197 7MNRAS.178. .451B 25 73 The remarkableSeyfertgalaxyMarkarian231 Received 1976June7 Mon. Not.R.astr.Soc.(1977)178,451-466 D. A.Allen,R.E.FosburyandM.V.Pension Astronomy, UniversityCollegeLondon A. BoksenbergandR.F.CarswellDepartmentofPhysics Australia Carnegie InstitutionofWashington,Pasadena,California91109,USA and Anglo-AustralianObservatory,POBox29,Epping,NewSouthWales2121, Royal GreenwichObservatory,HerstmonceuxCastle,Hailsham,SussexBN271RP W. L.SargentHaleObservatories,CaliforniaInstituteofTechnology, blackbody distributionfromhotstars.Eitherofthesemodelshasintrinsic presented. Thecontinuumcanberepresentedbyafv^v''powerlawbut Summary. Spectra,scansandadirectelectronographofMarkarian231are in opticalluminosity.Theobservedemissionspectrumcanbesynthesized tion aseitherareddened/¿-constantspectrumor10000K the presenceofopticalandinfraredabsorptionfeaturesfavoursinterpreta- (« -10cm")emissionregion.ThepresenceofCa ntripletemissionandno redshift of12600±200km/s.Exceptforthe[On],whichmustbeemitted from blendedfeaturesofHi,Her,[On],Can,Fen,[Fen]andNinata absolute magnitude,M—25.1,placingMarkarian231amongthequasars in theemissionregionarelarge(r//~100).There arethreeabsorptionline X3889 at6250km/sandSystemIIIfeaturingthe A'-line andmembersofthe corresponding HandKemissionshowsthatthe optical depthsinHandK from aseparatelow-densityregion,theselinesindicatehigh-density of Heiataredshift~8000km/s,SystemIIrepresented byZMinesand redshifts —SystemIcontainingresonancelinesof CanandNaiX3889 lines aremuchbroaderthantheabsorption seeninhigh-redshift stars theSystemIIIlinesrepresentabsorption spectraofthosestars.The Balmer seriesat12900km/s.Iftheobservedcontinuum isemittedbyhot by fluorescenceifthe ions experiencetheunreddenedradiationof quasars. ThedoubletratioforHandKinSystem Iindicatesalowoptical from theemissionlines.The calciumandironemissionlinescanbeproduced depth T//-0.6alongour Uneofsighttothenucleusincontrastresult of Markarian 231arepresented. continuum source.Two possible configurationsforthenuclearcomponents e y © Royal Astronomical Society • Provided by theNASA Astrophysics Data System 197 7MNRAS.178. .451B 73 large numberofSeyfertgalaxies(Sargent,unpublished)showsthatthereisagradation relatively narrow:inclass2SeyfertsboththeBalmerandforbiddenlinesarebroad lines areweakorabsent.Well-knownexamplesofthelattertypeIZw0051+12and which theyareinvisibletothoseindominatethespectrumandforbidden the strengthofbroadFenemissionfeaturesamongclass1Seyfertsfromobjectsin in thespectraofclass1Seyferts(e.g.Boksenbergetal1975b).Spectrophotometrya (~ 1000km/s).Otherbroadlines,inparticularthoseofHei,nandFen,areoftenseen Most SeyfertgalaxiesfallintotwoclassesdefinedbyKhachikian&Weedman(1973).Inthe 452 A.Boksenbergetal. tinuum sourceintheSeyfertnucleus,itwasdecidedtostudyamemberofclassFen spectrum appearstocomefromadense(n£10cm")regionclosethenon-thermalcon- that ofthequasar3C273(Baldwin1975a;Boksenbergetal.1975a).Sincebroad-line II Zw2130+09(Sargent1968,1970)andthespectraoftheseobjectssuperficiallyresemble class 1objectstheBalmerlinesareverybroad(~10000km/swide)andforbidden region couldbedetermined. emission objectsinsomedetailtoseeifthephysicalconditionsin,andgeometryof,this excess (e.g.Rieke&Low1972)andasilicateabsorptionat10/im(Allen1976, include unusuallyredUBVcoloursforaSeyfertnucleus(Weedman1973),stronginfrared emission withtheBalmerseriesandX3727of[On]alsopresentataredshift12300km/s. Adams &Weedman(1972)andby(1972).TheyhadfoundextremelystrongFen showing Balmerlineabsorptionat12500km/s.OtherpeculiarfeaturesofMarkarian231 the H,Kand/MinesX3889ofHeiataredshift7900km/ssecondsystem In additiontheyshowedthatMarkarian231hasastrongabsorptionlinespectrumfeaturing references therein). I Introduction The opticalappearanceofMarkarian231isillustratedinFig.1—thisfromaPDSmicro- whose natureisatpresentunknown.Visualobservations atthetelescopeshowthat 4-cm S20electronographiccamera(McMullan,Powell&Curtis1972)mountedatthe 2 Observationaldata e nucleus isunresolved. densitometer scanofanunfiltereddirectelectronograph.ItwasobtainedusingtheRGO red (XX5660—11020),theotheron1975June3/9 witharesolutionof20Athroughoutthe the Cassegrainfocusof200-inHaletelescopeon PalomarMountain.Onescanwastaken south-west. Thereisashield-likestructureonthe westernsideofthegalaxy,resembling 231 tobeagalaxywithbrightnucleusandsinglespiralarmrunningclockwisefromthe Cassegrain focusofthe40-intelescopeWiseObservatory,Israel.ItshowsMarkarian the UnitSpectrograph. Differentgratings wereusedgivingarange ofresolutionsbetween College London,ImagePhoton CountingSystem(IPCS)mountedatthe camera focusof Oke &Schild(1970).Thescansused10-arcsec diameter circularapertureswithasky on 1974April21/22witharesolutionof20Ain the blue(XX3150-5710)and40Ain aperture 40arcsecawayin rightascension.Thelowerresolutionscanisillustrated inFig.2. observed range.Theywerereducedtofluxesbased ontheabsolutecalibrationofaLyraeby telescope atHerstmonceux during1975February,AprilandJuneusing theUniversity features ininteractinggalaxies,whosesouth-west endleadstoanon-stellarcondensation The objectchosen,Markarian231,hadpreviouslybeenstudiedspectroscopicallyby © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Scans wereobtainedontheMulti-ChannelSpectrophotometer (MCSP)(Oke1969)at Spectra ofthenucleus Markarian231werealsoobtainedonthe98-inIsaac Newton 197 7MNRAS.178. .451B black. Thethinlinetotheeastofpictureisanemulsiondefect. by /4=2.1(dashedline)and anf=constantspectrumreddenedbyA2.3(solidline) areshown. particular theinfraredcalcium tripletinemission.Continuarepresentinga10000Kblackbody reddened bands are20Aintheblue and40Ainthered.Prominentspectralfeaturesareidentified —notein levels arespacedby0.05inopticaldensityonIlfordL4emulsionandrepeatthesequencewhite,grey, Figure 1.DirectelectronographofMarkarian231inunfilteredlightusinganS20tube.Thethreegrey Figure 2.AscanofMarkarian 231obtainedwiththeMultichannelSpectrophotometer (MCSP).The V vy © Royal Astronomical Society •Provided bytheNASA Astrophysics DataSystem E 30‘ The SeyfertgalaxyMarkarian231 N 453 197 7MNRAS.178. .451B 454 A.Boksenbergetal. The MCSPdataconfirmthatthecontinuumofMarkarian231isremarkablyredfora lay 58arcsecawayinRA.Fig.3showsoneofthelow-dispersionspectraandidentifies Seyfert galaxyasfoundbyWeedman(1973).However,thecontinuumcannotbeexplained length regionscoveringthestrongabsorptionlines:X3889,7/andKinFig.4D-lines in Fig.5.Botharefromdatatakenwithadispersionof50Â/mm. the scansdescribedabove.Figs4and5shownormalizedhigherdispersionspectraofwave- elsewhere (Boksenberg1972;Boksenberg&Burgess1973;etal1975a).The dures, methodsofwavelengthcalibrationandskysubtractionhavealreadybeendescribed lengths coveredbyeachspectrumaregiveninTable1.TheIPCS,theobservationalproce- strongest featuresintherange3300—6900Â.Ithasbeennormalizedtoabsoluteunitsusing observing aperturesforthespectrawere1arcsecdecby3RAandskyaperture 5 A(210Â/mm)and0.4Â(16Â/mm).Fulldetailsofthedispersionsusedwave- 3 Thecontinuum Figures. Thespectrumofthe nucleusofMarkarian231takenatanoriginaldispersion of210A/mm multiplets refertopermitted transitionsinFell.Bracketedromannumeralsindicate theabsorption (vfp) usingthescanofFig. 2. Strongemissionandabsorptionfeaturesareidentified -thenumbered system towhich thelinebelongs. (~ 5Aperchannel)withthe ImagePhotonCountingSystem(IPCS).Itisnormalized toenergyunits © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Table 1.DetailsofspectraMarkarian231. Date April 3/4 April 2/3 February 9/10 (1975) April 4/5 April 5/6 April 4/5 June 10/11 April 5/6 April 6/7 WAVELENGTH INANGSTROMS Dispersion (A/mm) 210 210 150 150 50 50 50 50 16 Wavelength range (A) 4600- 3200- 5450- 3500- 4300 4900 5250 3300 3930 -8100 -5850 -5300 -4230 -7200 -6350 -7200 -4550 -7100 197 7MNRAS.178. .451B 25 lines, inparticularHeIA.3889andCallHK,thedoublestructureK3727[OII]emission. Figure 4.ThenormalizedIPCSspectrumofthenucleusMarkarian231showingvariousabsorption The opticaldispersionwas50A/mmandthelinesareidentifiedasinFig.3. Figure 5.ThenormalizedIPCSspectrumofthenucleus Markarian231showingtheabsorptionlineof by apopulationoflate-typestarsasinanelliptical galaxy.Theoverallshapeoftheenergy Na ID.Theoriginaldispersionwas50A/mmandthelines are identifiedasinFig.3. elliptical (Oke&Sandage 1968).Moreoverthereisnosigninthedataofabsorption features continuum ofMarkarian 231 between6000and10000Âismuchgreater than thatofan distribution isquitedifferentfromthatofsuchagalaxy alone,inparticulartheslopeof like MgibandtheG-band normallyseenstronglyinspectraofsuchgalaxies. shows that, if apowerlawspectrum isappropriate,theobserved fluxf&v~'.There is v © Royal Astronomical Society • Provided by theNASA Astrophysics Data System A fittothecontinuanear thetwoendsofspectralrangecoveredin MCSPdata The SeyfertgalaxyMarkarian231455 197 7MNRAS.178. .451B 60A 456 A.BoksenbergQtú. A =2.3.ThisinterpretationhastheadvantageofbringingMarkarian231intolinewith (Wampler &Oke1967;1972;Sargent,unpublished). ultraviolet scanpointswell,runlowintheregionbetween4000and6000Â.Even by a^=constantspectrumsufferingabsorptionfromnormalgalacticlawwith natural toexpectadditionalsourcesofradiationatlongwavelengthsfromcoolerstars,free- green regionbetterbutrunslowattheinfraredendofspectrumnearl/xm.Becauseitis power lawsare20-30percenttoolow.Analternativemodelforthecontinuuminvolving other Fenemissionobjectslike3C273andIZw0051+12whichhaveflatcontinua reddened hotstars,specificallya10000Kblackbodyby^4~2.1,fitstheblue- absorption andtheopticallines—spectrumcanequallywellbefitted be abletodistinguishthetwopossibilities. as thesourceofopticalradiationweseefromnucleusMarkarian231.Thetwo free emissionorthesourceoflong-wavelengthinfraredflux,hotstarsmaybepreferred thermal) sourceisnotyetruledouthowever:testsbasedonvariabilityorpolarizationmay possible fitstothecontinuumareillustratedinFig.2.Thealternativeofapower-law(non- ‘windows’ betweenemissionlinesatwavelengths4750and5100Âintherestsystem considerable evidenceforhighabsorptioninMarkarian231however—boththesilicate v Markarian 231wouldhavetheopticalluminosityofmanyquasars(Sandage1972);Rieke& luminosity ofthegalaxywillbediscussedfurtherinSections4and6. lute visualmagnitudeoftheSeyfertgalaxyisAf=—23.0:if>42.1then25.1and sity thatshouldbeascribedtoMarkarian231.Evenwithoutallowingforreddeningtheabso- Low (1976)noteasimilarhighinfraredluminosity.Thenatureofthecontinuuminand 4 Emissionlines V is showninFig.6which thesynthesisissuperposedonFig.3.Thederived lineintensities Osterbrock 1974).ThesewerefittedtoHa,Bßand A5876.Thewidthsused,inkm/s,were are listedinTable2. Nonetheless asatisfactory fitcanbeobtainedthroughoutmostofthespectrum. Thebestfit strengths byMeinel,Aveni&Stockton(1969),the dataofWarner(1967)andobservations is tosynthesizethespectrum,varyingafewparametersuntilsatisfactoryfit scanner ontheAnglo-Australian telescope.Notalltheintensitiesarereliably determined, z*F°. Theratiosofindividuallinesoriginatingfrom eachuppertermweredeterminedfron: to measuretheirintensitiesindividuallyoridentifyweakfeatures.Anattractiveapproach Since theemissionlinesinMarkarian231arebroadandbadlyblended,itisimpracticable of thestarsHD37974,CD-52°9243andHDE 316285madewithanimage-dissector vV Gaussian profilesadopted.Awidthof3300km/sfor theFenlinesseemedappropriate,bul parameter forintensityandonewidth,acaseBdecrementisassumedboth(e.g objects (Boksenbergetal.1975a).TheBalmerseriesandHeiareeachcharacterizedbyone observed spectrumisproduced.TheobviousionspresentinemissionareHi,HeFenand seem broaderthanthisfigure.Adifferentintensity parameterwasrequiredforeachuppei 4200 forHiand6200Hei,thesebeingthefull widthsathalfmaximum(FWHM)ofthe term oftheFenlinesused.The.importantupper termswerez?,Z)°,F°,zD°and this figurecouldbeashighfortheBalmeremission. Inparticularthelinesofmultiplet42 four sources:theRevisedMultipletTable(Moore 1959),thecompilationofemission-lim [O n],andthesewereusedforthesynthesis.Allarecommonfeaturesinotherextragalactic © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Both thereddenedandunreddenedpowerlawcontinua,whilstfittinginfrared Both modelsdemandquitehighabsorptionswhichwouldincteasetheintrinsiclumino- Several implications ofthesynthesisdeserve mention. 197 7MNRAS.178. .451B 2s superimposed onthedataofFig.3. law finallyappliedtomakeitmatchthedata.Theselectionofcontinuumfluxdistribu- in aclassicalshellspectrum,itseffectonHawould bemuchmoremarkedthanisobserved. emission redshift.Thesearealsoseeninourdata and arediscussedinthenextsection.The body fortheunderlyingcontinuum.Bothcontinuaareverysimilarovermostof for radialvelocitydeterminations.Foranormalradiative Balmerdecrement,Hßshouldbe rather broaderemissionfeature;theyarenotthe two linesthemselvesandcannotbeused thought tobetheselinesaregeneratedbyanabsorption superposedonamuchstrongerand Adams (1972)reportedabsorptionsatH7toH13 atvelocitiessomewhatinexcessofthe 4.2 BALMERLINES lines toostrong.Agoodfitisfoundbyadoptingeitherf=constantora10000Kblack- An underlyingcontinuumwasselected;theemissionlineswereaddedtoitandareddening 4.1 THECHOICEOFCONTINUUM that thissupportsthenotion thatearly-typestarsemitthecontinuumwe observe. Forthe trived. Balmerabsorption isprobablyanintrinsicpropertyofthecontinuum sourceitselfso tion wereproducedbymaterialinterveningbetween theemissionregionandobserver,as almost twicetheheightofobservedfeaturein theabsenceofabsorption.Ifthisabsorp- 300±100km/s higherredshiftthantheemission. The twopeakswhichmightotherwisebe Hj3/Fe iiX4924blendcanbematchedonlyif there isalsostrongabsorptioninHj3at observed range.Thustheemissionlinesappeartobereddenedalongwithcontinuum. tion andthecorrespondingreddeningdeterminerelativelineintensities.Ifobserved absorbing materialliesbetween thecontinuumsourceandemissionregions seemscon- Instead, itmustactonthe underlyingcontinuumonly.Thegeometryofa galaxyinwhich continuum, fv~',isadopted,thefitpoor,redlinesbeingtooweakandblue Figure 6.AsyntheticspectrumgeneratedasdetailedinthetextusinglineintensitiesofTable2 synthesis, Balmer lineabsorptionofFWHM 2300km/sredshifted by300km/srelativeto the v v 16 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The SeyfertgalaxyMarkarian231457 197 7MNRAS.178. .451B 458 A.Boksenbergetal. Table 2.LineandmultipletintensitiesinMarkarian231. HI Species He I Ca II Fe II Notes toTable2: [Oil] Ni II adopted profilesofthe line absorptioncoefficientsofHß—H10wereGaussian andhad emission lineswasappliedtothecontinuumprior totheadditionofemissionlines.The may bepresent. the otherswerefittedto thespectrum.AsmallcontributiontoHefrom Ca nabsorptior central opticaldepths0.6, 0.3,0.22,0.19,0.16,0.14and0.12respectively. Thefigurefoi Ha couldnotbedetermined fromtheobservationsindependentofemission intensity © Royal Astronomical Society • Provided by theNASA Astrophysics Data System (3) Correctedfortheabsorptionlinescentred300km/sto theredofemissionline. (2) UsedtocalibratetheH0=100intensityscaleinterms ofcgsunits. (1) Notdeducedfromsynthesis-measureddirectly MCSPdata. 3 3 3 unclassified 154 153 unclassified unclassified Multiplet 2P°—3D ht H6 H0 Ha 2P°-4D 2S-3P° 40 28 27 26 25 42 29 IF 46 43 48 38 37 35 14 49 55 74 6 1 Wavelength of 4861 4340 (A) 4101 4471 6563 strongest line 5876 4461 4179 4233 8542 3889 4629 5001 3727 4584 5133 3193/4 3277 4731 3783 5018 6516 6084 3825 5317 5276 5535 5101 6456 6384 6318 3748 3814 3407 intensity 615 Observed (H0 =1OO) 100 129 103 154 50 33 16 40 21 16 10 47 56 96 33 13 26 32 69 20 18 14 4 4 9 3 6 7 8 6 3: 1: 142 (10"ergcm' s') Flux Notes 210 34 44 11 17 35 33 19 14 53 11 11 24 11 5 5 3 4 7 3 1.4 2 1 1.4 2 9 6 0.5 7 2 3 1: (3) (3) (1), (2),(3) (3) (1) 197 7MNRAS.178. .451B 67-3 2 67-373 43 2 Multiplet 4ofNinseemstobepresentintheIPCSdata.ThetwostrongestlinesareXX3407 4.3 OTHEREMISSIONLINES members oftheBalmerseriesorpresenceemissionincontinuum.The low inthespectralrange3700—3900Â.Thiscouldrepresenteitherabsorptionbyhigher electron density,«,intherange10—10cm. around 4230—4300Âand4400—4430Â.Thesecanmosteasilybeexplainedbythe and 3769,thesemightalsoexplainunidentifiedfeaturesin3C273:the‘3760Â’feature strongest linesof[Fen],similarwidthtotheFenandthereforeprobablyoccurringin described byBoksenbergetal(1975a)—tentativelyidentifiedthereinasTinandthe explicitly foronlythelowermembersofseries;secondrelatestodetection the sameregion.NosuitableanalysisexistsforFen,butweaknessof[Fen]suggestsan ‘X3405’ featurefoundbyBaldwin(1975a).Moreimportantaredeficienciesinthefit first suggestionisofinterestinviewtheBalmerlineabsorptionwhichhasbeenincluded this indicatesamassofgas~2000A/®involumeequaltothatspherewithradius Balmer continuumemissioninanumberoflow-redshiftquasarsbyBaldwin(1975b). 0.1 pc.Theradiusoftheemissionlineregionwillcoursebegreaterthanthisiffilling be understoodonlyifthenebulaisveryopticallythickinHandK,particularth~100.It upper stateasthetripletemissionandshould,accordingtobranchingratio,be100times One majorpuzzlehoweveristhelackofHandKemissionwhichcomesfromsame feature intheMCSPdatacanbeidentifiedwithinfraredcalciumtriplet(3dD—4pP°). factor islessthan1.OutsidethespectralrangecoveredbyIPCSdata,oneunequivocal narrower thantheotheremissionlinesandon50Â/mmIPCSdatainFig.3itisclearly lie underneaththeatmosphericA-bandandnousefullimitontheirstrengthcanbeobtained. would bepossibletodeducethedensityfromCanlinesifforbiddenX7291 2.2 x10cm(iftheHubbleconstantis50km/(sMpc)).Ifemittinggashasn~cm', gas inMarkarian231fairlyfarfromthenucleusis beingdrivenawayfromthecentreof Adams (1972)reportsthisemissionlinetobespatiallyextended;plainlyitarisesina which, takentogetherwiththeabsenceofred[On]doublets,indicates«<10cm'. stronger. Thisproblemisalreadyknowntoariseingalacticobjects(e.g.Herbig1975).Itcan be mapped. galaxy atvelocitiesof400km/sasaresulttheviolent activitythere.Thisnotioncouldbe and 7323(4sS—MD)couldbemeasured.Unhappilyasaresultoftheredshifttheselines tested ifthevelocityfieldfrom[On]emission intheotherpartsofgalaxywereto seen tobedoubleatredshiftsof11900and12700km/s.Oneinterpretationwouldthat e be termedhere‘SystemF,ataredshift~8000km/s. Thehighestresolutionspectrum The fourstrongestabsorptionlinesinMarkarian231 formanabsorptionsystem,whichwill different regionofthegalaxyfromthatemittingotherlines.InfactX3727isalso obtained withtheIPCScoversthreestrongabsorptions ofCanHandKHeiX3889in Sargent 1975).Inthoseobjects theabsorptionlinesariseonlyfromground orexcitedfine 5 Absorptionlines System I.Theyareshown inFig.7replottedonavelocityscale.Itisclearat oncethatthese structure statesandare usually verynarrow,manybeinglessthan50km/s broad.Also are ofadifferentnature from thoseseeninmosthigh-redshiftquasars(e.g. Boksenberg& strong lines usually gotozerointensity attheircentres.Bycontrast inMarkarian231 all e e The majorrespectinwhichthesynthesisdiffersfromoriginaldataisthatlatter © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The intensityofHßfoundinthesynthesisleadstoanemissionmeasureJnldV The forbiddendoubletX3727of[On]seeninMarkarian231isalow-densityfeature The SeyfertgalaxyMarkarian231459 197 7MNRAS.178. .451B 460 a metastable level23eVabovethegroundstate.These linesmoreresemblethoseseenin four ofthestrongabsorptionlinesarebroad(~1500km/s) andnoneisdeeperthan60per There issomeevidenceintheX3889,HznàKabsorptions forstructurewith‘components’ NGC4151 byKraft&Anderson(1969)whichare ~600km/sbroadandincludeX3889. Figure 7.TheprofilesoftheH,Kand\3889linesplottedonsamevelocityscalewithH at redshiftsof7420,7760,7900,8060and8320km/s. cent oftheobservedcontinuumattheirdarkestpart. InadditiontheHeilinearisesfroma is identifiedwithAandZ) taredshiftof6250km/s.Thisweshallcall‘System IF.Possible line absorptionfoundbyAdams(1972)isdoublewith aseparationwhichisappropriateifit fused bythedoubletnatureoftransitionwith aseparationwhichcorrespondsto lines displacedupwardsby100and200countsrespectively.Positionsofthepossiblevelocitycom- wide. the Earthwithrespectto theemissionredshift.Thelinesareresolvedand about 150km/s to Can.Thecloudproducing theSystemIIabsorptionhasavelocity-6500km/s towards absorption duetoHeiX3889 maybepresentinthissystem(fig.7)butnone isdetecteddue 300 km/s.Ourdatadohowevershowthattheweak absorptiontotheblueofmainD- ponents mentionedinthetextaremarked.Thesedatawereoriginallytakenatadispersionof16A/mm. 2 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System If anysimilarcomponentstructureispresentinthe SystemIZMineabsorptionitiscon- A. Boksenbergetal. 197 7MNRAS.178. .451B 12 u + in Section4fallshigherthanthedatajustlongwardofBalmerlimitalsosupports already beendiscussed;itwillhereafterbereferredtoasSystemIII.Thereareabsorptionsat Adams’ claims.Figs2and3alsoshowthattheA-lineofCa11ispresentinabsorption evidence forotherheliumabsorptions.Table3givestheredshiftdeducedfromstrongest H7, HßandHevisibleinFigs23.Adams(1972)claimedtoseeH8H10H13also: members ofthesystem:meanvalueis12900±160km/sindicatinganemissionlinered- the presentdatasuggestpresenceofH8andH10factthatsynthesisdiscussed broad. Itisnotpossiblefromourdatahowevertodistinguishbetweenarelativelysharp Gaussian profileandthebroadwingwhichwouldbepresentifSystemIII shift of12600±200km/s.Thesynthesis(Section4)showstheSystemIIIabsorptionstobe absorptions werestellar. Adams identifiedwithH8containsacontributionfromX3889ofHerbutthereisno System III.TheHeabsorptionisblendedwiththeCA11//-line.Itpossiblethatfeature emit theobservedcontinuum.Theequivalentwidthsareindeedcompatiblewithluminosity ? Balmer lineabsorptionisseeninNGC4151(Kraft&Anderson1969)presumablyagainst The observedluminosityof1OZ,indicatesatotalmassAstarsemittingthecontinuum class III—VAstars,andtheirwidthsmustbetheresultofvelocitydispersionstars. the non-thermalsourcesothatSystemIIIlines couldalsooriginateinanintervening ~ 200pc,correspondingto0".2ontheskywhichwouldnotberesolved.Onotherhand the continuumsourcealoneandnotemission-line region. ~lOAf. Thevelocitydispersiondeducedfromthesynthesisthenimpliesalengthscale also inSystemIIarguesstronglyfavourofclosely associatingtheabsorbingcloudswith cloud inasimilarway.Ascommentedabove,however, suchacloudwouldhavetoabsorb being illuminatedbyadiluteradiationfieldwith aratherhighcolourtemperature.Itis us atvelocities-4800and-6500km/swithrespect tothemeanemission-lineregion.The the nuclearactivityinMarkarian231.Thetwosystems thenrepresentmaterialapproaching fact thatHe,CaandNa°coexistwithinindividual absorbingcloudssuggeststhatthegasis excited byresonance fluorescencesuggests thattheseabsorbing cloudsareageometrically in theshellspectraofshell-stars (Underhill1966).Thepossibleoriginofthis diluteradiation in theOrionNebulaTrapezium starsandtheHe1,Ca11Na1absorptions sometimesseen interesting todrawtheanalogy herewiththeX3889absorptionsseen(Wilson etal1959) field isdiscussedinSection 6. 0 0 Balmer lineabsorptionatavelocityof200km/sinexcesstheemissionredshifthas © Royal Astronomical Society • Provided by theNASA Astrophysics Data System In Section4,argumentswereadvancedthattheSystemIIIlinesaroseinhotstars The presenceofabsorptionfromahigh-lyingmetastable levelinSystemIandpossibly This conclusiontakentogether withthepresenceofcalciumtripletemission probably Table 3.TheSystemIIIabsorptionredshift. \4340 Ht Line \4101 H6 \3969 He+H \ 3889H8+HeI \3933 K Mean Standard errorofmean Observed wavelength 4532 (A) 4139 4280 4102 4050 The SeyfertgalaxyMarkarian231461 Redshift (km/s) 13 270 13 090 12 850 12 890 12 420 12 900 ±160 197 7MNRAS.178. .451B 14 1 21 H andK.Thusinspiteoftheresultfromemissionlinesthatregion is notblackinitscentre.Eithertheabsorbingclouddoescoversourceorsomeother shows thatthe/Minesaremuchmoreopticallythick.TheDtoX3302/3ratiois15.0, optical depthinthesefeaturesalongthelineofsightbydoubletratiomethod(Strömgren the continuum.TheequivalentwidthsofCanHandKinSystemIcanbeusedtoinfer ness ofthecloudisorder10cm. source oflight(e.g.X5876emissionorstarlight)fromamoreextendedregionispresent. that thedoubletratiois1.7,th~0.6andradialvelocitydispersionparametersof would producethebroadeningofemissionlinesashasbeenhypothesizedbyOke& 462 A.Boksenbergetal. from thecalciumlines.Inviewofhighvaluetfound,itisinterestingthat/Mine optical depthisverylarge,alongthelineofsightviewedfromEarthitmuchless. absorbers (theDopplerwidth)is330km/sgenerallysimilartotheobservedlinewidthsof Sargent (1968),Anderson(1970)andothers. continuum deducedinSection3.IfthedensitythiscolumnisHq-IOthick- absorption ofabout2mag.Thusthecolumndensitiestieinwellwithresultsfrom this indicatesaminimumhydrogencolumndensityof2x10cm‘andvisual tively. Ifthegasdoingabsorbinghasnormalabundancesandadust-to-gasratio Td =31andtheDopplerwidthis210km/s:lastinquitegoodagreementwithresult favoured sampleofthebroademissionlineregion.Therelativevelocitiesclouds 1948). PuttingthevaluesofequivalentwidthsfromTable4intoStrömgren’s2shows d 1352 Table 4liststheequivalentwidthsofabsorptionUnesandfluxtheyextractfrom The deducedcolumndensitiesofGanandNaiare8x101.3xcm“respec- A similaranalysisusingtheratioofNaiD-linestoX3302/3blendforSystemI © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Table 4.Strengthsofabsorptionlines. System Line Note: nr II I ★ System IIIequivalent widthswithrespectto thetruecontinuumonlyfrom thesynthesis. A 3968Call A. 3889 A. 3970 A. 6563 A. 3835 A. 3889 \3933 A. 3968 A.4101 A. 3889HeI A. 5890Nai A.4340 A. 4861 A. 5896Nai \3302/3 Nai A. 3889HeI \4226 Cal Ä.7665 KI A. 3798 \3933 Call \5890/6 Nai HI Ca II Call HI HI He Ij 1 HI HI HI HI HI (60) <0.5 <3 (A) Equivalent width 35. 15. 18 5. 4.: 4.: 6. 4.: 0.8 0.3: 1. 0.7 6.1 6.3 3.6 1.2: _1421 (10erg cm's") Flux extractedfromcontinuum <3 <0.4 16 0.2: 0.7 0.6 0.5: 2.3 3.8 3.8 197 7MNRAS.178. .451B 13-2 132 would betemptingtosupposethatanon-thermal sourceexistsinMarkarian231which pletely obscuredinourdirectionsothatifviewed atadifferentangleitwouldmoreclosely provides theultravioletradiationrequiredtoexcite theemissionlines,butwhichiscom- thermal continua.Ifitwereacceptedthattheobserved continuumcamefromstars,thenit source, onewouldnotethatotherobjectswith strongFenemissionhaveclearlynon- but ontheinterveningcloudmodelequivalentwidthofKindicatesaCancolumn resemble, say,3C273orIZw0051+12. the resonancelinesisonlymodestlygreaterin other directions,andthemultiplet42 be increasedifthefluorescencetakesplaceinunreddenedradiationfield.Thenthereare absorbed astobeinvisiblefromEarth.Although thereisnodirectevidenceforsucha fluorescence withasomewhatthickerX2343line. the multiplet42)emissionisalsocausedbyfluorescence(Wampler&Oke1967)thena most otherdirectionstheHandKlinesabsorbedallcontinuumradiationoverarange density inexcessof10cm. show calciumtripletemission (Sargent,unpublished))donothavestrongH andKabsorp- adequate photonspresenttoexplainboththecalciumfluorescence,ifopticaldepthin reddened bythesameamountaslines,energyabsorbedinresonancelinesmay strength eveniftheexcitinglineX2343iscompletelyabsorbedfor4000km/s. Markarian 231. absorbed andemitted.It is interestingtonotethatotherFenemissionobjects (whichalso to theultravioletthereisinsufficientfluxproducemultiplet42linesatobserved similar calculationshowsthatwithanyreasonableextrapolationoftheobservedspectrum viewed throughasmallercolumnthanispresentintheemissionlineregion.However,if emission andabsorptionlinesalreadysuggeststhatthecontinuousobservedis Markarian 231.Forexamplethedifferencebetweenvaluesofthdeducedfrom this cloudalso.TheNaicolumndensityinmustexceedabout2x10cm". tinuum topowerthetripletemission.Moreover,ifitisthoughtthatFen(inparticular only abouthalftherequirednumberofphotonscanbeextractedfromobservedcon- of velocities4000km/s(thedifferencebetweenSystemIandtheemissionredshift),still energy isemittedintheinfraredtripletthanappearstobeabsorbedresonancelines. tion linesthussuggesting animbalanceinthefluorescenceprocess samesenseas Earth orthecalciumionscouldbeabsorbingadifferentlessabsorbedcontinuumsource. The absorption,however,couldbehigherinotherdirectionsthantheonesampledfrom optical depth.DetailedinspectionofTables2and4showsthataboutafactor7more collisions contributetothetripletemission,thisinnowayaffectsargumentsforhigh suggests thatfluorescenceplaysanimportantpartinproducingthelineemission.Evenif 6.1. FLUORESCENCE 6 Discussion The simultaneouspresenceofcalciumtripletemissionandresonanceline(HmàK)absorption Possibly allthesereasons playsomepartinexplainingtheimbalancebetween energy Alternatively therecouldbeafurthersourceof radiation seenbythegaswhichisso A columndensityfromtheSystemHIlinesismeaninglessiftheyareformedinhotstars The DiandZ>linesinSystemIIareessentiallyequalindicatingahighopticaldepth Because oftheevidencehowever(Section4)thatcontinuumobservedatEarthis These suggestionstieinwithotherdeductionsalreadymadeaboutconditions © Royal Astronomical Society • Provided by theNASA Astrophysics Data System 2 The SeyfertgalaxyMarkarian231463 197 7MNRAS.178. .451B modest obscuration. magnitudes. However,thecontinuumsourceseenfromEarthisviewedthrougharelatively particularly suggeststhatmuchoftheskyatnucleusiscoveredwithcloudswith>l^10 triplet emissionandsilicateabsorptionseemsquiteconclusive.The 464 A.Boksenbergetal. model showninFig.8(a).Thisincludestheunobservednon-thermalsourceinferredfrom The evidenceforveryhighcolumndensitiesofmaterialinMarkarian231fromthecalcium 6.2 GEOMETRY the appearanceofotherobjectsinSection6.1. jumble ofindependentcloudswithafewrandomdirectionscoveredlesswellthanmost. has asitsaxisofsymmetrypointingnearlytowardsEarth—Fig.8(b)—ortherecouldbea v tunnel tothenucleus.Thechoicebetweenthesetwodependsonhoworganizedis two differentvaluesoftheabsorptioncanbereconciledifweseethroughaholeordown overall nebulasurroundingthenucleus.Forexamplecouldbeathickdiskwhich Figure 8.(a)Apossiblegeometrical layoutofthenuclearregionMarkarian231inwhich thesourceof thermal source. the observedcontinuumistaken tobehotstars,(b)As(a)iftheobservedcontinuum comes fromanon- If thehot-starmodelforcontinuumisacceptedthenoneledtowardstypeof © Royal Astronomical Society • Provided by theNASA Astrophysics Data System For themodelinwhichobservedcontinuumcomesfromnon-thermalsource 197 7MNRAS.178. .451B absorptions formanouterpartoftheemission-lineregionandrepresentathinnerportion line region.Markarian231isextremeinthesensethatnosharpforbiddenemission galaxies ofthebroadpermittedlinesandnarrowforbiddenisrelatedtopropor- 6.3. IMPLICATIONSFOROTHEROBJECTS of itthroughwhichAisonly~2mag. indicates thathigh-optical-depthregionsexisttheretoo.Thusobjectslike3C273and Allen &Fosbury,unpublished).Thisindicateslargerholesinthedenseregionwhichties except for[On]isseen.The(/maynotbeproducedasaresultoftheultravioletradiation tion ofskyseenfromthecentralsourcethatiscoveredbydensematerialbroad Boksenberg etal(1975b)suggestedthattherelativeprominenceindifferentSeyfert not behidden.PossiblyobjectslikeM82,NGC253andCenAarelow-luminosityexamples tion lines).Nonethelessthecalciumtripletemissionintheseobjects(Sargent,unpublished) with theevidencethattheyareobservedessentiallyunobscured(i.e.flatspectra,noabsorp- absorbed inthedenseinnerregion. from thenucleusatall,sothatinMarkarian231allLymancontinuumphotonsmaybe it isaquasarandthatotherclose-byexamplesarealsohidingfromusbehindabsorbing activity mightbefoundintheopticalregionatall!Theinfraredemissionhowevercould and emissionlinesinMarkarian231weremuchmoreabsorbed,noevidencefornuclear directions. improved byspectrawithhighersignal-to-noise. Thepossibilitythatotherabsorption The datapresentedinthispaper,particularlythatrelatingtotheabsorptionlines,couldbe the nuclearregionsarecompletelyhiddenasviewedfromEarth,althoughultraviolet of suchcases.InadditionsomeClass2SeyfertslikeNGC1068mightrepresentcaseswhere IZw0051 +12mightshowsomeofthecharacteristicsMarkarian231ifviewedfromother more accurateequivalentwidthforX3302/3would beuseful,SystemIIabsorptionsat clouds intheirparentgalaxies. flux mightleakouttootherdirectionsexcitetheemissionlinespectrum. should beexamined.Thiscouldstudiedat50A/mm orslightlyhigherresolution. systems couldbepresentatthe/Mines answeredbylongerintegrationatabout 7 Futurework galaxies. Iftherearevery brighthiddenSeyfertnucleiinotherwisenormalgalaxies thismay nucleus totallyobscured at opticalwavelengthsshouldbeinvestigatedbyinfrared surveysof and Kshouldbesearchedfortheprofilesof the BalmerlineabsorptionsinSystemIII point ofview.ThesimultaneouspresenceofD-line andX3889absorptionsinSystemsI distinguish betweenthetwopossiblemodelsfor opticalcontinuum. 50Â/mm. AdditionallytheblueregionfromX3302/3 toHycouldbebetterobserved—a w be thebestway offindingthem. II isnotproperlyunderstood. In eithercase,asarguedinSection5,thecloudswhichproduceSystemIandII © Royal Astronomical Society • Provided by theNASA Astrophysics Data System In factmostotherFengalaxiesdoshowweak[Om]lines(Boksenbergetal1975a; Can moreextremeobjectsthanMarkarian231befound?Iftheultravioletcontinuum The highluminosityofMarkarian231putsitinthequasarclass.Itistemptingtosuggest Searches forvariabilityandpolarizationofthenucleus arehighlydesirableinorderto This paperhasonlytouchedonaproperanalysis of theabsorptionlinesfromaphysical Finally thesuggestionthat theremaybeotherobjectslikeMarkarian231 but withthe The SeyfertgalaxyMarkarian231465 197 7MNRAS.178. .451B 466 A.Boksenbergetal. Valuable assistanceatthetelescope,inpreparationofequipmentandreduction We aregratefultoourtimeassignmentpanelsforgenerousallocationsoftelescopetime. 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