauseitisotherwiseunconstrained relation, and the K-band magnitude (from Seymouretal. 2007)
(although varying it between reasonable values does not signifi- alongwithacruderedshiftcorrection.Thisassumesthatsuchre-
cantly alter the inferred value of the column density). Adding an lationships hold to out to redshift z = 1.88 which is not nec-
iron lineredshifted from6.4keV improves thefit from 1.4 for 4 essarily true, but radio galaxies at lower redshifts tend to be the
d.o.f. to 0.2 for 3 d.o.f., i.e. by ∆χ2 = 5. The redshift is mea- hosts of similarly massive black holes (e.g. Liuetal. 2006) so
suredasz =0.45+0.02.Thereareatotalof261counts(159back- this estimate seems reasonable. Using these estimates for black
−0.08
ground)inthe0.2−8keVband,meaningthatthederivedquantities hole mass, 6C0905+39 appears to fall into the high-Eddington
are somewhat uncertain; however, as Fig. 1 shows there are very regime(Lbol/LEdd > 0.1) sotheappropriatebolometriccorrec-
few photons below 3keV, this source must be a highly obscured tion is 55±13 which implies that the bolometric luminosity of
AGN. The 2− 10keV band observed (absorbed) flux is FX = 6C0905+39isLbol =1.0±0.4×1047ergs−1.Thismeansthat
3.1+5.7×10−14ergcm−2s−1andtheabsorption-correctedX-ray itisaverypowerfulsource.Comparedtootherpowerful,obscured
−1.0
luminosityinthesamebandisLX =1.3+−00..32×1044ergs−1. quasars(seeFig.3,assumingthatthemeasuredcolumndensityis
accurate), 6C0905+39 is amongst the most powerful, (X-ray de-
fined)type2quasarscurrentlyknown.
Recent research hasshown that high-Eddington objects tend
4 DISCUSSION
not to be jetted (e.g. Panessaetal. 2007; Sikoraetal. 2007 and
Thespectralfittingofthenucleusof6C0905+39presentedinSec- Maoz 2007). However, 6C0905+39 does not seem to be follow
tion3.2showsthatamodel consistingofanobscured quasar nu- this trend as it is an extremely powerful source which is host to
cleuswithacontaminatingpower-lawfromthelobesisagoodfit aradiosourcewhichspans945kpcontheskyandappearstostill
tothe data. Itscolumn density and luminosity clearlymeans that beactivelyfed.Itsgiantsizeindicatesthatithasbeencontinuously
thisisaCompton-thickquasar. activeforatleastthepasttenmillionyears.Suchhigh-redshiftgiant
Seymouretal.(2007)haveperformedaSpitzersurveyof69 radiogalaxiesmayactasbeaconsforextremelypowerfulAGN.
high-redshiftradiogalaxiesandfindthatthemonochromaticlumi-
nosityof6C0905+39at8µmisL8µm ∼5×1044ergs−1.They
donothavemid-infrareddatafor6C0905+39,butinthemajority
5 CONCLUSIONS
ofcaseswheretheyhavelongerwavelengthdata,thespectralen-
ergydistributions(SED)oftheirhigh-redshiftradiogalaxiescon- Our XMM-Newton spectrum of 6C0905+39, which is one of the
tinuetorise,oftenbyonetotwoordersofmagnitude.Observing highest redshiftgiant radiogalaxiesknown, shows ittocontaina
6C0905+39atthesewavelengthsisimportanttoconfirmitsnature Compton-thickquasar.6C0905+39isoneofveryfewsuchsources
asaCompton-thickquasar. currentlyknownandoneofevenfewerathighredshift.Itislikely
Radio galaxieshavelong been supposed toberadioquasars tobeahigh-Eddington sourcewithabolometricluminosityof∼
oriented closer to the plane of the sky. Given its immense linear 1047ergs−1,makingitoneofthemostpowerfulsourcesknown.
sizeofnearly1Mpc,6C0905+39islikelytolieclosetotheplane
of theskyand so,following theunificationmodel, weareseeing
thecorethroughthetorusandthuswewouldexpectthesourceto 1 wheretheEddingtonratioisdefinedasthebolometricluminositydivided
beCompton-thick.Thisalsomeansthatbeamingisunimportant. bytheEddingtonluminosity
(cid:13)c 0000RAS,MNRAS000,000–000
5
ACKNOWLEDGEMENTS
MCEacknowledgesPPARCforfinancialsupport.ACFandKMB
thanktheRoyalSociety.MCEthanksPoshakGandhiforproviding
thedatatorecreatehisfigure(Fig.3)sothat6C0905+39couldbe
includedandRanjanVasudevanforhelpfuldiscussions.
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