The Southern 2MASS AGN Survey: spectroscopic follow-up with 6dF

20 Pages·2010·1.43 MB·English

CSIRO PUBLISHING
PublicationsoftheAstronomicalSocietyofAustralia,2010,27,302–320 www.publish.csiro.au/journals/pasa
The Southern 2MASSActive Galactic Nuclei Survey:
Spectroscopic Follow-up with Six Degree Field
FrankJ.MasciA,E,RocM.CutriA,PaulJ.FrancisB,BrantO.NelsonA,F,
JohnP.HuchraC,D.HeathJonesD,MatthewCollessD,
andWillSaundersD
AInfraredProcessingandAnalysisCenter,Caltech100-22,Pasadena,CA91125,USA
BAustralianNationalUniversity,ACT0200,Australia
CHarvard-SmithsonianCenterforAstrophysics,Cambridge,MA02138,USA
DAnglo-AustralianObservatory,POBox296,Epping,NSW1710,Australia
ECorrespondingauthor.Email:[email protected]
FPresentaddress:VermontAcademy,SaxtonsRiver,VT05154,USA
Received2010January10,accepted2010May12
Abstract: TheTwoMicronAll-SkySurvey(2MASS)hasprovidedauniformphotometriccatalogtosearch
forpreviouslyunknownredactivegalacticnuclei(AGN)andQuasi-StellarObjects(QSOs).Wehaveextended
thesearchtothesouthernequatorialskybyobtainingspectrafor1182AGNcandidatesusingthesixdegree
field(6dF)multifibrespectrographontheUKSchmidtTelescope.Thesewerescheduledasauxiliarytargets
for the 6dF Galaxy Redshift Survey.The candidates were selected using a single color cut of J K >2
s
−
toKs!15.5andagalacticlatitudeof b >30◦.432spectrawereofsufficientqualitytoenableareliable
| |
classification.116sources( 27%)weresecurelyclassifiedastypeIAGN,20asprobabletypeIAGN,and
∼
57asprobabletypeIIAGN.Mostofthemspantheredshiftrange0.05<z<0.5andonly8( 6%)were
∼
previouslyidentifiedasAGNorQSOs.OurselectionleadstoasignificantlyhigherAGNidentificationrate
amongstlocalgalaxies(>20%)thaninanyprevious(mostlyblue-selected)galaxysurvey.Asmallfraction
ofthetypeIAGNcouldhavetheiropticalcolorsreddenedbyopticallythindustwithA <2magrelativeto
V
opticallyselectedQSOs.Ahandfulshowevidenceofexcessfar-infrared(IR)emission.Theequivalentwidth
(EW)andcolordistributionsofthetypeIandIIAGNareconsistentwithAGNunifiedmodels.Inparticular,the
EWofthe[Oiii]emissionlineweaklycorrelateswithoptical–near-IRcolorineachclassofAGN,suggesting
anisotropicobscurationoftheAGNcontinuum.Overall,theopticalpropertiesofthe2MASSredAGNare
notdramaticallydifferentfromthoseofoptically-selectedQSOs.Ournear-IRselectionappearstodetectthe
mostnear-IRluminousQSOsinthelocaluniversetoz 0.6andprovidesincentivetoextendthesearchto
≃
deepernear-IRsurveys.
Keywords: galaxies:active—quasars:general—infrared:general—surveys
1 Introduction
2002)hasemployedarangeofmulticoloropticalselection
Muchofourknowledgeaboutthedistributionandprop- techniques with relaxed constraints on morphology to
erties of active galactic nuclei (AGN) has come from searchforQSOstoredshiftsof 6.Thishasreducedthe
≃
samplesthatareflux-limitedatblueopticalwavelengths. biasrelativetothesimpleUV-excesscriteriausedinearly
Thisisbecausetheirspectralenergydistributions(SEDs) surveys,buthasnoteliminatedit.
generallyexhibitaUVfluxexcess.Suchsurveyscanbe AcompletecensusofAGNovercosmictimeisessen-
veryefficientandcompletedowntotheirbluefluxlimit, tial for understanding galaxy formation and evolution
e.g.,theLargeBrightQuasarSurvey(LBQS;Hewettetal. since the properties of the central black hole and host
1995).However,anysurveywithabluefluxlimitwillbe galaxy are intimately linked (see reviews in Ho 2004).
relativelybiasedagainstobjectswhoseintrinsicemission ThefractionofAGNmissingfromopticallyselectedsam-
peaksatotherwavelengths,e.g.,near-infrared(IR)emis- ples,bothasafunctionofredshiftandluminosity,isan
sionfromthehostgalaxy(Mascietal.1998;Bennetal. importantparameter.Thisisexpectedtoberelatedtothe
1998). Alternatively, the optical/UV can be partially or dutycycleofblack-holefuelingandtimescalesforregu-
heavily absorbed by dust, as revealed by radio, near-to- latingstarformation.Valuesarecurrentlyveryuncertain
mid IR, and X-ray surveys (Webster et al. 1995; Gregg andrangefrom15%togreaterthan50%(e.g.,Richards
et al. 2002; Polletta et al. 2007; Donley et al. 2008). etal.2003;Glikmanetal.2004,2007;Brownetal.2006).
The Sloan Digital Sky Survey (SDSS; Richards et al. This uncertainty is due to: selection of the appropriate
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TheSouthern2MASSActiveGalacticNucleiSurvey 303
comparisonsampleofopticallyselectedAGN;properties (FIRST)radiocatalog(Greggetal.2002;Glikmanetal.
ofthe‘unbiased’sampleandhowrepresentativeitis;and 2007).TheserevealedanumberofextremelyredQSOs
difficulties in quantifying the amount of bias (e.g., dust athighredshift,someshowingstrongevidencefordust.
extinction). Barkhouse & Hall (2001) studied the 2MASS colors of
QSOsthatwereidentifiedatotherwavelengths,primar-
1.1 Near-IRSelection ily from the Veron-Cetty & Veron (2000) catalog, and
Warrenetal.(2000)showedthatitispossibletoconstruct Georgakakisetal.(2009)usedmid-andfar-IRobserva-
a complete K-band–limited QSO sample by combining tions of red 2MASS AGN to infer their relationship to
opticalandnear-IRphotometry.Thisexploitsthecharac- luminousIRgalaxies(LIRGs).Mostofthesestudieshow-
teristic K-band excess seen in QSOs compared to stars everusedsingleband2MASSdetections(inJ,H,orKs)
and has been termed the KX-selection method.A num- andthereforecouldhavemissedmanyredAGN.Further-
ber of small pilot surveys have employed KX-selection more,nonewereabletoprovideacensusofAGNinthe
andvariantsthereof(e.g.,Croometal.2001;Sharpetal. 2MASScatalog.
2002; Jurek et al. 2008; Smail et al. 2008). The largest Auniformunbiasedsurveyof2MASSAGNwascar-
isbeingcompiledfromtheUKIDSSLargeAreaSurvey riedoutbyFrancisetal.(2004)byselectingcandidates
covering 12.8deg2 (Maddox et al. 2008). This survey in the southern hemisphere covering 12.56deg2 using a
cliunrerdenAtlGyNr∼epwoirtthsaKsurfa1c7eadnednszit<yo3f.≃T1h5eydeegs−ti2mfoartebdrotahda-t mniofidcearnattleycreodlourcecdutcoonftJam−inKatsi>on1f.r2o.mThfiosrecgorloourncdutstsairgs-.
50%weremissed≤bySDSS,thereforerevealingalarge Spectroscopicfollow-uprevealedthat 1.2%werebroad-
∼populationofredQSOs. lineAGNand 4%weregalaxieswi∼thSeyfert2nuclei.
∼
AlthoughtheKXmethodisturningupmanyluminous Themainfindingswerethat:(i)thetypeIAGNarepre-
redQSOstohighredshift,thesituationisdifferentforless dominately at low redshifts (z!0.3) and contamination
luminouslocalAGN(e.g.,Seyfertnuclei).Theseareusu- from host-galaxy light would make them hard to find
allynotfoundbycolorselectionsincehostgalaxylight inopticallyselectedQSOsamples(e.g.,theSDSS);and
candominatetheirbroadbandcolors.Historically,theyare (ii)theincidenceoftypeIIAGNamongstlocalgalaxiesis
foundbytakingopticalspectraofthenuclearregionsof higherthanusualwhencomparedtoblue-selectedgalaxy
largesamplesofgalaxies(e.g.,Hoetal.1997).Manyof samples.Theresultsofthisstudyarecomplementaryto
thesesamplesareflux-limitedatblueopticalwavelengths thosepresentedhereandwillbediscussedinmoredetail
andhencesubjecttobias.Themostsignificantbiasisthat later.
thebluelightisdominatedbyrecentstarformationwhich Theabovestudiesindicatethat2MASSissensitiveto
overwhelmsanyemissionfromacentralAGN.Anear-IR thelocalAGNpopulation.Theluminosityofbroad-lined
selected sample of local galaxies using color criteria QSOs,particularlyinthelocaluniverse,isonlyslightly
analogous to that used in KX methods will reduce this greaterthanthatoftheirhostgalaxies(e.g.,Hao&Strauss
bias. 2004).Onlyasmallamountofdustextinctionisneeded
Thelargestnear-IRsurveytodateistheTwoMicronAll tomis-identifythemorturnthemintotypeIIAGN.This
SkySurvey(2MASS;Skrutskieetal.2006).Therehave is in contrast to the heavily obscured, high-luminosity,
been several studies that searched forAGN in 2MASS. and usually unresolved QSOs at high redshift. There-
ThefirstwasthatfromCutrietal.(2002)inthenorthern fore,eventhoughnear-IRcolor-selectedsurveysareless
equatorialsky.TheyselectedsourceswithJ K >2and biased than those in the optical/UV, they are still some-
s
K 15.51 at galactic latitude b >30 . S−pectroscopic whatbiasedagainstthereddestQSOs.Theydohowever
s ◦
foll≤ow-up revealed that 75% w| e|re previously uniden- havetheadvantagethatcandidatesselectedinthenear-IR
tifiedAGN, with 80%∼of these associated with broad will be bright enough to allow spectroscopic follow-up
emission-line(type∼I)AGN—i.e.,Seyfert1sandQSOs— withrelativeease.
and the remainder were narrow-line (type II) AGN — It is worth mentioning why a red J Ks color cut
−
typicallySeyfert2s,typeIIQSOsandliners.Theyspanned has proven so efficient at findingAGN in the relatively
a redshift range of 0.03<z<2.52 with a median of shallow2MASSPointSourceCatalog(PSC).Thereare
0.22, therefore, the AGN were predominately local. three reasons. First, contamination from halo-giant and
T∼he extrapolated surface density of allAGN types was disk-dwarf stars is very much eliminated, as the major-
∼se0le.5ct7eddeAg−G2N, saigtnthifiecasanmtlyehKighmeragthnaitnudthe.atAofsigonpitfiiccaalnlyt 2itMyhAaSvSeJco−lorKsso!fg0a.l7a5xi(eFsraarnecipsreedtoaml.i2n0a0te4l)y.Sbleuceo,nwd,itthhea
s
fractionalsoshowedunusuallyhighpolarizationproper- medianJ Ks 1.1atz!0.1anda1-σ upperlimitof
− ∼
ties (Smith et al. 2002) and very weak X-ray emission J Ks<1.6atz 0.2(Jarrettetal.2000;Jarrett2004).
− ≃
(Wilkesetal.2002). Thiscolorreddensrapidlyforgalaxiesathigherredshifts
Other2MASSstudiesinvolvedcross-correlatingwith duetothek-correction,approachingJ Ks 2atz 0.4.
− ≃ ∼
theFaintImagesoftheRadioSkyatTwenty-centimeters However,typicalL∗galaxiesatsuchredshiftswillbewell
belowthedetectionlimitofthe2MASSPSC.Contami-
1KsissimilartotheKfilterbutcuttingoffatashorterredwavelength nation from non-active galaxies is therefore expected to
tominimizethermalemission. besmalloverall.Thethirdreasonismotivatedbythefact
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304 F.J.Mascietal.
thatanextremalcolorcutofJ K >2hasbeenshown magnitudecutwasinitiallyimposed.Ofmatches, 0.7%
s
− ≃
todiscriminateredAGNfromUV/optically-selectedones resulted in multiple (ambiguous) associations and were
inlargesamplesofknownAGN(Barkhouse&Hall2001; excluded. This yielded 6386 matches with an overall
Cutri et al. 2002). For example, 2MASS detects a large position-differencedispersionofσ 0.6arcsec.Asample
≃
fraction( 75%)oftheLBQSQSOsinallthreenear-IR of2260candidateswasthencompiledbyselectingsources
∼
bands (J, H, and K ). Most of these have J K <2. withSuperCOSMOSmagnitudesofb 18andr 17.
s s J F
− ≤ ≤
Therefore, J K >2 isolates the reddest subset of the Theselimitswererequiredtoobtaingoodsignal-to-noise
s
opticallysele−ctedpopulationandislikelytoprobemany ratiospectra(S/N"10pixel−1)andenablereliableiden-
more.Itisalsointerestingtonotethatnearlyallknown tifications.ThereliabilityoftheSuperCOSMOSoptical
QSOsatz!0.5haveJ Ks>1.2(Francisetal.2000; detections to these magnitude limits is expected to be
−
Barkhouse&Hall2001). >99.9%(Hamblyetal.2001b).Thesingleb andr band
J F
Inthispaper,weextendtheworkofCutrietal.(2002) photometryhasanaccuracyofσ 0.3magwhereas,due
∼
and Francis et al. (2004) to search for additional red to specifics of the calibration procedure, b r colors
J F
−
2MASSAGNinthesouthernequatorialsky.Weassem- are expected to have an accuracy of σ!0.12mag (for
bledarelativelyunbiasedsampleofredAGNcandidates details, see Hambly et al. 2001b). The 2260 candidates
using a color cut of J K >2 on the 2MASS Point were then proposed for follow-up with the 6dF multifi-
s
−
SourceWorkingDatabaseandthenusedthebrute-force brespectrograph.1182wereeventuallyallocatedfibers,
capabilitiesofthesixdegreefield(6dF)multiobjectspec- mainlyassecondarytargetsduringschedulingofobser-
trograph to obtain spectra of a subsample. We utlized vations for the 6dFGS (Jones et al. 2004). Our objects
theefficientmappingstrategyofthe6dFGalaxySurvey weredistributedoveraneffectivenon-contiguousareaof
(6dFGS)withourcandidatesselectedassecondarytargets 1592deg2.
∼
intheprogram. Itisimportanttonotethatourinitialsampleofcandi-
OursampleandtargetselectionaredescribedinSec- dates(withJ K >2)wasselectedfromanearlyversion
s
−
tion2.Observationsanddatareductionaredescribedin of the 2MASS Point Source Working Database. Subse-
Section3,andspectralclassificationsinSection4.Prop- quentrecalibrationandselectionofalternateobservations
ertiesofthenewlydiscoveredAGNandcomparisonsto ofsomeofthesesourcesforinclusioninthefinal2MASS
opticallyselectedQSOsamplesarediscussedinSection5. PSCresultedinsomeofthemhavingcolorsJ K <2.
s
−
ConclusionsaregiveninSection6.Weassumeaconcor- In the end, the majority of sources classified as AGN
dancecosmologywithH0 70kms−1Mpc−1,"m 0.3, hadJ Ks"1.5,with 20%satisfying1.5 J Ks 2
= = − ∼ ≤ − ≤
and" 0.7.Allmagnitudes,unlessotherwisespecified, accordingtophotometryinthepublic-releasePSC.Uncer-
#
=
arebasedontheVegasystem. tainties in the J Ks colors were typically !0.16mag
−
(1-σ).
2 SampleandTargetSelection
3 ObservationsandReduction
Candidateswereinitiallyselectedfromthe2MASSPoint
SourceWorkingDatabaseusingthefollowingcriteria:a Spectrawereobtainedoverthecourseofthe6dFGSdur-
colorcutofJ K >2;K 15.5;detectionsinallthree ing2001–2006usingtheUKSchmidtTelescopeandthe
s s
− ≤
bands (J, H, and K ); galactic latitude b >30 ; and 6dF spectrograph (Watson et al. 1998; Saunders et al.
s ◦
excludingaregionof 170deg2 covering|th|eLargeand 2001). Fordetailsonthe6dFGSobservingstrategy,see
∼
Small Magellanic Clouds. Previously identified sources Jones et al. (2004, 2009). The primary sample for the
were not omitted. This yielded 16977 candidates in an 6dFGS was drawn from the 2MASS Extended Source
effectiveareaof 20400deg2overthewholesky.These Catalog(XSC;Jarrettetal.2000).Seventeenadditional
∼
criteriadefinethe‘master’catalogofredAGNcandidates, (secondary)extragalacticsamplesweremergedwiththe
andwereusedinnorthernhemispherefollow-upstudies primary sample (see table 3 in Jones et al. 2009). Dur-
byCutrietal.(2002). ingsurveydesign,theseweregivenpriorityindicesand
NotethatadetectionintheH-bandwasincludedfor our initial sample of 2260 candidates had a complete-
reliability.TheredAGNcandidateselectioncriteriawere nessincoverageof91.7%.Thisgave 6AGNcandidates
∼
originally devised during the early stages of the survey per 6dF on average, although not all 6dFGS fields con-
before many of the source quality metrics were mature. tainedourtargetsbecauseofthedifferentgalacticlatitude
Sources detected in all three survey bands were known constraints.
to be the most reliable. Therefore, three-band detection The 6dF multifiber spectrograph was able to record
wasincludedwiththetwo-bandcolorlimittominimize up to 120 simultaneous spectra over a 5.7 field. Each
◦
sample contamination by spurious sources with unusual fiber has a projected diameter of 6.7arcsec on the sky.
colors. The2MASSpositionswereaccurateto 0.5arcsecand
≤
Southernequatorial(δ< 0 )AGNcandidatesfromthe therefore light losses due to fiber positioning errors
◦
mastercatalogwerepositionmatchedagainsttheSuper- wereexpectedtobesmall.Forthe10th–90thpercentile
COSMOS Sky Survey database (Hambly et al. 2001a). range in redshift for the extragalactic identifications,
A match radius of 4arcsec was used and no optical z 0.15–0.45,thefiberdiametercorrespondstophysical
≃
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TheSouthern2MASSActiveGalacticNucleiSurvey 305
scales of R≃17.5 to 38.7kpc h−701. This means the
6dF spectra sampled light from entire galaxies, and not
necessarilytheirnuclei.
Each spectrum was taken using V and R gratings,
whose outputs were later spliced to cover the effective
wavelengthrange: 3900–7500Å.TheobservedS/Nwas
typically 3–10pixe∼l 1, with >10pixel 1 being nominal
− −
due to the brightness of our sources. Spectra with low
S/Nwereprimarilyduetopoorobservingconditions.The
spectral resolution was typically R 1000 throughout,
∼
corresponding to emission-line full width at half max-
ima (FWHM) of 4–8Å over the observed wavelength
∼
range.Thisenabledustoresolverest-framevelocitiesof
"205kms−1overtherangez 0.15–0.45,sufficientfor
≃
AGNidentificationandclassification.
The data were reduced, spectra extracted, and wave-
length calibrated using a modified version of the Two
degreeFieldDataReduction(2dFDR)packagedeveloped
forthe2dFGalaxyRedshiftSurvey.Detailsofthereduc-
tionaredescribedinJonesetal.(2004)andproductsfrom
thefinaldatarelease(DR3;2009April)aredescribedin
Jones et al. (2009).The flux calibration was very crude Figure1 KsbrightessdistributionofproposedAGNcandidates,
sourceswithobservedspectra,andsourcesforwhichwesecured
inthatthesameaveragespectraltransferfunction(derived
a reliable spectral identification. The top horizontal axis shows
onceusingacoupleofstandards)wasassumedforevery theapproximatecompleteness( numberspectraobserved/number
=
6dF observation for all time. The spectra are therefore candidates)forseveralmagnitudebins.
not of spectrophotometric quality. This severely limited
ourclassificationprocess,e.g.,usingemissionlineratios
(see Section 4). The spectra were corrected for atmo-
spheric absorption and emission features. However in
somecases,imperfectsky-subtractionhaslefttheimprint
ofthebrightestskylines.
Quality flags were assigned by the semi-automated
6dFGSredshiftdeterminationpipeline(seeSection4for
details).Almost all redshifts were visually inspected by
the6dFGSteam.QualityflagsintherangeQ 1–4were
assignedinthefinalpubliccatalog2.Q 4r=epresentsa
=
very reliable redshift where, typically, the median spec-
tralS/Nwas 10pixel 1.Q 3wasassignedto‘likely’
−
∼ =
redshiftandQ 2to‘tentative’redshiftwithspectrawar-
=
ranting further examination. We visually examined all
spectra with quality flags Q 2, although the majority
≥
of usuable spectra had Q 4 and a handful had Q 3.
= =
Duetothefaintnessofthesourcesingeneral,750ofthe
1182spectraobservedwereofsuchpoorqualitythatno
classificationwaspossible.Classificationsweretherefore Figure2 J Kscolordistributionofsourceswithobservedspec-
−
securedfor432spectra. tra,sourcesforwhichwesecuredareliablespectralidentification,
andthoseidentifiedastypeIandtypeIIAGN.
Figure 1 shows: the number of proposed AGN can-
didates (using the optical/near-IR constraints defined in
Section2);thenumberof6dFspectraobserved;andthe seenintheCutrietal.(2002)sampleof704candidates
numberwithsecurespectralidentificationsasafunction withfollow-upopticalspectroscopy.Figure2showsthat
ofK magnitude.Thedearthofcandidatesinthefaintest wearenotsignificantlybiasedagainstidentifyingsources
s
bin,15<K <15.5,isduetoacombinationofouroptical withthereddestJ K colors.Infact,thespectraliden-
s s
−
magnitudelimitsandtheoriginalJ K >2cutoff.This tification rate as a function of J K is approximately
s s
− −
cutoffimpliesJ"17forKs>15andhenceafractionof uniform.
sources are expected to fall below the J-band flux limit Figure1showsthatthenumberofspectraobserveduni-
andbeexcludedfromthecandidatelist.Thisdropwasalso formallysamplestheK distributionofcandidates,i.e.,the
s
completenessisapproximatelyuniform.However,thereis
2Accessedviahttp://www.aao.gov.au/6dFGS/. relativelyhigherincompletenessinthenumberofspectra
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306 F.J.Mascietal.
Table 1. Rest wavelength regions used in emission-line
measurements
Line Centera Continuumintegration Lineintegration
(Å) limits(Å) limits(Å)
[Ciii] 1908.73 1800–1850,1970–2010 1860–1955
Mgii 2798.75 2650–2700,3000–3050 2750–2850
[Oii] 3728.48 3650–3700,3770–3815 3710–3740
Hβ 4862.68 4740–4840,4880–4940 4845–4880
[Oiii] 5008.24 4880–4940,5030–5100 4990–5025
Hα 6564.61 6400–6520,6630–6700 6550–6572
[Nii] 6585.28 6400–6520,6630–6700 6572–6594
[Sii] 6725.48b 6630–6700,6745–6845 6700–6745
aAsdefinedinVandenBerketal.(2001).
bAverageofdoublet[Sii]λλ6718.29,6732.67.
432spectrawereofsufficientqualitytoenableaclassi-
Figure3 bJ Kscolordistributionofsourceswithobservedspec- ficationofsomesort,butnotnecessarilyanunambiguous
tra,sourcesfo−rwhichwesecuredareliablespectralidentification, identification.Allspectrawereshiftedtotheirrestframe
andthoseidentifiedastypeIandtypeIIAGN. usingredshiftsdeterminedbythe6dFGSprogram.
4.1 Emission-LineDiagnostics
identified at the faintest magnitudes, 14.5<K <15.25.
s
The incompleteness in this range is 70% with respect Weassembledadatabaseofemission-linediagnosticsfor
tothenumberofspectraobserved.T∼hisisprimarilydue all the good quality spectra to assist with the identifi-
to the faintest sources generally having poorer quality cations.The diagnostics included line fluxes, equivalent
spectra.Theseareexpectedtobeneartheopticalmagni- widths(EWs),anddispersionvelocities.Thesewereesti-
tudelimitimposedforspectroscopy,17!bJ 18.Infact, matedbyfittingVoigtprofilesandtheunderlyingcontinua
theintroductionofanopticalmagnitudecut≤isexpected wereapproximatedbylinearlyinterpolatingstraight-line
tohavebiasedthespectralsampletowardsblueroptical fitsoneithersideofeachline.Linefluxeswerethendeter-
colors in general. Comparing the relative deficit in the minedbyintegratingthefluxinthefittedprofilesabovethe
numberoffaint(K >14.5)sourcestotheoriginalcan- continuum level. Dispersion velocities were determined
s
didate K distribution (from Cutri et al. 2002) with no fromtheFWHMofthelines.Thelinesofinterestandthe
s
optical magnitude limit imposed, we estimate we have effectivewavelengthregionsusedtodefinethecontinuum
lost"35%ofcandidatesduetothisopticalcut.Amajority andlineintegrationlimitsareshowninTable1.
ofthesemissedcandidates(withb >18)haveb K ThefitprofstaskinIRAFwasusedfortheautomated
J J s
colors 4–6. However, the spectrally-observed b −K measurementoflinediagnostics.Thisincludedtheabil-
J s
distribu≃tion(Figure3)showsthatwearenotcompl−etely itytodeblendcloselyseparatedlines,e.g.,Hαand[Nii].
biasedagainstidentifyingthereddestsources.Whencom- An important input parameter for the fitprofs task is an
paredtothecolorsofopticallyselectedQSOs,the2MASS estimate of the 1-σ uncertainty per pixel. This allows
AGNhaveatailextendingtomoderatelyreddercolors(see the program to compute uncertainties in each of the fit-
Figure10andSection5.2formoredetails). tedlineparameters.Sincethefittingwasnon-linear,this
was accomplished using a Monte Carlo simulation.The
spectraluncertaintywascomputedbyfirstselectingarel-
4 Classification
atively clean region in the rest frame common to each
All spectra were initially classified using the semi- spectrum,i.e.,devoidofstrongemissionandabsorption
automated6dFGSclassificationsoftware,whoseprimary lines.Weselectedtherestwavelengthrange5050–5400Å.
purposewastodetermineaccurateredshifts.Thisisamod- Wethenfittedastraightlinetothedatainthisregionusing
ifiedversionoftherunzsoftwareusedforthe2dFGalaxy arobust(outlier-resistant)regressionmethodbasedonthe
RedshiftSurvey(Collessetal.2001).Itused13spectral conceptof‘M-estimation’(Huber1981).Theuncertainty
templatestoidentifyspectrausingline-fittingandcross- was then estimated using the median absolute deviation
correlationtechniques.Ingeneral,thisprogramproduced in the residuals from the fit, and is also robust against
veryreliableredshiftsforallgalaxies,althoughwasless potentialoutliers:
reliable at separating out the variousAGN classes from
late-andearly-typegalaxies,andstars.Allspectrawere σ 1.4826median p fit p , (1)
i i
≃ {| − { }|}
visually inspected to determine whether a poor-quality
spectrumflaggedbythe6dFGSsoftwarewasworthyof where p is the value of the ith pixel in the 1-D spec-
i
furtherexamination. trumandfit p isthefittedvalue.Thisquantityisscaled
i
{ }
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TheSouthern2MASSActiveGalacticNucleiSurvey 307
Figure4 Distributionofrest-frameHαandHβequivalentwidths Figure5 LineratiodiagramforgalaxieswithtypeIAGNclassifi-
forallgalaxieswiththebestqualityspecta.Wearesensitivetoall cationsremoved.Diamondsandfilledcirclesare‘probable’typeII
equivalentwidthstotherightoftheverticaldottedline(>5Å). AGNandstar-forminggalaxies,respectively,allat 0.2dex(1-σ)
≥
fromtheclassificationboundaryofKewleyetal.(2001,solidline).
Crossesarecomposites(classifiedasunknowngalaxies).Errorsin
suchthatitconvergestothestandard-deviationofaGaus- thelineratiosaretypically0.2dex(1-σ).
sian in the limit of a large sample. For spectra where
therange5050–5400Åincludedorfelloutsidethelong
wavelength end (λ ) after shifting to the rest frame, AGN; type II AGN; starburst or late-type star-forming
red
a wavelength range of λ 250 λ λ 20Å was galaxies; early-type galaxies; stars; and unknown emis-
red red
− ≤ ≤ −
usedinstead.Inallcases,thisensured>20pixelsforthe sionlinegalaxies.Theseclassesandthecriteriausedto
noisecomputation. identifythemareasfollows.
WeestimatedthesmallestEWwearesensitivetoby Spectra with broad Hα and/or Hβ emission lines
examining the dispersion in Hα and Hβ EWs of all the exceeding 1000kms 1 (FWHM), or with other broad
−
galaxies(andpotentialAGN)withthebestqualityspectra permitted lines present, e.g., [Ciii] or Mgii for z"1
(Q 4).Figure4showsthesedistributions.EWmeasure- andz"0.4,respectively,wereclassifiedastypeIAGN.
=
mentsclusteredaroundzeroarelinelessgalaxieswherean Included in this criterion are S/N 2.5 on the FWHM
≥
identificationwouldbehighlyunreliable,ifatallpossible. measurementandaHαorHβEW>5Å.Spectrainwhich
We found we should be sensitive to galaxies with rest- knownbroadlinescouldbediscernedbyeyebutwererel-
frameEWineitherHαorHβof"5Å.Givenourobserved ativelynoisy,i.e.,withfluxS/N<2.5wereclassfiedas
wavelengthrange,eitheroftheselinesareexpectedtobe ‘probable’typeIAGN.
observedatz!0.5andthereforewereusedasconstraints Non-type I AGN spectra were classified using line
intheidentificationprocessbelow. ratios involving good measurements in either of the
LinefluxesandEWswerealsomeasuredinteractively followinglinepairs:([Oiii],Hβ)or([Nii],Hα)or([Sii],
byintegratingthelinefluxesdirectlyfromthe1-Dspectra. Hα)or([Oiii],[Oii]).Wefirstattemptedaclassification
These were in excellent agreement, to within measure- using the diagnostic diagrams of Kewley et al. (2001,
ment error, with the profile-derived fluxes from above. 2006), which are based on the classic BPT diagrams of
Alllineswerevisuallyinspectedandunreliablefluxmea- Baldwin et al. (1981). Figures 5 and 6 show the tradi-
surementsflagged.Thesewereprimarilylinesthatwere tionalline-ratiodiagramsusingourgoodqualityspectra
contaminatedbyastrongskylineoratmosphericabsorp- (withpre-classifiedtypeIAGNremoved)andwhereall
tionband.Ourfinalemissionlinedatabaseretainedlines fourlineshadfluxS/N 2.5.Unfortunately,allfourlines
≥
with fluxes "2.5σ, and lines that were clearly discern- ineitherFigure5or6wereonlysimultaneouslyvisible
ablebyeyeincaseourautomatedmeasurementofσwas (andwithgoodS/N)in 8%(23/296)ofthegood-quality
≃
overestimated. Unreliable σ estimates occured in 7% non-type I spectra. Furthermore, the errors in the line
≃
ofthespectra.Notethatwedidnotcorrecttheemission ratios( 0.2dex,1-σ)weretoolargeforthebulkofthese
≃
linefluxesforanyunderlyingabsorption(e.g.,fromstellar spectratobereliablyclassified.Wethereforedeclaredthe
photospheres)sinceastudyusingsimilarspectrafrom2dF fewtypeIIAGNandstar-forminggalaxiesthatcouldbe
byFrancisetal.(2004)foundthiseffecttobenegligible. classifiedusingthismethod(atdistances 1-σ fromthe
≥
A first pass examination of the spectra together with classificationboundaries)tobeprobableidentifications.
initial classifications provided by the 6dFGS program For the remaining 273 sources with good qual-
motivated us to define six broad object classes: type I ity data, and where only one of the above line pairs
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308 F.J.Mascietal.
Earlytypegalaxieswereidentifiedthroughthecharac-
teristic 4000Å break, a signature caused by the dearth
of hot and young (usually O- and B-type) stars and
strong heavy-metal absorption by stellar photospheres.
Starswereisolatedbyfirstensuringthattheirradialveloc-
itieswere!150kms−1.Theirspectrawerethenmatched
totemplatesfromtheELODIEstellarlibrary(Moultaka
etal.2004).ThemajoritywereKandMdwarfstars.All
othergalaxy-likespectrawithHαorHβEW>5Åbutnot
fittingtheabovecriteria—e.g.,withsinglelinemeasure-
ments,orcompositeslyingwithin1-σ ( 0.2dex)ofthe
±
type II/starburst classification boundaries in Figure 5 or
6—wereclassifiedas‘unknowngalaxies’.
4.2 ResultsSummary
Table 2 summarizes our source classifications.We only
includeoursecureidentificationsforthetypeIAGN,and
probable identifications (as described in Section 3) are
Figure6 LineratiodiagramforgalaxieswithtypeIAGNclassifi- classedas‘unknowngalaxies’.Alsoincludedarestatistics
cationsremoved,butwith[Sii]intheabscissa.Diamondsandfilled
for type I and type IIAGN from the Cutri et al. (2002)
circlesare‘probable’typeIIAGNandstar-forminggalaxies,respec-
northern 2MASSAGN survey and Francis et al. (2004)
tively.ThesolidlineistheAGN/star-formationdiscriminatorfrom
Kewleyetal.(2001).Crossesarecomposites(classifiedasunknown southern 2MASSAGN survey.The latter is broken into
galaxies).Errorsinthelineratiosaretypically0.2dex(1-σ). twocolorcuts.TheCutrietal.(2002)studyhasatleast
twicethedetectionratefortypeIAGN.Thiscouldbedue
was available (predominately when z"0.1), a galaxy todeeperspectroscopicfollow-upoffaintersingletargets
was classified as a ‘probable’ type II AGN if either intheirstudy.Itispossiblethatasignificantfractionofour
of the following was satisfied: log([Oiii]/Hβ)>0.3 faintertargetsatK >14.5(e.g.,Figure1)couldnotbe
s
or log([Nii]/Hα)> 0.2 or log([Sii]/Hα)> 0.35 (e.g., securedaretypeIAGNthroughspectralidentification.Itis
− −
Zakamska et al. 2003) or log([Oiii]/[Oii])>0 (e.g., alsointerestingtonoteinTable2thatthereisatendency
Fraquelli & Storchi-Bergmann 2004; Kewley et al. for the type I AGN identification rate to increase with
2006). Combined with any of these, we also required J K color.
s
a rest-frame FWHM([Oiii])>300kms 1, FWHM(Hα −Figure 7 shows a sampling of spectra for the new
−
or Hβ)<1000kms 1, and Hα or Hβ EW>5Å. The type IAGN, primarily those with the highest redshifts.
−
FWHM([Oiii])limitwasincludedtoimprovetherelability Two of the sources are at z>1: 2MASS J21571362-
oftypeIIidentificationswhenonlyonelinepairwasavail- 4201497withz 1.321and2MASSJ10012986-0338334
=
able. For comparison, Zakamska et al. (2003) assumed withz 1.389.Thelatterhasbeenclassifiedasaprobable
FWHM([Oiii])>400kms 1. We assumed 300kms 1 typeIA=GNduetoalowspectralS/N,althoughgivenits
− −
since the distribution for FWHM([Oiii]) for their entire relativelyhighredshift,itismostlikelyaQSO.
typeIIsamplefallsoffsharplyat<300kms 1.Intheend, Table3liststhesecure(T1)andprobable(PT1)typeI
−
thislimitmadelittledifferencetothetypeIIidentification AGN identifications.There are 116 classified asT1 and
statistics. 20 as PT1. Previous or alternative names as listed in
Probablestarburst/late-typestarforminggalaxieswere the NASA Extragalactic Database (NED) are given. Of
classified using the negation of these single line ratios the136typeIAGN,8( 6%)werepreviouslyclassified
∼
with some buffer to allow for flux errors, i.e., those as either ‘AGN’, ‘QSO’or ‘AGN/QSO?’in NED. Four
with: log([Oiii]/Hβ)<0.2, or log([Nii]/Hα)< 0.3, or of these are in the SDSS QSO sample. This implies a
−
log([Sii]/Hα)< 0.4,orlog([Oiii]/[Oii])< 0.2. majorityarenew,previouslyundiscoveredAGN.Interest-
− −
ItisimportanttonotethatthetypeIIAGNidentified ingly,tenofourtypeIAGNwerepreviouslydetectedin
usingtheabovesinglelinepairscouldbecontaminatedby X-ray by the ROSAT All-Sky Survey (RASS). Table 4
low-metallicityemission-linegalaxiesorliners,especially liststhetypeIIAGN,allofwhichareclassifiedasprob-
atlowredshift.Furthermore,thenon-spectrophotometric able using the methods described in Section 3. None of
natureofourspectracouldinvalidatesomeidentifications thetypeIIAGNwerepreviouslyclassifiedasAGN-like,
made using widely separated lines (e.g., the pair [Oiii], and only one was detected in X-ray by the RASS. Of
[Oii]). These classifications are therefore very tentative the previous classifications available in NED, a major-
giventhequalityofourdata.Hence,wedeclarealltype ityofourtypeIandIIAGNarelistedasgalaxy-likeand
IIAGN identifications quoted in this paper to be prob- extendedintheoptical(SuperCOSMOSdigitizedplates)
able. Follow-up with higher S/N spectral observations, ornear-IR(2MASSAtlasImages).Atleast30%arealsoin
preferablywithbettercalibratedthroughputasafunction the2MASSExtendedSourceCatalog(XSC;Jarrettetal.
ofwavelength,willbeneededforconfirmation. 2000).Thisisexpectedgiventhedepthofoursample.
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TheSouthern2MASSActiveGalacticNucleiSurvey 309
Table2. Sourceclassificationsandcomparisonstopreviousstudies
Thisstudy Cutrietal.(2002) Francisetal.(2004) Francisetal.(2004)
(J Ks!1.7) (J Ks>2.0) (J Ks>1.2) (J Ks>1.8)
− − − −
Numberclassified 432 664 1304 66
Stars(%) 23(5.3)a 66(9.9) 330(25.3) 6(9.1)
Unknowngalaxies(%) 193(44.7) … … …
Early-typegalaxies(%) 17(3.9) … … …
SB/late-typegalaxies(%) 26c(6.0) … 106(8.1) …
TypeIAGN(%) 116b(26.8) 385(57.9) 14(1.1) 4(6.0)
TypeIIAGN(%) 57c(13.2) 100(15.0) 23(1.8) 0(0.0)
aValuesinparenthesisarepercentagesofthe‘numberclassified’(firstrow).
bThe20probableidentificationsareexcludedhereandclassifiedas‘unknowngalaxies’.
cWedeclareallthesetobeprobableidentifications.
Figure7 Rest-framespectraofsomenewtypeIAGNoverlayedwithemissionlinestypicallyfoundinQSOspectra.Thespectraforall
objectslistedinTables3and4canbeviewedbyqueryingthe6dFpublicdatabase:http://www.aao.gov.au/6dFGS/.
5 Discussion AGNareatlowredshifts,withonlytwoatz>0.7.There
are six securely identified AGN (four type I, two type
This section reviews the properties of our 2MASS
II) at z<0.05.This is 24% (6/25) of all secure galaxy-
AGN and compares them to those of AGN/QSOs dis-
like spectral identifications (including unknown types)
covered in optical surveys. We explore their redshift,
in this redshift range. This is a lower limit since some
luminosity, photometric and line EW distributions. Our
objects classified as ‘unknown’ could be type II AGN.
primarybenchmarkandcomparisonsampleofoptically-
Even removing our ‘probable’ type II AGN, the AGN
selected AGN/QSOs is the SDSS Quasar Catalog Data
fraction is still relatively high. For comparison, Hao &
Release 5 (DR5; Schneider et al. 2007). This cata-
Strauss (2004) find 4% of 15200 SDSS-detected
log contains 2MASS matches to 9824AGN, all within ∼ ≃
galaxiesatz<0.05harborAGN(mostlySeyferts).Anear-
2arcsec.
lierstudybyHuchra&Burg(1992)foundAGN(including
LINERs) in 3.4% of a sample of 2399 nearby blue-
5.1 RedshiftandLuminosityDistributions ≃
selectedgalaxies.Thesestudiesarenotafaircomparison
Our 2MASS AGN span the range 0.01!z!1.38 as sincethegalaxieswereopticallyselected.Itwouldbeof
shown in Figure 8a. The median z is 0.27 and 0.21 interesttodeterminethefractionofSDSSgalaxieswith
≃ ≃
fortypeIandtypeIIAGN,respectively.Amajorityofour activenucleiforacolorcutofJ K >2.
s
−
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310 F.J.Mascietal.
3 4 0
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