Astronomy&Astrophysicsmanuscriptno.6168 (cid:13)c ESO2008 February5,2008 ⋆ Narrow–line AGN in the ISO–2MASS Survey C.Leipski1,⋆⋆,M.Haas1,H.Meusinger2,R.Siebenmorgen3,R.Chini1,H.Drass1, M.Albrecht4,B.J.Wilkes5,J.P.Huchra5,S.Ott6,C.Cesarsky3,andR.Cutri7 1 AstronomischesInstitutRuhr–Universita¨tBochum(AIRUB),Universita¨tsstraße150,44780Bochum,Germany 2 Thu¨ringerLandessternwarteTautenburg(TLS),Sternwarte5,07778Tautenburg,Germany 3 EuropeanSouthernObservatory(ESO),Karl–Schwarzschild–Str.2,85748Garching,Germany 7 4 InstitutodeAstronom´ıa,UniversidadCato´licadelNorte(UCN),AvenidaAngamos0610,Antofagasta,Chile 0 5 Harvard–SmithsonianCenterforAstrophysics(CfA),60GardenStreet,Cambridge,MA–02138,USA 0 6 HERSCHELScienceCentre,ESA,Noordwijk,POBox299,2200AGNoordwijk,TheNetherlands 2 7 IPAC,CaliforniaInstituteofTechnology(Caltech),770SouthWilsonAvenue,Pasadena,CA–91125,USA n Received;accepted a J ABSTRACT 9 2 Context.Along–standingchallengeofobservational AGNresearchistofindtype2quasars,theluminousanalogues ofSeyfert–2 galaxies. 1 Aims.Wesearchforluminousnarrow–linetype2AGN,characterisetheirproperties,andcomparethemwithbroad–linetype1AGN. v Methods.Combining the ISOCAMparallel survey at 6.7µm with 2MASS, we have selected AGN via near–mid–infrared colours 0 causedbythehotnucleardustemission.Weperformedspectroscopyintheopticaland,forasubsetofthesample,alsointhemid– 4 infraredwithSpitzer. 8 Results.Wefindninetype2AGNatredshift0.1 < z < 0.5,threeofthemhaveevenquasar–like[O]luminosities.Atthegiven 1 redshiftandluminosityrangethenumberoftype2AGNisatleastashighasthatoftype1s.Atz>0.5wedidnotfindtype2AGN, 0 probablybecausethehottestdustemission,stillcoveredbytheNIRfilters,isobscured.Theopticalspectraofthetype2hostgalaxies 7 showyoungandoldstellarpopulations.Onlyoneobjectisanultraluminousinfraredgalaxywithstarburst.The5–38µmspectraof 0 thetwotype2sourcesobservedshowastrongcontinuumwithPAHemissioninonecaseandsilicateabsorptionintheothercase. / h Conclusions. Thenear–mid–infraredselectionisasuccessfulstrategytofindluminoustype2AGNatlowz.Theobjectsexhibita p largerangeofpropertiessothatitisdifficulttoinferdetailsbymeansofpopularSEDfittingwithsimpleaveragetemplates. - o Keywords.Galaxies:active–quasars:general–Infrared:galaxies r t s a 1. Introduction didateshavebeenfoundingalaxieswithnarrowpermittedemis- : v sionlinesandhigh[OIII]λ5007equivalentwidths(Zakamskaet According to the AGN unification scheme type 2 quasars are i al.2003)andwereconfirmedbyspectropolarimetry(Zakamska X misalignedtype1s,sothatthecentralpowerhouseishiddenbe- etal.2005). r hindadustytorusseenedge-on(Antonucci1993).Whileamong An alternative approach is to look for the characteristic a radio–loudAGNSpitzerspectroscopycouldshowthatthepow- near– and mid–infrared reemission of the hiding dust heated erful FRII radio galaxies contain quasar–like nuclei (Haas et by the strong radiation field of the AGN. While several such al. 2005, Ogle et al. 2006), the detection of radio–quiettype 2 searches were started using 3.6–8.0 µm data from the Spitzer quasarsrequiresotherstrategies.HardX–raysurveysturnedout SpaceTelescope(e.g.Lacyetal.2004a),at24µm(e.g.Alonso– to be successful (Norman et al. 2002), but may be hampered Herrero et al. 2006), and as a combination of 24µm and radio by considerable extinction (Vignali et al. 2004). In the optical, data(Martinez–Sansigreetal.2005,Weedmanetal. 2006),we usingtheSloanDigitalSkySurvey(SDSS),type2quasarcan- have utilised the ISO–2MASSAGN survey.The results on the 24 type 1 AGN have been presented by Leipski et al. (2005). ⋆ Based onobservations made withESOtelescopesat LaSillaand Here, we report about the nine type 2 AGN. We use a Λ– Paranal(IDs072.B-0144,075.A-0345,and075.A-0374),withthe4-m cosmologywithH = 71kms−1,Ω =0.27,andΩ =0.73 0 matter Λ BlancotelescopeatCerroTololoInter–AmericanObservatory(CTIOis throughoutthepaper. partoftheNationalOpticalAstronomyObservatories,whichareoper- atedbytheAssociationofUniversitiesforResearchinAstronomy,un- dercontractwiththeNationalScienceFoundation),withthe2.2-mtele- 2. Data scopeattheCentroAstrono´micoHispanoAlema´n(CAHA)CalarAlto, operated jointly by the Max–Planck Institut fu¨r Astronomie and the WecombinedtheISOCAMParallelSurveyat6.7µm(Cesarsky InstitutodeAstrof´ısicadeAndaluc´ıa(CSIC),withthe1.9-mtelescope et al. 1996, Siebenmorgen et al. 1996, Ott et al. 2006) with at the South African Astrophysical Observatory (SAAO), and with 2MASS (Skrutskie et al. 2006) in order to search for AGN the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with largely independent of dust obscuration. At high galactic lati- NASA. tudes (|b|>20deg) the ISOCAM Parallel Survey covers a total ⋆⋆ Present address: University of California, Santa Barbara, effective area of ∼10deg2. We initially considered the ∼ 3000 CA–93106,USA;[email protected] sources down to F6.7µm ∼ 1mJy with MIR, NIR, and optical 2 C.Leipskietal.:Narrow–lineAGNintheISO–2MASSSurvey J0441 J0441 1.0 [OIV] 1.0 0.8 0.8 malized flux 0.6 [NeII] [NeIII] 0.6 νmalized Fν nor 0.4 [NeV] PAH 0.4 nor 0.2 0.2 [NeV] 0.0 0.0 J1232 J1232 1.0 1.0 [SIV] 0.8 0.8 malized flux 0.6 0.6 νmalized Fν nor 0.4 silicates 0.4 nor 0.2 0.2 0.0 0.0 3000 4000 5000 6000 7000 5 10 15 20 25 30 λ [Å] λ [µm] rest rest Fig.1.SpectraoftwoISO–2MASStype2AGN.Theopticalspectraareplottedinalinearfluxscale,theMIRspectraareshownas νF . ν photometry available. Then 77 AGN candidates were selected the orders were merged using custom IDL procedures. During bytheirredcolours,H−K >0.5andK −LW2(6.7µm)>2.7 this process no significant offsets between the different orders s s (Vegasystem).Thecolourcriteriaweredeterminedbycompar- wererecognized. isonwithsuitablesamplesofvariousastronomicalsources.The details of the ISO–2MASS sample selection and the begin of thespectroscopicfollow–upobservationsaredescribedbyHaas 3. Resultsanddiscussion etal.(2004).Meanwhile,wecompletedtheopticalspectroscopy Fig.8 shows the optical spectra of the 9 type 2 AGN. They fortestingtheAGNnature.SourceswithaBalmeremissionline were distinguished from the emission–line galaxies in the di- FWHMs>3000kms−1 are identified as type 1 AGN and pre- agnostic line–ratio diagrams (Baldwin et al. 1981), using the sentedinLeipskietal.(2005). dividing lines of Kewley et al. (2001; Tab.2). This procedure Theopticalspectraofthetype2AGNwereobtainedatsev- yields nine type 2 AGN (Tab.1). Three of the type 2 objects eral telescopes Tab.1). The instrument setups have been cho- canberegardedastype2QSOswith[O]luminositiesgreater sen to provide a reasonable combination of spectral resolution than 3 · 108L (according to the criterion of Zakamska et al. ⊙ and wavelength coverage. All spectra were obtained in long– 2003; Tab.1). In addition to the red H − K > 0.5 sources, s slit modeusingslit widthsof1–1′.′5′′, dependingonthe seeing the ISO–2MASS sample contains 56 blue sources with H − conditions.The data reductionand analysis was performedus- K < 0.5 and K − LW2(6.7µm) > 2.7. For 24 of them we s s ingE/Mversion04FEBpl1.0withstandardproceduresin- haveobtainedspectroscopy(randomlychosen).Onlyonesource cludingbiassubtraction,flatfielding,cosmicrayremoval,wave- (J17582331+5125419)turnsouttobeatype2AGN.Itmayac- lengthandfluxcalibration. tuallybelongtotheredsample,sinceithasonlyanupperlimit Wehaveobtainedlow–resolution5–38µmspectrausingIRS inH and,thus,alowerlimitH−K >0.41(Tab.1). s (Houcketal.2004)aboardtheSpitzerSpaceTelescope(Werner None of the sources is listed in NED as X–ray source et al. 2004) for a random sub–sample of 10 out of the 77 IR- and only two have detected radio emission (∼1 mJy from selectedAGNcandidates.Duringcompletionoftheopticalspec- FIRST for J12324114+1112587 and 38 mJy from NVSS for troscopyoftheentiresample,twoofthesourcesobservedwith J11095861−3720374). IRS turned out to be type 2 AGN. Their MIR spectra are pre- The emission–line fluxes of the type 2 AGN were deter- sented in this paper. The IRS integration times were 2×60s in mined by fitting gaussians to major emission lines. We did not SL1 and SL2 and 2×120s in LL1 and LL2. The two different correct the spectra for the underlying stellar continuum. Thus, nodpositionsweresubtractedtoremovethebackgroundandthe for objectswith strongyoung-to-intermediateage stellar popu- resultingframeswereaveraged.Theaveragedspectrawere ex- lations (J0441, J1109) the fluxes especially of Hβ are likely to tractedandcalibratedusingSwithstandardproceduresand beunderestimated.Whenweaccountforthiseffect,itdoesnot C.Leipskietal.:Narrow–lineAGNintheISO–2MASSSurvey 3 Table 1. Parameters of the ISO–2MASS type 2 AGN. NIR magnitudesare taken from the 2MASS PSC (Skrutskie et al. 2006). We used the following spectrographs: FORS1 with grism GRIS 300V at ESO–VLT, EMMI with grism Gr#2 at ESO–NTT, the R–CSpectrographwithgratingKPGL1-1attheCTIOBlanco4-mtelescope,CAFOSwithgrismG200attheCAHA 2.2-mtele- scope,andthegratingspectrographwithGR#8attheSAAO1.9-mtelescope. 2MASS(J2000.0) redshift J H Ks F6.7µm L[Oiii] L[Oii] telescope nominalresolution mag mag mag [mJy] [108L ] [108L ] Å/px ⊙ ⊙ J01520465+2232015 0.113 16.613 16.159 15.642 0.89 0.13 – CAHA 4.59 J02251432−2437154 0.104 15.790 15.125 14.519 2.92 0.35 – SAAO 2.30 J04411405−3734369a 0.236 16.629 15.521 14.797 3.96 4.98 0.85 NTT 3.54 J11095861−3720374 0.173 16.521 15.613 14.863 2.00 0.38 0.07 VLT 2.64 J12324114+1112587 0.249 16.772 15.474 14.261 7.12 1.35 0.18 NTT 3.54 J14563296−0847490 0.079 16.460 15.882 15.374 1.69 0.04 0.02 NTT 3.54 J17582331+5125419 0.201 16.742 >16.180 15.774 1.26 0.79 – CAHA 4.63 J19110553+6742507 0.171 >17.755 15.683 14.483 3.73 11.28 – CAHA 4.59 J22090602−3257505 0.425 16.701 15.778 15.134 1.92 13.76 3.81 CTIO 1.21 a DetectedwithIRAS:F =100mJy,F =950mJy,F =1600mJy. 25µm 60µm 100µm changetheclassificationoftheobjects.Thedominantsourceof 3.1.1. J0441 uncertainty in deriving the line fluxes is the placement of the TheopticalspectrumofJ0441(Fig.1,upperleft)showsablue continuum next to the emission line. Following Bennert et al. continuumwithprominentopticalemissionlines.Theemission (2006a,2006b)wethereforeestimatedtheerrorsofthefluxmea- lines include e.g. strong [Ne] emission, what is commonly surements as the productof the root–meansquare deviation of used as a tracer for an active nucleus. Besides the emission the localcontinuumandthe FWHM of the line.We assumeda lines,thespectrumofJ0441clearlyshowsabsorptionlines,es- minimumerrorin the flux measurementof at least5% though. pecially higherorderBalmer absorption(Fig.2). The spectrum ThefluxvaluesandthecorrespondingerrorsaregiveninTab.3. indicatesastrongcontributionofamoderatelyyoung(fewhun- dredmegayears)stellarpopulation.Thisyoungpopulationissu- 3.1.Type2properties perposed on an older population (indicated by the presence of absorptionfeatureslikeCaIIK;Fig.2). The ISO–2MASS type 2 AGN show a wide range of proper- Thatstarformationisstillongoingissupportedbythestrong ties. This is not surprising because according to the unifica- far–infrared (FIR) emission visible in the Spitzer MIR spec- tion scheme any details of the host galaxy can be well dis- trum(Fig.1,upperright).Thestarformationisalsoindicatedby cerned in type 2 AGN since the outshiningnucleusis shielded the presence of PAH emission. This emission, althoughclearly by the dust torus. In the following we discuss two type 2 ob- detected, is of low equivalent width due to a strong NIR and jects in detail for which optical as well as MIR spectroscopy MIR continuumprobablycaused by the AGN. Compared with is available (J04411405−3734369, hereafter called J0441 and thestarburstgalaxyM82(Sloanetal. 2003),thecontinuumof J12324114+1112587, hereafter called J1232). Both share the J0441 is stronger at 5–20 µm, but similar at 20–30 µm, while prominent optical emission lines from the narrow–line region equivalentwidthsof the PAH featuresare muchlargerin M82 (NLR),butthesignaturesofthehostgalaxiesareratherdifferent (Fig.3).Theprominenttype2AGN/ULIRGMrk273hasinter- (Fig.2). mediatePAHstrengthandaverysteepslopetowardsFIRwave- lengths,mostlikelyduetovigorousstarbursts(Fig.3).J0441is theonlyISO–2MASStype2AGNwhichwedetectedonIRAS– Table 2. Diagnostic flux ratiosand reddeningscalculated from thevaluesgiveninTab.3. 0.15 2MASS(J2000.0) log[Oiii] log[Nii] A x J01520465+2232015a 1.3H8β −0.H2α4 >9V.35 d flu 0.10 J02251432−2437154 0.96 −0.28 0.61±0.10 ze J04411405−3734369b 1.04 0.03 <2.76 ali J11095861−3720374b 1.22 −0.11 <5.05 rm 0.05 o J12324114+1112587 0.84 −0.43 2.41±0.04 n J0441 J14563296−0847490 0.55 −0.19 3.12±0.12 0.00 [OII] H9 Ca K Hδ 0.15 J17582331+5125419 0.98 −0.08 1.03±0.25 x u J19110553+6742507 0.80 −0.91 1.18±0.01 fl J22090602−3257505 0.85 – – ed 0.10 [OII] z a ForthissourcetheHβlinecouldnotbeidentifiedwithinthenoise. mali 0.05 WeonlygiveanupperlimitontheHβfluxand,thus,alowerlimit or J1232 onA . n 0.00 CN Ca K V b TheseobjectsshowBalmerabsorption.Sincewedonotcorrectfor 3000 4000 5000 6000 7000 thestellarpopulation,theHβfluxisalowerlimitand,thus,theA λ [Å] V rest isgivenasanupperlimit. Fig.2.ZoomedopticalspectraofJ0441andJ1232. 4 C.Leipskietal.:Narrow–lineAGNintheISO–2MASSSurvey Table3.Fluxesofmajoremissionlines.Allfluxesaregivenin10−16ergs−1cm−2.Theerrors(∆)aregivenin%. 2MASS(J2000.0) [O] ∆([O]) Hβ ∆(Hβ) [O] ∆([O]) Hα ∆(Hα) [N] ∆([N]) λ3727 % λ4861 % λ5007 % λ6563 % λ6583 % J01520465+2232015 – – <0.66 – 15.75 6 25.52 5 14.85 9 J02251432−2437154 – – 5.42 20 49.67 5 19.83 7 10.39 13 J04411405−3734369 19.88 5 >10.58 5 117.10 5 69.01 5 73.43 5 J11095861−3720374 3.23 6 >1.07 15 17.89 5 >12.95 5 10.04 5 J12324114+1112587 3.70 10 4.04 6 28.11 5 24.01 5 8.82 5 J14563296−0847490 4.73 20 3.15 13 11.11 5 22.65 5 14.51 5 J17582331+5125419 – – 2.80 25 26.55 5 11.46 7 9.46 8 J19110553+6742507 – – 85.71 5 546.35 5 365.36 5 44.53 5 J22090602−3257505a 22.74 13 11.59 15 82.14 5 – – – – a WhilethisobjectlacksmeasurementsofHαand[N]usedforthediagnosticline-ratiodiagrams,itshigh[O]/Hβand[O]/[O]ratiosas wellasitsredshiftand[O]luminositystronglysuggeststhatthisobjectsharboursanAGN. ADDSCANs yielding L ∼5 · 1012L according to Sanders 3.1.2. J1232 FIR ⊙ et al. (1996, their Tab.1). Thus, J0441 qualifies as a type 2 In the optical, J1232 displays a red continuum (Fig.1, lower AGN/ULIRG. Its MIR luminosity at 15µm of νL (15µm) = ν 2.31·1045ergs−1 placesthisobjectamongMIRstrongsources left).Thereareonlyveryfewclearabsorptionlinespresentwith CaII K being the mostprominent(Fig.2). No clear absorption thataremostlikelyAGNpowered(Ogleetal.2006). signature can be identified in the spectrum of J1232, neither a Notably,theMIRspectrumresemblesthatoftheFIRlumi- Balmerjumpnora4000Åbreak.Thisindicatesthatmoststel- nousdustrichbroad–linequasarMrk1014(Armusetal.2004; larfeaturesmayhavebeendilutedbya non–stellarcontinuum. Fig.4) and possibly is an intermediate case between Mrk1014 SinceCaIIKisvisibleandhigherorderBalmerabsorptionlines and Mrk273, both showing FIR signatures of dust-enshrouded arenot,itisunlikelythattheyoungstellarpopulationisstrongin starburstsaccompanyingtheAGNtovaryingdegree.Inthecom- thisobject.Thisissupportedbytheflat15–30µmspectrumand parison with the type 1 source Mrk1014 (Fig.4) the spectra the low IRAS–ADDSCAN 3σupperlimit (F < 180mJy), 60µm match well at wavelengths greater than ∼ 12µm. Below this i.e. little amounts of dust heated by young stars (Fig.1, lower wavelengththetype2sourcehasasignificantfluxdeficitwhich right).The15µmluminosityofνL (15µm)=1.06·1045ergs−1 ν steadilyincreasestowardsshorterwavelengths.Wewilladdress demonstrates that the MIR emission is powered by a hidden thisissuefurtherin§3.2. AGN(Ogleetal.2006). Measuringtheemission–linefluxesintheMIRspectrumof [Ne] is not detected in the optical whereas it was even J0441(Tab.4)andcomparingthefluxratioswiththosegivenin strongerthan[O]inJ0441.Thiscanbecausedbyobscuration Sturm et al. (2002) we see that this source falls between AGN of the regionswhere [Ne] is emitted, i.e. the inner regionsof andstarburstdominatedobjects.Whileitmaythereforebeclas- the NLR with the necessary high–energyphotons. In compari- sifiedasacompositeobjectfromtheMIRlineratiosalone,the sonwithJ0441,the[Ne]emittingregionwillbemorecompact opticallineratios,the[O]luminosity,andtheMIR(15µm)lu- becauseofthelesspowerfulcentralengine(astracedbye.g.the minosityisactuallydominatedbytheAGNinthistype2QSO. MIRluminosityorthe[O]luminosity). Remarkably,weseebroademission–linecomponentsatthe basesofHαandHβ(Fig.2),indicatingthatJ1232hastobeclas- sified as a type 1.8 according to Osterbrock (1977). This also 0.5 0.4 type 1 100 0.3 y y J J m 0.2 10 type 2 0.1 M82 Mrk273 J0441 J0441 Mrk1014 0.0 1 5 10 15 20 25 30 4 5 6 7 8 9 10 20 30 λ [µm] λ [µm] rest rest Fig.3. MIR spectra of J0441, M82, and Mrk273, all scaled to Fig.4.Comparisonof a type1 AGNwith a type 2AGN in the matchthefluxofJ0441at20µm. MIR.Mrk1014wasscaledtomatchthefluxofJ0441at25µm. C.Leipskietal.:Narrow–lineAGNintheISO–2MASSSurvey 5 indicatesthatthecentralregionoftheAGNiseitherviewedun- sidesthecontinuum(Fig.1).Theestimateddepthofthesilicate deranintermediateanglebygrazingthetorusedgeandallowing featureis sensitive to the placementof the continuum.Varying someemissionfromtheBLRtobevisibleorthatlightfromthe this placement allows us to estimate an uncertainty of the ob- BLRisscatteredintothelineofsight.Ifscattered,thenweex- scuration derived from the silicate trough. We get a range of pectto see also the featurelesscontinuum(FC) fromtheactive A ≈5.7−6.7usingA ≈17A (Kru¨gel2003)or,alter- V,MIR V 9.7µm nucleus(e.g. Cid Fernandeset al. 2004). In this case, the scat- natively,A ≈6.2−7.3usingA =18.5±2.0A (Draine V,MIR V 9.7µm teredFCdilutestheabsorptionlines,especiallyinthebluewhere etal.2003). the FC is stronger and the scattering efficiency is larger. But Since the estimated A is only an average value for the V,NLR the optical continuum of J1232 is red despite the contribution NLR, the difference in the A valuesindicatesthat the absorp- V ofanintrinsicallyblueFCsuggestingconsiderabledustredden- tionfortheNLRgasandtheMIRcontinuumtakesplaceatdif- ing. This is confirmedby an emission–lineratio of Hα/Hβ ∼ 6 ferent regions in the galaxy. Since most of the NLR emission correspondingtoAV,NLR ∼2.4(seeTab.2). comesfromscaleslargecomparedtotheMIRcontinuumemit- We estimate A also from the 9.7µm silicate absorption in ter, we suggest that the silicate absorption mainly arises from V theMIRspectra,almosttheonlyclearlydiscernablefeaturebe- dustwhichisconcentratedtowardsthecentreofthegalaxy.The NLRemissionandtheMIRcontinuumisadditionallyabsorbed byambientdustin thehostgalaxy.Suchadustdistributionfits Table 4. Optical and mid–infrared neon fluxes for wellwithourfindingsthattheemissionofhighlyionisedgasin J04411405−3734369. All fluxes in 10−16ergs−1cm−2 with theinnerpartsoftheNLRissignificantlyobscuredbydust. errorsoftypically∼10%. 3.2.Comparisonoftype1andtype2objects [Ne] [Ne] [Ne] [Ne] [Ne] [Ne] 3426Å 3869Å 12.8µm 14.3µm 15.5µm 24.3µm Forsurveysrelyingonisotropicproperties,e.g.using178MHz 23.9 17.5 147.0 79.5 266.0 113.0 radiofluxes(Laingetal.1983,Spinradetal.1985)orFIRdata (Keel et al. 2005), the number ratio of type 1 to type 2 AGN Note:Fluxfor[O]λ25.9µm:244.0. turnsout to be ∼ 1. Surveysin the MIR using the four Spitzer IRAC filters find also a ratio of ∼ 1 (e.g. Lacy et al. 2007). At redshiftz<0.5wefind12and9type1andtype2ISO-2MASS- 16 AGN, respectively (Fig.5). At higher redshift(z > 0.5) in fact nonarrow–lineISO-2MASS-AGNhasbeenfound,but12type 1AGN. Thedeficitoftype2 AGNsuggeststhat,in contrastto 15 the originalexpectations,our near-mid-infraredAGN selection strategyismoreaffectedbyextinctionandhencenotisotropic. g] Tounderstandthisbehaviourinmoredetail,wefirstconsider a m 14 theoriginofthedifferentpartsoftheSEDoftype2objects.The [ K s dust emission seen in the NIR most likely originates at the in- ner surface of the obscuring torus that is heated by the central 13 engine.Intype2objectsthetorusisthoughttobeinclinedsuch type−1 thatthecentralengineishiddenfromourdirectview.Whileat z < 0.5a sufficientamountofwarm–to–hotdust(T ∼ 1000K) type−2 12 may be visible, at z > 0.5 only the very hottest (T ∼ 1500K) andinner–mostdustisobservableintheNIRfilters.Theextinc- 0.1 1.0 redshift z tionofthemostcentralandhottestdustemissionbythetorusor thetorusedgeleadstoalossinmagnitudeandthesourceshifts Fig.5.K versuszdistributionfortheISO–2MASSAGN. s 10.00 10.00 Lsun Lsun 80 1.00 80 1.00 1 1 n n iOIII] iOIII] L[ 0.10 L[ 0.10 type−1 type−1 type−2 type−2 0.01 0.01 0.1 1.0 −22 −23 −24 −25 −26 −27 −28 redshift z M Ks Fig.6.[O]λ5007luminosityversusredshift. Fig.7.[O]λ5007luminosityversusabsoluteK magnitude. s 6 C.Leipskietal.:Narrow–lineAGNintheISO–2MASSSurvey belowourmagnitudecutoff(K ∼ 15.5mag).Also,therelative 4 s contributionofthehostgalaxyincreasesatshorterwavelengths, J0152 leadingto a bluer H −K colourfora redshiftedsource.Since s none of the 24 spectroscopically studied ISO–2MASS sources 2 with H −K < 0.5 and properdetectionin H and K is a type s s 2 AGN, we concludethat the NIR magnitudelimit is the main 0 reason for the lack of high–z type 2 AGN in the ISO–2MASS sample. 4 J0225 If the hottestdustemission in type 2sis affected by obscu- ration, then such AGN have to be intrinsically more luminous 2 than type 1 objects to be selected by our survey. We test this picture by assuming that the luminosity of the [O] emission lineisagoodmeasureforthetotalluminosityofthecentralen- 0 gine.Thiswaspossiblefortheninetype2sandforsix type1s 10 withz<0.5andK >14mag.The[O]luminositydistribution J0441 s showsnodifferencebetweentype1andtype2AGN(Figs.6and 7).However,fromFig.6weseethatthetype2sourceshavesys- 5 tematically higher [O] luminositiesat a given redshift.If the [O]luminosityisinfactproportionaltothetotalluminosityof 0 theAGN,thismeansthatourtype2sourcesaremoreluminous 2 than equally distant type 1s. This is in fact remarkable if parts J1109 oftheNLRemissionisindeedobscuredassuggestedbyHaaset al.(2005).Thentype2AGNwouldtendtoshowless[O]than 1 type 1 sources of comparable intrinsic luminosity. This would furtherincreasethediscrepancyobservedhere,sincewewould underestimatethe[O]luminosityand,thus,thetotalluminos- 0 3 ityofourtype2sources. J1232 In addition, the Ks luminosity at a given L[Oiii] is system- atically lower for type 2 sourcesthan for type 1s (Fig.7). This 1.5 supportstheideathattype2AGNsufferfromobscurationalso in the NIR. To compensatefor obscuredradiationthey need to have a more luminous central engine to reach our magnitude 0 limit.Performinga linearleast–squarefittoeitherdistributions 2 J1456 in Fig.7 we estimate the difference between the two types of AGN in M to ∼1.3mag at a given [O] luminosity. These Ks 1.3magin K correspondsroughlyto A ∼ 11.6mag(Rieke& 1 s V Lebofsky1985). The difference of type 1s and type 2s in the NIR/MIR can 0 also be seen in Fig.4. The MIR spectra of a type 1 and a type 2 2 QSO are similar at MIR to FIR wavelengthsbutshow an in- J1758 creasingdiscrepancyforλ < 12µm.Thetype2spectrumhasa considerablefluxdeficittowardstheNIR comparedto thetype 1 1thatisassumedtohavea largelyunobscuredline-of-sightto- wardstheinnermostregionsoftheAGN.However,torusmodels predict obscuration even at longer MIR wavelengths (e.g. Pier 0 50 andKrolik1992)andobservationsindicatethatthetorusisstill J1911 affectingradiationat24µm(Shietal.2005)andbecomesopti- callythinatλ>60µm(Keeletal.1994;Shietal.2005).Thatthe 25 type1andtype2MIRspectralooksimilarevenatwavelengths assmallas20µmindicatesthatatthesewavelengthsthedustis ratherheatedbyextendedstarbursts/starformationthancoming 0 fromanuclearandpotentially(self–)obscuringtorus.Notethat 10 around 18µm the type 1 object shows additional flux over the J2209 type2source,likelyduetosilicateemissionpoweredbytheac- tive nucleus. At shorter MIR wavelengths the dust emission is 5 poweredbytheAGNandtheobscurationofthetorusisclearly visible.AsshownabovethistrendcontinuestowardtheNIR. 0 3000 4000 5000 6000 7000 4. Conclusions wavelength WepresentastudyofNIR/MIRselectedtype2AGNfoundvia Fig.8.Spectraofthe9type2AGNintherestframe.Allfluxes the ISO–2MASS AGN survey. The optical spectra of the ob- jectsrevealdifferenttypesofhostgalaxies.Atleastthreesources areshownin10−16ergs−1cm−2Å−1. are sufficiently luminous to be classified as type 2 QSOs. The C.Leipskietal.:Narrow–lineAGNintheISO–2MASSSurvey 7 SpitzerMIRspectraoftwosourcesshowverydifferentfeatures suggestiveofastrongstarburstcontributioninonecaseanddis- playing continuum emission with strong silicate absorption in theother. IncomparisonwiththeISO-2MASStype1sourceswefind acleartrendfortype2sourcestobeobscuredevenintheNIR. This obscuration seems to continue into the MIR wavelength rangeaswell. The flux deficit in the NIR compared to intrinsically lumi- nous type 1 objects is identified as the reason for the lack of type2sourcesatz > 0.5inoursurvey.Thisalso resultsin our detected type 2 sourcesbeingintrinsically more luminousthan type1sourcesatcomparableredshifts. Acknowledgements. This work was supported by Sonderforschungsbereich SFB591“UniversellesVerhaltengleichgewichtsfernerPlasmen”derDeutschen Forschungsgemeinschaft, and by Nordrhein–Westfa¨lische Akademie der Wissenschaften.WearegratefultoVassilisCharmandarisforkindlyproviding uswiththeIRSspectrumofMrk1014.Wethanktheanonymousrefereeforhis detailedsuggestions. 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