Active Galactic Nuclei in the mid-IR. Evolution and Contribution to the Cosmic Infrared Background

1.1 MB·English

by  I. Matute

Astronomy&Astrophysicsmanuscriptno.matute06˙15mu˙AGNLF (cid:13)c ESO2008
February5,2008
Active Galactic Nuclei in the mid-IR
⋆
Evolution and Contribution to the Cosmic Infrared Background
6
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0 I.Matute1,2,F.LaFranca2,F.Pozzi3,4,C.Gruppioni4,C.Lari5,G.Zamorani4
2

n
1 Max-PlanckInstitutfu¨rextraterrestrischePhysik(MPE),Giessenbachstraβe,Postfach1312,D-85741Garching,Germany
a
J e-mail:[email protected]
2 DipartimentodiFisica,Universita`degliStudi“RomaTre”,ViadellaVascaNavale84,I-00146Roma,Italy
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1 e-mail:[email protected]
3 DipartimentodiAstronomia,Universita`diBologna,ViaRanzani1,I-40127Bologna,Italy

1 4 INAF,OsservatorioAstronomicodiBologna,ViaRanzani1,I-40127Bologna,Italy
v 5 INAF,IstitutodiRadioastronomia(IRA),ViaGobetti101,I-40129Bologna,Italy
5
5
ReceivedJune28,2005;acceptedDecember30,2005
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1
ABSTRACT
0
6
0 Aims.Westudytheevolutionoftheluminosityfunction(LF)oftype-1andtype-2ActiveGalacticNuclei(AGN)inthemid-infrared,andthen
/ derivethecontributionoftheAGNtotheCosmicInfraRedBackground(CIRB)andtheexpectedsourcecountstobeobservedbySpitzerat
h
24µm.
p
Methods.Weusedasampleoftype-1andtype-2AGNselectedat15µm(ISO)and12µm(IRAS),andclassifiedonthebasisoftheiroptical
-
o spectra.Localspectraltemplatesoftype-1andtype-2AGNhavebeenusedtoderivetheintrinsic15µmluminosities.Weadoptedanevolving
r smoothtwo-powerlawshapeoftheLF,whoseparametershavebeenderivedusinganun-binnedmaximumlikelihoodmethod.
t
s Results.WefindthattheLFoftype-1AGNiscompatiblewithapureluminosityevolution(L(z) = L(0)(1+z)kL)modelwherekL ∼2.9.A
a smallflatteningof thefaint (L < L∗ ) slopeof theLFwithincreasingredshift isfavouredbythedata. Asimilar evolutionary scenariois
: 15 15
v foundforthetype-2populationwitharatekL rangingfrom∼1.8to2.6,dependingsignificantlyontheadoptedmid-infraredspectralenergy
i distribution. Alsofor type-2 AGN aflatteningof theLF withincreasing redshift issuggested by thedata, possibly caused bythe lossof a
X
fractionoftype-2AGNhiddenwithintheopticallyclassifiedstarburstandnormalgalaxies.Thetype-1AGNcontributiontotheCIRBat15µm
r is(4.2–12.1)×10−11 Wm−2sr−1,whilethetype-2AGNcontributionis(5.5–11.0)×10−11 Wm−2sr−1.WeexpectthatSpitzerwillobserve,down
a
toafluxlimitofS =0.01mJy,adensityof∼1200deg−2type-1and∼1000deg−2type-2opticallyclassifiedAGN.
24µm
Conclusions.AGNevolveinthemid-infraredwitharatesimilartotheonesfoundintheopticalandX–raysbands.Thederivedtotalcontri-
butionoftheAGNtotheCIRB(4-10%)andSpitzercountsshouldbeconsideredaslowerlimits,becauseofapossiblelossoftype-2sources
causedbytheopticalclassification.
Keywords.cosmology:observations–infrared:galaxies–galaxies:active–surveys–galaxies:evolution
1. Introduction of the luminosity (the luminosity function, LF hereafter) can
providefundamentalcluestoexplainthepresentdayuniverse
ActiveGalacticNucleus(AGNhereafter)areexpectedtohave
(e.g.Ballandetal.2003;Mencietal.2004;Granatoetal.2004;
playedanimportantroleintheformationandevolutionofthe
DiMatteoetal.2005).
galaxiesintheUniverse.Anexampleistheobservationofthe
correlationbetweenthemassofthecentralblackholes(M ) The LF will not only provide informations on the demo-
BH
andthemassofthebulges(Magorrianetal.1998)ortheveloc- graphicsof AGN, butwill also place constraintson the phys-
itydispersionofgasandstarsinthebulgesofgalaxies(M –σ ical model of AGN, the origin and accretion history into su-
BH
relation;Ferrarese&Merritt2000;Tremaineetal.2002).Thus, permassive black hole and the formation of structures in the
themeasureofthehistoryofthedensityofAGNasafunction earlyuniverse.Moreover,themeasureoftheAGNLFoverthe
whole spectral range will provide information on the relative
⋆ Data presented here are based on observations by the Infrared importanceofaccretionpowerintheoverallenergybudgetof
Space Observatory (ISO). ISO is an ESA project with instruments
theuniverse.
funded by ESA member states (especially the PI countries: France,
Germany,theNetherlandsandtheUnitedKingdom)andwiththepar- AGN have been historically classified into two groups
ticipationofISASandNASA. (type-1 and type-2), dependingon the presence or absence in
2 Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR
their optical spectra of broad emission lines (FWHM>1200 Shupeetal.1998)anddeepbutsmallISOfieldsintheHubble
kms−1).TheunifiedmodelforAGNassumesthatthetwotypes DeepField North (HDFN;Ausseletal. 1999a,1999b),South
of sources are intrinsically the same (Antonucci et al. 1993), (HDFS; Oliver et al. 2002) and the Canada-France Redshift
andthattheobserveddifferencesintheopticalspectraareex- Survey(CFRS;Floresetal.1999)thatprovidedonlyahandful
plainedbyanorientationeffect.Thepresenceofobscuringma- ofobjects.
terial(torusor warped disc) aroundthe centralenergysource Therefore,duetothesmallnumberofhighredshiftobjects
and its orientation respect to the observer could preventa di- available,theshapeandevolutionofthemid-IRLFoftheseob-
rect view of the central region around the nuclei responsible jectsathighredshiftwaslargelyunknown.Onlyrecently,large
forthebroadlineemissionobservedintheopticalband.AGN and highly complete 15µm spectroscopic samples, based on
arethenclassifiedintwogroups:theun-obscured(type-1)and theEuropeanLargeAreaISOSurvey(ELAIShereafter;Oliver
obscured(type-2)sourcesdependingwhethertheline-of-sight etal.2000),havebeenreleased(Rowan-Robinsonetal.2004;
intersects or not the obscuringmaterial. However,nowdaysit LaFrancaetal.2004;FLF04hereafter).Basedontheanalysis
seems clear that the orientation-dependentmodel is probably of the ISO observations in the ELAIS-S1 field, Matute et al.
justa0th-orderapproximationtothetruenatureofAGN(e.g. (2002, M02 hereafter) derived the first estimate of the type-1
Bargeretal.2005;LaFrancaetal.2005). LF in the mid-IR (15µm) finding an evolution similar to the
ThemajoreffortstounderstandtheevolutionofAGNhave ones observed in the optical (Croom et al. 2004) and X–rays
beenhistoricallyconcentratedatopticalwavelengthsandinthe (e.g. Hasinger et al. 2005; La Franca et al. 2005). The evo-
lastyearshaveproducedthelargestcompilationsofAGNfrom lution of the LF for normaland starburst galaxies detected at
the Two Degree Field (2dF; Boyle et al. 2000; Croom et al. 15µminthesouthernELAISfields(S1&S2)hasbeenderived
2004) and the Sloan Digital Sky Survey (SDSS; York et al. anddiscussedbyPozzietal.(2004).
2000;Haoetal.2005).Howevertheopticalbandshaveproved Inthispaperweinvestigatetheseparateevolutionoftype-
to be veryinefficientin the selection of obscuredsourcesand 1 and type-2AGN in the mid-IRandtheir contributionto the
the assembly of unbiased samples of type-2 objects is diffi- CIRB using: a) the local IR population uncovered by IRAS,
cult.Inthelastyearsmanynewwindowshavebeenopenedat b) deep observations from ISO fields and, c) the most recent
variouswavelength bandsfor the observationof high redshift spectroscopic identifications of 15µm sources in the southern
galaxiesandAGN:sub-mm(SCUBA),Infrared(IRAS,ISOand fields of the ELAIS Survey (Pozzi et al. 2003; FLF04)1. The
Spitzer) andthe X–ray(ROSAT,XMM andChandra).Thanks AGNsampleusedinourworkisdescribedinsection2.Section
to their smaller dependence on dust obscuration if compared 3introducesthegeneralmethodusedtoderivetheparameters
to the optical surveys, the Infraredand the hard X–ray wave- oftheluminosityfunctionandpresentstheresults.Insection4
lengthshaveprovedtobemuchmoreefficientinthedetection wediscussourresultsandcomparethemwiththeLFsderived
oftype-2sources. inthemid-IRbypreviousanalysisofmid-IRobservations.The
X–ray observations have been so far consistent with contributiontotheCIRBandthepredictedcountsinthemid-IR
population synthesis models based on the 0th-order unified Spitzerbandat24µm arecomputedanddiscussedinsections
AGN scheme (e.g. Comastri et al. 1995) or its modifications 5and6.Section7summarisesthemainresults.
(Pompilio et al. 2000; Gilli et al. 2001; Ueda et al. 2003; La AcosmologywithH = 75kms−1 Mpc−1 andΩ = 0.7,
o λ
Franca et al. 2005). The hard spectrum of the X–ray back- Ω =0.3wasadoptedfortheworkpresentedinthispaper.
m
groundisexplainedbyamixtureofabsorbedandunabsorbed
AGN,evolvingwith cosmictime.Accordingtothesemodels,
mostAGNspectrashouldbeheavilyobscuredasthelightpro- 2. Thesampleandsourceclassification
ducedbyaccretionisabsorbedbygasanddust.
The mid-IRselected AGN sample usedin thiswork hasbeen
For this reason an important mid-infrared (4-40µm) ther-
extractedfrom:(i)the15µmELAISfieldsS1andS2 (Lariet
mal emission is expected from reprocessed radiation by gas
al.2001;Pozzietal.2003),(ii)the15µmdeepISOsurveysin
and dust grains directly heated by the central black hole
the HDFN (Aussel et al. 1999a, 1999b), HDFS (Oliver et al.
(e.g. Granato et al. 1997; Oliva et al. 1999; Nenkova et al.
2002)andCFRS(Floresetal.1999),(iii)thelocalIRAS12µm
2002). AGN could be in this case important contributors to
sampleofRMS.
the Cosmic Infrared Background (CIRB). Indeed, IRAS has
The 15µm catalogue in the ELAIS S1 field has been re-
observed strong mid-IR emission in all local (z < 0.1) AGN
leased byLarietal. (2001).Itcoversan areaof∼4 deg2 cen-
(Mileyetal.1985;deGrijpetal.1985;Neugebaueretal.1986;
tred at α(2000) = 00h34m44.4s, δ(2000) = −43o28′12′′ and
Sandersetal.1989).Completeandlargelyunbiasedsamplesof
includes462 mid-IRsourcesdown to a flux limit of 0.5 mJy.
AGNhavebeenproducedat12µm(Rush,Malkan&Spinoglio
Mid-IR source countsbased on this catalogue have been pre-
1993,RMShereafter)and25µm(Shupeetal.1998)basedon
sentedanddiscussedbyGruppionietal.(2002).
IRASobservations.Thesesamplesprovideanimportantunder-
The analysis presented here is restricted to the more reli-
standingoftheinfraredemissionfromlocalgalaxiesandafirm
ablesubsampleof406sourcesasdescribedbyFLF04.About
basetocomparethepropertiesofgalaxiesinthelocaluniverse
80% of these sources have been optically identified on CCD
with the high redshift populations uncoveredby ISO (z ≤ 1)
exposuresdowntoR∼23,whilespectralclassificationhasbeen
andmostrecentlybySpitzer(z≤2).
StatisticalstudiesofAGNinthemid-IRhavebeenbasedon 1 Data and related papers about the ELAIS southern survey are
largebutlocalsamplesfromtheIRASobservations(e.g.RMS, availableat:http://www.fis.uniroma3.it/∼ELAIS S
Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR 3
when available, depending on the redshift of the source (e.g.
log([OIII]/Hβ)>0.5andlog([NII]/Hα)>-0.4).
In total, the southern ELAIS (S1+S2) AGN sample in-
cludes27(25+2)type-1and25(23+2)type-2AGNrepresent-
ing∼16%(52/320)oftheidentifiedinfraredsourcesand∼24%
(52/221) of the extragalactic population. Their distribution in
theredshift–R-magnitudespaceisshowninFig.1.Theplotted
linesrepresentalinearfit(±1σ)tothedata(dottedfortype-1
AGN and dashed for type-2). Type-1 sources are detected up
toz∼3andshowaquasiredshift-independentandnarrowop-
tical magnitudedistribution(R = 18.76±1.01).On the other
hand, type-2 sources show a larger spread of optical magni-
tudes(R =18.45±1.62)andarathersteepdependenceonthe
redshiftuptoz∼1.
Fig.1. DistributionofELAIS-S1type-1and-2AGNinthez−R-magnitudeplane.
Bestfitlinearsolution(±1σ)inthelog(z)-Rspaceareshownasdottedlinesfortype-1 Constraintsonthefainterpopulationareprovidedbydeep
AGNandasdashedlinesfortype-2AGN.TheR-magnitudeandredshiftdistributionsare
shownintheleftandbottompanelsrespectively. 15µmobservationsintheHDFN(Ausseletal.1999a,1999b),
HDFS (Oliver et al. 2002) and in the 1452+52 field of the
CFRS(Floresetal.1999).Thesefieldshaveafluxlimitabout
obtained for 90% of the optically identified sample. As dis-
anorderofmagnitudedeeper(S ≃0.1mJy)thantheELAIS-
cussedbyFLF04,duetoadifferentmid-IRfluxlimitcoverage 15
Sfieldsbutwith skycoverages100to500timessmaller. The
oftheELAIS-S1field,thetotalareahasbeendividedintotwo
multiwavelengthfollow-upinthesefieldshasidentifiedasmall
regions: the central and deepest region of S1 (S1-5) reaching
fractionofsourcesasAGN(bothtype-1andtype-2).
mid-IRfluxes(S hereafter)of0.5mJy,andtheouterregion
15
(S1-rest) with a 0.9 mJy flux limit. The S1-5 area is spectro-
scopically complete at the 97% level down to R=21.6, while In the HDFN, 41 15µm sources have been detected over
S1-rest completenessreaches the 98% level down to R=20.5. an area of 21.5arcmin2. The sample is complete downto 0.1
In total 116 sources (29% of the total mid-IR sample) do not mJy (Aussel et al. 1999a).4 of these sources, with fluxes be-
have a spectroscopic identification due to incompleteness of tween0.1and0.45mJy,arefoundtobeAGNbyAlexanderet
thefollow-uportothelackofopticalcounterpartbrighterthan al.(2002),equallydividedbetweenthetwoclasses(Alexander
R=23.Adetaileddescriptionoftheopticalidentification,spec- et al., privatecommunication).In the HDFS we use the AGN
troscopicclassification,size andcompletenessfunctionofthe identified by Franceschini et al. (2003) from a sample of 59
differentareasusedintheELAIS-S1samplearepresentedand ISOCAMsourceswithS15≥0.1mJyselectedoveranareaof
discussedbyFLF04. 19.6 arcmin2. They have detected 2 secure AGN, one type-1
The S2 field is a smaller and deeper area centred at (S15 = 0.288mJy)andonetype-2(S15 = 0.518mJy).Theob-
α(2000) = 05h02m24.5s, δ(2000) = −30o36′00′′ covering ∼ servations in the CFRS 1452+52 field cover an area of ∼100
0.12deg2.Thisregionincludes43sourceswithS/N>5downto arcmin2.41sourceswithS15 ≥0.35mJyandwithS/N≥4have
S =0.4mJy(Pozzietal.2003;Rowan-Robinsonetal.2004). been selected (Flores et al. 1999). The spectroscopic follow-
15
PhotometryinthewholefieldisavailableintheU,B,IandK′ uphasidentifiedonetype-1(S15=0.459mJy)andonetype-2
bands down to 21.0, 24.5, 22.0 and 18.75 respectively (Vega (S15=1.653mJy)source.Intheabovementionedfields,afrac-
magnitudes). 39 infrared sources have a counterpart in the I tionofthetype-2sourceswerenotclassifiedaccordingtotheir
band. 22 of them have a spectroscopic identification, while 8 opticalspectrabutonthebasisoftheshapeoftheirmultiwave-
sources are classified as stars since they are associated with lengthspectralenergydistribution(SED).Forthisreasononly
bright (I<14) point-like sources in the optical catalogue. To theselectedtype-1populationhasbeenusedinouranalysis.
avoid large incompletenessdue to the optical follow-up,only
sourceswithopticalcounterpartsbrighterthanI=20.6werese- Finally,wehavecombinedtheISOCAMdatawiththelocal
lected.Downtothisopticallimit,thesampleconsideredinS2 IRASsampleofAGNfromRMS.TheRMSCatalogueisahigh
isthencompleteatthe95%spectroscopiclevelforsourceswith galactic latitude(|b|≥25o), colourselected (S /S ) and
60µm 12µm
S >0.7mJy. flux limited (S >0.22Jy) sample extractedfromthe IRAS
15 12µm
Classification for AGN dominated sources in the ELAIS 12µmFaintSourceCatalog,Version2(Moshiretal.1991).To
fields was based on their optical spectral signatures. Sources avoid completeness uncertainties in the mid-IR sample, only
showingbroademissionlineprofiles(restframeFWHM>1200 sourceswithS ≥300mJywereselected(seeRMS).Into-
12µm
kms−1) were classified as type-1. Type-2 sources were se- tal 41type-1and50 type-2sourcesarefoundwithinthis flux
lected following classic diagnostic diagrams (e.g. Tresse et range.Thephotometricandspectroscopicfollow-upoftheop-
al. 1996; Osterbrock 1989; Veilleux & Osterbrock 1987) ticalcounterpartsisconsideredtobe∼100%completeforthese
that included one or more of the following line ratios: sourcesandhasbeenusedinouranalysisasrepresentativeof
[NII]/Hα, SII/(Hα+[NII]), OI/Hα, [OIII]/Hβ and [OII]/Hβ themid-IRselectedpopulationofAGNinthelocaluniverse.
4 Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR
Table 1. Mean redshift and Luminosity (±1σ) of the mid-IR Unlike type-1 sources, the mid-IR SED of type-2 objects
samples variesgreatly.Theyrangefromstarburst-likegalaxiesSEDs,as
inthecaseofCircinus(Sturmetal.2000),showingaweakcon-
Sample IRAS ISO tinuum, prominentemission from PAH moleculesand a deep
<z> <logL15 >(a) <z> <logL15> silicateabsorptionfeatureat∼10µm,tomorepower-likeSEDs
Type-1 0.03±0.02 43.87±0.87 1.23±0.71 45.22±0.78 dominatedbyhotdustdirectlyheatedbytheactivenucleus,as
Type-2 0.02±0.01 43.66±0.68 0.28±0.20 43.90±0.69 inNGC1068(Sturmetal.2000).Asaconsequence,themid-
IRSEDofNGC1068andCircinusgalaxies(Fig.2)wereused
(a) 12µmluminositiesfromRMShavebeenconvertedto15µmusingtheadoptedSED.
InparticulartheCircinusSEDwasadoptedfortype-2AGN(see§3.1). asrepresentativeoftwoextremecasesofobscuredAGNinthe
mid-IR. In order to derive the optical k-correction for type-2
sources,anaveragespectrumwasproducedintheopticalband
(3500-7000Å)usingourbesttype-2spectra(Fig.5inFLF04).
InFigure3weshowthedistributionsintheL −zplaneof
15
thetotalsampleusedinouranalysis,whileTable1summarises
themeanredshiftsandmid-IRluminosities,andcorresponding
1σdispersion,measuredforthelocal(IRAS)andhighredshift
(ISO)sources.
Fig.2. Adoptedmid-IRspectralenergydistributions(SEDs)fortype-2sources(top
panel)andthecorrespondingk-correctionsasafunctionofredshiftcomputedforthe
ISOCAM-LW3filter(bottompanel). Fig.3. DistributionintheL15−zspaceofthetotalsampleofmid-IRtype-1and2
AGN.Themid-IRluminosityfortype-2sourceshasbeencomputedapplyingtheCircinus
k-correction(seetext,§3.1).Thedotted,dashedanddot-dashedlinesrepresenttheflux
limitsat0.1,1.0and300mJyrespectively.
3. TheEvolutionofAGN
3.1.TheMethod The adopted shape of the luminosity function (LF) is a
smoothtwo-powerlawoftheform:
In order to compare local sources with a higher redshift se-
lectedpopulationandto probechangeswith cosmictime itis dΦ(L ,z) Φ∗
15 = (1)
necessary to compare their intrinsic physical properties. Our dlogL (L /L∗ )α(z)+(L /L∗ )β
15 15 15 15 15
sample has been selected in the mid-IR and identified in the
opticalband,thereforerest-frameluminosities, in the form of where we have introduced a dependence on z in order to
νL ,at15µm(L )andintheR-band(L )werecomputedus- allowachangeintheslope(α)ofthefaintendoftheluminosity
ν 15 R
ingwellknownSEDsfortype-1and2sources. function.Wehaveassumedaluminosityevolutionoftheform:
The compilation of Elvis et al. (1994) of 47 QSO, in the
range of 1–20µm, was used as representative of the mid-IR L15(z)=(LL1155((0zc)u(t1)+z)kL ffoorrzz>≤zzccuutt (2)
emissionforthetype-1population.ThiscompositeSEDagrees
very well with spectroscopic observations of type-1 sources wherez istheredshiftcutbeyondwhichtheluminosityfunc-
cut
performedbyISOinthemid-IR(Claveletal.2000;Spoonet tionisassumedtoremainconstant.
al. 2002),showinga strongpower-likecontinuum,veryweak Theparametersforthe luminosityfunctionandthe evolu-
or no PAH emission bands and no evidence of the 10µm sil- tion have been derived using an un-binned, maximum likeli-
icate absorptionfeature.Optical luminosities for type-1AGN hood method (Marshall et al. 1983), as described by Matute
were computedusing the R-bandk-correctionby Natali et al. etal.(2002).Thespectro-photometricfollow-upoftheELAIS
(1998),whichtakesintoaccountthelargeeffectofbroadlines southernfieldswasnotdeepenoughtoprovideanidentification
enteringandleavingthepassband. and a classification for all the sources detected in the mid-IR
Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR 5
sample. A factor depending on redshift and mid-IR luminos- form:α(z) = α exp(−α ∗z)+α . Ajustificationforthis
1 2 3
ity was therefore introduced in order to take into account the formisgiveninthefollowingparagraphs.
fractionof objectsnotidentified,dueto the lack ofan optical - Afixedvalueforz equalto2.0inourparameterisationof
cut
counterpartor to incompletenessof the spectroscopic follow- theLF. Ourrelativelysmallnumberofsources,especially
up. We will refer to this factor as the optical−completeness athighz,doesnotallowtosamplecorrectlytheexistenceof
factor,Θ(z,L). It representsthe probabilitythat a source with thisredshiftcutoff.Wehavethereforeassumedabehaviour
agivenredshiftandmid-IRluminosityL hasanapparentR- inthemid-IRsimilartowhatisalreadyknownfortheevo-
15
bandmagnitudewithinthespectroscopiclimitsofthesamples, lutionofAGN in the optical(Boyleet al. 2000,Croomet
and was derived taking into account both the average L /L al.2004)andX–ray(Miyajietal.2000,2001;LaFrancaet
15 R
ratio of the sources and its observed natural spread. For any al.2005;Hasingeretal.2005),suggestingvaluesforz in
cut
given mid-IR luminosity L , this probability has been com- therange1.7-2.5.
15
putedassumingagaussiandistributionoflog(L /L )centred - A universe volume between z=0 and z=4, while the
15 R
atitsmeanvalueandwithasigmaequaltotheobservednatu- space density was integrated in the luminosity interval
ralspreadof the L /L relation(see nextsectionsfordiscus- logL =[42,47].
15 R 15
sionsontheadoptedmeanvaluesandspreadsoflog(L /L ))
15 R
2.Then,thefunctiontobeminimisedcanbewrittenas: In total, 72 (27 ELAIS + 41 RMS + 4 ISO-Deep) type-1
AGNwereusedtoderivetheLF.Thebestsolutionsfoundfor
N
S=−2 ln[Φ(z,L)]+2 Φ(z,L)Ω(z,L)Θ(z,L)dVdzdL (3) each parameter(±1σconfidencelevels) andthe 2D-KSprob-
i i
Xi=1 ZZ dz abilities are presentedin Table 2 in the rows labeled ”A” and
”B”forthefixedandvariablefaintslopemodelsrespectively.
where Ω(z,L) is the effective area coveredby each subsample
InFigure4weshowtheobservedspacedensitydistribution
at 15µm, L=L and the index i runs over all the objects of
15 and the best fit models in two redshift intervals: z = [0,0.2],
a given class. For the ELAIS-S1 sources the area coverageis
where the IRAS sources dominate, and z = [0.2,2.2],mainly
presentedas a functionof flux by FLF04. Area coveragesfor
populatedby ISO sources.The largeintervalat highredshifts
thedeeperISOfields(HDF-N,-SandCFRS)aredescribedin
(z = [0.2,2.2]) was chosen only for representation purposes,
theprevioussection.TheareasampledbyRMSisconsidered
toassureasignificantnumberofobservedsourcesineachlu-
equalto 0forS < 300mJyandequalto22191.80deg2 for
12
minosity bin. In order to correct for evolution within the red-
S ≥300mJy(seeRMSfordetails).TherepresentativeSEDs
12
shift intervals, the observedspace density distribution is plot-
wereusedtoconverttheRMS12µmfluxesinto15µmfluxes.
ted following La Franca et al. (1997). The expected number
Confidenceregionsforeachparameterhavebeenobtained
of sources given by the model in each bin (Nmodel) is com-
bycomputing∆S(=∆χ2)atanumberofvaluesaroundthebest
puted and compared to the observed number of AGN in the
parameter(S ), while allowingthe otherparametersto float
min bin (Ndata). The ratio Ndata/Nmodel for each bin is then multi-
(see Lampton, Margon & Bowyer 1976). A 68% confidence
plied for the value of the LF at the correspondingcentral lu-
region for any single parameter corresponds to ∆S = 1. The
minosity and redshift value of the bin. The plotted error bars
goodness of the fit has been verified using the bidimensional
correspondto the 1σPoisson distributionand were estimated
Kolmogorov-Smirnovtest(2D-KShereafter)intheL –zspace
15
followingGehrels(1986).Spacedensityupperlimitsaregiven
(seePeacock1983;Fasano&Franceschini1987).Thenormal-
wheresourcesareexpectedbythemodelbutnotobserved.The
isationfactorΦ∗ isdeterminedinsuchawaytoreproducethe
1σ dispersion for the LF was computed according to the 1σ
observedtotalnumberofsources(ISO+IRAS).
uncertaintiesofitsparameters,andrepresentedasalight-grey
shadedareainFig.4.
3.2.TheEvolutionoftype-1AGN Besidethenaturalstatisticaluncertaintiesduetothelimited
numberofsourcesusedinouranalysis,ourfittingtechniqueis
For type-1 sources, the opticalcompletenessfactor,Θ(z,L ),
15
affected by possible errors due the assumed average value of
has been estimated using the mean value log(L /L )=0.23
15 R
the L /L ratio used to correct for the spectroscopic incom-
with a 1σ dispersion of 0.25 (from the sample of Elvis et 15 R
pletenessofthesamples.Inorderto estimatetheseuncertain-
al. 1994). These values are in rough agreement with the de-
tieswehaverecomputedtheLFbestfitsolutionsassumingtwo
rivedvaluefortheIRASlocalsub-sampleoftype-1AGNfrom
extremecasesforthelog(L /L )ratio.Inparticular,wehave
Spinoglioetal.(1995;0.05witha1σspreadof0.13).Thisra- 15 R
usedasaveragevalueoflog(L /L ) ourbestestimate(0.23)
tioanditsspreadshowsnosignificantdependenceonL over 15 R
15
plus or minus the observed 1σ dispersion (0.25). Even under
more than 4 decades of mid-IR luminosity and we assumed
these extremeassumptions,the parametersofthe LFresultto
themtobeconstantwithredshift.
be within the errors quoted in Table 2. The dark-greyshaded
In order to find the best fit solutions to the LF we have
areasinbothpanelsofFig.4showthecorresponding1σrange
considered:
of uncertainty introducedin the estimate of the space density
- 2 different behaviours for the faint slope of the LF: a) a oftype-1AGN.Asitisclearlyseeninthefigure,thestatistical
fixedvalue(α=α(0))and,b)adependenceofαonzofthe errorsduetothelimitednumberofsourcesusedinouranalysis
2 Asimilarapproachwasadoptedinthepreviousestimatesofthe dominateovertheuncertaintiesduetotheassumptionsonthe
MIRtype-1AGNLFbyMatuteetal.(2002),andoftheX-rayAGN averagevalueofthelog(L15/LR)ratio.Forexample,theuncer-
LFbyLaFrancaetal.(2002,2005) taintyintroducedintheestimateddensityoftype-1AGNbythe
6 Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR
Table2.ParametervaluesofthefitoftheLuminosityFunction
Model α α α β logL∗ a k z logΦ∗b CIRB(%)c P
1 2 3 15 L cut 2DKS
(I) (II) (III) (IV) (V) (VI) (VII) (VIII) (IX) (X) (XI)
Type-1AGN
A 0.00(fixed) 0.00(fixed) 1.08−+00..1019 2.29−+00..1366 44.45−+00..0088 2.78−+00..1283 2.00 −5.78±0.03 12.1−+27..57(4.5−+12..09) 0.11
B 0.69+0.07 5.92+0.17 0.64+0.07 2.76+0.21 44.78+0.07 2.87+0.27 2.00 −6.26±0.03 4.2+1.1(1.6+0.3) 0.35
−0.08 −0.12 −0.11 −0.26 −0.06 −0.15 −1.0 −0.4
Type-2AGN(NGC1068SED)
C 0.00(fixed) 0.00(fixed) 0.78−+00..1150 2.67−+00..3451 44.15−+00..2210 1.79−+00..5373 ... −4.77±0.03 9.2−+36..31(3.4−+11..62) 0.18
D 0.90+0.12 6.00∗∗ 0.00∗∗ 2.66+0.29 44.11+0.17 2.04+0.70 ... −4.69±0.03 5.5+2.6(2.0+1.0) 0.33
−0.14 −0.25 −0.16 −0.60 −1.5 −0.8
Type-2AGN(CircinusSED)
E 0.00(fixed) 0.00(fixed) 0.76−+00..1143 2.73−+00..3432 44.15−+00..2204 2.50−+00..5505 ... −4.75±0.03 14.6−+58..14(5.4−+13..91) 0.16
F 0.87+0.12 6.00∗∗ 0.00∗∗ 2.68+0.26 44.10+0.13 2.55+0.52 ... −4.66±0.03 7.7+3.0(2.9+1.1) 0.49
−0.14 −0.24 −0.12 −0.54 −2.1 −0.8
aL∗15correspondstoνLνat15µmandisgiveninergs−1.
bThenormalisation,logΦ∗,isgiveninMpc−3mag−1.
cContributionνIνtotheCIRBat15µminunitsof10−11Wm−2sr−1.Inparenthesiswereportthe%oftheCIRBproducedbythefitgivenaCIRBvalueof2.7nWm−2sr−1.
∗∗Thevaluesoftheseparametersreachedthephysicallimitimposedinourminimisationprocess.Theseconstraintswereimposedtothemodelsinordertonotproduceunphysical
solutionsathighredshiftduetothelowstatistics.Therefore,noerrorsarereportedfortheseparameters.
amount (∼1σ), but systematically over 5 luminosity bins, the
numberofthelowluminosity(L (z) < L∗ )andhighredshift
15 15
(z=1.2) type-1 AGN. This is the main reason why a depen-
denceofthefaintslopeontheredshiftwasincluded.Theintro-
ductionofthisdependencetranslatesintoaluminositydepen-
dentluminosityevolution(LDLE) forthe faintpartof the LF
(modelBinTable2).Inthiscase,abetteragreementbetween
themodelandthedataisobtained(Fig.4,toppanel)asshown
bytheincrementofthe2D-KSprobability(0.35vs.0.11).
Themeasuredrateofluminosityevolution(k )resultstobe
L
ratherindependentfromtheadoptedrelationbetweenαandz.
Thebestfitvalues,k ∼2.7−2.9,aresimilar,withintheerrors,
L
to those alreadyfoundforthese objectsin the opticalandX–
ray wavelengths, where the evolution rate is between 2.5 and
3.5(LaFrancaetal.1997;Boyleetal.2000;Miyajietal.2000
&2001;Croometal.2004;LaFrancaetal.2005;Hasingeret
al. 2005). Therefore,according to this analysis, no difference
is found in the evolution of mid-IR selected type-1 AGN in
comparisontotheonesselectedintheopticalandX–rays.
Fig.4. Type-1AGNlocal(z=0.1,continuousline)andhighredshift(z=1.2,dash– The 15µm integral counts and the observed redshift dis-
dottedline)bestfitstothe15µmLuminosityFunctionassumingaredshiftdependentfaint
slope(Fit“B”,toppanel)andaPLEmodel(Fit“A”,bottompanel).Opencirclesrepresent tribution for type-1 AGN can now be comparedwith the pre-
theobservedspacedensityofthelocalpopulation(dominatedmainlybyRMSsources) dictions derived from the best fit models. The results of this
whilefilledcirclesrepresenttheobservedspacedensityofthehighredshiftpopulation
(dominatedbyELAISsources).Poissonianerrorsat1σconfidencelevelareshown.The comparisonare presentedin theupperrightandleftpanelsof
dark-greyshadedareas(quitenarrowandthusbarelyvisible)representtheuncertainty Figure5. The bottom panels show the corresponding relative
introducedbytheassumptionsusedtoestimatetheopticalincompletenessofthesamples,
whilethelight-greyshadedareascorrespondtothe1σerrorsofthebestfitparametersof errorsas derivedfrom the 1σ uncertaintieson the best fit pa-
theLF(see§3.2). rametersoftheLF.
The expected integral counts (Fig. 5, left) given by both
assumedL /L relationis∼10%atL ∼ L∗ ,whilethestatis- models (dashed line for model “A” and continuous line for
15 R 15 15
ticaluncertaintyduetothelimitednumberofsourcesislarger model “B”) provide a good representation of the observed
than 20%. For these reasons, in order to compute the errors data (shadedareas)downto ∼2mJy.Atfluxesbelow∼2 mJy
over the parameters of the LF and any derived quantity (e.g. the variable slope model agrees better with the ELAIS data,
integralcountsandredshiftdistributions),weneglectedtheun- whilethePLEmodelismorerepresentativeofthefainterpop-
certainties related to the assumptions on the average value of ulation in the HDF and CFRS fields. At the faintest fluxes
thelog(L /L )ratio. (S = 0.1 mJy) the variable slope model underpredicts the
15 R 15
We observethatalthoughtheoverallfitbythePLEmodel observedcountsby a factor ∼2. The uncertaintieson the pre-
(model A in Table 2, Fig.4 bottom panel) is in reasonable dictedcountsatthisflux(∼20%)arenotlargeenoughtojustify
agreement with the observed data, it overpredicts by a small theobserveddiscrepancy.
Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR 7
Fig.5.Left)Mid-IRintegralcountsfortype-1AGN.ShadedareasshowtheRMSandELAISobservedcounts,whilesymbols(triangle,staranddiamonds)representtheISO-Deep
surveys.Thethicksolidlinegivesthepredictedcountsfromthevariableslopefitmodel(“B”).Expectationsfromthefixedfaintslopemodel(“A”)areplottedasadashedline.All
errorsintheobserveddistributionsarequotedatthe1σlevelandgivenbyPoissonstatistics.Thebottompanelshowthe1σrelativeerroronthepredictedcountsduetotheuncertainties
onthebestfitparametersoftheLF(seetext).Linetypeshavethesamemeaningasintheabovepanel.Right)Redshiftdistributionfortype-1AGN.Thehistogramrepresentsthetotal
numberofobservedsourcesineachredshiftinterval.ShadedregionsoutlinetheISOCAMcontribution,whiletheopenhistogramaccountsforthecontributionfromIRASsources.
Thelinesarethepredictionsfromthebestfitmodel(“B”):thickdash-dottedforIRAS,thickdashedforISOpopulation,whilethecontinuousthicklinegivestheirsum.Theexpected
distributionsforthe“A”modelareplottedasabovebutusingthinlines.Thebottompanelshowstherelativeerrorsofthepredictionsfortheentiremid-IRpopulation(IRAS+ISO)only.
We also find an overall good agreement between the red- shownthatsucharelationexistsandfoundittobevalidforthe
shift distribution of the observed data and the model predic- wholemid-IRpopulation(excludingtype-1sources)overmore
tions(Fig.5,right).Thelargestdiscrepancybetweenthemod- than4decadesofmid-IRluminosity.Inordertoderivethisre-
els occurs at the lowest redshift bin (z = [0,0.5]) where the lationship, optical identifications from RMS, ELAIS-S fields,
fixedfaintslopemodel(thinlines)overpredictsthenumberof HDFNorth(Ausseletal.1999a,1999b)andSouth(Mannetal.
sourcesobservedinthe ISOCAM fieldsbya factor∼3 (4ob- 2002;Franceschinietal.2003)wereused.Asopposedtotype-
servedvs.∼12expected),whilethevariablefaintslopemodel 1 sources, the ratio (L /L ) has a dependence on L which
15 R 15
providesabetterrepresentationoftheobserveddistribution. canbeexpressedinthelinearform
In summary, although the variable faint slope model pro-
log(L /L )=0.47logL −5.02,
vides a better overall fit to the LF and reproduces better 15 R 15
the z-distribution, it underestimates the faint mid-IR counts
where luminosities are expressed in solar units (see FLF04,
(S <0.5mJy) by a factor of ∼4. As both models (“A” fixed
15 Fig.13).Thedatashowanintrinsic1σdispersionof0.32.This
&“B”variablefaintslope)arestatisticallyacceptable,wecon-
equationimpliesthatL increases∼3timesfasterthanL .The
15 R
cludethatgiventhepresentlyavailablestatisticsthetwomod-
relationworksforbothnearbyanddistantobjects(fromz=0up
elsareequivalentwithintheerrors.
toz=0.7)and,therefore,hasbeenusedtoestimatethefraction
M02 measured for the first time the evolution of type- of sources lost due to the spectroscopic limit of the different
1 AGN based on a preliminary mid-IR catalogue from the surveys,i.e.theΘ(z,L )termusedintheminimisationofthe
15
ELAIS-S1field.Thevaluesfoundhereforthefixedfaintslope
Sfunction(seeeq.3).
LF (model“A”) are in agreementwith what alreadyfoundby The LF was computed in the luminosity range logL =
15
M02 within ∼1σ errors. The only exception is for the bright [42,47]and in the redshiftintervalz = [0,0.7].A total of 75
slope (β),where the herederivedvalue is flatter (2.29+0.36 vs.
−0.16 sources(25fromELAISand50fromRMS) wereusedtode-
2.89+0.29). This difference can be understood if we take into
−0.26 rivetheLFshapeandevolution.BestfitvaluesfortheLFpa-
accountthelargeuncertaintiesinthebrighterluminosityinter-
rameters,aswellasthecorresponding2D-KSprobabilities,can
vals(inbothM02andoursample)duetothesmallnumberof
be foundin Table2 for each of the two representativemid-IR
sources.Whilethe2D-KStestgivesa11%probabilitythatthe
k-correctionsassumed.
oursampleofAGNiswellrepresentedbyanon-evolvingfaint
For type-2 AGN, we followed the same fitting sequence
slope (model“A”), this probabilitydecreases to 5% if we use
as for type-1 AGN. The results for a fixed faint slope (PLE
theparametersvaluesreportedbyM02(Table1inM02).
model)arefoundonTable2onrowslabeled“C”(fortheNGC
1068 k-correction)and “E” (for the Circinusk-correction).In
both cases, the value of the 2D-KS test (P =0.16 & 0.18
3.3.TheEvolutionoftype-2AGN 2DKS
respectively)doesnotrejectthe PLE model,butan important
Asfortype-1AGN,arelationbetweentheintrinsic15µmlu- sourceoverpredictionisagainpresentinthemodelifcompared
minosity(L )andtheopticalluminosity(L )fortype-2AGN to the observed data for the low luminosity and moderate-z
15 R
isrequiredtocorrectfortheopticalincompleteness.FLF04has (z=[0.1,0.6])partofthesample.Abetterfitisfoundbyassum-
8 Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR
Fig.6.ObservedandbestfitLFfortype-2AGNat<z>=0.05(continuousline)and<z>=0.35(dash–dottedline).Thesolutionswitharedshiftdependentfaintslope,assuminga
NGC1068(“D”;leftpanel)andCircinus(“F”;rightpanel)k-corrections,areshown.SymbolsandshadedareasasinFig.4.Duetothesmallredshiftintervalsused,andforthesake
ofclarity,theplotshavebeendividedintotwopanelsforthelow-redshift(top)andintermediate-zsources(bottom).Atintermediate-z,thethinlineshowsthelow-zbestfit.
inganevolvingfaintslopeoftheLFparameterisedasspecified the one measured for type-1 sources (k ∼ 2.8 ± 0.3). On
L
in§3.2.Inthiscase,inordertoavoidunphysicalsolutionsdue the other hand, an evolution rate k ∼ 2.5± 0.5 is obtained
L
to the low statistics at high redshift,the α parameterwas al- if the Circinus SED is assumed, closer to the one measured
2
lowedtovaryintherange[0,6],whiletheα parameterwasal- for type-1 AGN. These differences in k are caused by the
3 L
lowedtovaryintherange[0,2].Indeed,bothparametersfound strongerk-correctionintroducedbytheCircinusSED(Fig.1),
a solution at the edgesof the allowed ranges. The results can which generates higher intrinsic luminosities in the redshift
befoundinTable2andFigure6,fitslabeled“D”and“F”for range [0.1,0.6] compared to those given by the NGC1068 k-
theNGC1068andCircinusk-correctionsrespectively. correction.Atvariance,theimpactoftheassumedSEDonall
Fig.6showstheobservedandpredictedspacedensitydis- theotherfittedparametersissmall(seeTable2&Fig.6).
tribution of the type-2 AGN in two representativeredshiftin- ForeachbestfitLFofthetype-2AGN,thepredictedcounts
tervals, low-z (z = [0,0.1])where the IRASsources dominate andredshiftdistributionswerederivedandcomparedwiththe
and intermediate-z (z = [0.1,0.6]), mainly populated by ISO observeddata.Predictionsat15µmfortheintegralcountsare
sources.Theoverallbestsolutionfoundforthe two represen- given in the left panel of Fig. 7, while the derived redshift
tative SED, NGC1068 (left panel, model “D”) and Circinus distribution is shown in the right panel of the same figure.
(rightpanel,model“F”),areplotted.Theobservedspaceden- In both panels of Fig. 7 a thick-dashed line represents the
sity,errors,upperlimitsandthe1σconfidenceregionsforthe Circinus fixed slope model (“E”), a thick-continuousline the
LF have been computed and plotted as described in §3.2. As Circinus variable slope model (“F”), a thin-dashed line the
observed for type-1 AGN, the errors introduced by the low NGC1068fixedslopemodel(“C”)andathick-continuousline
statistics (light shaded area) are larger than the ones due to theNGC1068variableslopemodel(“D”).
theassumptionsontheL15/LRrelation(darkshadedarea).For A good agreement,within the errors, between the models
example, in this case, while the error introduced by the as- andthedataisobserved.Atvariancewiththesolutionsfound
sumed L15/LR relation in the determination of the density of forthetype-1AGN(§3.2),althoughahigher2D-KSprobabil-
type-2AGN is ∼20%at the breakluminosity(L∗ ) of the LF, ityisobtainedusingavariablefaintslopemodel,nomajordif-
15
the limited statistics introduce, instead, a ∼60% uncertainty. ferencesare observedin the integralcountsdown the faintest
Therefore, also for type-2 AGN, in order to compute the er- ELAIS flux (∼1mJy; logS ∼ 0). The differences between
15
rorsontheLFparametersandonanyderivedquantity(counts the various models only become significant at fainter fluxes
and redshift distributions) the uncertainties introduced by the (S ≤ 1mJy), especially at the fluxesreachedby the Spitzer
15
assumptionsonthevalueoftheL15/LR relationhavebeenne- space observatory (S24 ∼ 0.01mJy; see §6). Indeed, the rel-
glected. ative errors are rather uniform, with mean values around 30-
The rate of evolution k found for type-2 AGN depends 50%. Therefore, the higher observed density of the deep ISO
L
stronglyontheassumedSED.IfanNGC1068k-correctionis data(afactor∼10)isstatisticalsignificant.However,asprevi-
adopted, the rate of evolution is k ∼ 2.0 ± 0.5, lower than ouslymentioned(§2),thehigherdensityofAGNfoundinthese
L
Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR 9
Fig.7.Left)Observedandpredictedintegralcountsfortype-2sourcesusingthethetworepresentativeSED:NGC1068(thin-dashedforthe“C”modelandthin-continuousforthe
“D”model)andCircinus(thick-dashedforthe“E”modelandthick-continuousforthe“F”model).Right)Observedandpredictedtype-2AGNredshiftdistributionforthedifferent
modelsmentionedbefore.Linesasinleftpanel.SymbolsandshadedareainbothpanelsasinFig.5.Errorsreportedinthelowersectionofbothpanelshavebeencomputedand
plottedasdescribedinFig.5.
fieldsispartlyduetothe classificationmethodused,whichis usedintheminimisationprocess.Ifthesuggestedflatteningis
notonlybasedonapureopticalspectralclassification.Thepre- reallyduetoanincompletenesseffect,thenthepossiblymissed
dictedredshiftdistributionsagree,withintheerrors,inthered- objectsareexpectedtofollowanoptical-mid-IRrelationdiffer-
shift interval where the LF was computed (z = [0,0.7]). The entfromtheonemeasuredfortheidentifiedfractionofthehigh
different behaviour observed at z > 0.7 is understood by the redshiftISOsourcesandinthelocaluniversebyIRAS(z<0.1)
differentk-correctionsobtainedfromtheadoptedSED.Thees- andadoptedinourestimateofΘ(z,L ).Inthecaseoftype-1
15
timatederrorsrangefromafactor∼1.3atlowredshiftto∼2.0 AGN,iftheaveragevalueoftheassumedlog(L /L )relation
15 R
at the limiting redshiftused to fit the LF, and increase drasti- isincreasedby2.5timestheobserveddispersionoftherelation
callywhenthepredictionsareextrapolatedathigherredshifts. (i.e.∼0.86insteadof0.23),∼7additionalsourceswouldbeex-
pectedatlow luminosityandhighredshifts,thusavoidingthe
needforaflatteningoftheLF.ThishigherL /L valuecould
15 R
3.4.ThemissingfractionofAGN
correspond,forexample,tosourceswithasignificantlylarger
amount of gas and dust with respect to the local samples of
Intheprevioussectionswehaveseenthataflatteningwithred-
Elvisetal.(1994)andSpinoglioetal.(1995).Fortype-2AGN,
shift of the faint slope of both type-1 and type-2 luminosity
therelationderivedbyLaFrancaetal.(2004)impliesthatfor
functionsisslightlyfavouredbythedata.Thisflatteningwould
thelowluminosity(L < L∗ )andmoderateredshift(z∼0.1–
disappear if ∼7 additional type-1 AGN, in the luminosity- 15 15
0.5) sources, all the possible opticalcounterpartsshould have
redshift range logL =[42,45] and z=[0.2,3.2], and ∼6 addi-
15 been observed. Only the high luminosity (L > L∗ ), higher
tionaltype-2AGNwithinlogL15=[42,44]andz=[0.1,0.7]were 15 15
redshift(z>0.5)sourcesareaffectedbyincompleteness.Also
found.We briefly discussnowtwo possibleoriginsofincom-
in this case a significant higher value of the L /L relation
pleteness for the mid-IR AGN sample and their effects on 15 R
wouldberequiredinordertoavoidtheneedforaflatteningof
the observed LF: i) objects not identified due to the spectro-
theLF.
photometriclimitofcompletenessofthesurveyand,ii)possi-
blemisclassificationoftheobjects. A second source of incompleteness could be caused by a
The spectroscopic incompleteness at faint optical mag- spectroscopicmis-classificationofthesources.Theincreasein
nitudes of the ELAIS sample is represented by the factor themeanredshiftoftheISOpopulationwithrespecttothelo-
Θ(z,L ) (§3.1).The correctionsintroducedthroughthisterm calIRASsample(see Table1) impliesthata largerfractionof
15
are represented in Fig. 8 as a function of redshift and mid- the host galaxy light is collected for a given size of the slit
IR luminosity. The shaded areas in the figure outline the re- used for the optical spectroscopic identifications. The conse-
gion where type-1 (top) and type-2 sources (bottom), fainter quence is a dilution of AGN signatures that affects the mea-
thanthespectro-photometriclimitofthesurveys,areexpected sured equivalentwidths (EW) and line ratios (see e.g. Moran
to be found. A darker area in the plot indicates that a higher etal.2002).Theeffectislessimportantfortype-1sourcesdue
fraction of correction has been applied (i.e. highernumber of tothepresenceofeasilyidentifiedbroademissionlinescaused
lostsources)anditspansfrom∼1%(light-greyarea)to∼30% by a directview of the centralenergysourcewith a low level
(darkest area) of the total sky area available in a given inter- of obscuration. The situation is more complicated for type-2
valofredshiftandluminosity.Thecorrectioncomputedcorre- AGN since their higher degree of obscuration suppresses an
spondsto∼4type-1and∼1type-2sourcesintheL −zinterval importantfractionoftheopticallightfromthenuclei.Thehost
15
10 Matute,LaFranca,Pozzietal.:TheEvolutionofAGNinthemid-IR
Table 3.Parametervaluesofthefitofthemid-IRLuminosity
Functions
Type-1 Type-2 All
α β logL∗ α β logL∗ α β logL∗
15 15 15
RMS(a) 0.0 2.1 42.8 0.0 2.5 43.2 0.3 2.1 43.4
Xu03(a,b) - - - - - - 0.3 1.7 43.4
Here(low-z)(c) 1.3 2.8 44.8 0.9 2.7 44.1 - - -
Here(int-z)(c,d) 0.7 2.8 45.4 0.0 2.7 44.6 - - -
(a)Luminositieshavebeenconvertedto15µmusingtheadoptedSEDs.
(b)FromXuetal.2003.
(c)Bestfitsolutionsfortype-1(model“B”)andtype-2(CircinusSED,model“F”).
(d)Intermediateredshift,z=0.6.
manyas10to25sources,classified intheopticalasstarburst
ornormalgalaxies,mayharbouranAGNthatcancontributeto
theobservedmid-IRflux.Ifthisisthecase,thenumberoftrue
type-2AGNmaybehigherbyafactor∼1.4−2.0thanthatused
inthisanalysisandthiswouldhaveimportantconsequencesfor
theobservedLFanditsevolution.
We conclude that the observed flattening with redshift of
theLFforlowluminosityAGNcanbeexplainedifweassume
thattheincompletenessintheELAISsamplewasnotproperly
modeled,fortwopossiblereasons:(i)theunidentifiedfraction
of sources may have a L /L ratio different from that of the
15 R
identifiedAGNor,(ii)morelikely,arelevantfractionoftype-
2 AGN could have been mis–classified due to dilution of the
opticalnuclearspectrabythehostinggalaxy.Thesesourcesof
incompletenessareprobablyalmostnegligiblefortype-1AGN
(especially the last one), while our measure of the density of
Fig.8.Luminosity–redshiftdistributionofthetype-1(top)andtype-2(bottom)AGN. type-2AGNhasinsteadtobeconsideredalowerlimit.
Shadedareasrepresentsthefractionofincompletenessduetotheopticalspectroscopic
limits.Contoursrangefrom75%(black)to95%(light-gray)ofthetotalarealostand
havea5%stepbetweenthem.Fortype-1AGN∼4missedsourcesareexpectedwithin
z=[0.1,1.2].Mostmissedtype-2AGNareonlyexpectedwithlogL15 >44.5atz>0.7,
aredshiftrangeoutsidethecurrentanalysis(seetext).Only∼1missedtype-2sourceis
expectedtoliewithinz=[0.3,0.5]andL15 = [43.5,45.0].Dottedlinesinbothpanels 4. Comparisonwithpreviousworks
showthreerepresentativefluxlimitsat1,4and300mJy.
4.1.TheIRAS12µmlocalluminosityfunction
galaxywouldthennaturallycontributewithalargerfractionto Alocalluminosityfunction(LLF)fortype-1andtype-2AGN
theobservedopticallightfromthe nuclearsource,sometimes was derived at 12µm by RMS. Their results can be directly
even dominating it. The effect is maximised for the intrinsi- compared to our findings at z=0. The values for the LF pa-
callyfaintAGN,whoseopticalfeatureswillbehighlydiluted rametersreportedbyRMSandthosecomputedbyusaresum-
intothehostgalaxyspectrum.Thiseffectcouldberelevantin marisedinTable3.
producingtheobservedflatteningathighredshiftofthelumi- Two parameters of the LF computed by RMS, the faint
nosityfunction. slope (α) and the luminosity break (L∗ ), differ significantly
15
ObservationswithXMMandChandrahaveuncoveredanot from our solutions for both the type-1 and the type-2 AGN.
negligiblenumberofAGNforwhichtheclassificationgivenby Thedifferenceonthevalueoftheluminositybreakcannotbe
theiropticalspectradoesnotcorrespondtotheX–rayclassifi- explained by the SEDs conversion factors from 12 to 15µm,
cationofthesource(e.g.Fioreetal.2000,2003;Bargeretal. since theyimply a small changein the intrinsic luminosityof
2005).Intheiranalysisofdeep(∼70Kseclong)XMMobserva- thesources(∆logL[12−15µm]∼0.0fortype-1and∼0.1for
tionsonELAISS1-5,LaFrancaetal.(inpreparation)findthat type-2 AGN). These discrepancies can instead be understood
the fraction of misclassified type-1 AGN appearsto be negli- ifwetakeintoaccountthat:(i)thefaintslopewasfixedapri-
gible.Vice-versa,inthecaseoftype-2AGN,theobservations ori in the fitting procedure by RMS; (ii) the strong effect of
withXMMhaverevealedthatabout10-20%oftheISOcounter- evolution in the different luminosity bins was not considered
partsopticallyclassifiedasno-AGN(i.e.normalandstarburst by RMS and, (iii) our best fit is obtained by taking into ac-
galaxies) show X–ray luminosity consistent with AGN activ- countnotonlythelocalIRASsourcesbutalsothehighredshift
ity.Thisfraction,inthecaseofELAIS-S,wouldimplythatas ISOCAMpopulation.