Constraints on the Star Formation Rate in Active Galaxies

1.3 MB·English

by  Minjin Kim

TOAPPEARINTheAstrophysicalJournal.
PreprinttypesetusingLATEXstyleemulateapjv.26/01/00
CONSTRAINTSONTHESTARFORMATIONRATEINACTIVEGALAXIES
MINJINKIM
AstronomyProgram,SEES,SeoulNationalUniversity,Seoul151-742,Korea
LUISC.HO
TheObservatoriesoftheCarnegieInstitutionofWashington,813SantaBarbaraSt.,Pasadena,CA91101
MYUNGSHINIM
AstronomyProgram,SEES,SeoulNationalUniversity,Seoul151-742,Korea
ToAppearinTheAstrophysicalJournal.
6
ABSTRACT
0
0 The[OII]λ3727emissionlineisoftenusedasanindicatorofstarformationrateinextragalacticsurveys,andit
2 canbeanequallyeffectivetracerofstarformationinsystemscontainingluminousactivegalacticnuclei(AGNs).
InordertoinvestigatetheongoingstarformationrateofthehostgalaxiesofAGNs,wemeasuredthestrengthof
n
a [OII]andotheropticalemissionlinesfromalargesample(∼3600)ofbroad-line(Type1)AGNsselectedfrom
J the Sloan Digital Sky Survey. We performeda set of photoionizationcalculations to help evaluate the relative
5 contributionofstellarandnonstellarphotoionizationtotheobservedstrengthof[OII].Consistentwiththerecent
1 studyofHo(2005),wefindthattheobserved[OII]emissioncanbeexplainedentirelybyphotoionizationfrom
theAGNitself,withlittleornoadditionalcontributionfromHIIregions. Thisindicatesthatthehostgalaxiesof

1 Type1AGNsexperienceverymodeststarformationconcurrentwiththeopticallyactivephaseofthenucleus.By
v contrast, we showthatthesampleof “Type2” quasarsselectedfromtheSloanDigitalSkySurveydoesexhibit
6 substantiallystronger[OII]emission consistentwithan elevatedlevelofstar formation,a resultthatpresentsa
1
challengetothesimplestformoftheAGNunificationmodel.
3
1 Subjectheadings:galaxies:active—galaxies:nuclei—galaxies:Seyfert—galaxies:starburst—quasars:
0 general
6
0
1. INTRODUCTION Additionalhintsforacloseconnectionbetweenstarformation
/
h andblackholeaccretioncomesfromthehighdegreeofchem-
It is now almost universally accepted that massive black
p ical enrichment in quasars as deduced from analysis of their
holesarecommonandthattheyplayanimportantroleinmany
- spectra(Hamannetal. 2004andreferencestherein).Lastly,the
o facetsofgalaxyevolution(seereviewsinHo2004).Numerous
hostgalaxiesofquasars,atleastinthenearbyUniverse,possess
r theoreticalmodelshavebeenproposedtoexplainthestrongob-
t a richsupplyofbothmoleculargas(Scovilleet al. 2003)and
s servedcorrelationsbetweenblackholemassandbulgeproper-
a dust(Haasetal. 2003),andthushavethepotentialforsustain-
ties(Magorrianetal. 1998;Gebhardtetal. 2000;Ferrarese&
: ingongoingorfuturestarformation.
v Merritt2000). Whileeveryoneagreesthatthegrowthofblack
While there is little doubt that the above observationsindi-
i holes mustbe closely linkedwith galaxyformation(e.g., Silk
X cate that AGN activity and star formation do often coincide
&Rees1998;Kauffmann&Haehnelt2000;Begelman&Nath
r 2005; Robertson et al. 2006), there is no consensus as to ex- withinthesamegalaxy,oneofthemainchallengesininterpret-
a ingthisevidenceinaphysicalcontextliesinthedifficultyofes-
actlyhowthetwoverydifferentprocessesinvolved—accretion
tablishingadirect,causalconnectionbetweenthetwophenom-
andstarformation—arereallycoupled.Aretheywellsynchro-
ena. An importantstep forward was recently achieved. From
nized,ordoesoneprocessprecedetheother,andifso,whatis
an analysis of a large sample of narrow-line (Type 2) AGNs
thetimelag?Whentheblackholeisactivelygrowing,doesthe
selected from the Sloan Digital Sky Survey (SDSS; York et
feedbackfromtheactivegalacticnucleus(AGN)actuallytrig-
al. 2000), Kauffmann et al. (2003) showed that, unlike low-
gerorinhibitstarformationinthehostgalaxy?Theseimportant
luminosity sources (Ho et al. 2003), more powerful AGNs
issuesareunlikelytobesettledthroughtheoreticalspeculations
oftenexhibitstellar absorptionfeaturesindicativeof youngto
ornumericalsimulationsalone.Someempiricalguidancefrom
intermediate-agestars. Althoughmoredifficulttoascertaindue
observationswouldbewelcomed.
tothestrongcontaminationfromthebrightnucleus,Kauffmann
In recent years, there has been mounting evidence that
etal. showthatthespectralsignaturesforyoungstarsseemto
AGN activity and starburst activity are often intermixed. The
persistevenamongbroad-line(Type1)AGNs.1 Mostcrucially,
mostcommonlycitedexampleoccursinultraluminousinfrared
they find, for their primary sample of Type 2 objects, that the
galaxies, whose dominant energy source has been much de-
fractionofyoungstarsinthecentralregionofthehostgalaxies
bated(e.g.,Genzeletal. 1998). Detailedstudiesofindividual
increases with increasing AGN luminosity, providing, for the
activegalaxieshaverevealedasignificantpopulationofyoung
first time, tantalizing evidence for a causal link between star
starsinanumberofinstances,eitherthroughstellarabsorption
formationandaccretion.
features(e.g.,Boissonetal. 2000;Canalizo&Stockton2000;
It is important to recognize, however, that the SDSS re-
CidFernandesetal. 2001)orcolors(e.g.,Jahnkeetal. 2004).
sults constrain only the post-starburst population, with ages
1Wewillusetheterms“Type1”and“Type2”AGNstorefertobroad-lineandnarrow-lineobjects,respectively. Wheneverpossible,wewillrefrainfromusing
theterms“Seyferts”or“quasars,”which,forhistoricalreasons,carryartificialluminositycriteria.
1
2 KIM,HO&IM
∼ 108- 109 yr. There is no information on the presence of
younger,ionizingstars(ages<107 yr). SincetheType2AGN
∼
sample was identified using standard optical emission-line ra-
-24
tiosdesignedtoisolateaccretion-dominatedsources,byselec-
tion it excludes any sources with a sizable contribution from
HIIregions. Ontheotherhand,itisclearlyofsignificanceto
constraintheongoingstarformationrate(SFR)inAGNs,inor- -22
dertoassesstheextenttowhichthetwoprocessesarecoeval.
AGN lifetimes are uncertain, but current best estimates range
g)
from106 to108 yr(Martini2004),preciselyintheunexplored a
m
regimeofinterest. (Mg -20
Estimating SFRs in AGNs presents significant complica-
tions, since the AGN, by definition, dominates the integrated
output of the source. Nearly all of the standard measures of
-18
SFRsemployedforinactivegalaxies(e.g.,Kennicutt1998)are
rendered useless by emission from the AGN itself. There is,
however, one important exception: the [OII] λ3727 line. As
discussed by Ho (2005), [O II], an emission line commonly -16
used in spectroscopic surveys to estimate SFRs in galaxies at
redshiftsz > 0.4, shouldbe an equallyeffectivetracer of HII 0.0 0.1 0.2 0.3
∼
z
regionsinSeyfertsandquasars. Insuchhigh-ionizationactive FIG. 1.— Thedistributionofg-bandabsolutemagnitudesasafunctionof
galaxies,the[OII]emissionintrinsictothenarrow-lineregion redshiftforthesample. Themagnitudesare“PSF”magnitudes,asdefinedby
(NLR) of the AGN is both observed and predicted to be low SDSS,andtheyhavebeencorrectedforGalacticextinction.
(e.g., Ferland & Osterbrock 1986; Ho et al. 1993b). If high-
ionization AGNs experiencesubstantiallevels of ongoingstar et al. 2002), whose spectra (taken with 3′′-diameter fibers)
formation, the integrated contribution from H II regions will cover ∼ 3800- 9200 Å with an instrumental resolution of
boost the strength of the [OII] line (comparedto, say, [OIII] λ/∆λ≈1800- 2100. This paper makesuse of data from the
λ5007,whichcanbelargelyascribedtotheAGNitself). From Third Data Release (DR3) of SDSS (Abazajian et al. 2005),
a survey of the existing literature on [OII] measurements on fromwhichwehaveselectedatotalof4086objectsflaggedby
quasars, Ho (2005)concludesthat their SFRs are surprisingly the spectroscopic pipeline as “AGN” that have a redshift con-
modest. Moreintriguingly,forasubsetoflow-redshiftquasars fidencelargerthan0.9andaredshiftz<0.3. Theredshiftcut
withavailablemoleculargasmeasurements,Hofindsthattheir ensures that the diagnostically important [SII] λλ6716, 6731
starformationefficiencies(SFRperunitgasmass)appeartobe doublet falls comfortably within the bandpass. After inspec-
abnormally low compared to either normal or infrared-bright tion of the spectra, we discarded 498 that either had gaps in
galaxies.Hesuggeststhatthisunusualbehaviormaybeaman- the spectralcoverageor hadat leastone of themain emission
ifestationofAGNfeedbacksuppressingstarformation. linescorrupted,leavingafinalsampleof3588objects.Figure1
The results of Ho (2005) regarding SFRs in quasars were showsthe distributionofabsolute(“PSF”) g-bandmagnitudes
based largely on [O II] strengths deduced from composite as a function of redshift. The magnitudes were corrected for
spectra (ensemble averages) constructed from quasar surveys. Galactic extinction, using the extinction values of Schlegel et
While the conclusions are statistically robust, it would be de- al. (1998) and the extinction curve of Cardelli et al. (1989),
sirable to revisit this problem from actual measurements of a butwe did notapplya K-correctionbecause of the ambiguity
large, well-defined sample of individual objects. This is ac- introduced by the uncertain amount of galaxy contamination.
complished in this paper, where we make use of an extensive Since local field galaxies have an absolute g-band magnitude
sampleofType1AGNsselectedfromtheSDSStoinvestigate of M∗ =- 20.44- 5logh≈- 21.2 (Blanton et al. 2003), it is
thepropertiesnotonlyof[OII],butalsoanumberofotherdi- clear that some of our objects are significantly contaminated
agnosticallyimportantnarrowopticalemission lines. We per- by host galaxy emission. We simulated the expected magni-
form a new set of photoionizationcalculationsto demonstrate tude due to K-correctionby addingdifferentrelative fractions
that the observed strength of [OII] emission is entirely con- ofquasarandgalaxylight,whichweapproximate,respectively,
sistentwithstandardAGNphotoionization. Theseresultssup- usinga compositequasarspectrum(VandenBerketal. 2001)
port and strengthen the conclusion of Ho (2005) that Type 1 andanearly-typegalaxytemplate(Colemanetal. 1980). For
AGNsexperienceverylimitedongoingstarformation.Bycon- an AGN contributionof 40% or more at 5100Å, we find that
trast, weshowthatType2 quasarsexhibitmarkedlyenhanced the expected K-correction is less than 0.4 mag. Since the ex-
[OII]emission,andhencepresumablyelevatedSFRs. Thisre- act absolute magnitudesare not critical for our study, we will
sultseemstoviolatethebasicorientation-dependentunification neglectithereafter.
modelforAGNs.
We adopt the following cosmological parameters: H =
100h=71kms- 1Mpc- 1,Ωm=0.27,andΩΛ=0.75(Sperge0let 3. SPECTRALFITTING
al. 2003). The spectra of Type 1 AGNs are generally very complex.
Figure 2 illustrates some spectra from our sample. While in
2. SAMPLESELECTION
this study we are primarilyinterested in the narrow-linespec-
Ourgoalistoinvestigatethenarrow-linespectrumofalarge, trum,whichissomewhateasiertomeasure,wewilldescribein
homogeneous sample of broad-line (Type 1) AGNs. We uti- greaterdetailourmethodforanalyzingtheentirespectrum.We
lize the database from SDSS (Stoughton et al. 2002; Strauss illustrate an example of spectral fitting in Figure 3. After re-
STARFORMATIONINAGNs 3
movingtheGalacticextinction,thespectrumistransformedto effectivetemplatetomodel[NII]λλ6548,6583andthenarrow
therestframeusingtheredshiftmeasuredbytheSDSSpipeline. componentofHα.Inobjectswithsufficientlystrong[SII](S/N
A full decomposition of the continuum spectrum requires fit- >3),wefiteachofthe[SII]lineswithamulti-Gaussianmodel
∼
ting of four components. First, we allow for the presence of (usually just two suffice), holding fixed the known separation
a galaxy component. As mentioned in §2, we expect galaxy of the two lines. This [SII] template is then used as a model
contaminationto be importantin the lower-luminosityobjects for[NII]andnarrowHα. Theseparationofthe[NII]doublet
in our sample. While more sophisticated methods of galaxy is held fixed, as is its flux ratio (2.96). In practice, we often
modeling have been attempted (e.g., Ho et al. 1993a, 1997a; find it necessaryto also fix the position of the Hα component
Greene & Ho 2004), Greene & Ho (2005b)find thata simple relativeto[NII].Incaseswhere[SII]hasS/N<3,wehaveno
∼
scalingandsubtractionofavelocity-broadenedK-giantstarare alternativebuttouse[OIII]λ5007asatemplate,eventhough
usuallysufficientforType1AGNswiththesignal-to-noisera- weknowthatthisisnotentirelyadequatebecause[OIII]often
tio (S/N) of SDSS. As in Greene & Ho (2005b), we use the hasablue,asymmetricwingthatisgenerallyabsentfrom[SII]
equivalent width (EW) of the Ca II K λ3934 absorption line (Greene & Ho 2005a). As in Greene & Ho (2005a, 2005b),
asaguidetothelevelofgalaxycontaminationandhowmuch we model each of the [OIII] λλ4959, 5007 lines with a two-
starlight to subtract. These authors find that an EW of 1.5 Å componentGaussian, andweconstrainthenarrowcomponent
forCaIIKcorrespondstoagalaxycontributionof∼10%.We of Hβ to the narrowcomponentof Hα, keepingthe flux ratio
omittheK-giantcomponentforEW(CaIIK)<1.5Å,andwe fixedtotheCaseB′valueofHα/Hβ =3.1(Osterbrock1989).2
scaleitappropriateforEWslargerthanthisvalue. Second,the The broadcomponentsof Hα and Hβ are fitted with multiple
ultravioletandopticalcontinuumcontainsaprominentfeature- Gaussiancomponents.
less,nonstellarcomponent,whichpreviousstudies(e.g.,Fran- Thevastmajority(∼95%)ofoursampleyieldedunambigu-
cis et al. 1991; Vanden Berk et al. 2001) have approximated ousdetectionofthestrongestemissionlinesconsideredinthis
bya doublepowerlaw, witha spectralbreakat∼5000Å.We study ([OII], [OIII], Hα, and [NII]). The success rate, how-
ever,waslowerforHβ (66%),anditwasdisappointinglylow
find, however, that after accountingfor the galaxycomponent
the second, long-wavelengthpower-law componentis usually (44%)for[OI] and[SII],which, inadditiontobeingweaker,
alsolieinthereddestportionoftheSDSSbandpass,wherethe
unnecessary,andthuswemodelthefeaturelesscontinuumus-
effectiveS/Nisdecreasedbysystematicnoiseduetoimperfect
ing a single power law. Third, following Grandi (1982), we
removalofnight-skyemission,telluricabsorption,anddetector
include the contribution from a Balmer continuum, assuming
thatit is opticallythick andemitted bya uniformtemperature fringing. Weseta3σ upperlimitforthenondetectionsbyas-
sumingthatthelinehasaGaussianprofileequaltothatofthe
ofT =15,000K;wedonotattempttomodelthehigher-order
e
averageofthe detectednarrowlinesandanamplitude3times
Balmer lines. Lastly, we modelthe “pseudo-continuum”gen-
thelocalrmsofthelocalcontinuum.
erated by the plethora of broad and blended Fe II multiplets,
which,forourpresentapplication,areparticularlytroublesome
fortheregioncontainingtheHβand[OIII]emissionlines.Fol-
4. RESULTS
lowingstandardpractice(Boroson&Green1992),weremove
the Fe blends by scaling and broadening an Fe template gen- 4.1. [OII]and[OIII]Luminosities
erated from the spectrumof I Zw 1, kindlyprovidedby T. A.
Figure 4 showsthe distributionsof [OII] λ3727and[OIII]
Boroson. (Theregionbetween3082Åand3685Åisnotmod- λ5007 luminosities for our sample. The [O II] luminosities
eled because of a gap in the current Fe template.) After sub- range from ∼ 1040 to 3×1042 ergs s- 1, with an average of
tractingthegalaxycomponent,wesimultaneouslyfitthequasar hL i=5.2×1040ergss- 1. Forcomparison,weoverplotthe
[OII]
spectrum with the power-law continuum, Balmer continuum, sample of Type 2 AGNs studied by Kauffmannet al. (2003),
andFetemplate.Weperformthefitoverthefollowingregions, which, in terms of narrow-line emission, is approximately a
whichare devoidofstrongemissionlines: 3550–3645,4170– factor of 2 less luminous than our sample. As discussed in
4260,4430–4770,5080–5550,6050–6200,and6890–7010Å. Ho (2005), this level of [OII] emission, taking conservative
Withapureemission-linespectrumathand,thenexttaskis assumptionsaboutAGNcontaminationandcorrectionsforex-
tofitthelines(seebottomofFig. 3). Sincethecontinuumnear tinctionandmetallicity,correspondstoSFRsofonly∼0.5M⊙
∼3700Åisstillnotcompletelymodeled(weneglectedhigher- yr- 1. Figure 5 shows the luminosity of [OIII] plotted versus
orderBalmerlinesintheBalmercontinuummodelandtheFe thelineratio[OII]/[OIII].The[OII]/[OIII]ratiospansawide
template is incomplete), we have decided to fit [OII] λ3727 range, in the extreme by nearly 2 orders of magnitude from
withasingleGaussianabovealocallydefinedcontinuum.This ∼ 0.06 to ∼ 5, but most cluster around ∼ 0.2- 1. Assum-
procedure, although crude, should be adequate for our pur- ingthat[OIII]isproducedentirelybytheAGNandisagood
poses. WenotethatGreene&Ho(2005a)findthat[OII],atthe tracerofAGNpower(e.g.,Kauffmannetal. 2003),itisclear
S/NofSDSS,isusuallywell-fittedbyasingleGaussian. How- that the [OII]/[OIII] ratio decreases strongly with increasing
ever,becauseofthestrongconfusionwiththebroad-lineregion, [OIII]strength.Forcomparison,wealsooverplotthesampleof
thelinesaroundHαandHβ—andespeciallythenarrowcom- Kauffmannetal. (2003)toillustratethatthelower-luminosity
ponentsoftheselinesthemselves—requiremorecomplextreat- Type2objectsalsoobeyroughlythesamepattern.Thistrendis
ment. Previousstudies(Ho et al. 1997b; Greene& Ho 2004, easytoexplaininthecontextofAGNphotoionizationmodels,
2005b)findthatthe[SII]λλ6716,6731doubletservesasavery inwhichthe[OII]/[OIII]ratiovariesstronglyasafunctionof
2NotethatthisconstrainteffectivelyassumesthattheNLRexperiencesnegligibleinternalextinction. Wehaveexperimentedwithleavingfreetheamplitudeof
thenarrowcomponentofHβ.TheresultingdistributionofBalmerdecrementissharplypeakedatavalueofHα/Hβ=3.3,suggestingthatanyinternalextinctionis
evidentlyquitemodest(AV ≈0.2mag). However,aminorityoftheobjectshaveextremelytiny,unphysicalHα/Hβratios,resultingfromsevereoverestimationof
theHβstrength. SincethemajorityoftheobjectsseemtobewellcharacterizedbysmallamountsofinternalextinctionintheirNLR,wehavedecidedthatitwould
beprudenttofixtheHα/HβratiototheCaseB′value. Asanaside,itisinteresting,andprobablynotcoincidental,thatGreene&Ho(2005b)findthattheBalmer
decrementsofthebroad-lineregionintheirsampleofSDSSType1AGNsalsoindicatelittle,ifany,internalextinctionalongthelineofsight.
4 KIM,HO&IM
150
Ha +[N II]
SDSS J152108.92+504006.9
[O III]
100
[O II]
fl
Hb
[O I] [S II]
50
0
60
SDSS J162553.48+363410.9
40
fl
20
0
600
SDSS J021359.79+004226.7
400
fl
200
0
120
SDSS J115234.99−000542.7
80
fl
40
0
105
SDSS J031722.16−065343.0
70
fl
35
0
4000 4500 5000 5500 6000 6500
Rest Wavelength (Å)
FIG. 2.— Samplespectraofourobjects,chosentospanarangeinS/Nand[OII]strength. Therestframeflux, fλ= fλ,obs(1+z)3,isinunitsof10- 17ergss- 1
cm- 2Å- 1.
STARFORMATIONINAGNs 5
250
(a)
200
150 [O III] Ha +[N II]
fl
100 [O II] Hb
[O I] [S II]
50
0
50
40
30
Power law
fl
20
10 BaC Galaxy
0
Fe
−10
20
10
fl
0
−10
3500 4000 4500 5000 5500 6000 6500
Rest Wavelength (Å)
120 120
(b) (c)
100 100
80 80
60 60
fl
40 40
20 20
0 0
10 10
00 00
fl −10 −10
4800 4900 5000 5100 6450 6550 6650
Rest Wavelength (Å) Rest Wavelength (Å)
FIG. 3.— Exampleofspectraldecompositionforoneofthesampleobjects(SDSSJ161141.95+495847.9). Therestframeflux, fλ= fλ,obs(1+z)3,isinunitsof
10- 17ergss- 1cm- 2Å- 1.(a)Thetoppanelshowstheoriginalspectruminfullscale.Themiddlepanelgivesamagnifiedviewoftheoriginalspectrum,withsome
keyemissionlineslabeled,alongwiththedifferentcontinuumcomponentsincludedinthefit[featurelesspowerlaw,Balmercontinuum(BaC),galaxy(K-giant
star),andtheFetemplate]. Thecontinuum-subtracted emission-linespectrumisshowninthebottompanel. NotethattheFetemplatedoesnotcovertheregion
between3082and3685Å,andtheBaCcomponentdoesnotproperlymodelthehigher-orderBalmerlinesintheregion3650–3800Å,resultinginstrongresiduals
inthecontinuumnear[OII].Toillustratethedetailedfitstotheemissionlines,weshowanexpandedviewoftheregionssurrounding(b)Hβand[OIII]λλ4959,
5007and(c)Hαand[NII]λλ6548,6583.
6 KIM,HO&IM
1.0
(a) (b)
0.8
N
d 0.6
e
z
ali
m
or
N 0.4
0.2
0.0
39 40 41 42 43 39 40 41 42 43
log (L / ergs s−1) log (L / ergs s−1)
[O II] [O III]
FIG. 4.— Relativedistributionof(a)[OII]and(b)[OIII]luminosity. Thewidthofeachbinis0.25dex. ThesolidlinerepresentstheType1AGNsfromour
sample,andthedashedlinerepresentstheType2AGNsfromKauffmannetal.(2003).TheType2objectshavebeencorrectedforinternalextinction,asgivenin
Kauffmannetal.
[OII]/[OIII]ratiowouldeitherremainconstantorrisewithin-
0.5 creasingL .
[OIII]
4.2. PhotoionizationModel
Tobetterconstraintheoriginofthenarrow-linespectrumin
0.0
general,andofthe[OII]lineinparticular,weperformedanew
set of photoionization calculations using the code CLOUDY
O III]) (cvloeursdioonf9a4g.0iv0e;nFdeernlasnitdyeatndale.le1m9e9n8t)a.laFbournadnanocpetiicllaulmlyinthaitcekd
O II] / [ −0.5 btiyonasnofinstpautitstiiocnailzainndgtshpeermctraulmeq,uCiliLbOriuUmDYandsoplrvoedsuctheesaesqeulaf--
g ([ consistentmodeloftherunoftemperatureandionizationasa
o
l functionofdepthintothecloud.Forsimplicity,weassumethat
thecloudhasauniformdensity,hasaplane-parallelgeometry,
−1.0
and is dust-free. We calculated a grid of models by varying
threemainparameters: thehydrogendensity(102 cm- 3 ≤n≤
106cm- 3),theionizationparameter(10- 4≤U ≤10- 1),andthe
shape of the ionizing continuum (α = - 0.5,- 1,- 1.5,- 2, and
−1.5 - 2.5). The ionizing continuum has the basic shape described
39 40 41 42 43
byHoetal. (1993b),wherethevariationisparameterizedbya
log (L / ergs s−1)
[O III] power-lawindexα between 10µm and 50 keV. We adoptthe
FIG. 5.— The[OII]/[OIII]ratiovs. [OIII]luminosityforType1AGNs solar abundancesas givenin Grevesse & Anders(1989),with
(largeblackdots)andType2AGNs(smallbluedots;fromKauffmannetal. theexceptionofnitrogenwhoseabundanceisincreasedtotwice
2003). Thesolidanddottedlinesgivetheordinaryleast-squaresbisectorre- thesolarvalue,assuggestedbyStorchi-Bergmann(1991).
gressionfortheType1andType2objects, respectively. Thelargetriangle
Webeginbyexaminingoursampleusingthethreediagrams
marks the location of the sample of Type 2 quasars from Zakamska et al.
(2003),usingtheaveragevalueofthe[OIII]luminosityandthe[OII]/[OIII] proposedbyVeilleux&Osterbrock(1987),whichhavethead-
ratiomeasuredfromtheircompositespectrum.ThevaluesfromKauffmannet vantageofbeinginsensitivetoextinctioncorrections.Ascanbe
al.andZakamskaetal.havebeencorrectedforinternalextinction. seenin Figure6, themajorityofthesampleof Type1objects
iswellcoveredbyourgridofmodels, spanningthefollowing
oftheionizationparameter3(seebelow),butitisdifficultto range of parameters: - 2.0≤ α≤- 1.0, 10- 3.3 ≤U ≤10- 2.0,
reconcileinapicturewhereinthestrengthofstarformationin- and102cm- 3≤n≤104cm- 3. Thedensityestimateisnotcon-
creases with increasing AGN activity. In such a scenario, the sistent from one diagram to another because the NLR very
3Theionizationparameter,denotedbyU,isdefinedtobetheratiooftheionizingphotondensitytothedensityofhydrogen.
STARFORMATIONINAGNs 7
2.0
1.5 10−2 10−2
10−2
) 10−2.5 10−2.5 10−2.5
b7/H 1.0 10−3 U 10−3 U 10−3 U
0
0
5
lO III] 0.5 10−3.5 10−3.5 10−3.5
log ([ 0.0 −1.5 −1 −1.5−1 −1.5−1
−0.5 −2 a −2 a −2a
− − −
2. (a) 2. (b) 2. (c)
5 5 5
−1.0
−2.0 −1.5 −1.0 −0.5 0.0 −1.0 −0.5 0.0 0.5 −1.0 −0.5 0.0 0.5
log ([O I] l 6300/Ha ) log ([N II] l 6583/Ha ) log ([S II] ll 6716, 6731/Ha )
FIG. 6.— Lineratios ofType1AGNs(smalldots)andType2quasars(largetriangle, fromcompositespectrum ofZakamskaetal. 2003)comparedwith
photoionizationmodels. Thelargecrossmarksthelocationoftheaveragevalueoftheupperlimitsinoursample,whichareconsistentwiththedetections. The
threeVeilleux&Osterbrock(1987)diagramsplot[OIII]λ5007/Hβvs.(a)[OI]λ6300/Hα,(b)[NII]λ6583/Hα,and(c)[SII]λλ6716,6731/Hα. Twogrids
areshown,representingn=102cm- 3(solidblackline)andn=104cm- 3(dashedblueline).EachgridshowsmodelsforU=10- 2,10- 2.5,10- 3,and10- 3.5(topto
bottom)andα=- 2.5,- 2,- 1.5,and- 1(lefttoright). TheshadedregionrepresentsthelocusofHIIregions,ascalculatedfromthestarburstmodelsofKewleyet
al.(2001).
1.5
10−2
10−2.5 (a) 0.0 (b)
U 10−3.5 −1
1.0 10−3 007) −0.5 U a
) 5
b007/H lO III] 10−3 −2.5
lO III] 5 0.5 −1 10−3.5 6300/[ −1.0 10−2 10−2.5
g ([ −1.5 lO I] −1.5
lo a g ([
0.0 −2 lo
−2.0
−2.5
−0.5 −2.5
−1.5 −1.0 −0.5 0.0 0.5 1.0 −1.5 −1.0 −0.5 0.0 0.5 1.0
log ([O II] l 3727/[O III] l 5007) log ([O II] l 3727/[O III] l 5007)
FIG.7.— LineratiosofType1AGNs(smalldots)andType2quasars(largetriangle,fromcompositespectrumofZakamskaetal.2003,withinternalextinction
correctionapplied)comparedwithphotoionizationmodels. Thelargecrossmarksthelocationoftheaveragevalueoftheupperlimitsinoursample,whichare
consistentwiththedetections. Diagnosticdiagramsshowing(a)[OIII]λ5007/Hβvs.[OII]λ3727/[OIII]λ5007and(b)[OI]λ6300/[OIII]λ5007vs.[OII]
λ3727/[OIII]λ5007.Twogridsareshown,representingn=102cm- 3(solidblackline)andn=104cm- 3(dashedblueline).EachgridshowsmodelsforU=10- 2,
10- 2.5,10- 3,and10- 3.5(lefttoright)andα=- 2.5,- 2,- 1.5,and- 1(bottomtotop).TheshadedregionrepresentsthelocusofHIIregions,ascalculatedfromthe
starburstmodelsofKewleyetal.(2001).
8 KIM,HO&IM
likely has a wide range of densities, and the various forbid- lines,andwehavegeneratedanewsetofphotoionizationmod-
dentransitionshavedifferentcriticaldensities(e.g.,Filippenko els to interpret them. Our analysis indicates that the princi-
&Halpern1984). Nonetheless,theaboverangeofparameters palnarrowemission linesof oursample, including[OII], can
corresponds well to previous estimates made using the same beentirelyexplainedbyAGNphotoionizationassumingfairly
versionofCLOUDY(Nagaoetal. 2001). standardparameters.Thereisnoevidenceforanyexcess[OII]
Having established that our sample of Type 1 objects has emission arising from a separate origin, such as HII regions.
NLRpropertiesthatcanbe readilyaccommodatedusingstan- Thus, the observed [OII] strengths can be used to set a limit
dardAGNphotoionization,wenextturntotwodiagnosticdia- on the ongoingSFR in these systems. However, as discussed
gramsthathighlightthe[OII]line. Figure7comparestheline byHo,anumberofuncertaintiescomplicatetheSFRestimates
ratio [OII] λ3727/[OIII] λ5007, which is a sensitive indica- basedonthe[OII]line. Theseincludetheunknownmagnitude
tor of the ionizationparameter, to [OIII] λ5007/Hβ and [OI] ofdustextinction,thepossibleinfluenceofmetallicitycorrec-
λ6300/[OIII] λ5007 (Baldwin et al. 1981; Ferland & Netzer tions,andpreciselyhowmuchoftheobserved[OII]strengthto
1983). These diagramshave the advantageof beingrelatively attribute to star formation. For illustrative purposes, we make
insensitive to abundance, although[OII]/[OIII] is affected by threesimpleassumptions(seeHo2005foramoredetaileddis-
extinction.Thebasicconclusionfromthisfigureisthattheob- cussion): (1) that the amount of dust extinction in AGN host
served line ratios, and in particular the [OII] strength, can be galaxiesiscomparabletothatdeducedformoderatelyactively
well reproduced with AGN photoionization with roughly the star-forminggalaxies(A ≈1mag;e.g.,Hopkinsetal. 2003);
V
same rangeofparameterspreviousdeducedfromtheVeilleux (2)thatthemetallicityistwicesolar(e.g.,Storchi-Bergmannet
& Osterbrock diagrams (Fig. 6). In other words, there is no al. 1998);and(3)thatone-thirdoftheobserved[OII]strength
needtoinvokeanyadditionalsourceapartfromtheAGN,such comes from H II regions. For a typical [OII] luminosity of
as star formation, to account for the observed range of [OII] 5.2×1040ergss- 1 (Fig. 4a),theempiricalcalibrationofKew-
emission. ley et al. (2004) yields a SFR of merely 0.5 M yr- 1. This
⊙
Toreinforceourconclusionthatstarformationmakesaneg- estimateisconsistentwith,butevenlowerthan,thanthatgiven
ligiblecontributiontothenarrow-linespectrumofourobjects, byHo(2005). Toputthisestimateinperspective,wenotethat
wesuperimposeonFigures6and7thetheoreticalphotoioniza- theSFRsofnearbynormalspiralgalaxies,includingtheMilky
tionmodelsofHIIregionscalculatedbyKewleyetal. (2001). Way,rangefrom∼1to3M⊙yr- 1(Solomon&Sage1988).The
These models explore a range of input parameters meant to situationinAGNsisthustrulyunusual.
representrealisticconditionsencounteredinstarburstgalaxies. Unlikethelow-redshiftquasarsconsideredbyHo(2005),our
Whilethe[OII]/[OIII]and[OIII]/HβratiosofHIIregionsand current sample of SDSS AGNs has no available information
our sample of AGNs overlap substantially (Fig. 7a), the star- on its cold gas content. However, the unbiasedCO surveyby
burstmodels, evenunderthe mostextremeconditions, cannot Scovilleetal. (2003)showsthatopticallyselectedquasars,not
accountforthestrengthofthelow-ionizationtransitions([NII], unlike those studied here, are quite gas-rich. This is not un-
[SII],andespecially[OI])observedintheAGNs. expected,consideringthatthenuclei,byselection,arequiteac-
tive,andthereforeaccretingatafairlyhighrate.Thus,although
5. DISCUSSION theevidenceiscertainlylessdirect,wecanreasonablysurmise
thattheobjectsstudiedherealsomayalsopossessabnormally
5.1. AGNFeedback
lowstarformationefficiencies.
Ho(2005;seealsoHo2006)recentlyproposedthatthe[OII] How can this finding be reconciledwith Kauffmannet al.’s
λ3727 emission line can effectivelytrace ongoingstar forma- (2003) result that luminous AGNs show a preponderance of
tion in the host galaxies of luminous, high-ionization AGNs post-starburstactivity? Apossiblesolutionistoinvokeanevo-
suchasSeyfertsandquasars.The[OII]linehaslongbeenused lutionary scenario, not unlike that proposed by Sanders et al.
asaSFRindicatorinspectroscopicsurveysofdistantgalaxies, (1988), in which a starburst phase precedes the optically re-
butpreviouslyithadnotbeenappliedtosystemscontainingac- vealed quasar phase. From the age estimates given in Kauff-
tivenuclei.Fromareviewoftheextantliteratureon[OII]mea- mannetal.,thetimelagseemstobeapproximately108 to109
surementsinquasars,HocametotheconclusionthattheSFRs years. The starburst phase terminates not because all of the
in their host galaxies are quite modest, being no greater than gas has been transformed into stars, nor because it has been
∼1- 10M⊙ yr- 1. Thisresultwassomewhatunexpectedcon- completelyexpelledbyAGNorsupernovafeedback,sinceev-
sideringtherecentevidencefortherebeingasubstantialpost- identlyplentyofgascoexistsduringthequasarphase, butbe-
starburstpopulationinAGNs(Kauffmannetal. 2003).Perhaps causetheactivenucleussomehowpreventsthegasfromform-
evenmoresurprising,Hofound,fromamorelimitedsampleof ing stars. Througha combinationof radiative heating and ki-
low-redshiftquasarswithavailableCOmeasurements,thatnot neticenergyinjection,anAGNisexpectedtosignificantlyalter
only are the SFRs low but that the star formation efficiencies the thermodynamicalstate of the gas in the circumnuclearre-
(SFRperunitgasmass)arealsolow,suggestingthattheAGN gionsofgalaxies(Begelman1993;DiMatteoetal. 2005),but
somehowinhibitsstarformation. precisely how this results in suppression of star formation re-
Apartfromthesmallsampleoflow-redshiftobjectsthathad mainstobeelucidated.
individual spectroscopic data, Ho’s more general conclusions
regardingthequasarpopulationasawholewasbasedonpub-
5.2. PossibleEvidenceforStarburstActivityinType2
lishedmeasurementsof[OII]strengthsderivedfromcompos-
Quasars
ite,orstatisticallyaveraged,spectra.Toplacetheseresultsona
moresecurefooting,wehaveundertakenamajortasktohomo- Zakamska et al. (2003) have drawn attention to a class of
geneouslymeasure the [OII] line in a largesample of Type 1 narrow-lineAGNsfromtheSDSS whose[OIII] linestrength,
AGNs. Apart from [O II], we also simultaneously measured when translated into continuum luminosities, place them in
a number of other diagnostically important narrow emission the league of quasars. This, along with the detection in
STARFORMATIONINAGNs 9
Table 1. Narrow Emission-line Ratios of Type 1 AGNs and Type 2 Quasars
Group Hα/Hβ [OII]/[OIII] [OIII]/Hβ [OI]/Hα [NII]/Hα [SII]/Hα
Type1AGNs 3.3a 0.27 6.06 0.16 0.71 0.44
Type2Quasars 4.1 0.75 5.29 0.14 1.06 0.58
aThe median value obtained from our spectral fit when the amplitude of Hα and Hβ were left
unconstrained;see§3.
Note. —ThelineratiosfortheType1AGNsarethemedianvaluesofthedetectedobjects. The
upperlimitsareconsistentwiththesevalues. ThelineratiosfortheType2quasarsweremeasured
by Zakamska et al. (2003) from the composite spectrum; they have been corrected for internal
extinction.
some sources of polarized broad-line emission through spec- moderatelyhealthystarburst.Incidentally,wenotethatadusty,
tropolarimetry (Zakamska et al. 2005) and heavy photo- gas-richenvironmentcharacteristicofstarburstsmaynaturally
electric absorption through X-ray observations (Ptak et al. accountforthehigherinternalextinctionmentionedabove.
2006), has prompted the identification of these objects with Thepossibleexistenceofsignificantongoingstarformation
the long-sought“Type2 quasars”anticipatedfromthe the ba- in Type2 quasars, in contrastto their stark absencein normal
sic orientation-dependent unification model of AGNs. If the (Type1)quasars,presentsanobviouschallengetotheconven-
Zakamskaetal. objectstrulyareintrinsicallyidenticaltonor- tional orientation-dependent model of AGN unification (An-
mal,high-luminosityType1AGNs,butsimplyviewedfroma tonucci 1993). Type 2 objects are not intrinsically the same
line of sight obscured from the nucleus, then the narrow-line as Type 1 objects, but instead may represent an earlier evolu-
spectrum of the two groups, assumed to be isotropic, should tionaryphase,perhapssimilartothescenariodiscussedin§5.1,
besimilar. Weweresurprisedtodiscoverthatthisappearsnot whereinthe centralengineis eithernotyet fullydevelopedor
to be the case. Comparison of the line strengthsfor our Type elseliesburiedbyextended,dustyregionsofstarformation.In
1 objects with those tabulated for the composite SDSS spec- this context, the optically selected Type 2 quasars closely re-
trum of Zakamskaet al. (2003)revealsthat both groupshave semble those ultraluminous infrared galaxies that host both a
similarlineintensityratios,withtwonoticeableexceptions(see strongstarburstandahighlyobscuredAGN(e.g.,Genzeletal.
Table1). First,theType2sourceshaveahigherBalmerdecre- 1998;Gerssenetal. 2004).
ment, and hence a higher inferredinternal extinction for their Observations of radio-loud AGNs have suggested that the
NLR, comparedto the Type 1 sources. Recall fromour spec- [OIII]-emittingregion,unlikethatproducing[OII],maynotbe
tralfitting (§3)that, when allowedto varyfreely, the majority entirely isotropic (Jackson & Browne 1990; Hes et al. 1996).
ofoursourcesyieldedan Hα/Hβ ratioof3.3, tobe compared IfthisisgenericallytrueofallAGNs,thenthe[OII]/[OIII]ra-
with Hα/Hβ = 4.1 forthe Type2 composite. If interpretedin tioisorientation-dependent,anditmayaccountforthehigher
terms of dust extinction assuming an intrinsic Balmer decre- ratiosinType2quasarscomparedtoType1quasars. Thisex-
mentof3.1andaGalacticextinctioncurve,thistranslatesinto planation,however,isunsatisfactorybecauseFigure5showsno
aninternalextinctionofA =0.2magversusA =0.9mag,re- evidencethatthispatternholdsforlower-luminositysources.
V V
spectively. Second,andmorepertinenttothecentralthemeof
this study, we find that the Type 2 quasars exhibit an anoma-
louslyenhanced(extinctioncorrected)[OII]/[OIII]ratio. This
6. SUMMARY
point is illustrated in Figures6 and 7, where we highlightthe
locationoftheZakamskacompositeinrelationtothelocusof We present a detailed analysis of the narrow-line spectrum
theType1sources. Whiletherelativelyhigh[OII]/[OIII]ratio ofalarge,homogeneoussampleofbroad-line(Type1)AGNs
cannormallybeattributedtoasomewhatlowerionizationpa- selectedfromSDSS,withtheprimaryaimofusingthestrength
rameter,wenotethatthisexplanationisinseriousconflictwith of the [OII] λ3727 line to constrain the ongoing star forma-
thehighluminosityofthesesources. Asdiscussedin§4.2,the tion rate in the host galaxies. Comparing the measurements
[OII]/[OIII] ratio varies strongly with [OIII] luminosity. For with a new set of photoionization models calculated using
L ≈3×1042 ergss- 1,theaveragefortheZakamskaetal. the code CLOUDY, we find that the principal narrow optical
[OIII]
sample,Figure5predictsthatthe[OII]/[OIII]ratioshouldbe lines, including [O II], can be readily reproduced using con-
∼0.06- 0.1. By contrast, the Type 2 quasar composite gives ventionalAGNphotoionizationparameters. Thisplacesstrong
[OII]/[OIII] = 0.75, nearly an orderof magnitudelarger than constraintsonanyadditionalstarburstcontributiontothe[OII]
thepredictedvalue.Wespeculatethattheenhanced[OII]emis- strength, with resulting limits on the inferred star formation
sion in the Type 2 quasars comes from star formation. If the rate. Consistent with the recent study of Ho (2005), the host
observed [O II]/[O III] ratio in Type 2 quasars is ∼10 times galaxiesofType1AGNsevidentlyexperienceverymodeston-
larger than expected for their [OIII] luminosity, then the ex- goingstarformation. On theotherhand,thesample“Type2”
cess [OII] luminosityof ∼2×1042 ergss- 1, adoptingthe as- quasarsidentifiedfromSDSS(Zakamskaetal. 2003)exhibits
sumptionsspecified in §5.1, correspondsto an estimated SFR enhanced [OII] emission, which we argue arises from signif-
of ∼20 M⊙ yr- 1. This level of star formation qualifies as a icant ongoing starburst activity. We propose an evolutionary
scenariothatcanaccountfortheseobservations.
10 KIM,HO&IM
We would like to thank T. A. Boroson for sending us the I M.I. acknowledge the support from the grant No. R01-2005-
Zw1irontemplate,G.J.Ferlandformakingavailablethecode 000-10610-0provided by the Basic Research Program of the
CLOUDY,L.J.Kewleyformakingavailablehermodelcalcu- KoreaScience&EngineeringFoundation.TheworkofL.C.H.
lationsofHIIregions,andananonymousrefereeforahelpful was supported by the Carnegie Institution of Washington and
critiqueofourwork.WearegratefultotheentireSDSScollab- byNASAthroughgrantsfromtheSpaceTelescopeScienceIn-
orationforprovidingaccesstotheirinvaluabledatabase. M.K. stitute(operatedbyAURA,Inc.,underNASAcontractNAS5-
has been supported in part by the BK 21 program. M.K. and 26555).
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