X-rays as dominant excitation mechanism of [Fe ii] and H2 emission lines in active galaxies

6.1 MB·English

by  Oli L. Dors Jr

Mon.Not.R.Astron.Soc.000,1–10(0000) Printed20January2012 (MNLATEXstylefilev2.2)
ii
X-rays as dominant excitation mechanism of [Fe ] and H emission
2
lines in active galaxies
2
Oli L. Dors Jr.1⋆, Rogemar A. Riffel2, Mo´nica V. Cardaci3,4,5, Guillermo F. Ha¨gele3,4,5,
1
0 Aˆngela C. Krabbe1, Enrique Pe´rez-Montero6, Irapuan Rodrigues1
2
1UniversidadedoValedoPara´ıba,Av.ShishimaHifumi,2911,Cep12244-000,Sa˜oJose´dosCampos,SP,Brazil
n
2UniversidadeFederaldeSantaMaria,Av.Roraima,1000,Cep97105-900,SantaMaria,Brazil
a
3ConsejoNacionaldeInvestigacionesCient´ıficasyTe´cnicas(CONICET),Argentina.
J
4FacultaddeCienciasAstrono´micasyGeof´ısicas,UniversidadNacionaldelaLaPlata,PaseodelBosques/n,1900LaPlata,Argentina.
9 5DepartamentodeF´ısicaTeo´rica,C-XI,UniversidadAuto´nomadeMadrid,28049Madrid,Spain.
1 6InstitutodeAstrof´ısicadeAndaluc´ıa(CSIC),POBox3004,18080Granada,Spain

]
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20January2012
C
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h
p ABSTRACT
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o
We investigatethe excitationmechanismsof near-infrared[Feii] andH emissionlines
r 2
t observedinActiveGalacticNuclei(AGNs).Webuiltaphotoionizationmodelgridconsider-
s
ingatwo-componentcontinuum,oneaccountsfortheBigBumpcomponentpeakingat1Ryd
a
[ and anotherrepresentsthe X-ray source that dominatesthe continuumemission at high en-
ergies.Photoionizationmodelsconsideringas ionizingsource a spectralenergydistribution
1
obtainedfromphotometricdataoftheSy2Mrk1066takenfromtheliteraturewereconsid-
v
ered.Results of these modelswere comparedwith a largesample of observationallong-slit
6
and Integralfield Unit (IFU) spectroscopy data of the nuclear regionfor a sample of active
4
objects. We found that the correlation between the observational [Feii]λ1.2570µm/Paβ vs.
0
H λ2.1218µm/Brγiswellreproducedbyourmodelsaswellastherelationshipsthatinvolve
4 2
. theH2emissionlineratiosobservedinthespectroscopicdata.Weconcludethattheheatingby
1
X-raysproducedbyactivenucleicanbeconsideredacommonandveryimportantmechanism
0
ofexcitationof[Feii]andH .
2 2
1 Keywords: galaxies:Seyfert–galaxies:ISM–infrared:galaxies
:
v
i
X
r
a 1 INTRODUCTION 2009; Mu¨llerSa´nchezetal. 2009; RamosAlmeidaetal. 2009;
Rodr´ıguez-Ardilaetal.2005,2004;Daviesetal.2007).
The excitation of the Narrow Line Region of Seyfert (Sy) galax-
The H can be excited by two mechanisms: (i) fluorescent
ies can reveal how radiation and mass outflows from the nu- 2
excitation through absorption of soft-UV photons (912–1108Å)
cleus interact with circumnuclear gas. In particular, near-infrared
in the Lyman and Werner bands, existing both in star-forming
(hereafter near-IR) observations are a powerful tool to investi-
regions and surrounding the Active Galactic Nuclei (AGNs)
gatethisissue,becausetheobscuration–whichcanaffecttheop-
(Black&vanDishoeck 1987) and (ii) collisional excitation due
tical morphology of the emitting gas region– is less important
to the heating of the gas by shocks, the interaction of a radio
at these wavelengths (Mulchaeyetal. 1996; Ferruitetal. 2000).
jet with the interstellar medium (Hollenbach&McKee 1989), or
Relevant emission-linesinthenear-IR include[Feii]λ1.2570µm
by X-ray photons from the central AGN (Maloneyetal. 1996).
and λ1.6440µm, Hi lines such as Paβ, and H at λ1.9576µm,
2 Several studies based on intensity-line ratios (Riffeletal. 2010;
λ2.1218µm, and λ2.3085µm, which can be used to map the
Storchi-Bergmannetal.2009;Rodr´ıguez-Ardilaetal.2005,2004)
gas kinematics and excitation (e.g. Riffel&Storchi-Bergmann
have shown that collisional excitation processes dominate the H
2011b; Riffeletal. 2010). Nevertheless, the dominant excita- 2
emissionsurroundingAGNs.However,whichisthedominantpro-
tion mechanisms of the [Feii] and H emission lines in the
2 cessisanopenquestion.Veilleuxetal.(1997),usingJandK-band
central regions of active galaxies are still unclear and have
spectraofasampleof33Sy2galaxies,foundthatshocksassoci-
been the subject of several recent studies (e.g. Riffeletal.
ated with nuclear outflows are a likely source of both [Feii] and
2010, 2008, 2006; Storchi-Bergmannetal. 2009; Hicksetal.
H emissionratherthancircumnuclearstarbursts,assuggestedby
2
Quillenetal.(1999).
Forthe[Feii]emission,the[Feii]λ1.2570µm/Paβlineratio
⋆ E-mail:[email protected] isgenerallyusedtoinvestigatethemainmechanismofexcitation.
2 Dorset al.
Thevalueofthislineratioiscontrolledbythequotientofthevol- Knopetal.(2001),andRiffeletal.(2006).Thissamplecomprises
umesof partiallyandfullyionizedgasregions, with[Feii] emis- long-slitdataof13Sy1and21Sy2galaxies,alongwith1Quasar.
sion being excitedin thepartiallyionized gas (Mourietal.1990, Theintensitiesofthenear-IR[Feii]andH emissionlinesobserved
2
1993;Rodr´ıguez-Ardilaetal.2005;Riffeletal.2010,2008,2006; intheseobjectswerecompared withourphotoionization models.
Storchi-Bergmannetal. 2009). Such zones in AGNs are created Wealsousedthe[Oiii]λ5007Å/Hβand[Oi]λ6300Å/Hαlinein-
by X-ray emission (e.g. Simpsonetal. 1996) and/or shock heat- tensityratiosofabout600000emission-linegalaxieslistedinthe
ing of the gas by mass outflows from the nuclei which interact MPA/JHUDatacatalogueoftheSloanDigitalSkySurveyDR7re-
with the ambient clouds (e.g. Forbes&Ward 1993). This prob- lease(availableathttp://www.mpa-garching.mpg.de/SDSS/DR7/).
lemwasaddressedbyMourietal.(2000),whocomparedtheval-
uesofthelineratios[Feii]λ1.2570µm/Paβand[Oi]λ6300Å/Hβ
predictedbymodels,consideringphotoionizationandshockheat- 2.2 IFUdata
ing, with those observed in a sample of AGNs and Starburst
For this study we selected two Sy1 galaxies, Mrk1157 and
galaxies. These authors pointed out that in AGNs, X-ray heat-
NGC4151, and two Sy2galaxies (ESO428-G14 and Mrk1066).
ing is the most important [Feii] excitation mechanism. However,
AllofthemwerepreviouslyobservedbyourgroupusingtheIFU
Rodr´ıguez-Ardilaetal. (2004), using near-IR spectroscopy of a
spectrographs of the Gemini telescopes. We selected these ob-
sample of galaxies obtained with the Infrared Telescope Facility,
jectsbecausebothJ-andK-bandspectroscopicdataareavailable.
found that X-ray excitation is enough to explain the H emis-
2 The observations of Mrk1066, Mrk1157, and NGC4151 were
sion and part of the [Feii] emission observed in Sy1 galaxies,
performed using the Near-IR Integral field Spectrograph (NIFS;
but fails to explain the emission of these elements in Sy2. For
McGregoretal.2003)onGeminiNorth,whileESO428-G14was
these objects, a combination of shocks and circumnuclear star-
observed withtheGemini Near Infra-RedSpectrograph (GNIRS;
formation is required to explain these emissions. Moreover, it is
Eliasetal.1998)onGeminiSouth.
notclearwhetherthe[Feii]andH areexcitedbythesamemech-
2
anism.Rodr´ıguez-Ardilaetal.(2004)foundacorrelationbetween
the[Feii]λ1.2570µm/PaβandtheH λ2.1218µm/Brγratios,indi-
2
3 PHOTOIONIZATIONMODEL
catingthatbothsetsoflinesmaybeoriginatedbyasingledominant
mechanism. However, high spatial resolution spectroscopy data Toanalysethe[Feii]andH excitationmechanisms,webuiltagrid
2
fromIntegralFieldUnit(IFU)ofactivegalaxiesindicatethatthe ofmodelsusingthephotoionizationcodeCloudy/08(Ferlandetal.
H2 andthe[Feii]emittinggashavedistinctfluxdistributionsand 1998),andthenwecomparedthelineintensityratiospredictedby
kinematics,withtheformerbeingconsideredatracerofthefeed- themwiththoseobserved.Thespectralenergydistribution(SED)
ingoftheAGNandthelatteratracerofitsfeedback(Riffeletal. oftheionizingsourceusedasinputfortheCloudycodewasatwo-
2010,2009,2008;Storchi-Bergmannetal.2009;Hicksetal.2009; component continuumrangingfrom∼ 1015Hzto∼ 1021Hz.The
Mu¨llerSa´nchezetal.2009).Thisresultindicatesthatthelinesof shapeofthisSEDissimilartotheoneobservedintypicalAGNs
these elements are formed in distinct regions. Although several for that range. The first is the Big Bump component peaking at
workshaveinvestigatedtheexcitationoriginsofH2andthe[Feii], 1Ryd with a high-energy and an infrared exponential cutoff and
itisstillunknownwhetheracommonmechanismcanexcitethese thesecondonerepresentstheX-raysourcethatdominatesathigh
elements. Fortunately, a large number of near-IR data of AGNs energiesandischaracterized byapower lawwithanindexα =
x
arecurrentlyavailableintheliterature,whichenablesanextensive −1.Itsnormalizationwascomputedtoproducetherequiredvalue
comparisonwithmodelsyieldingamorereliableconclusionabout oftheopticaltoX-rayspectralindexα .Thisindexdescribesthe
ox
thelikelydominantexcitationmechanismoftheseemissionlines. continuumbetween2keVand2500Å (Zamoranietal.1981).We
In this paper, we combined near-IR data of Sy galaxies ob- assumedthedefaultvalueoftheCloudycodeα =−1.4,because
ox
tained with IFU and long-slit spectroscopy with photoionization thatisabouttheaverageoftheobservedvalues,whicharebetween
modelstoinvestigatetheoriginoftheH2and[Feii].InSection 2, -1.0and-2.0,fortheentirerangeofobservedluminositiesofAGNs
wedescribetheobservationaldatausedintheanalysis.Themod- (Milleretal.2011;Zamoranietal.1981).
ellingproceduresarepresentedinSect.3.InSect.4,thediagnostic The cosmic ray emission was considered inthe models as a
diagrams used to compare the observational data withour model secondionizingsource.Cosmicraysheattheionizedgasandpro-
predictionsaredescribed.Resultsanddiscussionarepresentedin duce secondary ionizations in the neutral gas, which mostly in-
Sects.5and6,respectively.Aconclusionoftheoutcomeisgiven creasetheintensitiesoftheH emissionlines.Weassumedavalue
2
inSect.7. oftheH ionizationrateof10−15s−1,whichisaboutthesamerate
2
foundbyMcCalletal.(2003)foragalacticlineofsight.Itisworth
notingthatthevalueofthecosmicrayratemustbeestimatedob-
2 OBSERVATIONALDATA jectbyobject.Forexample,Suchkovetal.(1993)foundforM82
acosmicrayrateseveraltimeslargerthantheoneintheMilkWay.
Wecompiledfromtheliteratureobservational dataofthenuclear
GammarayobservationsofthestarburstNGC253byAceroetal.
regionofactivegalaxiesinthenear-IRandopticalspectralrange
(2009)indicateacosmicrayratethreeordersofmagnitudelarger
obtained with long-slit and IFU spectroscopy. The selection cri-
thanthatfortheMilkyWay.Also,moleculardataofstarforming
terion was the presence of bright infrared emission lines in their
galaxies,suchasArp220,showevidenceforextremelyhighcos-
spectra.Thesedataaredescribedbelow.
micrayratesyieldedbytheUVemissionfromsupernovaremnants
(Meijerinketal.2011).
We computed a sequence of models assuming an electron
2.1 Long-slitdata
density N = 104 cm−3, ionization parameter U in the range
e
Near-IRemissionlineintensityratiosof35activegalaxieswereob- −4.0 6 logU 6 −1.0 defined as U = Q /4πR2nc, where Q
ion S ion
tainedfromRodr´ıguez-Ardilaetal.(2004),Reunanenetal.(2002), isthenumberofhydrogenionizingphotonsemittedpersecondby
.... 3
Table1.Fe/OandO/Hgasphaseabundancesassumedinthemodels.
Metallicity(Z/Z⊙) 12+log(O/H) log(Fe/O)
2 9.0 -1.47(a1) -1.94(a2) -2.24(a3)
1 8.69 -1.77(b1) -2.24(b2) -2.77(b3)
0.5 8.38 -2.15(c1) -2.54(c2) -2.76(c3)
the ionizing source, R is the Stro¨mgren radius (in cm), n is the
S
particle density (in cm−3), and c is the speed of light. The cho-
senrangeofthesevaluesforUistypicalofnarrow-lineregionsof
Sy galaxies (e.g. Ferland&Netzer 1983). The H emission lines
2
are very dependent on the electron density value assumed in the
models. For example, when N varies from 103 to 105 cm−3, the
e
logarithm of the H λ2.1218µm/Brγ emission line intensity ratio
2
spanabout2.6dex.ThevalueN =104cm−3,assumedinourmod-
e
els,isameanvaluefromthoseconsideredbyMourietal.(2000).
Weconsideredinourmodelsthreevaluesof12+log(O/H)=8.38,
8.69,and9.00,whichcorrespond tovaluesofthemetallicity0.5,
1, and 2 times the solar value published by AllendePrietoetal.
(2001).Theabundancesofothermetalsinthenebulawerescaled
linearly to the solar metal composition through the comparison Figure1.Diagnostic diagram showingtheobservational datatakenfrom
of the oxygen abundances, with the exception of the N and Fe theliterature(seeSect.2)andresultsfromthegridofphotoionizationmod-
els(seeSect.3).Solidlinesconnectcurvesofiso-Z,whiledottedlinescon-
abundances. Thenitrogen abundance was taken from therelation
nectcurvesofiso-U.Thevalues oflogU andZ areindicated. Thethree
log(N/O)=log(0.034+120O/H)ofVila-Costas&Edmunds(1993).
differentlinesforeachZcorrespondtothedifferentassumedvaluesofthe
TheFe/OabundanceratiohasalargescatterforafixedO/Hvalue
Fe/Oasindicatedbythelabels(seeTable1).Circles,squares,andstarrep-
(Izotovetal. 2006) and its value is uncertain because the Fe and
resentSy1,Sy2,andquasar data, respectively. Thetypical errorbar(not
O abundances in grains are poorly known (Peimbert&Peimbert shown)oftheemissionlineratiosisabout10%.
2010).Thus,wevariedtheFe/Oabundanceratiobyabout0.7dex
oneachmetallicity.
The presence of internal dust was considered and the grain H2λ2.247µm/λ2.121µm (Fig. 2) — Mouri (1994) proposed
abundances (vanHoof et al.2001) werealsolinearlyscaled with these diagrams to separate gas emission yielded by shocks from
the oxygen abundance. To take into account the depletion of re- emission caused by fluorescence. The drawback in using the
fractoryelementsontodustgrains,theabundancesofMg,Al,Ca, H2λ1.957µm/λ2.121µm ratio is that the H2λ1.957µm may be
Ni,andNawerereducedbyafactorof10,andSibyafactorof2 affected by telluric bands of H2O and CO2, or blended with the
relativetoadopted abundances ineachmodel inaccordance with [Siiv]λ1.963µmemissionline(Rodr´ıguez-Ardilaetal.2005).
Garnettetal.(1995).InTable1,theO/HandFe/Oabundanceval- • [Oiii]λ5007Å/Hβ vs.[Oi]λ6300Å/Hα(Fig.3)—Thisdi-
uesofthegasphaseassumedinthemodelsareshown.Themodel agramwassuggested byBaldwinetal.(1981)toseparateobjects
oftheH moleculedescribedbyShawetal.(2005)andthemodel accordingtotheirprimaryexcitationmechanisms,i.e.(a)photoion-
2
of the Fe+ ion described by Verneretal. (1999), which consider ization by stars, (b) photoionization by a power law continuum
371 energy levels, were assumed in our computations. The outer source or (c) shock heating. In particular, the [Oi]λ6300Å/Hα
radius of the modelled nebula is that where the temperature falls lineratioisgreatlyincreasedbythepresenceofshockinggas,even
below1000K. whenithaslowvelocities(e.g.Allenetal.2008).
4 DIAGNOSTICDIAGRAMS 5 RESULTS
We used four diagnostic diagrams containing predicted and ob- 5.1 Integratedspectra
servedemissionlineratiosofthe[Feii],H ,[Oiii],and[Oi]which
2
In Figs. 1 and 2 we show the first three diagnostic diagrams de-
aredescribedbelow.
scribedabovecontainingtheresultsofourgridofphotoionization
• [Feii]λ1.2570µm/Paβ vs. H λ2.1218µm/Brγ (Fig. 1) modelsandthedatasample.Sy1,Sy2andquasararerepresented
2
— Diagnostic diagram suggested by Larkinetal. (1998) and bydifferentsymbols.FortheIFUdata,theemissionlineratiosrep-
Rodr´ıguez-Ardilaetal. (2004) to separate galaxies accord- resented in these Figs. were estimated by integrating the spaxels
ing to their level of nuclear activity. Recently, Riffeletal. insideacentralapertureof0.5′′×0.5′′foreachgalaxy,withexcep-
(2010) constructed this diagram with spatially resolved tion of ESO428-G14 for which an aperture of 0.75′′×0.75′′ was
IFU data of an AGN. Typical values for the nucleus of considered.ThesevaluesarepresentedinTable2.
Sy galaxies are 0.6 .[Feii]λ1.2570µm/Paβ. 2.0 and InFig. 1, wecan see that almost all theobservational ratios
0.6 .H λ2.1218µm/Brγ. 2.0 (Rodr´ıguez-Ardilaetal. 2005). arewithintheparameterspacedefinedbyourgridofphotoioniza-
2
This[Feii]/PaβisverydependentontheFe/Oabundancewhilethe tionmodels. Alowermetallicitythanthoseassumed inour mod-
H emissionlinesaredependentontheionizationparameter. els is required to reproduce the data of the galaxies out of the
2
• H λ1.957µm/λ2.121µm and H λ2.033µm/λ2.223µm vs. grid.Theobservedcorrelationbetween[Feii]λ1.2570µm/Paβand
2 2
4 Dorset al.
Figure3.[Oiiiλ5007Å/Hβvs.[Oiλ6300Å/Hαdiagnosticdiagram.The
yellowlineseparatesobjectsionizedbymassivestarsfromthosecontaining
activenucleus(Kewleyetal.2001).Blue,greenandredsolidlinesareasin
Fig.1.Pointsrepresentemission-linegalaxieslistedintheMPA/JHUData
Figure 2. As in Fig. 1 for H2 emission lines. The arrow indi- catalogueoftheSloanDigitalSkySurveyDR7release(seeSect.2).
cates the direction in which the ionization parameter increases. Circles,
squares, and star represent Sy1, Sy2, and quasar data, respectively. The
hatchedarearepresents theregionoccupied byshockmodelresultsfrom appeartohavetheX-raysasmainionizingsourcewhileforthere-
Hollenbach&McKee(1989). mainingonesacompositeionizationbyX-raysandshockcanbe
considered.
Fig.3showsthe[Oiii]λ5007Å/Hβvs.[Oiλ6300Å/Hαdiag-
Table2.IntegratedlineratiointensitiesofIFUdata nosticdiagram.InthisFig.wecanseethattheobservationaldataof
AGNsarewelldescribebyourmodels.Ifourmodelsusethelower
values of the ionization parameter (logU < −3.5; these models
Object [Feii]λ1.2570µm/Paβ H2λ2.1218µm/Brγ arenotshown),wecanextendtheparameterspacetoincludethe
ESO428-G14 0.75 1.10
objectsthathavevaluesofthelogarithmofthe[Oiii]λ5007Å/Hβ
Mrk1066 0.52 0.96
ratiolowerthanzero.Asinthecaseofthe[Feii]λ1.2570µm/Paβ
Mrk1157 0.73 2.24
andH λ2.1218µm/Brγdiagnosticdiagram(Fig.1),theparameter
NGC4151 0.45 0.26 2
spaceofthemodelswithsolarmetallicityalmostcontainsthatof
theZ = 0.5Z models.
⊙
H λ2.1218µm/Brγisexplainedbyanincreaseinmetallicityand
2
ionizationparameter.Noteworthythattheparameterspacedefined
5.2 IFUdata
by the models built using Z = 0.5Z isalmost completely con-
⊙
tainedintheonedefinedbythemodelsbuiltusingthesolarmetal- Weplotthe[Feii]λ1.2570µm/PaβandH λ2.1218µm/Brγ diag-
2
licity. nosticdiagramforeachspaxelofourfourobjectswithourmodel
In the case of the diagnostic diagrams that only involve H results (see upper panels of Figs. 4 and 5). In these Figs., the
2
line ratios (Fig. 2), the photoionization models are slightly de- spaxel data are separated by their ionization mechanism accord-
pendent on the assumed metallicities, covering almost the same ing to the place in the diagnostic diagram, with different colours
parameter space, and strongly dependent on variations in the for each mechanism. The different ionization mechanism zones
ionization parameter. Taking into account the observational er- are delimited in the Figs. by dashed-lines, following the work of
ror bars, our models are in good agreement with the observed Rodr´ıguez-Ardilaetal.(2004).Thespaxelsshowingtypicalvalues
H λ2.033µm/λ2.223µmandH λ2.247µm/λ2.121µmratios(up- ofstarbursts,Seyferts,andlow-ionizationnuclearemission-linere-
2 2
perpanel).Ontheotherhand,inthelowerpanelofFig.2canbe gions(LINERs)arerepresentedbygreenopencircles,blackfilled
noticedlargerdispersionoftheobservationaldatawhichisnotwell circles, and redcrosses, respectively. Withthe samecolour code,
reproduce bythemodels.Thisdispersioncouldbetheresultofa weshowthespatialpositionofeachspaxelintheIFUfieldofview
contaminationofthemeasurementsoftheH λ1.957µmemission (seelowerpanelsofFigs.4and5).Ourmodelscompletelyrepre-
2
line intensities due to a blend with the [Siiv]λ1.963µm line (as senttheregionoccupiedbySeyfertandLINERsdata.
explained above). Therefore, the predicted H λ1.957µm intensi-
2
tiesaresomewhatlowerthantheobservedones.InFig.2,wealso
show the area occupied by the theoretical intensities of the line
6 DISCUSSION
ratio H λ2.247µm/λ2.121µm from shock models performed by
2
Hollenbach&McKee (1989). These authors computed emission- The excitation mechanism of the near-IR emission lines of the
line spectra of steady interstellar shocks in molecular gas con- [Feii] and H in active galaxies have been the subject of sev-
2
sidering velocities from 30 to 150 km/s and particle densities of eral works. For example, Mourietal. (2000) compared results of
103−106cm−3.Wecanseethatmostoftheobjectsofoursample models considering photoionization by X-rays and shock heating
.... 5
Figure4.Toppanels:[Feii]λ1.2570µm/Paβvs.H2λ2.1212µm/Brγline-ratiodiagnosticdiagramforMrk1157(left)andESO428-G14(right).Thedashed
linesdelimitregionswithratiostypicalofStarbursts(greenopencircles),Seyferts(blackfilledcircles)andLINERs(redcrosses).Blue,greenandredsolid
linesareasinFig.1.Bottompanels:spatialpositionofeachspaxelintheIFUfieldofviewfromthediagnosticdiagram.
with observational data of AGNs and starburst galaxies. These For our models, we assumed an incident continuum whose
authors built their models considering large ranges in shock ve- shapeisgivenbytwocomponents,aBigBumpandanX-raypower
locities, gas density, metallicity, and different ionizing continua. law, varying the Fe/O abundance. With these models that do not
Mouri and collaborators showed that the [Feii] emission is en- consider shock heating, we are able to explain the observational
hanced when a partially ionized zone is produced by photoion- data.Nevertheless,wedonotexcludesomecontributionbyshock
ization by X-rays (described by a power-law) and shock heating. heating to the [Feii] emission. Comparing our models with the
These two processes can be discriminated by the electron tem- SDSSDR7emission-linegalaxies(Fig.3),weareabletodescribe
perature of the [Feii] region: 8000K in heating by X-rays and the[Oi]/HαlineratioobservedinAGNs.Thisdiagramcannotbe
6000 K inshock heating. Comparing the electron temperature of usedtodiscriminate[Feii]excitationmechanism,nevertheless,we
the [Feii] region estimated by Thompson (1995) for NGC4151 musttakeintoaccountthatthe[Oi]/Hαlineratioisshocksensitive.
(8000 < T < 12000K)withtheirmodels,Mourietal.(2000) Hence,althoughshockcontributionintheionizationofFecannot
e
showed that, at least for this galaxy, it indicates that X-rays are beruledout,modelsconsideringacontinuumdescribedbyaBig
themoreimportantmechanismtoyieldthe[Feii]flux.Theseau- BumpandanX-raypowerlawastheionizationsourcecanalsore-
thorsarrivedtothesameconclusionusingthe[Oi]λ6300Å/Hαvs. producethe[Feii]emissionlinesaswellasthebehaviourofshock
[Feii]λ1.2570µm]/Paβ diagnostic diagram. A similar result was sensitiveemissionlinessuchas[Oi]λ6300Å.AnalysingIFUob-
also obtained by Jackson&Beswick (2007) by analysing J-band servationsoftheSygalaxyNGC4151,Turneretal.(2002)found
spectraofthreeSy2galaxies. thatthe[Feii]emissionmainlyarisesinthevisiblenarrow-linere-
6 Dorset al.
Figure5.AsinFig.4butforMrk1066(left)andNGC4151(right).
gioninwhichthedominantexcitationmechanismisthephotoion- wherehigh-velocitygasmotionsheatandacceleratethismolecule;
izationbycollimatedX-rayemissionfromthenucleus.Olivaetal. and (3) X-ray illumination, where hard X-ray photons penetrate
(2001)pointedoutthatinregionswhereshocksarethedominant deep into molecular clouds, heating large amounts of molecular
mechanismtheiron-basedgrainsaredestroyedbutthephosphorus gasresultingintheH emission(seeRodr´ıguez-Ardilaetal.2004,
2
is not, yielding a larger [Feii]λ1.2570µm/[Pii]λ1.188µm line- and references therein). Rodr´ıguez-Ardila and collaborators used
ratiointensity thanthat observed in theregions dominated by X- thediagramsshowninFig.2tocompareobservationaldataof22
ray.Inordertoverifythis,inFig.6weshowahistogramcontaining objectswithmodelsconsideringathermalemission,anon-thermal
thisobservedlineintensityratiofor17Seyfertgalaxies,5Sy1and UVexcitation,athermalUVexcitation,andamixtureofthermal
12Sy2,takenfromJackson&Beswick(2007),Riffeletal.(2006), and low-density fluorescence. These authors found that for 4 ob-
and Olivaetal. (2001). It can be seen that the [Feii]/[Pii] for Sy jectstheexcitationmechanismisclearlythermal,whileforthere-
galaxiesrangesfrom1.5to6(withameanvalueof2.7).Themean maining objects a mixing with a non-thermal process cannot be
value of this ratio is about 20 for SNRs, which indicate that the discarded,eventhoughtheresultspointouttoadominantthermal
emittinggashasrecentlypassedthroughafastshock(Olivaetal. mechanism.
2001).Therefore,theseresultsconfirmthatshockshavelittleinflu- ToanalysetherelativeweightoftheX-rayemissionwithre-
enceonthe[Feii]emission. specttotheothermodelcomponents(mainlywithfluorescenceand
RegardingtheH ,thismoleculecanbeexcitedviathreedis- UVphotons),notonlyfortheH emissionbutalsoforthe[Feii],
2 2
tinct mechanisms: (1) UVfluorescence, where photons withλ > wemademodelsfixingallparameterswiththeexceptionoftheα
ox
912ÅareabsorbedbytheH moleculeandthenre-emitted,result- value (see Fig. 7), which is related to the X-ray power law nor-
2
inginthepopulationofvariousvibro-rotationallevels,(2)shocks, malization(seeSection§3).WeassumedZ=Z andlogU=−2.5
⊙
.... 7
Sy 1
Sy 2
4
3
N
2
1
0
0 1 2 3 4 5 6
[FeII]1.257µm/[PII]1.88µm
Figure6. Histogramshowingthe[Feii]/[Pii]emissionlineintensityratios
ofasampleofobjectscollectedfromtheliterature.
sincethemodelsbuiltusingthesolarmetallicityandthisvalueof
theionizationparametercoveralmostalltheparameterspaceoccu- Figure 7. Model results using solar metallicity, Ne=104cm−3, logU=-
piedbytheobservationaldata(seeFig.1).Takingintoaccountthe 2.5andvaryingonlytheαoxparametertoseetheinfluenceoftheX-rayson
α definition(Tananbaumetal.1979),whichfixestheBigBump the[Feii]λ1.2570µm/PaβandH2λ2.1218µm/Brγemissionlineratios.To
ox
delimittheregionoccupiedbytheAGNswefollowRodr´ıguez-Ardilaetal.
parameters, adecrement oftheα valueimpliesthat theamount
ox (2004).
oftheX-raysemittedbythesourcedecreases.InFig.7wecansee
thatourmodelswithα = −1.4reproducewelltheobservational
ox
AGNdata.Nevertheless,whenweuselowervaluesofthisparame-
13.6 eV 1 keV 70 keV
ter,theratiospredictedbythemodelsgooutoftheregiontypically
occupied bytheAGNs(Rodr´ıguez-Ardilaetal.2004).Therefore,
ourmodelsfavourthescenariosuggestedbyMaloneyetal.(1996), 16
wemheitrteedthaetXH-2ramyowleacvuelleenegmthisssfiroonmisthmeaciennltyragloAvGerNne.dThbiysaplhsootocanns -2m]
c
be inferred from the dependence of the H2 emission lines on the -1g s 14
ionizationparameterU. er
To verify if shock models can fit the observational data, we F) [ν
compared shock model results by Hollenbach&McKee (1989) ν
g( 12
with our sample (see Fig. 2). Only few observational points are o
l
located in the area occupied by these shock models and, even in
thesecases,modelsconsideringX-raysalsodescribethedata.
On the other hand, varying in our models the H ioniza- 10
2
tion rate by cosmic rays by a factor of 200, we found that the
H2λ2.1218µm/Brγ line ratio only increases by about 0.15dex, 8 10 12 log(ν1)4 [Hz] 16 18
whichshowsthattheadditionalionizationbycosmicrayshaslittle
influenceontheH emissionlines. Figure 8. Spectral energy distribution at the Schwarzschild radio
2
A simple scenario where both [Feii] and H emissions are (10−5pc) of the Sy1 galaxy Mrk1066 used as the photoionization
2
mainlyduetotheX-raycontinuumcomingfromtheactivenucleus source for some models of this galaxy. We assumed a galaxy dis-
tance of 50Mpc (Mouldetal. 2000). The photometric data were taken
hasalsobeenproposedbyotherauthors.Forexample,Blietzetal.
fromDressel&Condon(1978),Moshiretal.(1990),deVaucouleursetal.
(1994) and Knopetal. (2001) showed that X-rays from the nu-
(1991), Beckeretal. (1991), Douglasetal. (1996), Condonetal. (2002),
cleus can heat the gas located in the narrow line region driving
Skrutskieetal. (2003), Braatzetal. (2004), Guainazzietal. (2005),
the [Feii] and H emission. Because 98% of the iron is tied up
2 Mun˜oz-Mar´ınetal.(2007),andCardamoneetal.(2007).
industgrains,thisprocessmustfreetheironthroughdustdestruc-
tionandyetnotdestroytheH molecules(Rodr´ıguez-Ardilaetal.
2
2004). These authors computed the emergent [Feii]λ1.2570 µm Longinottietal. 2007; Bianchietal. 2009; Krongoldetal. 2009;
andH λ2.1218µmfluxusingtheX-raymodelsbyMaloneyetal. Cardacietal. 2011; Corraletal. 2011, and references therein),
2
(1996) and compared their predictions withobservational dataof provide information about the continuum shape and the particu-
seven objects. They found that X-ray heating can only explain lar spectral features of the AGNs in this wavelength range. For
a fraction of the [Feii] and H emission, and they stated that Mrk1066,wecomparedtheresultsobtainedusingthissimplesce-
2
the discrepancy found can be alleviated if the emitting gas is lo- nario that only involves a continuum modelled by a Big Bump
cated closer than the distance adopted in their models. The X- and an X-ray power law with those obtained using its intrinsic
ray data, provided by the XMM-Newton and Chandra space tele- SED.WebuilttheobservationalSEDtakingthephotometricdata
scopes,andtheirdetailedanalysis(seee.g. Piconcellietal.2005; from the NASA/IPAC Extragalactic Database (NED), following
8 Dorset al.
by Riffeletal. (2006, 2008, 2010) and Storchi-Bergmannetal.
(2009), respectively. However, these authors did not reach con-
clusiveresults.Forexample,Riffeletal.(2006)suggestedthatthe
[Feii]excitationinESO428-G14ismainlyduetoshocks.Never-
theless, the detailed analysis performed in the present work con-
fronting our models with the IFU data shows that X-rays are a
morereliabledominant excitationmechanism even inthecaseof
ESO428-G14.
7 CONCLUSIONS
In this work we show that a photoionization model grid built by
adopting a continuum source characterized by two components,
oneaccountingfortheBigBumpcomponentpeakingat1Rydand
theotherdescribingtheX-raysemission,isabletoreproducethe
[Feii]andH infraredemissionlinesofasampleofAGNs.Testing
2
theinfluenceoftheX-raysontheintensityoftheseemissionlines,
wefoundthatadecrement intheX-raycontentofthecontinuum
source translatesinto aweakening of these lines, and themodels
areno longer compatiblewiththeobservations. Thisimpliesthat
the heating by the X-ray emission from the active nuclei can be
Figure9.ComparisonbetweenthegridmodelresultsshowninFig.1(solid
consideredasthemostimportantmechanismofexcitationforthe
lines)andthemodelsbuiltconsideringthesemi-empiricalSEDofMrk1066
IRemissionlinesoftheseelements.
showninFig.8(dashedlines).
Cardacietal.(2009).ToenhancethenumberofpointsoftheSED ACKNOWLEDGMENTS
asneeded by Cloudy, weperformed alinear interpolation among
Wearegratefultothereferee,NealJackson, forathoroughread-
thesemi-empiricalpoints(seeFig.8).Webuiltanewgridofpho-
ingofthemanuscriptandforsuggestionsthatgreatlyimprovedits
toionization models under the same assumptions of abundances,
clarity.
ionization parameters and density, but only for one value of the
Based on observations obtained at the Gemini Observatory,
Fe/Oratioforeachmetallicity.InFig.9thepredictionsofourmod-
whichisoperatedbytheAssociationofUniversitiesforResearch
elsusingtheSEDofMrk1066andthemodelresultspresentedin
inAstronomy, Inc., under a cooperative agreement withthe NSF
Fig.1assumingthesameFe/Oabundance astheMrk1066 mod-
onbehalfoftheGeminipartnership:theNationalScienceFounda-
els are shown. The model results derived using the two different
tion(UnitedStates),theScienceandTechnologyFacilitiesCoun-
ionizingsourcesaremostlyinagreement.
cil (United Kingdom), the National Research Council (Canada),
The semi-empirical SED of Mrk1066 includes not only the
CONICYT (Chile), the Australian Research Council (Australia),
rangecoveredbytheCloudymodelbutalsotheradioandIRwave-
Ministe´rio da Cieˆncia e Tecnologia (Brazil) and south-eastCYT
lengths. Hence, the agreement between solid and dashed lines in (Argentina). This research has made use of the NASA/IPAC Ex-
Fig. 9 only indicates that the assumed multicomponent model is
tragalactic Database (NED) which is operated by the Jet Propul-
a good representation of the AGN continuum when studying the
sionLaboratory,CaliforniaInstituteofTechnology,undercontract
[Feii]andH emission.
2 with the National Aeronautics and Space Administration. OLD
Recent resolved integral field spectroscopy of the central
and ACK are grateful to the FAPESP for support under grant
region of active galaxies shows that the ionized (in particu- 2009/14787-7 and 2010/01490-3. MC and GH aregrateful tothe
lar the [Feii] emitting gas) and the molecular (traced by the SpanishMinisteriodeCienciaeInnovacio´nforsupportundergrant
H emission) gas have distinct flux distributions and kinemat-
2 AYA2010-21887-C04-03, and the Comunidad de Madrid under
ics. The molecular component is more restricted to the plane of grantS2009/ESP-1496(ASTROMADRID).EPMisgratefultothe
the galaxies and the ionized one extends to high latitudes above SpanishMinisteriodeCienciaeInnovacio´nforsupportundergrant
it, which is in most cases co-spatially with the radio jet (e.g.,
AYA2010-21887-C04-02, andtheJuntadeAndaluc´ıaundergrant
Riffeletal. 2006, 2008, 2009, 2010; Riffel&Storchi-Bergmann
TIC114.
2011a,b; Storchi-Bergmannetal. 2009, 2010). Usually the [Feii]
hasanenhancementinfluxandvelocitydispersioninregionssur-
roundingtheradiostructure,suggestingthattheradiojetplaysan
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