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THEASTROPHYSICALJOURNAL,516:917,1999May11999.
TheAmericanAstronomicalSociety.
Allrightsreserved.
PrintedinU.
S.
A.
(RADIO-QUIETREDQUASARSDONG-WOOKIM1ChungnamNationalUniversity,Taejon,305-764,SouthKoreaANDMARTINELVISHarvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138Received1998May11;accepted1998December8ABSTRACTWehaveperformedasuccessfultargetedsearchforapopulationofred,radio-quiet,andprobablyabsorbedquasars.
Radio-quiet,opticallyredROSATPSPCX-raysourcesbrighterthan1]10~13ergscm~2s~1weresearchedforred(O[E[2.
0,O20)counterpartsintheAutomatedPlate-measuringMachinecatalogofPalomarSkySurveyobjects.
Of45objectsforwhichweobtainedadequatefollow-upopticalspectroscopy,wehavefoundsevenredquasars,vewithTheirredshiftsrangeaopt\[2.
from0.
06to0.
31,andtheirluminositiesaremoderate,lyingonthequasar/Seyfertboundary.
Theseredquasarsstrengthenthecaseforaradio-quietpopulationthatisthecounterpartoftheradio-loudredquasarsfoundbySmith&SpinradandWebsteretal.
Unidentied,faintersourcescouldincreasethefractionofredquasarsbyuptoafactorof7.
Fortheredquasarsfoundhere,theHa/Hbratios,opticalslope,andX-raycolorsallindicatethattheyareabsorbedbyratherthanhavingintrinsicallyredAVD2spectra.
ThisamountofobscurationseemstohideD1%7%ofquasarsatagivenobserveduxorD3%20%whentheiruxesarecorrectedtotheirintrinsicvalues.
Thissizeofpopulationisconsistentwithearlierlimits,withpredictedvaluesfromComastrietal.
,andiscomparablewiththeratefoundamongradio-loudquasars.
Alargepopulationofmoreheavilyabsorbedfainterquasarsequal(AV\5),insizetothebluepopulationcouldexistwithoutviolatingexistingupperlimits,whichisinaccordwiththeComastrietal.
predictions.
Subjectheadings:dust,extinctionquasars:generalX-rays:galaxies1.
INTRODUCTIONQuasarsarethecanonical""ultravioletexcessobjects(Sandage1965).
Yetredquasarshavebeenfoundinradio-selectedsamplesbySmith&Spinrad(1980)andWebsteretal.
(1995).
Websteretal.
(1995)proposedthatalargefrac-tion,perhaps80%,ofradio-loudquasarsmighthavebeenhiddenasredobjects.
Moreover,thecurrentlyfavoredexplanationsforthecosmicX-raybackgroundinvokeapopulationofheavilyobscuredactivegalacticnuclei(AGNs)5timesmorecommonthantheunobscuredpopu-lation(Comastrietal.
1995).
Ifthesmallnumberofknownredquasarsreallyisthe""tipoftheicebergofalarge,evendominant,quasarpopulation,thentheconsequenceswouldbeinteresting:theoverallAGNpopulationandsothemassiveblackholepopulationmaybe5timeslargerthanhadbeenthought;obscuredquasarswouldbealong-livedevolutionaryphase(seeSandersetal.
1988),orallquasarsmaybehiddenalong80%ofpossiblelinesofsight;andredquasarsmaycontributeimportantlytothecosmicX-raybackground.
TheWebsteretal.
conclusioniswidelydis-puted.
Boyle&diMatteo(1995),Stickeletal.
(1996),andBennetal.
(1998)allarguethatanymissingpopulationmustbesmallerandperhapsinsignicant,whileGunn&Shanks(1998)disagree.
HerewepresentanX-raybasedsurveytargetedexplicitlyatredAGNstondradio-quietredquasars.
Radio-loudredquasarsarerelativelyeasytond,sincetheradioemissionisunaectedbyabsorbinggasordustand,inlow-frequencysurveys,usuallycomesfromthelargeradiolobesthatliewelloutsideanyobscuringmaterialin1AlsoatHarvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138.
thehostgalaxy.
Anexplicit,albeitsmall-scale,searchforradio-andX-rayloudbutopticallyquietquasars,whichshouldincludereddenedquasars,was,however,notsuc-cessful(Kollgaardetal.
1995).
Radio-quietredquasarsaremuchhardertond,althoughtheymightbeexpectedtobemuchmorecommon.
Inthenormalunreddenedpopulation,radio-quietquasarsoutnumberradio-loudquasars10to1(e.
g.
,theExtendedMediumSensitivitySurvey,Stockeetal.
1991;thePalomarGreencatalog[PG],Kellermanetal.
1989).
However,mostopticalquasarsurveysareactivelybiasedagainstndingredquasars.
SincethesesurveysprimarilysearchforUV-brightobjects(e.
g.
,Markarian,Lipovetsky,Markarian,&Stepanian1987;PG,Schmidt&Green1983;theLargeBrightQuasarSurvey,Hewett,Foltz,&Chaee1995;theHamburgQuasarSurvey,Engelsetal.
1998),theyareblindtoredobjects.
Asaresult,opticalboundsonhowlargeapopulationofradio-quietredquasarsmightexistareweak.
X-rayselectionprovidesawayofselectingredquasarsefficiently:hardX-rays(210keV)penetrateeventensofmagnitudesofopticalextinctionwithminimalabsorption.
EventhelowerenergybandofROSAT(0.
52.
5keV)isnotstronglyaectedbyopticalextinctionofuptoD2mag.
2Astrophysicsmightbeagainstus,sincealthoughbluequasarsareoverwhelminglyX-rayloud(Avni&Tanan-baum1986),redonesmightbeintrinsicallyX-rayfaint.
Fortunately,however,radio-loudredquasarsareknowntobeX-raysources(Bregmanetal.
1985;Elvisetal.
1994),sothisisunlikelytobeaproblem.
2ForstandardMilkyWaycompositionanddust-to-gasratio(Bohlin,Savage,&Drake1978;Seaton1979)thePSPCcountrateisreducedbyafactorof1/eatatomscm~2)forapower-lawAV\1.
7(NH\3]1021photonindexof2.
0,allat0redshift.
910KIM&ELVISVol.
516Completeux-limitedX-raysurveyshavefoundsomeredquasarsorAGNs(Stockeetal.
1982;Kruper&Canizares1989;Puchnarewiczetal.
1996,1997).
InparticulartheROSATInternationalX-Ray/OpticalSurvey(RIXOS)survey(Puchnarewicz&Mason1998)hasidentiedasmallsampleofredquasars.
AspartofamoregeneralsearchforminoritypopulationsofX-raysources,wehaveusedtheROSAT1983)(TrumperarchiveofpointedPSPC(Pfeermanetal.
1987)datatodesignanefficientsearchstrategyexplicitlytargetingredquasars.
TheROSATpointedarchiveprovidesuswithatenfoldincreaseinthenumberofX-raysourcesfrombeforeROSAT(toabout70,000)andreachesmorethan10timesfainterthantheROSATAll-SkySurvey(Vogesetal.
1996).
Bycarefullyselectingasmallnumberofinterestingobjects,wecanmakeanefficientsearchforaradio-quietpopulationofredquasars.
Inthispaperwendasubstantialpopulationofradio-quietredAGNsonthequasar/Seyfertluminositybound-ary.
2.
SAMPLESELECTIONMostROSATsourcesathighGalacticlatitudeareblue,unobscuredAGNs(e.
g.
,Boyle,Wilkes,&Elvis1997;Schmidtetal.
1997).
WeusethisfacttoefficientlyselectagainstsuchobjectsandsoisolateanyredAGNthatmaybeintheROSATarchive.
AsacompendiumoftheX-raysourcesfoundbytheROSATPSPCwehaveusedthe""WGACATcatalog(whichisnamedafteritsauthors,White,Giommi,&Angel-ini1995).
ThiscatalogwasgeneratedfromROSATPSPCpointedobservationsusingaslidingcell,detectalgorithm.
Thismethodissensitivetondingpointsourcesbutcanalsondspurioussourceswhereextendedemissionispresent.
WGACATincludesaqualityagthatnotessuchdubiousdetectionsbasedonavisualinspectionoftheelds.
WehaveonlyusedX-raysourceswithhighdetectionqualityinordertoexcludethespurioussources.
FromtheWGACATcatalogwehaveselectedsourcesbythefollow-ingcriteria:1.
X-raybrightergscm~2s~1)toallow(fX[10~13follow-upobservationswithotherX-raytelescopes;2.
welldetectedwithasignal-to-noiseratiogreaterthan10andaWGACATqualityag,DQFLAG,greaterthan5;3.
withinr\18@fromthedetectorcentertoprovidegoodpositions;4.
athighgalacticlatitude(obo[20)tominimizethefractionofGalacticstars(whicharealsored);5.
notwithin2@ofthetargetposition(atwhichpointthesourcedensityreachesthebackgroundlevel)toselectonlyrandom,serendipitous,sources;6.
northofdecl.
\[18inordertohavetwobandmea-surementsintheAutomatedPlate-measuringMachine(APM)catalog(McMahon&Irwin1992),hencegivinganarchivalopticalcolor;and7.
unidentied,withWGACATclass\9999andnoSIMBADorNASAExtragalacticDatabaseidentication.
3Therstsixcriteriaselected1624sources.
SincethesesourceswereselectedpurelyontheirX-rayproperties,theyformawell-denedsamplefromwhichtostudytheinci-denceofminorityX-raypopulations,includinganyradio-quietredquasars.
Addingtherequirementthatasourcebeunidentiedleft940X-raysourcesthatcouldbeexaminedforhavingredopticalcounterparts.
WethensearchedtheAPMcatalogofobjectsdetectedonthePalomarSkySurveyforopticalcounterpartstotheunidentiedX-raysources.
Tondcounterparts,weusedasearchradiusof26A,whichcorrespondstoabout95%con-denceforX-raysourceswithin18@ofthePSPCdetectorcenter(Boyleetal.
1995a).
Ofthese,881sourceshadAPMcatalogcounterpartsbrighterthanthelimitingmagnitudesO\21.
5andE\20.
(Theremaining""blankeldsarethe3Althoughonly""unidentiedsourceswereselected,one(1WGAJ1118.
0]4505;Table1)turnedouttobeaknownSeyfert1(Badeetal.
1995).
Fig.
1aFig.
1bFIG.
1.
(a)OpticallyidentiedEMSSsourcesintheplane.
Dierentsourcesaremarkedbydierentsymbols.
Oursource-selectioncriteriaaox-(O[E)(O[E[2,areseenasalowerrightbox.
(b)Sameas(a)butforourROSATsamples.
Thelineshowshowaslightlystrictercriterionyieldsaaox\1.
8)aoxlargerfractionofredquasars.
No.
1,1999RADIO-QUIETREDQUASARS11subjectofanotherstudy;Elvis,Kim,&Nicastro1999.
)Amongthese881,manyhadonlyO-bandmagnitudes(suggestingthattheyareblue),leaving575withO[Ecolorsavailable.
ThecombinationoftheROSATX-rayuxandtheAPMmagnitudesallowsustocreatearoughclassicationoftheX-raysourcesinoursample.
ThetwoPalomarO(blue)andE(red)magnitudesareclosetoJohnsonBandCousinsR,respectively(Greggetal.
1996).
FromtheX-rayuxandthetwoopticalmagnitudeswecanconstructatwo-colordiagramofisdenedbythepower-lawindexaox-(O[E).
aoxbetween2keVand2500(Tananbaumetal.
1979;StockeAetal.
1991).
ThisdiagramallowsustoselectredobjectsandthenreducetheGalacticstellarpopulationamongtheseredobjectsbyselectingtheX-rayloudpopulation.
HerewemakeuseoftheobservationthatinstellarsourcestheX-rayuxforagivenopticaluxismuchweakerthaninAGNsandclusters(e.
g.
,Maccacaroetal.
1988).
Figure1ashowsEinsteinExtendedMediumSensitivitySurvey(EMSS)X-raysources(Stockeetal.
1991)intheplane.
aox-(O[E)ThisplotclearlyillustratesthedistinctionbetweenGalacticstellarsourcesandextragalacticsources.
BasedontheEMSSsourcedistribution,wedividedtheROSAT-APMsourcesonthesameplane(Fig.
1b).
TheO[EAPMcolorsarenotasaccurateastheEMSSvalues,whicharebasedonCCDphotometry.
Asaresult,thespreadofobservedcolorsiswider(Fig.
1b),andtherewillbesomeblueobjectsintheredzoneandviceversa.
Tocreateourlistofredquasarcandidates,werstexcludedthe128sourceswithbecausetheyarelikelytobeGalacticaox[1.
8,stars.
Thenweexcludedanother360sourceswithbluecolors,O[E\2,becausetheyaremostlikelyjustnormal,blue,unobscuredAGNs.
Thisresultsinanalsampleof87X-raysourcesdenedbythelowerrightcornerofFigure1bintheplane.
aox-(O[E)Oursampleof87opticallyredX-rayloudsourcesisamere0.
1%oftheD70,000WGACATsources.
ThefractionofX-raysourcesthatmayberedquasars,however,ismuchlarger:D5%oftheinitialX-rayselectedsample,D15%oftheunidentiedsourceswithAPMcolors,andD20%ofX-raybrightobjectswithAPMcolorsthatwillprimarilybeAGNs.
However,otherclassesofX-raysourcethanredquasarscaninhabitthisregionoftheplane:forexample,aox-(O[E)rst-rankedellipticalgalaxiesindistantclustersofgalaxies.
Opticalspectroscopyisneededtondredquasars.
Wehavetakenspectrafor51ofthe87redquasarcandidates,asisdescribedinthenextsection.
3.
OBSERVATIONS3.
1.
OpticalSpectroscopyAtypicalX-rayerrorcirclecontainsjust12opticalobjectsintheAPMcatalog.
Sincewehaveselectedagainstbluecounterparts,webeganbyobservingthebrightestredcounterpart.
Iftwoobjectswerepresent,wealignedthespectrographslittoobtainspectraofbothatonce.
IfthistherstspectradidnotndanAGN,wethenobservedthenextfaintest,ifpresent.
Sincethedensityof(blue)AGNsatB\21isonlyD0.
005pererrorcircle(e.
g.
,Zitellietal.
1992),weexpectonlya1in4chanceofAGNcoincidenceinthe51spectra,sostoppingonceanAGNisfoundwillnotproduceasignicantnumberoffalseidentications.
WeperformedopticalspectroscopywiththeMultipleMirrorTelescope(MMT)on1997March1315,withtheFredLawrenceWhippleObservatory(FLWO)60Atele-scopeon1996November1617and1997February1213,andwiththeCerroTololoInter-AmericanObservatory(CTIO)60Atelescopeon1997February35.
Weusedlong-slitaperturesof2@@3@@]180@@andgratingswith300gpm.
Thespectralresolutionsare6and9fortheMMTand60AAtelescopes,respectively.
Wavelengthcoverageisabout35008000Wetookbias,domeat,andtwilightskyA.
frameseachnight,andthecorrespondingcorrections(biassubtraction,atelding,andilluminationcorrection)wereappliedseparatelytoeachnightofdata.
Atleasttwostan-dardstarswereobservedeachnightforspectrophotometriccalibration.
Theobservingconditionswerenotphoto-metric,exceptfortheCTIOrun,sotheabsolutecalibrationissubjecttoasignicantuncertainty.
However,relativeintensities(suchasalineintensityratioandanopticalpower-lawindex)areaccuratewithin20%,asisconrmedbymultipleobservationsofthesamesource.
Sixsourcesareofundeterminednaturebecausetheyaretoofaintandsogavespectraoftoopoorasignal-to-noiseratio.
3.
2.
ClassicationofSpectraOfthe45sourcesobservedatagoodsignal-to-noiseratio,wehaveidentiedsevenredquasars(Table1).
Theresultsforthesesevenredquasarsarepresentedinthispaper.
(Thefulldatasetwillappearelsewhere.
)Theredquasarsaremixedinwith18starsandasmallnumberofnormalbluequasars,narrowemissionlinegalaxies,andellipticalgalaxies(Table1).
Theellipticalgalaxiesarelikelytobebrightestclustergalaxies.
Wewillreportontheseseparately.
FortheremainingnineX-raysources,theredopticalcandidatewithintheerrorcircleturnedouttobeastar(mostlylatetype),butitisnotlikelythattheseredstarsarethecounterparts,becausetheirvaluesaretoolargeaoxforastar(seeabove;Maccacaroetal.
1988).
Theremainingopticalcandidatesarenotred,andhencewestoppedmakingfurtherobservations.
TheseninesourcesandthethreebluequasarsmeasurethebluecontaminationofthesampleandshouldnotbeconsideredaspartofthelistofredX-raycounterparts.
TheopticalspectraofthesevenredquasarsareshowninFigures2aand2b.
BroadlinesofHa,Hb,andMgIIareclearlyseeninthespectraaswellasbrightnarrowlines(e.
g.
,[OIII]j5007),makingtheAGNcharacteroftheobjectsunambiguous.
InTable2wetabulatesourceposition,red-shift,opticalmagnitudeandcolor,X-rayuxandX-rayTABLE1SUMMARYOFOPTICALIDENTIFICATIONSOFREDX-RAYSOURCECOUNTERPARTSMeasurementTotalRedQuasarsTooFaintNLXGEllipticalGalaxiesMStarsOtherStarsBlueQuasarsNotRed1.
8[aox[1.
6(O19)22100117102aox\1.
6(O\19)2966241037Total5176251813912KIM&ELVISVol.
516Fig.
2aFig.
2bFIG.
2.
(a)Observedspectraoftheveradio-quietredquasars(O[E[2mag,BroadlinessuchasHa,Hb,andMgIIareclearlyseenintheaopt[2.
0).
spectraaswellasbrightnarrowlines(e.
g.
,[OIII]j5007).
Thosestronglinesaremarkedinthegures.
SteepcontinuumslopesandveryweakHblinestrengthsindicateasignicantamountofdustextinctionmag),incontrastto(b),twoquasarspectrawithandrelativelystrongHblines.
(AV[2aopt\2.
0colors,andaswellasosetsbetweentheopticalandaox,X-raypositions.
InFigure1beachclassofidentiedsourcesisplottedintheplane.
Thedistributionofthesesourcesaox-(O[E)canbecomparedwiththeEMSSsourcesinFigure1a,conrmingthatmostX-raysourceswithlowareindeedaoxquasarsorgalaxyclusters.
Theselectiontechniquends7/51redquasars,i.
e.
,8%efficiency.
Aslightlystrictercri-terion,(insteadof1.
8),wouldhaveselectedredaox\1.
6quasarsmoreefficiently(Table2;Fig.
1b):onlyoneMstar(insteadof19stars)wouldhavebeenpresent,withonlyoneredquasarlost,i.
e.
,6/29,alittleover20%.
Oftheinitial87,71%(62)remainwhenistheboundary.
aox\1.
63.
3.
ObservedOpticalPropertiesoftheRedQuasarsWemeasuredtheopticalcontinuumslopesbyttingapowerlawtothecontinuumspectraafterexcludingthestrongemissionlines(Table3).
AlthoughallthesourcesTABLE2BASICPROPERTIESOFREDQUASARSNameRightAngleaDeclinationaOsetbzfXcasoftdahardeOO[Eaox(aopt\[2.
0)J2255.
5]0536225531.
005360111.
30.
06477.
741.
4590.
75016.
382.
441.
636J1234.
3]2614123421.
82613285.
00.
31202.
420.
8601.
71120.
552.
351.
180J1218.
1]2956121807.
12955214.
30.
15141.
620.
9591.
51219.
162.
081.
466J0909.
7]4302090943.
64302477.
00.
27481.
650.
8591.
41721.
382.
281.
114J1143.
6]5521114335.
55520214.
00.
14671.
520.
5351.
04019.
332.
331.
451[0.
9[aopt[[2.
0)J1051.
4]3358105128.
33358048.
00.
18292.
761.
2901.
29518.
272.
721.
515J1142.
6]4624114241.
24624213.
10.
115115.
750.
9261.
26316.
332.
441.
522aOpticalcoordinateinEquinoxJ2000.
bDierenceofX-rayandopticalpositionsinarcseconds.
cInunitsof10~13ergs~1cm~2.
dX-rayspectralindexin0.
10.
8keV.
eX-rayspectralindexin0.
82.
0keV.
No.
1,1999RADIO-QUIETREDQUASARS13TABLE3LINE,CONTINUUMPROPERTIES,ANDLUMINOSITIESOFREDQUASARSADDD4000HaNameaoptaFWHMfcLdM(O)M(E)L(X)43b(aopt\[2.
0)J2255.
5]05362.
39596229.
80.
54[21.
6[24.
11.
02J1234.
3]26142.
647184e5.
1e2.
70e[21.
1[23.
58.
19J1218.
1]29562.
3650115.
20.
56[20.
8[22.
91.
21J0909.
7]43022.
4321782.
30.
91[20.
0[22.
34.
29J1143.
6]55212.
1132235.
10.
51[20.
5[22.
91.
07([0.
9[aopt[[2.
0)J1051.
4]33580.
931797f7.
1f7.
10f[22.
1[24.
83.
07J1142.
6]46241.
41310481.
34.
90[23.
0[25.
46.
75aOpticalspectralindex,flPlaopt.
bInunitsof1043ergss~1.
cInunitsof10~15ergscm~2s~1.
dInunitsof1043ergss~1.
eMgII.
fHb.
wereselectedbasedonaredO[Ecolor,insomecasestheobservedopticalcontinuumshapeisrelativelyat.
Thisisbecauseofbothlineemissioncontributingtotheblueand/orredbandsanduncertaintiesontheOandEmagni-tudes,particularlywhentheobjectisfaint(M.
Irwin1996,privatecommunication).
Thepower-lawindex(FlDlaopt)rangesfrom[0.
9to[2.
6.
Asteepopticalcontinuum,isfoundinveofthesevenredquasars,whileaopt\[2,eventheremainingtwo,intermediateredquasarsareredderthanisfoundforUV-excessselected([1.
5\aopt\[1)quasars([0.
2^0.
8;Neugebaueretal.
1987).
Toillustratethespectraldierencesbetweenthesetwogroups,wedisplaythespectraseparatelyinFigures2a(steep)and2b(intermediate).
Inadditiontothedierenceincontinuumshape,thesetwogroupsalsodierintheirHblinestrengths(seeFigs.
2aand2b).
ThegroupwiththesteepopticalcontinuumhaveonlyweakHblinesornodetection,whereasthegroupwitharelativelyatcontinuumhavestrongerHblines.
SincetheratioofHatoHbissensitivetoopticalextinction,thissuggestsmorereddeninginthesteepslopegroupthantheintermediateslopegroup(Table3),whichisinaccordwiththeopticalcontinuumslopes.
ToquantifythiseectforthosequasarswithnodetectedHbline,weestimateditsupperlimitusingasimplemethodthatassumesaboxprolewithabaseequalto3000kms~1(themeanFWHMofdetectedHblines)andaheightequalto3timestheuctuationnoiseonthecontinuum.
Thisisaconservativemeasurement,becausethepeakofaGaussianprolewouldbemoreeasilydetectedthantheattopofaboxprole,particularlywhenthelinewidthisconsiderablylargerthanthespectralresolution.
MonteCarlosimula-tionsusingaGaussianlineproleassumingPoissonsta-tisticsshowthattheboxproleoverestimatestheupperlimitbyupto50%fortheadoptedlinewidth,whileitreproducesconsistentresultswhenthelinewidthiscompa-rablewiththespectralresolution.
ForthetwoobjectswhoseopticalspectradonotcovertheHaline,wehaveinsteadusedthe[OIII]/HbratioasameasureofrelativeHbstrength.
ForallvequasarswiththeHa/Hbratiosaopt\[2,aregreaterthan5,whilethe[OIII]/Hb[0.
8.
Thelineratiosofthetworemainingquasarsaresmaller(Table2),whichisconsistentwithlessreddeninginintermediate-slopeobjects.
Noneofthecharacteristicgalaxianstellarabsorptionfeatures4areseeninourspectra.
Moststrikingly,no4000Abreakisseeninanyoftheveredquasarsforwhichourspectracoverthatregion,includingallofthesteep-slopegroup.
TypicalvaluesofD(4000)5are11.
2,asisexpectedfromthemeasuredopticalslopes.
Thesecomparewithvaluesof2^0.
2fornormalEandS0galaxiesDressler&Shectman(1987).
Henceanystarlightcontinuumcontribu-tiontotheredquasarcontinuummustbeminor.
3.
4.
X-RayColorsoftheRedQuasarsTodeterminetheroughX-rayspectralpropertiesofthesevenredquasars,werstdouble-checkedinthePSPCimagesthatthesourceswerecleanlyseparatedfromanyconfusingsources,thenmeasuredtheirX-rayhardness(HR\H/M)andsoftness(SR\S/M)ratiosbasedonthecountratesinthestandardROSATPSPCbands:soft,S(0.
10.
4keV);medium,M(0.
40.
86keV);andhard,H(0.
872keV).
TheseratiosarethenconvertedtoeectiveX-rayspectralindices,and(Table2),tocorrectforthevariableasoftahardGalacticline-of-sightabsorptionandtheenergy-dependentpoint-spreadfunction.
(ThesearenotphysicalslopesbutshouldbeconsideredanalogoustoU[BandB[Vcolors;seeFioreetal.
1998foradetaileddiscussionoftheestima-tionandusageofeectiveX-rayspectralindices.
)Duetothelowsignal-to-noiseratioofX-raydata,individualspec-tralindicesarenotreliable.
However,thelocusoftheircolorsformsausefulindicatorofglobalX-rayproperties.
Wecomparethecolorsoftheredquasarswiththoseofnormalradio-quietquasarsinFigure3.
Thelargelledsymbolsaretheredquasarsreportedhere,whilethecloudofsmalldotsrepresentsradio-quietquasarsfromthesampleofFioreetal.
(1998).
Onaverage,theredquasarshavesmallerthanindicatingacutospectralasoftahard,shape.
[Thelinepairsaroundtheperipheryofthegure4CHG-bandj4304,MgIj5175,Ca]Fej5269,Najj5890,5896.
Dressler&Shectman5D(4000)\Fl(40504250A)/Fl(37503950A),(1987).
14KIM&ELVISVol.
516FIG.
3.
EectivesoftandhardROSATPSPCX-rayspectralindices(seetext)oftheredquasars(largelledsymbols)comparedwithnormalradio-quietquasars(smalldots;Fioreetal.
1998).
Steepopticalspectrum(a\[2)redquasarsareshownascircles,andintermediateopticalspec-trum([1.
5\a\[1)redquasarsareshownastriangles.
Thelinepairsaroundtheperipheryofthegureshowoutlinespectralshapesfortheirlocationsintheplane.
(ahard,ahard)showoutlinespectralshapesfortheirlocationsinthe(asoft,plane.
]Thisisconsistentwiththeirhavingthemoder-ahard)ateX-rayabsorptionexpectedfromtheiropticalproperties(Fioreetal.
1998).
4.
DISCUSSIONTheseobservationsshowthatapopulationofredAGNscanbeextractedefficientlyfromtheROSATpointedarchive.
Moreover,theredAGNswendareradioquiet.
NoneofthemisaradiosourceintheNRAOVLARadioSkySurveymJy;Condonetal.
1998),(f1.
4GHz\2.
5implyingGHz)]\2.
0,comparedwithRL\log[f(opt)/f(5forradio-loudquasars(Wilkes&Elvis1987).
2\RL\5TheagreementofX-raycolors,opticalcontinuumslope,andHa/Hbratioswiththesamevalueofobscuringdustandgasarguesfortheirbeingdustreddened(AVD12)objectsratherthaninstrinsicallyredcontinua.
Puchnarewicz&Mason(1988)discussasimilarpopu-lationof14candidatered,AGNsderivedfromtheaopt[2,RIXOSsample,whichextendstoseveraltimesfainterX-rayuxes.
TheRIXOSsamplewasselectedfromtheROSATpointedarchivebasedonX-rayuxalone.
(Thereisoneobjectincommonbetweenthetwosamples.
)TwooftheredRIXOSAGNsmaybeintrinsicallyred,andthreehaveclearreddening.
SothereiscurrentlyatotalofeightreddenedAGNsavailablefromROSAT.
TheredAGNs,bothfromthissampleandfromRIXOS,areborderlinequasar/Seyfertobjects.
TheobservedopticalluminositiesoftheredAGNsaremodest,lyingatthehighendofthetraditionalSeyfertluminosityrange(MB[[23mag;Veron-Cetty&Veron1984;Schmidt&Green1983):fromto[22mag.
6Thedereddened,intrinsicMO\[206Wehaveusedkms~1Mpc~1andH0\50q0\0.
luminosityofthesourcesislikelytobesignicantlyhigher,dependingontheamountofabsorption(thelowerlimitontheextinctionis23maginO).
Thisplacesthematwhichiswithinthequasarregime.
[25\MO\[22,Similarly,theobservedX-rayluminosityrangesfrom1.
3]1043to1.
2]1044ergss~1,whiletheintrinsiclumi-nositiesarelikelytobehigherbyaboutafactorof2,dependingontheintrinsicspectrumandtheamountofabsorptionpresent.
ThepreciseallocationoftheseAGNsasquasarsorSey-fertsisnotfundamentallyveryinteresting,sincethisisjustaconvenientlychosenvalueonacontinuousluminosityscale.
Forsimplicitywewillrefertothemasredquasarsfortherestofthispaper.
Itis,however,interestingtounder-standwhymuchmoreormuchlessluminousAGNswerenotfoundinthetworedquasarsearches.
Isthisaselectioneect,oristhereaphysicalpreferenceforredobjectstoclusterinalimitedrangeofluminosityWewillreturntothisquestionlater(°4.
3).
Theexistenceofaredquasarpopulationimmediatelyraisesimportantquestions.
Whatmakesthemredcom-paredwithusualbluequasarsHowcommonarethey,particularlyonceallowanceismadefortheirreduceduxduetoprobableobscurationIftheyareabsorbed,byhowmuchWhereistheabsorbingdustymaterialMightalarger,moreobscured,AGNpopulationexist4.
1.
HowCommonareRedQuasarsWecanaddresstherelativenumbersofredquasarsinaroughway.
Thepopulation,althoughaminority,isquitecommon.
Wefoundsevenredquasarsoutof45candidatesforwhichourspectroscopywasadequatetoproduceaclas-sication.
Assumingthatthe45werearandomsubsampleoftheoriginalsampleof87,thatsamplewouldproduce14redquasars.
Thisis2.
4%of575sourceswithopticalcolorsavailable.
Atouruxlimit,Stockeetal.
(1991)ndthat51%ofallX-raysourcesareAGNs.
Soaminimumof13.
5/288(4.
7%)AGNsinourunidentiedsampleareradio-quietredquasars(withO\20)atthissoftX-rayuxlevel.
TheBoyle&diMatteo(1995)upperlimitof9%ofCam-bridgeROSATSerendipitySurvey(CRSS)X-raysourcesbeingredquasarsisconsistentwiththeminoritypopulationofmoderatelyobscuredquasarswehavefound(AV\2)here.
Additionalredquasarscouldbehiddeninoursample.
Oursampleof45classiedspectraisnotarandomsub-sampleofthecandidatelist.
Figure1bshowsthatweprefer-entiallyselectedobjectswithi.
e.
,thebrighteraox[1.
3,objects(withB\21.
5).
Ifthesixobjectsforwhichweattemptedtogetspectraturnouttoberedquasars,thenthefractionofredquasarsamongthePSPCAGNscouldbealmostdoubleourrstestimate.
Thereareafurther36redcandidatesforwhichnospectroscopywasattempted.
Sothetrueoccurrencerateofredquasarsisuncertainbyafactorof7.
TondthefractionofredquasarsamongalltheAGNsinourX-ray,ux-limitedsample,wemustallowforthe165300blueAGNsintheidentiedsample,so1%7%ofthewholesoftX-rayAGNpopulationisred.
IftheredAGNsareobscured,thentheintrinsicrateofoccurrenceofredquasarshastobecalculatedrelativetotheirunreddenedparentpopulation.
ObscurationbyAV\reducestheirunobscuredX-rayuxesbyafactorofD2.
2SincehigheruxAGNsarerarer(theX-rayselectedAGNNo.
1,1999RADIO-QUIETREDQUASARS15logN/logSrelationhasaslopeof[3/2inthisuxrange;Hasinger1995;DellaCeccaetal.
1992),theredpopulationformsalargerfractionofthispopulation,D3%20%.
Usingthefractionofthesourcesattheintrinsicuxoftheredquasarsallowsustocompareourresultwithsamplesunaectedbyobscurationandwithmodelpredic-tions.
Comparisonswiththeobservedfrequencyofradio-loudredquasarswithbroademissionlinesfoundbySmith&Spinrad(1980;178MHz3CR)andStickeletal.
(1996;5GHz1Jysample)andwiththeComastrietal.
(1995)pre-dictionsarestraightforward.
Redquasarsarefoundin15%ofthe3CRsourcesampleand6%20%ofthe1Jy(Carillietal.
1998)sample.
Thesearecomparablewiththe3%-20%ofthe""intrinsicuxX-raypopulationthatwend,suggestingthattheradio-loudandradio-quietquasarpopulationshavesimilarlysizedpopulationsofmoderatelyobscuredquasars.
Theobscurationinthe3CRredquasarsisalsoaboutAV\2(Elvisetal.
1994;Economouetal.
1995;Rawlingsetal.
1995).
Forfourobjectsinthe1Jysample,Carillietal.
(1998)estimatelowerlimitsonfrom2to5basedonAVextrapolatingtheradio-infraredindextotheoptical.
However,sincesuchsteeplyrisingslopesarenotknownamongunobscuredquasars,theselimitsarelikelytobetoolarge,andvaluescomparablewiththe3CRestimatesareprobablyacceptable.
ForAGNswithcm~2,theComastrietal.
NH\1022(1995)modelpredictsthat26%willhave1021\NH\1022cm~2whichissomewhatlargerbutcomparable(AVD2),withthenumbersfoundhere.
Comastrietal.
predictfarlargernumbersofmoreobscuredobjects.
4.
2.
MoreObscuredObjectsMoreobscuredobjectsmayexist.
Puchnarewicz&Mason(1998)ndseveralobjectswithsteeperopticalcon-tinua,andFigure1bshowsseveralreddercandidatesandmanymoreX-rayloudcandidateswithnoopticalspectratodate.
Websteretal.
(1995)suggestedthatmaybeAV\5typicaloftheirredobjects,givingtheirputativeROSATcounterpartsinoursampleV\2325,whichisbelowthePalomarSkySurveylimit.
correspondstoacolumnAV\5of9]1021atomscm~2,whichwouldreduceROSATPSPCcountratesto15%oftheirunobscuredvalues.
Theseobjectswouldthenbehiddenasa6%minorityamongthemorecommon,lowerluminosity,unabsorbedquasarsiftheyhadthesameunobscuredspacedensityasnormalbluequasars.
Some9%ofourinitialX-rayselectedsampleare""blankeldobjects,i.
e.
,havenocounterpartonthePalomarSkySurveys.
Afractionofthesecouldbemoreheavilyobscuredredquasars.
Boyle&diMatteo(1995)ndthattheCRSSsamplecouldbemissingnomorethan9%(attheobservedux)inredquasars.
Subtractingthe1%ofmoderatelyobscuredquasarswehaveidentiedstillleaves8%thatcouldbehighlyobscured.
HencetheCRSSresultis,perhapssurprisingly,consistentwithasizeable,heavilyobscuredpopulation.
TheComastrietal.
(1995)modelpre-dictsacomparablepopulationofAGNswithNHD1022cm~2,whichis1.
1timeslargerthantheunobscuredpopu-lation.
Inoursampletheoccurrenceofredquasarsappearstobe4timeshigherforO[19mag(4/12)thanforO\19mag(3/32;Table2),althoughthenumberofsourcesissmall.
Suchatrendisexpectedifthequasarsareheavilyabsorbed.
Anincreaseinfrom3]1021to1]1022NHcm~2cutstheROSATuxbyafactorof2.
4butreddenstheVbandby3.
8mag(afactor33).
Sotheopticallyfaintersourcesmightwellberedder.
However,theRIXOSredquasars(Puchnarewicz&Mason1998)shownocorrelationofopticalslopewiththethreesteepestslopesareallinmV:thebrighterhalfofthesampleof14.
Gunn&Shanks(1998)havepointedoutthatwhileredshiftingtheultravioletintotheopticalincreasestheeectsofreddening,thecorre-spondingshiftofhardX-raysintothesoftROSATbanddecreasestheeectivenessofreddening.
Toestimateanaccuratefractionofthispotentialhiddenpopulationneedsalargersample,includingmoreabsorbed,fainterobjects.
Atevenlargercolumndensities(10231024cm~2)theComastrietal.
(1995)modelpredictsnearly4timestheunobscuredpopulation.
MoreheavilyobscuredquasarscouldbefoundinhardX-raysurveysfromASCA(Uedaetal.
1998)andtheBeppo-SAXHELLASsurvey(Fioreetal.
1999).
4.
3.
LimitedLuminosityRangeItisstrikingthatwhileROSATsurveysthataredenedsimplybyanX-rayuxlimitndAGNsspanningover3decadesinX-rayluminosity(e.
g.
,CRSS,Boyleetal.
1997;RIXOS,Puchnarewiczetal.
1996),bothRIXOSandthissurveyndredquasarsinonly1decadeofluminosity,andthisdecadeisthelowestoneinwhichRIXOSandCRSSAGNsarefound.
Atwo-tailKolmogorov-SmirnovtestshowsthatthechancethatredandnonredAGNsfromRIXOScomefromthesameluminositydistributionisonlyD2%.
ThissuggeststhatpredominantlylowerluminosityAGNsareobscured.
(NotethattheobservedamountofobscurationonlydecreasestheobservedX-rayluminositybyafactorofD2,andsodoesnotitselfcausethelowobservedluminosities.
)Similarsuggestionshavebeenmadebefore:Lawrence&Elvis(1982)foundthatonlyAGNsbelowergsLXD1044s~1(210keV)showedobscuration.
Occasionalexamplesofhighlyobscuredtype2(i.
e.
,narrow-line)quasarshavebeenreported(Stockeetal.
1982;Almainietal.
1995;Shanksetal.
1995),andcarefulsearcheshavefoundbroadHainmostcases(Halpern,Eracleous,&Forster1998),makingthemsimilartotheredquasarsfoundhere.
Search-esamongthefainterobjectsinoursampleandsearchesathigherenergies(e.
g.
,theBeppo-SAXHELLASsurvey;Fioreetal.
1999)willbemoreeectiveatndingahigh-luminosityredquasarpopulation.
TheComastrietal.
X-raybackgroundmodelsassumeluminosityfunctionsfortheobscuredobjectsthatareiden-ticaltothoseoftheunobscuredobjectsexceptfornormal-izationandsopredictshigh-luminosityredquasars.
IfinsteadobscuredAGNsoccurpreferentiallyatlowlumi-nosity,thiswillsubstantiallyaectthemodelpredictions.
Wewould,forexample,expecttheobscuredpopulationtobemorenumerousandatlowerredshift.
Ifthereisarealdecitofhigh-luminosityredquasars,thenonepossibilitytoexplainthislackmighthavebeenthatasanAGNbecamemoreluminous,thecontinuumionizedtheobscuringmedium,renderingittransparenttoX-rays.
However,ionizedabsorbersarealsomorecommonatlowerluminosities(Laoretal.
1994).
Somostlikely,high-luminosityAGNshavefewerlinesofsightwithinterveningmaterialregardlessofionizationstate.
(Interestingly,thisisinthesamesenseastheBaldwineect:thathigherlumi-nosityquasarshaveweakerCIII]j1909emissionlines.
)Any16KIM&ELVISVol.
516physicalmodelofaquasarwouldneedtoexplainthisdier-ence.
4.
4.
PhysicalPropertiesWecansayonlyalittleaboutthephysicalpropertiesoftheredquasarsfromthisdata.
TheconsistencyoftheopticalreddeningindicatorswiththeX-raycolorssuggeststhatthesameobscuringmaterialcoversbothemittingregionsandthatitliesoutsidethebroademissionlineregion.
Smith&Spinrad(1980)suggestedthattherednessofthered3CRquasarsisintrinsictothecontinuumemissionprocessbasedonthelackofanabsorptionfeatureatj\2200whichisatypicalcharacteristicofMilkyWayA,dust(e.
g.
,Bless&Savage1972).
Thedetectionof21cmHIabsorptiontowardalargefractionofthe1Jy(Stickeletal.
1996)redquasars(Carillietal.
1998)arguesfordustreddeninginthoseobjects.
Sinceoursampleofradio-quietquasarsisrelativelynearby(withredshiftsupto0.
3),wecannotcheckforthisfeaturedirectly.
However,thisexpla-nationisincontrasttotheobservedBalmerdecrementandtheX-raycolors.
Itispossiblethatthereddened3CRsourcescontaindusty,ionizedabsorbers,asisseenin3C212(Mathur1994;Elvisetal.
1994)andIRAS17020]4544(Komossa&Bade1998),wherethedustcompositionmaydierdependingon,forexample,thequasarcontinuumshape.
Ultravioletobservationsareneededtoinvestigatethej\2200featurebutareprobablyinfeasibleatApresent.
4.
5.
OtherRedAGNsSomepreviousstudieshaveconsideredredAGN-likeobjectsinX-raysurveys.
TheRIXOSsurvey(Puchnarewicz&Mason1998)foundthat9%(14/160)oftheirAGNswerered.
However,onlythreeoftheRIXOSsourceshaveBalmerdecrementsthatrequirereddening,sothetrueoccurrencerateofreddenedobjectsmaybesimilartowhatwehavefound.
ThefainteruxlimitoftheRIXOSsurveymayrendermoreabsorbedobjectsvisible.
Certainly,thesteeperopticalslopesofhalfthe([2.
5[aopt[[4.
0)RIXOSsamplesuggestgreaterreddening.
Kruper&Canizares(1989)studiedredAGNsinEinsteinX-rayselectedsamplesandindirectlyconcludedthattheseareredbecauseofthepresenceofhostgalaxies.
Bennetal.
(1998)arriveatasimilarconclusionforlow-frequency,selectedradio-loudquasars.
However,nogalaxianstarlightfeaturesareseeninourspectra,andthehostgalaxycannotexplaintheobservedBalmerdecrementsinoursample.
TheKruper&Canizaresobjectsarenotasredasoursamples,havingB[I\1.
52.
5mag.
IfwetakeR[Itobe0.
51.
0mag(thisistheR[IrangeofthesamplesintheirTable2),B[Rwouldbelessthan2.
0,ourdeningthreshold.
Infact,noneoftheirobjectswithmeasuredRmagnitudesexceedB[R\2.
0.
Thenarrow-lineX-raygalaxies(NLXGs)foundplenti-fullyindeepROSATsurveys(e.
g.
,Boyleetal.
1995a)arealsonormallyassumedtobeobscuredAGNs(e.
g.
,Hasingeretal.
1998;Schachteretal.
1998).
X-rayabsorbedNLXGscouldcontributesignicantlytothecosmicX-rayback-groundiftheyaremorecommonatfainterX-rayuxlevels,aswassuggestedbyMcHardyetal.
(1998;seealsoHasingeretal.
1998foracautionarynote).
AlthoughtheyhavesimilarX-rayluminositiestotheNLXGs,theX-rayselectedredquasarsarenotsimplythesamepopulation,however.
Theredquasarshavethenormal,broadopticalemissionlinesofquasars,whileNLXGshaveeithernoneoronlyextremelyweakones(Boyleetal.
1995b;Figs.
2aand2b).
Moreover,NLXGsusuallyexhibitblueopticalcontin-ua(forexample,O[E\2forveoutofsixNLXGsinCRSS;Boyleetal.
1997),whileredquasarshaveredopticalcontinua(O[E[2).
FurtherX-rayandopticalstudyoftheseobjectsmayletusunderstandwhethertheyaretwoseparatepopulationsorarerelatedby,e.
g.
,specialviewinggeometryorscatteringofabluecontinuum.
ThePalomarsurveyofthenucleiofbrightgalaxies(Ho,Filippenko,&Sargent1997)isbasedonasampleofgal-axiesselectedfortheirnonnuclearpropertiesandsoislessbiasedagainstndingredAGNsthanmostotheropticalsearchmethods.
ThePalomarAGNsarelow-luminosityAGNs,allowingustoseewhetherthehighincidenceofredAGNsatlowerluminositiescontinuestoincreaseatevenlowervalues.
ThePalomarsurveyndsthat18%(8/44)ofSeyfertnucleihavebasedontheirnarrow-lineAV[2Balmerdecrements(Hoetal.
1997).
However,almostalloftheseareLINERsortype2Seyferts,whichmayhavelargereddeningtowardthebroad-lineregion.
OnlyoneSeyfert(NGC7479)showsanyevidenceforabroad-linecom-ponent.
Broademissionlinesareextremelyhardtodetectattheseuxlevels,butthesuggestionisthatthemiddlingluminositiestowardthequasar/Seyfertborderlinearepar-ticularlypronetomoderateobscuration.
5.
CONCLUSIONSRadio-quietredquasarscanbefoundinsubstantialnumbers.
Theycompriseatleast1%,andpotentially7%,ofthesoftX-raypopulationinauxlimitedsurvey.
Correct-ingtheX-rayuxestotheirintrinsicvaluesputsthemamongbrighterAGNs,wheretheyform3%ofthepopu-lation.
Allowingforblankeldsources,asmuchas20%ofROSATselectedquasarsmayberedatagivenunobscuredux.
Thesizeofthispopulationisconsistentwithpreviousupperlimits,withtheComastrietal.
(1995)modelfortheX-raybackground,andwiththesizeoftheradio-loud3CRand1Jyredquasarpopulations.
Redquasarsseemtobepreferentiallylowerluminosityobjectsonthequasar/Seyfertborderlinebutnotathigherorlowerluminosities.
Suchabiasagainstobscuredhigh-luminosityobjectswouldaectX-raybackgroundestimatesforthispopulationandwouldneedexplaininginaphysicalmodelofquasars.
Westressthatthequasarswendhavebroadopticallines.
TheyarenotNLXGs,whichbycontrasthavepre-dominantlynarrowopticalpermittedlinesandbluecontin-ua.
Theopticalslopes,Ha/Hbratios,andX-raycolorsareallconsistent,withreddeningbyassumingstandardAVD2MilkyWaydustproperties.
Sothesameobscuringmaterialprobablycoverseachoftheemittingregions.
Asignicantpopulationofmorehighlyobscured(AV\5)quasarscouldwellexistandbeconsistentwiththeresultshere,withearlierROSATlimits,andwouldbeaspredictedbytheComastrietal.
(1995)model.
HardX-raysurveyswillsoonsettlethequestionofthesizeofanysuchpopulation.
Usingaminorrenementofthetechniquepresentedhere,redquasarscanbefoundwithhigh(20%)efficiencyintheROSATdata.
WethankL.
AngeliniforhelpingustouseWGACATandM.
IrwinforuseoftheAPMonlinecatalog.
WealsoNo.
1,1999RADIO-QUIETREDQUASARS17thankF.
FioreandF.
NicastroforprovidingtheirprogramtocalculateX-rayspectralindicesandF.
FioreonceagainforsupplyingtheX-rayslopesoftheradio-quietsampleinFigure3.
ThesupportbytheFLWO,MMT,andCTIOstasinsettingupandoperatingvariousinstrumentswereinvaluabletothisstudy.
TheHEASARC/GSFCPIMMSprogramwasusedtocalculatePSPCcountrates.
ThisworkwassupportedbyNASAgrantsNAG5-3066(ADP),NAG5-6078(LTSA),andNASAcontractNAS8-39073(ASC).
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TheAmericanAstronomicalSociety.
Allrightsreserved.
PrintedinU.
S.
A.
(RADIO-QUIETREDQUASARSDONG-WOOKIM1ChungnamNationalUniversity,Taejon,305-764,SouthKoreaANDMARTINELVISHarvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138Received1998May11;accepted1998December8ABSTRACTWehaveperformedasuccessfultargetedsearchforapopulationofred,radio-quiet,andprobablyabsorbedquasars.
Radio-quiet,opticallyredROSATPSPCX-raysourcesbrighterthan1]10~13ergscm~2s~1weresearchedforred(O[E[2.
0,O20)counterpartsintheAutomatedPlate-measuringMachinecatalogofPalomarSkySurveyobjects.
Of45objectsforwhichweobtainedadequatefollow-upopticalspectroscopy,wehavefoundsevenredquasars,vewithTheirredshiftsrangeaopt\[2.
from0.
06to0.
31,andtheirluminositiesaremoderate,lyingonthequasar/Seyfertboundary.
Theseredquasarsstrengthenthecaseforaradio-quietpopulationthatisthecounterpartoftheradio-loudredquasarsfoundbySmith&SpinradandWebsteretal.
Unidentied,faintersourcescouldincreasethefractionofredquasarsbyuptoafactorof7.
Fortheredquasarsfoundhere,theHa/Hbratios,opticalslope,andX-raycolorsallindicatethattheyareabsorbedbyratherthanhavingintrinsicallyredAVD2spectra.
ThisamountofobscurationseemstohideD1%7%ofquasarsatagivenobserveduxorD3%20%whentheiruxesarecorrectedtotheirintrinsicvalues.
Thissizeofpopulationisconsistentwithearlierlimits,withpredictedvaluesfromComastrietal.
,andiscomparablewiththeratefoundamongradio-loudquasars.
Alargepopulationofmoreheavilyabsorbedfainterquasarsequal(AV\5),insizetothebluepopulationcouldexistwithoutviolatingexistingupperlimits,whichisinaccordwiththeComastrietal.
predictions.
Subjectheadings:dust,extinctionquasars:generalX-rays:galaxies1.
INTRODUCTIONQuasarsarethecanonical""ultravioletexcessobjects(Sandage1965).
Yetredquasarshavebeenfoundinradio-selectedsamplesbySmith&Spinrad(1980)andWebsteretal.
(1995).
Websteretal.
(1995)proposedthatalargefrac-tion,perhaps80%,ofradio-loudquasarsmighthavebeenhiddenasredobjects.
Moreover,thecurrentlyfavoredexplanationsforthecosmicX-raybackgroundinvokeapopulationofheavilyobscuredactivegalacticnuclei(AGNs)5timesmorecommonthantheunobscuredpopu-lation(Comastrietal.
1995).
Ifthesmallnumberofknownredquasarsreallyisthe""tipoftheicebergofalarge,evendominant,quasarpopulation,thentheconsequenceswouldbeinteresting:theoverallAGNpopulationandsothemassiveblackholepopulationmaybe5timeslargerthanhadbeenthought;obscuredquasarswouldbealong-livedevolutionaryphase(seeSandersetal.
1988),orallquasarsmaybehiddenalong80%ofpossiblelinesofsight;andredquasarsmaycontributeimportantlytothecosmicX-raybackground.
TheWebsteretal.
conclusioniswidelydis-puted.
Boyle&diMatteo(1995),Stickeletal.
(1996),andBennetal.
(1998)allarguethatanymissingpopulationmustbesmallerandperhapsinsignicant,whileGunn&Shanks(1998)disagree.
HerewepresentanX-raybasedsurveytargetedexplicitlyatredAGNstondradio-quietredquasars.
Radio-loudredquasarsarerelativelyeasytond,sincetheradioemissionisunaectedbyabsorbinggasordustand,inlow-frequencysurveys,usuallycomesfromthelargeradiolobesthatliewelloutsideanyobscuringmaterialin1AlsoatHarvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138.
thehostgalaxy.
Anexplicit,albeitsmall-scale,searchforradio-andX-rayloudbutopticallyquietquasars,whichshouldincludereddenedquasars,was,however,notsuc-cessful(Kollgaardetal.
1995).
Radio-quietredquasarsaremuchhardertond,althoughtheymightbeexpectedtobemuchmorecommon.
Inthenormalunreddenedpopulation,radio-quietquasarsoutnumberradio-loudquasars10to1(e.
g.
,theExtendedMediumSensitivitySurvey,Stockeetal.
1991;thePalomarGreencatalog[PG],Kellermanetal.
1989).
However,mostopticalquasarsurveysareactivelybiasedagainstndingredquasars.
SincethesesurveysprimarilysearchforUV-brightobjects(e.
g.
,Markarian,Lipovetsky,Markarian,&Stepanian1987;PG,Schmidt&Green1983;theLargeBrightQuasarSurvey,Hewett,Foltz,&Chaee1995;theHamburgQuasarSurvey,Engelsetal.
1998),theyareblindtoredobjects.
Asaresult,opticalboundsonhowlargeapopulationofradio-quietredquasarsmightexistareweak.
X-rayselectionprovidesawayofselectingredquasarsefficiently:hardX-rays(210keV)penetrateeventensofmagnitudesofopticalextinctionwithminimalabsorption.
EventhelowerenergybandofROSAT(0.
52.
5keV)isnotstronglyaectedbyopticalextinctionofuptoD2mag.
2Astrophysicsmightbeagainstus,sincealthoughbluequasarsareoverwhelminglyX-rayloud(Avni&Tanan-baum1986),redonesmightbeintrinsicallyX-rayfaint.
Fortunately,however,radio-loudredquasarsareknowntobeX-raysources(Bregmanetal.
1985;Elvisetal.
1994),sothisisunlikelytobeaproblem.
2ForstandardMilkyWaycompositionanddust-to-gasratio(Bohlin,Savage,&Drake1978;Seaton1979)thePSPCcountrateisreducedbyafactorof1/eatatomscm~2)forapower-lawAV\1.
7(NH\3]1021photonindexof2.
0,allat0redshift.
910KIM&ELVISVol.
516Completeux-limitedX-raysurveyshavefoundsomeredquasarsorAGNs(Stockeetal.
1982;Kruper&Canizares1989;Puchnarewiczetal.
1996,1997).
InparticulartheROSATInternationalX-Ray/OpticalSurvey(RIXOS)survey(Puchnarewicz&Mason1998)hasidentiedasmallsampleofredquasars.
AspartofamoregeneralsearchforminoritypopulationsofX-raysources,wehaveusedtheROSAT1983)(TrumperarchiveofpointedPSPC(Pfeermanetal.
1987)datatodesignanefficientsearchstrategyexplicitlytargetingredquasars.
TheROSATpointedarchiveprovidesuswithatenfoldincreaseinthenumberofX-raysourcesfrombeforeROSAT(toabout70,000)andreachesmorethan10timesfainterthantheROSATAll-SkySurvey(Vogesetal.
1996).
Bycarefullyselectingasmallnumberofinterestingobjects,wecanmakeanefficientsearchforaradio-quietpopulationofredquasars.
Inthispaperwendasubstantialpopulationofradio-quietredAGNsonthequasar/Seyfertluminositybound-ary.
2.
SAMPLESELECTIONMostROSATsourcesathighGalacticlatitudeareblue,unobscuredAGNs(e.
g.
,Boyle,Wilkes,&Elvis1997;Schmidtetal.
1997).
WeusethisfacttoefficientlyselectagainstsuchobjectsandsoisolateanyredAGNthatmaybeintheROSATarchive.
AsacompendiumoftheX-raysourcesfoundbytheROSATPSPCwehaveusedthe""WGACATcatalog(whichisnamedafteritsauthors,White,Giommi,&Angel-ini1995).
ThiscatalogwasgeneratedfromROSATPSPCpointedobservationsusingaslidingcell,detectalgorithm.
Thismethodissensitivetondingpointsourcesbutcanalsondspurioussourceswhereextendedemissionispresent.
WGACATincludesaqualityagthatnotessuchdubiousdetectionsbasedonavisualinspectionoftheelds.
WehaveonlyusedX-raysourceswithhighdetectionqualityinordertoexcludethespurioussources.
FromtheWGACATcatalogwehaveselectedsourcesbythefollow-ingcriteria:1.
X-raybrightergscm~2s~1)toallow(fX[10~13follow-upobservationswithotherX-raytelescopes;2.
welldetectedwithasignal-to-noiseratiogreaterthan10andaWGACATqualityag,DQFLAG,greaterthan5;3.
withinr\18@fromthedetectorcentertoprovidegoodpositions;4.
athighgalacticlatitude(obo[20)tominimizethefractionofGalacticstars(whicharealsored);5.
notwithin2@ofthetargetposition(atwhichpointthesourcedensityreachesthebackgroundlevel)toselectonlyrandom,serendipitous,sources;6.
northofdecl.
\[18inordertohavetwobandmea-surementsintheAutomatedPlate-measuringMachine(APM)catalog(McMahon&Irwin1992),hencegivinganarchivalopticalcolor;and7.
unidentied,withWGACATclass\9999andnoSIMBADorNASAExtragalacticDatabaseidentication.
3Therstsixcriteriaselected1624sources.
SincethesesourceswereselectedpurelyontheirX-rayproperties,theyformawell-denedsamplefromwhichtostudytheinci-denceofminorityX-raypopulations,includinganyradio-quietredquasars.
Addingtherequirementthatasourcebeunidentiedleft940X-raysourcesthatcouldbeexaminedforhavingredopticalcounterparts.
WethensearchedtheAPMcatalogofobjectsdetectedonthePalomarSkySurveyforopticalcounterpartstotheunidentiedX-raysources.
Tondcounterparts,weusedasearchradiusof26A,whichcorrespondstoabout95%con-denceforX-raysourceswithin18@ofthePSPCdetectorcenter(Boyleetal.
1995a).
Ofthese,881sourceshadAPMcatalogcounterpartsbrighterthanthelimitingmagnitudesO\21.
5andE\20.
(Theremaining""blankeldsarethe3Althoughonly""unidentiedsourceswereselected,one(1WGAJ1118.
0]4505;Table1)turnedouttobeaknownSeyfert1(Badeetal.
1995).
Fig.
1aFig.
1bFIG.
1.
(a)OpticallyidentiedEMSSsourcesintheplane.
Dierentsourcesaremarkedbydierentsymbols.
Oursource-selectioncriteriaaox-(O[E)(O[E[2,areseenasalowerrightbox.
(b)Sameas(a)butforourROSATsamples.
Thelineshowshowaslightlystrictercriterionyieldsaaox\1.
8)aoxlargerfractionofredquasars.
No.
1,1999RADIO-QUIETREDQUASARS11subjectofanotherstudy;Elvis,Kim,&Nicastro1999.
)Amongthese881,manyhadonlyO-bandmagnitudes(suggestingthattheyareblue),leaving575withO[Ecolorsavailable.
ThecombinationoftheROSATX-rayuxandtheAPMmagnitudesallowsustocreatearoughclassicationoftheX-raysourcesinoursample.
ThetwoPalomarO(blue)andE(red)magnitudesareclosetoJohnsonBandCousinsR,respectively(Greggetal.
1996).
FromtheX-rayuxandthetwoopticalmagnitudeswecanconstructatwo-colordiagramofisdenedbythepower-lawindexaox-(O[E).
aoxbetween2keVand2500(Tananbaumetal.
1979;StockeAetal.
1991).
ThisdiagramallowsustoselectredobjectsandthenreducetheGalacticstellarpopulationamongtheseredobjectsbyselectingtheX-rayloudpopulation.
HerewemakeuseoftheobservationthatinstellarsourcestheX-rayuxforagivenopticaluxismuchweakerthaninAGNsandclusters(e.
g.
,Maccacaroetal.
1988).
Figure1ashowsEinsteinExtendedMediumSensitivitySurvey(EMSS)X-raysources(Stockeetal.
1991)intheplane.
aox-(O[E)ThisplotclearlyillustratesthedistinctionbetweenGalacticstellarsourcesandextragalacticsources.
BasedontheEMSSsourcedistribution,wedividedtheROSAT-APMsourcesonthesameplane(Fig.
1b).
TheO[EAPMcolorsarenotasaccurateastheEMSSvalues,whicharebasedonCCDphotometry.
Asaresult,thespreadofobservedcolorsiswider(Fig.
1b),andtherewillbesomeblueobjectsintheredzoneandviceversa.
Tocreateourlistofredquasarcandidates,werstexcludedthe128sourceswithbecausetheyarelikelytobeGalacticaox[1.
8,stars.
Thenweexcludedanother360sourceswithbluecolors,O[E\2,becausetheyaremostlikelyjustnormal,blue,unobscuredAGNs.
Thisresultsinanalsampleof87X-raysourcesdenedbythelowerrightcornerofFigure1bintheplane.
aox-(O[E)Oursampleof87opticallyredX-rayloudsourcesisamere0.
1%oftheD70,000WGACATsources.
ThefractionofX-raysourcesthatmayberedquasars,however,ismuchlarger:D5%oftheinitialX-rayselectedsample,D15%oftheunidentiedsourceswithAPMcolors,andD20%ofX-raybrightobjectswithAPMcolorsthatwillprimarilybeAGNs.
However,otherclassesofX-raysourcethanredquasarscaninhabitthisregionoftheplane:forexample,aox-(O[E)rst-rankedellipticalgalaxiesindistantclustersofgalaxies.
Opticalspectroscopyisneededtondredquasars.
Wehavetakenspectrafor51ofthe87redquasarcandidates,asisdescribedinthenextsection.
3.
OBSERVATIONS3.
1.
OpticalSpectroscopyAtypicalX-rayerrorcirclecontainsjust12opticalobjectsintheAPMcatalog.
Sincewehaveselectedagainstbluecounterparts,webeganbyobservingthebrightestredcounterpart.
Iftwoobjectswerepresent,wealignedthespectrographslittoobtainspectraofbothatonce.
IfthistherstspectradidnotndanAGN,wethenobservedthenextfaintest,ifpresent.
Sincethedensityof(blue)AGNsatB\21isonlyD0.
005pererrorcircle(e.
g.
,Zitellietal.
1992),weexpectonlya1in4chanceofAGNcoincidenceinthe51spectra,sostoppingonceanAGNisfoundwillnotproduceasignicantnumberoffalseidentications.
WeperformedopticalspectroscopywiththeMultipleMirrorTelescope(MMT)on1997March1315,withtheFredLawrenceWhippleObservatory(FLWO)60Atele-scopeon1996November1617and1997February1213,andwiththeCerroTololoInter-AmericanObservatory(CTIO)60Atelescopeon1997February35.
Weusedlong-slitaperturesof2@@3@@]180@@andgratingswith300gpm.
Thespectralresolutionsare6and9fortheMMTand60AAtelescopes,respectively.
Wavelengthcoverageisabout35008000Wetookbias,domeat,andtwilightskyA.
frameseachnight,andthecorrespondingcorrections(biassubtraction,atelding,andilluminationcorrection)wereappliedseparatelytoeachnightofdata.
Atleasttwostan-dardstarswereobservedeachnightforspectrophotometriccalibration.
Theobservingconditionswerenotphoto-metric,exceptfortheCTIOrun,sotheabsolutecalibrationissubjecttoasignicantuncertainty.
However,relativeintensities(suchasalineintensityratioandanopticalpower-lawindex)areaccuratewithin20%,asisconrmedbymultipleobservationsofthesamesource.
Sixsourcesareofundeterminednaturebecausetheyaretoofaintandsogavespectraoftoopoorasignal-to-noiseratio.
3.
2.
ClassicationofSpectraOfthe45sourcesobservedatagoodsignal-to-noiseratio,wehaveidentiedsevenredquasars(Table1).
Theresultsforthesesevenredquasarsarepresentedinthispaper.
(Thefulldatasetwillappearelsewhere.
)Theredquasarsaremixedinwith18starsandasmallnumberofnormalbluequasars,narrowemissionlinegalaxies,andellipticalgalaxies(Table1).
Theellipticalgalaxiesarelikelytobebrightestclustergalaxies.
Wewillreportontheseseparately.
FortheremainingnineX-raysources,theredopticalcandidatewithintheerrorcircleturnedouttobeastar(mostlylatetype),butitisnotlikelythattheseredstarsarethecounterparts,becausetheirvaluesaretoolargeaoxforastar(seeabove;Maccacaroetal.
1988).
Theremainingopticalcandidatesarenotred,andhencewestoppedmakingfurtherobservations.
TheseninesourcesandthethreebluequasarsmeasurethebluecontaminationofthesampleandshouldnotbeconsideredaspartofthelistofredX-raycounterparts.
TheopticalspectraofthesevenredquasarsareshowninFigures2aand2b.
BroadlinesofHa,Hb,andMgIIareclearlyseeninthespectraaswellasbrightnarrowlines(e.
g.
,[OIII]j5007),makingtheAGNcharacteroftheobjectsunambiguous.
InTable2wetabulatesourceposition,red-shift,opticalmagnitudeandcolor,X-rayuxandX-rayTABLE1SUMMARYOFOPTICALIDENTIFICATIONSOFREDX-RAYSOURCECOUNTERPARTSMeasurementTotalRedQuasarsTooFaintNLXGEllipticalGalaxiesMStarsOtherStarsBlueQuasarsNotRed1.
8[aox[1.
6(O19)22100117102aox\1.
6(O\19)2966241037Total5176251813912KIM&ELVISVol.
516Fig.
2aFig.
2bFIG.
2.
(a)Observedspectraoftheveradio-quietredquasars(O[E[2mag,BroadlinessuchasHa,Hb,andMgIIareclearlyseenintheaopt[2.
0).
spectraaswellasbrightnarrowlines(e.
g.
,[OIII]j5007).
Thosestronglinesaremarkedinthegures.
SteepcontinuumslopesandveryweakHblinestrengthsindicateasignicantamountofdustextinctionmag),incontrastto(b),twoquasarspectrawithandrelativelystrongHblines.
(AV[2aopt\2.
0colors,andaswellasosetsbetweentheopticalandaox,X-raypositions.
InFigure1beachclassofidentiedsourcesisplottedintheplane.
Thedistributionofthesesourcesaox-(O[E)canbecomparedwiththeEMSSsourcesinFigure1a,conrmingthatmostX-raysourceswithlowareindeedaoxquasarsorgalaxyclusters.
Theselectiontechniquends7/51redquasars,i.
e.
,8%efficiency.
Aslightlystrictercri-terion,(insteadof1.
8),wouldhaveselectedredaox\1.
6quasarsmoreefficiently(Table2;Fig.
1b):onlyoneMstar(insteadof19stars)wouldhavebeenpresent,withonlyoneredquasarlost,i.
e.
,6/29,alittleover20%.
Oftheinitial87,71%(62)remainwhenistheboundary.
aox\1.
63.
3.
ObservedOpticalPropertiesoftheRedQuasarsWemeasuredtheopticalcontinuumslopesbyttingapowerlawtothecontinuumspectraafterexcludingthestrongemissionlines(Table3).
AlthoughallthesourcesTABLE2BASICPROPERTIESOFREDQUASARSNameRightAngleaDeclinationaOsetbzfXcasoftdahardeOO[Eaox(aopt\[2.
0)J2255.
5]0536225531.
005360111.
30.
06477.
741.
4590.
75016.
382.
441.
636J1234.
3]2614123421.
82613285.
00.
31202.
420.
8601.
71120.
552.
351.
180J1218.
1]2956121807.
12955214.
30.
15141.
620.
9591.
51219.
162.
081.
466J0909.
7]4302090943.
64302477.
00.
27481.
650.
8591.
41721.
382.
281.
114J1143.
6]5521114335.
55520214.
00.
14671.
520.
5351.
04019.
332.
331.
451[0.
9[aopt[[2.
0)J1051.
4]3358105128.
33358048.
00.
18292.
761.
2901.
29518.
272.
721.
515J1142.
6]4624114241.
24624213.
10.
115115.
750.
9261.
26316.
332.
441.
522aOpticalcoordinateinEquinoxJ2000.
bDierenceofX-rayandopticalpositionsinarcseconds.
cInunitsof10~13ergs~1cm~2.
dX-rayspectralindexin0.
10.
8keV.
eX-rayspectralindexin0.
82.
0keV.
No.
1,1999RADIO-QUIETREDQUASARS13TABLE3LINE,CONTINUUMPROPERTIES,ANDLUMINOSITIESOFREDQUASARSADDD4000HaNameaoptaFWHMfcLdM(O)M(E)L(X)43b(aopt\[2.
0)J2255.
5]05362.
39596229.
80.
54[21.
6[24.
11.
02J1234.
3]26142.
647184e5.
1e2.
70e[21.
1[23.
58.
19J1218.
1]29562.
3650115.
20.
56[20.
8[22.
91.
21J0909.
7]43022.
4321782.
30.
91[20.
0[22.
34.
29J1143.
6]55212.
1132235.
10.
51[20.
5[22.
91.
07([0.
9[aopt[[2.
0)J1051.
4]33580.
931797f7.
1f7.
10f[22.
1[24.
83.
07J1142.
6]46241.
41310481.
34.
90[23.
0[25.
46.
75aOpticalspectralindex,flPlaopt.
bInunitsof1043ergss~1.
cInunitsof10~15ergscm~2s~1.
dInunitsof1043ergss~1.
eMgII.
fHb.
wereselectedbasedonaredO[Ecolor,insomecasestheobservedopticalcontinuumshapeisrelativelyat.
Thisisbecauseofbothlineemissioncontributingtotheblueand/orredbandsanduncertaintiesontheOandEmagni-tudes,particularlywhentheobjectisfaint(M.
Irwin1996,privatecommunication).
Thepower-lawindex(FlDlaopt)rangesfrom[0.
9to[2.
6.
Asteepopticalcontinuum,isfoundinveofthesevenredquasars,whileaopt\[2,eventheremainingtwo,intermediateredquasarsareredderthanisfoundforUV-excessselected([1.
5\aopt\[1)quasars([0.
2^0.
8;Neugebaueretal.
1987).
Toillustratethespectraldierencesbetweenthesetwogroups,wedisplaythespectraseparatelyinFigures2a(steep)and2b(intermediate).
Inadditiontothedierenceincontinuumshape,thesetwogroupsalsodierintheirHblinestrengths(seeFigs.
2aand2b).
ThegroupwiththesteepopticalcontinuumhaveonlyweakHblinesornodetection,whereasthegroupwitharelativelyatcontinuumhavestrongerHblines.
SincetheratioofHatoHbissensitivetoopticalextinction,thissuggestsmorereddeninginthesteepslopegroupthantheintermediateslopegroup(Table3),whichisinaccordwiththeopticalcontinuumslopes.
ToquantifythiseectforthosequasarswithnodetectedHbline,weestimateditsupperlimitusingasimplemethodthatassumesaboxprolewithabaseequalto3000kms~1(themeanFWHMofdetectedHblines)andaheightequalto3timestheuctuationnoiseonthecontinuum.
Thisisaconservativemeasurement,becausethepeakofaGaussianprolewouldbemoreeasilydetectedthantheattopofaboxprole,particularlywhenthelinewidthisconsiderablylargerthanthespectralresolution.
MonteCarlosimula-tionsusingaGaussianlineproleassumingPoissonsta-tisticsshowthattheboxproleoverestimatestheupperlimitbyupto50%fortheadoptedlinewidth,whileitreproducesconsistentresultswhenthelinewidthiscompa-rablewiththespectralresolution.
ForthetwoobjectswhoseopticalspectradonotcovertheHaline,wehaveinsteadusedthe[OIII]/HbratioasameasureofrelativeHbstrength.
ForallvequasarswiththeHa/Hbratiosaopt\[2,aregreaterthan5,whilethe[OIII]/Hb[0.
8.
Thelineratiosofthetworemainingquasarsaresmaller(Table2),whichisconsistentwithlessreddeninginintermediate-slopeobjects.
Noneofthecharacteristicgalaxianstellarabsorptionfeatures4areseeninourspectra.
Moststrikingly,no4000Abreakisseeninanyoftheveredquasarsforwhichourspectracoverthatregion,includingallofthesteep-slopegroup.
TypicalvaluesofD(4000)5are11.
2,asisexpectedfromthemeasuredopticalslopes.
Thesecomparewithvaluesof2^0.
2fornormalEandS0galaxiesDressler&Shectman(1987).
Henceanystarlightcontinuumcontribu-tiontotheredquasarcontinuummustbeminor.
3.
4.
X-RayColorsoftheRedQuasarsTodeterminetheroughX-rayspectralpropertiesofthesevenredquasars,werstdouble-checkedinthePSPCimagesthatthesourceswerecleanlyseparatedfromanyconfusingsources,thenmeasuredtheirX-rayhardness(HR\H/M)andsoftness(SR\S/M)ratiosbasedonthecountratesinthestandardROSATPSPCbands:soft,S(0.
10.
4keV);medium,M(0.
40.
86keV);andhard,H(0.
872keV).
TheseratiosarethenconvertedtoeectiveX-rayspectralindices,and(Table2),tocorrectforthevariableasoftahardGalacticline-of-sightabsorptionandtheenergy-dependentpoint-spreadfunction.
(ThesearenotphysicalslopesbutshouldbeconsideredanalogoustoU[BandB[Vcolors;seeFioreetal.
1998foradetaileddiscussionoftheestima-tionandusageofeectiveX-rayspectralindices.
)Duetothelowsignal-to-noiseratioofX-raydata,individualspec-tralindicesarenotreliable.
However,thelocusoftheircolorsformsausefulindicatorofglobalX-rayproperties.
Wecomparethecolorsoftheredquasarswiththoseofnormalradio-quietquasarsinFigure3.
Thelargelledsymbolsaretheredquasarsreportedhere,whilethecloudofsmalldotsrepresentsradio-quietquasarsfromthesampleofFioreetal.
(1998).
Onaverage,theredquasarshavesmallerthanindicatingacutospectralasoftahard,shape.
[Thelinepairsaroundtheperipheryofthegure4CHG-bandj4304,MgIj5175,Ca]Fej5269,Najj5890,5896.
Dressler&Shectman5D(4000)\Fl(40504250A)/Fl(37503950A),(1987).
14KIM&ELVISVol.
516FIG.
3.
EectivesoftandhardROSATPSPCX-rayspectralindices(seetext)oftheredquasars(largelledsymbols)comparedwithnormalradio-quietquasars(smalldots;Fioreetal.
1998).
Steepopticalspectrum(a\[2)redquasarsareshownascircles,andintermediateopticalspec-trum([1.
5\a\[1)redquasarsareshownastriangles.
Thelinepairsaroundtheperipheryofthegureshowoutlinespectralshapesfortheirlocationsintheplane.
(ahard,ahard)showoutlinespectralshapesfortheirlocationsinthe(asoft,plane.
]Thisisconsistentwiththeirhavingthemoder-ahard)ateX-rayabsorptionexpectedfromtheiropticalproperties(Fioreetal.
1998).
4.
DISCUSSIONTheseobservationsshowthatapopulationofredAGNscanbeextractedefficientlyfromtheROSATpointedarchive.
Moreover,theredAGNswendareradioquiet.
NoneofthemisaradiosourceintheNRAOVLARadioSkySurveymJy;Condonetal.
1998),(f1.
4GHz\2.
5implyingGHz)]\2.
0,comparedwithRL\log[f(opt)/f(5forradio-loudquasars(Wilkes&Elvis1987).
2\RL\5TheagreementofX-raycolors,opticalcontinuumslope,andHa/Hbratioswiththesamevalueofobscuringdustandgasarguesfortheirbeingdustreddened(AVD12)objectsratherthaninstrinsicallyredcontinua.
Puchnarewicz&Mason(1988)discussasimilarpopu-lationof14candidatered,AGNsderivedfromtheaopt[2,RIXOSsample,whichextendstoseveraltimesfainterX-rayuxes.
TheRIXOSsamplewasselectedfromtheROSATpointedarchivebasedonX-rayuxalone.
(Thereisoneobjectincommonbetweenthetwosamples.
)TwooftheredRIXOSAGNsmaybeintrinsicallyred,andthreehaveclearreddening.
SothereiscurrentlyatotalofeightreddenedAGNsavailablefromROSAT.
TheredAGNs,bothfromthissampleandfromRIXOS,areborderlinequasar/Seyfertobjects.
TheobservedopticalluminositiesoftheredAGNsaremodest,lyingatthehighendofthetraditionalSeyfertluminosityrange(MB[[23mag;Veron-Cetty&Veron1984;Schmidt&Green1983):fromto[22mag.
6Thedereddened,intrinsicMO\[206Wehaveusedkms~1Mpc~1andH0\50q0\0.
luminosityofthesourcesislikelytobesignicantlyhigher,dependingontheamountofabsorption(thelowerlimitontheextinctionis23maginO).
Thisplacesthematwhichiswithinthequasarregime.
[25\MO\[22,Similarly,theobservedX-rayluminosityrangesfrom1.
3]1043to1.
2]1044ergss~1,whiletheintrinsiclumi-nositiesarelikelytobehigherbyaboutafactorof2,dependingontheintrinsicspectrumandtheamountofabsorptionpresent.
ThepreciseallocationoftheseAGNsasquasarsorSey-fertsisnotfundamentallyveryinteresting,sincethisisjustaconvenientlychosenvalueonacontinuousluminosityscale.
Forsimplicitywewillrefertothemasredquasarsfortherestofthispaper.
Itis,however,interestingtounder-standwhymuchmoreormuchlessluminousAGNswerenotfoundinthetworedquasarsearches.
Isthisaselectioneect,oristhereaphysicalpreferenceforredobjectstoclusterinalimitedrangeofluminosityWewillreturntothisquestionlater(°4.
3).
Theexistenceofaredquasarpopulationimmediatelyraisesimportantquestions.
Whatmakesthemredcom-paredwithusualbluequasarsHowcommonarethey,particularlyonceallowanceismadefortheirreduceduxduetoprobableobscurationIftheyareabsorbed,byhowmuchWhereistheabsorbingdustymaterialMightalarger,moreobscured,AGNpopulationexist4.
1.
HowCommonareRedQuasarsWecanaddresstherelativenumbersofredquasarsinaroughway.
Thepopulation,althoughaminority,isquitecommon.
Wefoundsevenredquasarsoutof45candidatesforwhichourspectroscopywasadequatetoproduceaclas-sication.
Assumingthatthe45werearandomsubsampleoftheoriginalsampleof87,thatsamplewouldproduce14redquasars.
Thisis2.
4%of575sourceswithopticalcolorsavailable.
Atouruxlimit,Stockeetal.
(1991)ndthat51%ofallX-raysourcesareAGNs.
Soaminimumof13.
5/288(4.
7%)AGNsinourunidentiedsampleareradio-quietredquasars(withO\20)atthissoftX-rayuxlevel.
TheBoyle&diMatteo(1995)upperlimitof9%ofCam-bridgeROSATSerendipitySurvey(CRSS)X-raysourcesbeingredquasarsisconsistentwiththeminoritypopulationofmoderatelyobscuredquasarswehavefound(AV\2)here.
Additionalredquasarscouldbehiddeninoursample.
Oursampleof45classiedspectraisnotarandomsub-sampleofthecandidatelist.
Figure1bshowsthatweprefer-entiallyselectedobjectswithi.
e.
,thebrighteraox[1.
3,objects(withB\21.
5).
Ifthesixobjectsforwhichweattemptedtogetspectraturnouttoberedquasars,thenthefractionofredquasarsamongthePSPCAGNscouldbealmostdoubleourrstestimate.
Thereareafurther36redcandidatesforwhichnospectroscopywasattempted.
Sothetrueoccurrencerateofredquasarsisuncertainbyafactorof7.
TondthefractionofredquasarsamongalltheAGNsinourX-ray,ux-limitedsample,wemustallowforthe165300blueAGNsintheidentiedsample,so1%7%ofthewholesoftX-rayAGNpopulationisred.
IftheredAGNsareobscured,thentheintrinsicrateofoccurrenceofredquasarshastobecalculatedrelativetotheirunreddenedparentpopulation.
ObscurationbyAV\reducestheirunobscuredX-rayuxesbyafactorofD2.
2SincehigheruxAGNsarerarer(theX-rayselectedAGNNo.
1,1999RADIO-QUIETREDQUASARS15logN/logSrelationhasaslopeof[3/2inthisuxrange;Hasinger1995;DellaCeccaetal.
1992),theredpopulationformsalargerfractionofthispopulation,D3%20%.
Usingthefractionofthesourcesattheintrinsicuxoftheredquasarsallowsustocompareourresultwithsamplesunaectedbyobscurationandwithmodelpredic-tions.
Comparisonswiththeobservedfrequencyofradio-loudredquasarswithbroademissionlinesfoundbySmith&Spinrad(1980;178MHz3CR)andStickeletal.
(1996;5GHz1Jysample)andwiththeComastrietal.
(1995)pre-dictionsarestraightforward.
Redquasarsarefoundin15%ofthe3CRsourcesampleand6%20%ofthe1Jy(Carillietal.
1998)sample.
Thesearecomparablewiththe3%-20%ofthe""intrinsicuxX-raypopulationthatwend,suggestingthattheradio-loudandradio-quietquasarpopulationshavesimilarlysizedpopulationsofmoderatelyobscuredquasars.
Theobscurationinthe3CRredquasarsisalsoaboutAV\2(Elvisetal.
1994;Economouetal.
1995;Rawlingsetal.
1995).
Forfourobjectsinthe1Jysample,Carillietal.
(1998)estimatelowerlimitsonfrom2to5basedonAVextrapolatingtheradio-infraredindextotheoptical.
However,sincesuchsteeplyrisingslopesarenotknownamongunobscuredquasars,theselimitsarelikelytobetoolarge,andvaluescomparablewiththe3CRestimatesareprobablyacceptable.
ForAGNswithcm~2,theComastrietal.
NH\1022(1995)modelpredictsthat26%willhave1021\NH\1022cm~2whichissomewhatlargerbutcomparable(AVD2),withthenumbersfoundhere.
Comastrietal.
predictfarlargernumbersofmoreobscuredobjects.
4.
2.
MoreObscuredObjectsMoreobscuredobjectsmayexist.
Puchnarewicz&Mason(1998)ndseveralobjectswithsteeperopticalcon-tinua,andFigure1bshowsseveralreddercandidatesandmanymoreX-rayloudcandidateswithnoopticalspectratodate.
Websteretal.
(1995)suggestedthatmaybeAV\5typicaloftheirredobjects,givingtheirputativeROSATcounterpartsinoursampleV\2325,whichisbelowthePalomarSkySurveylimit.
correspondstoacolumnAV\5of9]1021atomscm~2,whichwouldreduceROSATPSPCcountratesto15%oftheirunobscuredvalues.
Theseobjectswouldthenbehiddenasa6%minorityamongthemorecommon,lowerluminosity,unabsorbedquasarsiftheyhadthesameunobscuredspacedensityasnormalbluequasars.
Some9%ofourinitialX-rayselectedsampleare""blankeldobjects,i.
e.
,havenocounterpartonthePalomarSkySurveys.
Afractionofthesecouldbemoreheavilyobscuredredquasars.
Boyle&diMatteo(1995)ndthattheCRSSsamplecouldbemissingnomorethan9%(attheobservedux)inredquasars.
Subtractingthe1%ofmoderatelyobscuredquasarswehaveidentiedstillleaves8%thatcouldbehighlyobscured.
HencetheCRSSresultis,perhapssurprisingly,consistentwithasizeable,heavilyobscuredpopulation.
TheComastrietal.
(1995)modelpre-dictsacomparablepopulationofAGNswithNHD1022cm~2,whichis1.
1timeslargerthantheunobscuredpopu-lation.
Inoursampletheoccurrenceofredquasarsappearstobe4timeshigherforO[19mag(4/12)thanforO\19mag(3/32;Table2),althoughthenumberofsourcesissmall.
Suchatrendisexpectedifthequasarsareheavilyabsorbed.
Anincreaseinfrom3]1021to1]1022NHcm~2cutstheROSATuxbyafactorof2.
4butreddenstheVbandby3.
8mag(afactor33).
Sotheopticallyfaintersourcesmightwellberedder.
However,theRIXOSredquasars(Puchnarewicz&Mason1998)shownocorrelationofopticalslopewiththethreesteepestslopesareallinmV:thebrighterhalfofthesampleof14.
Gunn&Shanks(1998)havepointedoutthatwhileredshiftingtheultravioletintotheopticalincreasestheeectsofreddening,thecorre-spondingshiftofhardX-raysintothesoftROSATbanddecreasestheeectivenessofreddening.
Toestimateanaccuratefractionofthispotentialhiddenpopulationneedsalargersample,includingmoreabsorbed,fainterobjects.
Atevenlargercolumndensities(10231024cm~2)theComastrietal.
(1995)modelpredictsnearly4timestheunobscuredpopulation.
MoreheavilyobscuredquasarscouldbefoundinhardX-raysurveysfromASCA(Uedaetal.
1998)andtheBeppo-SAXHELLASsurvey(Fioreetal.
1999).
4.
3.
LimitedLuminosityRangeItisstrikingthatwhileROSATsurveysthataredenedsimplybyanX-rayuxlimitndAGNsspanningover3decadesinX-rayluminosity(e.
g.
,CRSS,Boyleetal.
1997;RIXOS,Puchnarewiczetal.
1996),bothRIXOSandthissurveyndredquasarsinonly1decadeofluminosity,andthisdecadeisthelowestoneinwhichRIXOSandCRSSAGNsarefound.
Atwo-tailKolmogorov-SmirnovtestshowsthatthechancethatredandnonredAGNsfromRIXOScomefromthesameluminositydistributionisonlyD2%.
ThissuggeststhatpredominantlylowerluminosityAGNsareobscured.
(NotethattheobservedamountofobscurationonlydecreasestheobservedX-rayluminositybyafactorofD2,andsodoesnotitselfcausethelowobservedluminosities.
)Similarsuggestionshavebeenmadebefore:Lawrence&Elvis(1982)foundthatonlyAGNsbelowergsLXD1044s~1(210keV)showedobscuration.
Occasionalexamplesofhighlyobscuredtype2(i.
e.
,narrow-line)quasarshavebeenreported(Stockeetal.
1982;Almainietal.
1995;Shanksetal.
1995),andcarefulsearcheshavefoundbroadHainmostcases(Halpern,Eracleous,&Forster1998),makingthemsimilartotheredquasarsfoundhere.
Search-esamongthefainterobjectsinoursampleandsearchesathigherenergies(e.
g.
,theBeppo-SAXHELLASsurvey;Fioreetal.
1999)willbemoreeectiveatndingahigh-luminosityredquasarpopulation.
TheComastrietal.
X-raybackgroundmodelsassumeluminosityfunctionsfortheobscuredobjectsthatareiden-ticaltothoseoftheunobscuredobjectsexceptfornormal-izationandsopredictshigh-luminosityredquasars.
IfinsteadobscuredAGNsoccurpreferentiallyatlowlumi-nosity,thiswillsubstantiallyaectthemodelpredictions.
Wewould,forexample,expecttheobscuredpopulationtobemorenumerousandatlowerredshift.
Ifthereisarealdecitofhigh-luminosityredquasars,thenonepossibilitytoexplainthislackmighthavebeenthatasanAGNbecamemoreluminous,thecontinuumionizedtheobscuringmedium,renderingittransparenttoX-rays.
However,ionizedabsorbersarealsomorecommonatlowerluminosities(Laoretal.
1994).
Somostlikely,high-luminosityAGNshavefewerlinesofsightwithinterveningmaterialregardlessofionizationstate.
(Interestingly,thisisinthesamesenseastheBaldwineect:thathigherlumi-nosityquasarshaveweakerCIII]j1909emissionlines.
)Any16KIM&ELVISVol.
516physicalmodelofaquasarwouldneedtoexplainthisdier-ence.
4.
4.
PhysicalPropertiesWecansayonlyalittleaboutthephysicalpropertiesoftheredquasarsfromthisdata.
TheconsistencyoftheopticalreddeningindicatorswiththeX-raycolorssuggeststhatthesameobscuringmaterialcoversbothemittingregionsandthatitliesoutsidethebroademissionlineregion.
Smith&Spinrad(1980)suggestedthattherednessofthered3CRquasarsisintrinsictothecontinuumemissionprocessbasedonthelackofanabsorptionfeatureatj\2200whichisatypicalcharacteristicofMilkyWayA,dust(e.
g.
,Bless&Savage1972).
Thedetectionof21cmHIabsorptiontowardalargefractionofthe1Jy(Stickeletal.
1996)redquasars(Carillietal.
1998)arguesfordustreddeninginthoseobjects.
Sinceoursampleofradio-quietquasarsisrelativelynearby(withredshiftsupto0.
3),wecannotcheckforthisfeaturedirectly.
However,thisexpla-nationisincontrasttotheobservedBalmerdecrementandtheX-raycolors.
Itispossiblethatthereddened3CRsourcescontaindusty,ionizedabsorbers,asisseenin3C212(Mathur1994;Elvisetal.
1994)andIRAS17020]4544(Komossa&Bade1998),wherethedustcompositionmaydierdependingon,forexample,thequasarcontinuumshape.
Ultravioletobservationsareneededtoinvestigatethej\2200featurebutareprobablyinfeasibleatApresent.
4.
5.
OtherRedAGNsSomepreviousstudieshaveconsideredredAGN-likeobjectsinX-raysurveys.
TheRIXOSsurvey(Puchnarewicz&Mason1998)foundthat9%(14/160)oftheirAGNswerered.
However,onlythreeoftheRIXOSsourceshaveBalmerdecrementsthatrequirereddening,sothetrueoccurrencerateofreddenedobjectsmaybesimilartowhatwehavefound.
ThefainteruxlimitoftheRIXOSsurveymayrendermoreabsorbedobjectsvisible.
Certainly,thesteeperopticalslopesofhalfthe([2.
5[aopt[[4.
0)RIXOSsamplesuggestgreaterreddening.
Kruper&Canizares(1989)studiedredAGNsinEinsteinX-rayselectedsamplesandindirectlyconcludedthattheseareredbecauseofthepresenceofhostgalaxies.
Bennetal.
(1998)arriveatasimilarconclusionforlow-frequency,selectedradio-loudquasars.
However,nogalaxianstarlightfeaturesareseeninourspectra,andthehostgalaxycannotexplaintheobservedBalmerdecrementsinoursample.
TheKruper&Canizaresobjectsarenotasredasoursamples,havingB[I\1.
52.
5mag.
IfwetakeR[Itobe0.
51.
0mag(thisistheR[IrangeofthesamplesintheirTable2),B[Rwouldbelessthan2.
0,ourdeningthreshold.
Infact,noneoftheirobjectswithmeasuredRmagnitudesexceedB[R\2.
0.
Thenarrow-lineX-raygalaxies(NLXGs)foundplenti-fullyindeepROSATsurveys(e.
g.
,Boyleetal.
1995a)arealsonormallyassumedtobeobscuredAGNs(e.
g.
,Hasingeretal.
1998;Schachteretal.
1998).
X-rayabsorbedNLXGscouldcontributesignicantlytothecosmicX-rayback-groundiftheyaremorecommonatfainterX-rayuxlevels,aswassuggestedbyMcHardyetal.
(1998;seealsoHasingeretal.
1998foracautionarynote).
AlthoughtheyhavesimilarX-rayluminositiestotheNLXGs,theX-rayselectedredquasarsarenotsimplythesamepopulation,however.
Theredquasarshavethenormal,broadopticalemissionlinesofquasars,whileNLXGshaveeithernoneoronlyextremelyweakones(Boyleetal.
1995b;Figs.
2aand2b).
Moreover,NLXGsusuallyexhibitblueopticalcontin-ua(forexample,O[E\2forveoutofsixNLXGsinCRSS;Boyleetal.
1997),whileredquasarshaveredopticalcontinua(O[E[2).
FurtherX-rayandopticalstudyoftheseobjectsmayletusunderstandwhethertheyaretwoseparatepopulationsorarerelatedby,e.
g.
,specialviewinggeometryorscatteringofabluecontinuum.
ThePalomarsurveyofthenucleiofbrightgalaxies(Ho,Filippenko,&Sargent1997)isbasedonasampleofgal-axiesselectedfortheirnonnuclearpropertiesandsoislessbiasedagainstndingredAGNsthanmostotheropticalsearchmethods.
ThePalomarAGNsarelow-luminosityAGNs,allowingustoseewhetherthehighincidenceofredAGNsatlowerluminositiescontinuestoincreaseatevenlowervalues.
ThePalomarsurveyndsthat18%(8/44)ofSeyfertnucleihavebasedontheirnarrow-lineAV[2Balmerdecrements(Hoetal.
1997).
However,almostalloftheseareLINERsortype2Seyferts,whichmayhavelargereddeningtowardthebroad-lineregion.
OnlyoneSeyfert(NGC7479)showsanyevidenceforabroad-linecom-ponent.
Broademissionlinesareextremelyhardtodetectattheseuxlevels,butthesuggestionisthatthemiddlingluminositiestowardthequasar/Seyfertborderlinearepar-ticularlypronetomoderateobscuration.
5.
CONCLUSIONSRadio-quietredquasarscanbefoundinsubstantialnumbers.
Theycompriseatleast1%,andpotentially7%,ofthesoftX-raypopulationinauxlimitedsurvey.
Correct-ingtheX-rayuxestotheirintrinsicvaluesputsthemamongbrighterAGNs,wheretheyform3%ofthepopu-lation.
Allowingforblankeldsources,asmuchas20%ofROSATselectedquasarsmayberedatagivenunobscuredux.
Thesizeofthispopulationisconsistentwithpreviousupperlimits,withtheComastrietal.
(1995)modelfortheX-raybackground,andwiththesizeoftheradio-loud3CRand1Jyredquasarpopulations.
Redquasarsseemtobepreferentiallylowerluminosityobjectsonthequasar/Seyfertborderlinebutnotathigherorlowerluminosities.
Suchabiasagainstobscuredhigh-luminosityobjectswouldaectX-raybackgroundestimatesforthispopulationandwouldneedexplaininginaphysicalmodelofquasars.
Westressthatthequasarswendhavebroadopticallines.
TheyarenotNLXGs,whichbycontrasthavepre-dominantlynarrowopticalpermittedlinesandbluecontin-ua.
Theopticalslopes,Ha/Hbratios,andX-raycolorsareallconsistent,withreddeningbyassumingstandardAVD2MilkyWaydustproperties.
Sothesameobscuringmaterialprobablycoverseachoftheemittingregions.
Asignicantpopulationofmorehighlyobscured(AV\5)quasarscouldwellexistandbeconsistentwiththeresultshere,withearlierROSATlimits,andwouldbeaspredictedbytheComastrietal.
(1995)model.
HardX-raysurveyswillsoonsettlethequestionofthesizeofanysuchpopulation.
Usingaminorrenementofthetechniquepresentedhere,redquasarscanbefoundwithhigh(20%)efficiencyintheROSATdata.
WethankL.
AngeliniforhelpingustouseWGACATandM.
IrwinforuseoftheAPMonlinecatalog.
WealsoNo.
1,1999RADIO-QUIETREDQUASARS17thankF.
FioreandF.
NicastroforprovidingtheirprogramtocalculateX-rayspectralindicesandF.
FioreonceagainforsupplyingtheX-rayslopesoftheradio-quietsampleinFigure3.
ThesupportbytheFLWO,MMT,andCTIOstasinsettingupandoperatingvariousinstrumentswereinvaluabletothisstudy.
TheHEASARC/GSFCPIMMSprogramwasusedtocalculatePSPCcountrates.
ThisworkwassupportedbyNASAgrantsNAG5-3066(ADP),NAG5-6078(LTSA),andNASAcontractNAS8-39073(ASC).
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