Near-infrared spectra of Seyfert galaxies and line production mechanisms

0.89 MB·English

by  N. Jackson

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed5February2008 (MNLATEXstylefilev2.2)
Near-infrared spectra of Seyfert galaxies and line
production mechanisms
N. Jackson and R. J. Beswick
The University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire, SK11 9DL U.K.
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ABSTRACT

2 NewobservationsarereportedofJ-bandspectra(1.04µm–1.4µm)ofthreeSeyfert2
1 galaxies,Mkn34,Mkn78andNGC5929.Ineachcasethe spectralrangeincludesthe
near-infraredlinesof[Feii],[Pii],HeiandPaβ.EachSeyfertgalaxyhasaknownradio
1
jet, and we investigate the infrared line ratios of the nuclear and extended regions of
v
each galaxy compared to the radio structure. In Mkn 34 there is a clear indication of
4
an extranuclear region, probably coincident with a shock induced by the radio jet, in
8
3 which[Feii]isconsiderablyenhanced,althoughthenuclearemissionisalmostcertainly
1 the resultofphotoionizationbythe continuumofthe activenucleus.Similareffects in
0 extranuclear regions are seen in the other objects, in the case of Mkn 78 confirming
7 recent studies by Ramos Almeida et al. A possible detection of extranuclear [Pii]
0 emission suggests, if real, that photoionization by the active nucleus is the dominant
/ lineexcitationmechanismoverthewholesource,includingtheregionscoincidentwith
h
p the radio jet.
-
o Key words: galaxies:active–galaxies:individual:Mkn34–galaxies:individual:Mkn78
r – galaxies:individual:NGC5929– line formation – infrared:galaxies
t
s
a
:
v
i
X 1 INTRODUCTION generate UV and X-ray photons which can then ionize the
gas–indeed,thedifferencebetweentheobservedproperties
r
a Optical and near-infrared studies of nearby active galaxies of such “autoionizing” shocks and regions photoionized by
cangiveusalotofinformationaboutthephysicalprocesses a central active nucleus can be quite subtle and depend on
taking place in these objects. Seyfert galaxies, as relatively thedetails of thespectra of theincident photons.
nearbylow-luminosityactivegalaxies,canbestudiedinpar-
ticular detail. These objects contain emission lines within
More recently, Simpson et al. (1996) showed that the
their spectrum, including in general broad lines produced observed [Feii] line is also consistent with predominantly
within the inner parsec, close to the central black hole and
being produced by photoionization, and Mouri, Kawara &
accretion disk, and narrow lines whose region of produc-
Taniguchi(2000)conductedmoresophisticatedcalculations
tionmayextendoverakiloparsec.Inaddition,manySeyfert
suggestingthatelectroncollisionsinapartially-ionizedzone
galaxieshaveweakradioemission,ofteninalinearstructure
associated with a shock may be the production mecha-
correspondingtoanoutflowfromthecentralactivenucleus.
nism rather than grain dissociation. Based on such studies,
The physics of emission lines is of particular interest. Alonso-Herrero et al. (1997) propose the use of the ratio of
These are thought mostly to arise through photoionization, infrared [Feii] lines to the hydrogen recombination lines to
and many studies have been done which use sophisticated separate active galaxies from starbursts. Oliva et al. (2001)
photoionization models toreproduceemission linesin great studiedtheproblem of shock ionization versusphotoioniza-
detail.However,additionalphysicsmaybeneededtorepro- tion. They suggested that the[Pii]line is potentially useful
duce some lines. In particular, the near-infrared [Feii] line indisentangling thisproblem,becausesimilar physicalcon-
at 1.257µm has been suggested as a diagnostic of shock ex- ditions are needed to produce it to those which produce
citation, which may arise as dust grains are dissociated in [Feii], but unlike iron, phosphorus is not produced in de-
fastshocks(e.g.Forbes&Ward1993).Theideathatshocks struction of dust grains. A high Fe/P ratio is thus a pre-
may have general applicability to AGN spectra was argued diction of models in which dust grains are dissociated by
bySutherlandetal.(2003)andDopita&Sutherland(1995) shocks. Oliva et al. find that pure photoionization better
whosucceededinreproducingmanycharacteristicphotoion- fits their spectrum of the very bright Seyfert galaxy NGC
izedspectrausingshockmodels. Thisarisesbecauseshocks 1068. In fainter galaxies where [Pii] is not detected, even
2 N. Jackson and R. J. Beswick
lower limits on the Fe/P ratio may be useful in ruling out blackbody spectrum of 7200K for the Hipparcos F0V star
shock models if thespectra are sensitive enough. HIP36366(Perryman1997)andtemperaturesof7200Kand
AlthoughSeyfertgalaxieshavebeenstudiedextensively 7000K for the stars HIP56601 and HIP75411 respectively.
intheoptical, highsignal-to-noise observationsinthenear- The Pa5→3, 6→3 and higher order absorption bands were
infrared are rarer. A compilation has recently been pub- interactively edited out of the stellar spectrum before this
lished by Riffel, Rodr´ıguez-Ardila & Pastoriza (2006) in- process in order to avoid them appearing as emission lines
cludingmanyoftheirownJHK spectra(Rodr´ıguez-Ardila, in the divided spectrum.
Riffel&Pastoriza2005).Manyofthesespectrasuggestthat For deriving velocity profiles, successive rows of the
the line ratio of active galaxies have a [Feii]/Hβ ratio sug- sky-subtracted image have been extracted, continuum-
gestive of X-ray heating or regions of star formation. Re- subtracted using line-free regions immediately around each
cently,RamosAlmeidaetal.(2006)havepublishedinfrared line, and fitted with Gaussian profiles. Profile fitting has
spectra of one of the objects discussed here, Mkn 78, and been done by software written by us, using the Levenberg-
deducefrom observationsandsimulationsthatthepredom- MarquardtalgorithmasimplementedbyPressetal.(1992).
inantline-excitationmechanisminthenuclearregionispho- In addition to the infra-red data, some data from the
toionization bythehardUVcontinuumfrom theactivenu- Isaac Newton Group (ING) archive was extracted for the
cleus,althoughradio-inducedshocksmaycontributewithin Seyfert galaxy Markarian 34. These data were taken using
one of the radio lobes. theWilliam Herschel 4.2-m telescope on La Palma on 1998
In this work we choose objects in which linear radio December 17 using the ISIS two-arm spectrograph. On the
structureisclearlyseeninordertoinvestigatetheemission- red arm of this spectrograph the TEK-2 chip was used to-
line structure along the jet axis and look for evidence for gether with an R600R grating, using a 1′.′23 slit and a re-
shock excitation. If shocks are indeed an important part of sultingspectralresolutionofjustover0.2nm.Thedatawere
thephysics,theareawheretheradiojetisdepositingenergy co-added and fluxes of [Oi] and Hα derived assuming a flat
into the interstellar medium is an obvious place to look. spectral response over this wavelength range, because both
The spectra presented here are from deep exposures (1–2 theTEK-2 chip and theR600R grating havea flat spectral
hourswithan8-mclasstelescope)andthereforeallowusto response over thisrange towithin a few percent.
investigate both nuclearand off-nuclearemission.
2.2 Radio data
Complementary, mainly previously-published (Falcke, Wil-
2 OBSERVATIONS AND ANALYSIS
son&Simpson1998;Whittle&Wilson2004)publicdomain
2.1 Infrared and optical data radiodataforeachofthesethreesourceswereobtainedand
re-mappedforcomparison purposesfrom theNRAO’sVery
All infrared observations were conducted using the 8-m
Large Array (VLA) archive. In each case these data were
Gemini-NorthTelescope,MaunaKea,onthenightsof2005
observed at a wavelength 3.6cm and in the VLA’s highest
February17,February18andMay22usingtheNIRIimag-
resolution configuration, providing an angular resolution of
ingspectrographinf/6spectroscopymode.TheJgrismwas ∼0.2-0.3 arcsec which is directly comparable to the spec-
used with aJ-band order-sorting filter,giving awavelength
troscopic observations presented. A brief summary of the
range of approximately 1.05µm – 1.42µm, together with a
observingparametersis givenin Table1.Each data-set was
4-pixel slit which corresponds to 0′.′46 on the sky and a re-
calibrated using standard data reduction techniques within
solvingpowerofabout600(Hodappetal.2003).Mkn78was NRAO’s aips package where these data were edited before
observedfor2hourson2005February17,Mkn34for2hours
phasesolutionsfromanearbyreferencesourcewereapplied.
on2005February18andNGC5929for70minuteson2005
In each case the flux density scale was calibrated with re-
May22.Eachobservationwasdividedinto3×30secondex-
spect to3C286 using theBaars et al. (1977) scale.
posures, and each successive exposure was chopped up and
down the slit to make sky subtraction easier and minimize
effectsofbadpixels.Lampflatfieldexposuresandargonarc
exposureswerealsoobtainedforMkn34andMkn78.Inthe 3 RESULTS AND LINE DIAGNOSTICS
case ofNGC5929, noargon lamp spectrum isavailable and
3.1 Mkn 34 (z=0.0505)
in this case the atmospheric OH lines have been used for
wavelength calibration. Markarian 34 is a Seyfert 2 galaxy with a 2′.′4 extended ra-
Data reduction was performed using the Starlink fi- diostructurewhoseextendedopticallineemission hasbeen
garo package and began with combination of the flatfields studied in detail by Whittle et al. (1988) and Falcke, Wil-
and division of the data frames by a flatfield normalised in son & Simpson (1998). Its off-nuclear line emission shows
the spectral direction. A second-order polynomial fit to at a velocity profile with relatively broad [Oiii] emission ex-
least six arc lines was obtained which gave a maximum er- tending slightly beyond the radio lobes. There is a distinct
rorof0.1nmacrossthewavelengthrange.Thespectrawere difference between the velocity of the line emission near to
correctedforS-distortionusingthreespectraoftheHippar- thenucleusandthatbeyondtheradiolobes.Thenewinfra-
cos stars taken at different positions on the chip. The sky red spectrum was taken with the slit at a position angle
wasthensubtractedusingtheFigarotaskpolyskyandthe 158◦ East of North, coincident with the radio axis. Figure
spectraextracted usingtheoptimalextraction algorithm of 1 shows the image of the spectrum along the slit, showing
Horne(1986) using the Figaro task optextract. aclearrotation-typevelocity profileinallsecurely detected
Relative flux calibration was performed assuming a lines (Hei,[Feii], Paβ and Paγ).
Infra-red spectra of Seyferts 3
Table 1.SummaryofarchivalVLAobservations.
Source Date Phaseref. Timeonsource Originalpublication
Mkn34 1996Nov4 1030+611 166min Falcke,Wilson&Simpson1998
NGC5929 2003Jun20 1506+426 8min
Mkn78 1990Apr16 0716+714 340min Whittle&Wilson2004
HeI [FeII] -2.0 -1.5 -1.0 -0.5Relative di0s.t0ance (arcse0c.5) 1.0 1.5 2.0
NW SE
-0.6
Paγ Paβ Relative distance (arcsec)--00000.....42024
0.6
Figure 1. J-band spectrum of Mkn 34, showing the brightest
emissionlines.
[Pii]isdetectedatamarginal levelinthenuclearspec-
trum. In the nuclear region, the ratio [Feii]λ1.257/[Pii] is
3.2±1.0,whichisconsistentwiththerangeofvaluestypical
of nuclear photoionization (Oliva et al. 2001).
The radio map of Falcke et al. (1998) shows structure
similartothatseenearlierbyUlvestad&Wilson(1984)at6
and20cm,butwithhigherangularresolutionandsensitivity
shows the definition of the radio jet structure more clearly.
In particular the jet is resolved into several discrete knots
and a significant change in the projected position angle of
thetip of thesoutheastern jet is seen.
The most striking feature of thespectrum is thediffer-
Figure 2. Central panel: Velocity profile of Mkn34. Four lines
ent distribution between [Feii] and the other lines. Figure are shown, Hei (circles), Paγ (crosses), [Feii] (squares) and Paβ
2 illustrates the rotation curve and strength of the lines, (triangles). Linequantities have beenmeasuredby Gaussianfits
and it is clear that [Feii] has a bright knot about 1′.′2 to tosuccessiverowsofthespectralimage,andtheamplitudewithin
the SE. This is just behind the bright radio knot in the eachsequenceoflinefitsisproportionaltothesizeofthesymbol.
southeastern jet imaged here and by Whittle et al. (1988). Each successive line has been offset by 200 kms−1 for clarity.
Although the Paschen recombination lines are slightly en- Note the clearlydifferent pattern of the [Feii] emission fromthe
hanced in the off-nuclear regions compared to the nucleus, other lines; it is highly concentrated in region of the SE radio
theratioof[Feii]/Paβbecomesafactorof2–3strongeraway knot. The radio map (top panel) from data presented by Falcke
et al. (1998) isshown to the same scale, and rotated to the axis
from thenucleus,suggesting thatdifferentionization mech-
of the spectrograph slit. Thebottom panel shows the lineratios
anismsmaybeoperatinghere.TheHeilinedisplaysadiffer- comparedtoHeiof[Feii](solidline),Paγ (dashedline)andPaβ
entbehaviourtothehydrogenrecombinationlines,showing
(dotted line).
a smooth decrease in each direction away from the nucleus
withlittlesignofanyincreaseinstrengthintheradiolobes.
4 N. Jackson and R. J. Beswick
HeI [FeII] 0 2 4 6 mJy/bm
41 40 15.0 NGC 5929
Paβ 14.8
14.6
0)
0
0
2
N (J 14.4
O
TI
A 14.2
N
LI
C
DE 14.0
13.8
13.6
13.4
15 26 06.24 06.22 06.20 06.18 06.16 06.14 06.12 06.10 06.08 06.06
RIGHT ASCENSION (J2000)
Figure4.VLA3.6cmimageofNGC5929,showingfeaturessim-
ilartothemapofSuetal.(1996).Themapiscontouredatmulti-
plesof√2 0.146mJybm−1(3σ).Theimagehasbeenconvolved
withGauss×ianbeamof0′.′234 0′.′211withaofPA=74◦.
×
Figure3.J-bandspectrumofNGC5929,showingthebrightest
emissionlines.
ernradiolobe.Aroundthenortheasternradiolobe, thereis
some evidence of disturbed kinematics in the[Feii]line.
3.2 NGC5929 (z=0.00831)
3.3 Mkn78 (z=0.03715)
NGC5929 is a Seyfert galaxy with a radio jet extending
about0′.′7inPA∼60◦ eithersideofacompactnucleus(Su Mkn 78 has recently been the subject of an extensive op-
et al. 1996; Cole et al. 1998) and extended optical emission tical study with the Hubble Space Telescope by Whittle
partly coincident with the western radio lobe (Bower et al. & Wilson (2004), Whittle et al. (2004) and Whittle et al.
1994), strongly suggesting an interaction. Nuclear infrared (2005)specificallydirectedatdisentanglingshockionization
spectroscopy was performed by Simpson et al. (1996) who and photoionization. This Seyfert galaxy has extended ra-
clearlydetectedthe[Feii]lineandfoundittobemarginally dioemission, in a linear structureabout 2′′ long in an E-W
resolved.Thecurrentobservationshaveasmallerpixelscale, direction,andWhittle&Wilson(2004)haveimagedalarge
and we clearly see the resolution in this object, using a slit andcomplexextendedstructurein[Oiii]extending3′′ from
atthepositionangleoftheextendedopticalemissionprevi- the nucleus in each direction. In these observations the slit
ously seen (65◦). The nuclear spectrum is shown in Figure was aligned in PA 90◦ in the hope of detecting significant
3. extended structure in the infrared emission lines. The nu-
In the nuclear emission the [Pii] line is detected at clear spectrum of this object is shown in Figure 6, and the
just over the 3σ level; a previous spectrum reported by measurements of theextendedemission and velocity profile
Rodr´ıgues-Ardila, Riffel & Pastoriza (2005) and Riffel et are displayed in Figure 7.
al. (2006) did not detect this line. A Gaussian fit to the IntheopticalobservationsofWhittleetal.(2005),very
[Feii] and [Pii] lines was carried out, constraining the [Pii] complex extended optical line emission was detected in all
linewidthtobeequaltothatof[Feii].The[Feii]/[Pii]ratio major recombination and forbidden lines. In these infrared
is 4.3±1.1 in the nuclear region, which again is typical of observations, the structure is not well resolved either spa-
photoionizedregionsandconsiderablylessthanthevalueof tially or spectrally. However, the overall rotation curve is
∼20 expected for shock ionization (Oliva et al. 2001). The detectedinextendedemissioninthreemajoremissionlines,
[Feii]/Paβ ratio is approximately 1, in agreement with the Hei, [Feii] and Paβ, covering about 2′′ to the East of the
previous spectra. nucleus. Relatively little line emission is seen to the West,
Figure 4 shows theradio map; similar structureis seen which is also the direction of lower surface brightness in
to the earlier radio map of Su et al. (1996) and Cole et al. [Oiii]from the mapsof Whittle & Wilson (2004).
(1998)withacompactcoreandtwolobestructuresdetected. A recent detailed infrared study has been carried out
The most remarkable feature of the spectrum is the with theLIRISspectrograph on theWilliam HerschelTele-
wholesale displacement of the line emission from the cen- scope by Ramos Almeida et al. (2006). They find a nuclear
treofthegalaxy andofthevelocity profile(Figure5).This ratio of [Feii]/Paα which is consistent with photoionization
effectisextremearoundthesouthwesternradiolobe,where from the active nucleus. Moreover, they detect [Pii] in the
the [Feii] line strength peaks. The Hei line peak is also off- nucleus at a strength that is a factor of 1.82 lower than
set, although by less than the [Feii]. Optical studies with the [Feii]λ1.644 line. This ratio is also consistent with pho-
the Hubble Space Telescope (Bower et al. 1994) have also toionization, being similar to the ratio found in NGC1068
revealed [Oiii]emission which peaks around the southwest- by Oliva et al. (2001), and is inconsistent with shock mod-
Infra-red spectra of Seyferts 5
-2.0 -1.5 -1.0 -0.5Relative di0s.t0ance (arcse0c.5) 1.0 1.5 2.0 HeI [FeII]
SW NE
-0.6
Relative distance (arcsec)--00000.....42024 Paβ
0.6
Figure 6. J-band spectrum of Mkn 78, showing the brightest
emissionlines.
aregion inwhichthe[Feii](andPaγ)emission becomesex-
tremely faint and difficult to measure accurately. The Hei
line displays different behaviour; as in Mkn34, its strength
decreases smoothly away from the nucleus. The Paγ line,
although it can be followed for about 2′′ either side of the
nucleus, is not detected at high enough signal-to-noise to
describe its structurein detail.
4 IONIZATION MECHANISMS AND
DIAGNOSTICS
4.1 Nuclear emission: the [Feii]/[Pii]ratio
Figure5.Centralpanel:VelocityprofileofNGC5929.Twolines This paper adds to the total number of detections of
areshown,Hei(circles)and[Feii](squares).Linequantitieshave [Pii]λ1.188µm in the literature. Since its original detection
been measured by Gaussian fits to successive rows of the spec-
in NGC 1068 by Oliva et al. (2001) the line has been de-
tralimage,andtheamplitudewithineachsequenceoflinefitsis
tectedintheSeyfert1galaxyAkn564byRodr´ıguez-Ardila,
proportional to the size of the symbol. Each successive line has
beenoffsetby200kms−1 forclarity.Theradiomap(toppanel) Riffel & Pastoriza (2005), in Mkn 78 by Ramos Almeida et
isthesameasFig.4,butisshowntothesamescale,androtated al. (2006), and in a total of 11 galaxies in the compilation
totheaxisofthespectrographslit.Thebottom panelshowsthe of Riffel et al. (2006). Although very marginal, the detec-
variationofthelineratio[Feii]/Heiover thesamescale. tions of [Pii] in Mkn 34 and NGC 5929 suggest [Feii]/[Pii]
ratios of 3–4, in agreement with ratios determined in other
objects. Comparison with themodels presentedby Olivaet
els. However, in the extended emission associated with the al.(2001)suggeststhatthedominantionization mechanism
western radio lobe, they find that the [Feii] line strength is photoionization by the active nucleus rather than shock
increases by a factor ∼2 relative to the hydrogen recombi- excitation; fast shocks would be expected to disrupt dust
nationlines,suggestingacontributionfrominteractionwith grains and produceline ratios a factor of 10 higher.
the radio bow shock. We find a similar effect in our data,
withanenhancementin[Feii]whichpeaksat∼0′.′6fromthe
nucleus.However,theassociationofincreased[Feii]emission 4.2 Extended emission
with the radio structure is less obvious in this object than
4.2.1 Line diagnostic diagrams
in theother twoin thisstudy.In thewestern lobe thepeak
ofthe[Feii]doescoincidewiththeW-knotofradioemission There have been a number of investigations which have at-
seenathigherresolutionbyWhittleetal.(2004)andshown tempted to compare near-infrared line ratios with ioniza-
inthesuperposeduniformlyweightedimageinthetoppanel tioncodes.Manyhaveusedthecloudycode(Ferlandetal.
offigure7.Thestronger,easternradiojetisco-spatialwith 1998)forarangeofphysicalparameters,namely:densityof
6 N. Jackson and R. J. Beswick
-1.0-2.0 -1.5 -1.0 -0.5Relative dis0t.a0nce (arcse0c.)5 1.0 1.5 2.0 infraredspectrum,inparticularthehydrogenrecombination
W E lines. Also interesting is the optical [Oi]λ6300 line, as this
-0.8
-0.6 has a similar ionization potential to [Feii] and is produced
Relative distance (arcsec)--00000.....42024 amtitnholtuaeatctrhhnpteahsattrerrietvo[giOeaniolgidln]ye/ir-a[piFgotrnehnoioaidizs]nuetrdic[acPitnzoiigooif]n,si[esFh[Oeoswiucii]ik]bt.shjeuIextffcsctiecmirtatsaoinltfairstoruohnmbpesrthteotyafhson[eiPrtceidiaaili]sls.eaprArdervdovletpdahnaeeornstutaiianggenhges
0.6
and is difficult to measure with thesame instrument.
0.8
1.0 The effect of different physical parameters on the rela-
tivestrengthsofthe[Feii]andhydrogenrecombinationlines
(inparticularPaβ )canbesummarizedasfollows(Simpson
et al. 1996, Mouri et al. 2000; Ramos Almeida et al. 2006):
• Metal abundance changes the [Feii]/Paβ ratio by ap-
proximately thescaling of theabundance,forexample bya
factor of ∼6 from solar to Orion abundance.
• Neutralhydrogendensityhasrelativelylittleeffect,ex-
cept at very high densities when the forbidden lines are in
any case close to collisional de-excitation. For most values
of ionization parameter, increasing densities decreases the
relative strength of [Feii]very slightly.
• Increasing ionization parameter causes a slow decrease
in the [Feii]/Paβ ratio below U = 10−1.5, and a rapid in-
crease above this.
• The slope of the power-law has a drastic effect on the
[Feii]/Paβ ratio; a steepening by 0.4 (from −1.0 to −1.4)
typicallyhalvestheratio,andtheratiobecomesevensmaller
if a 40000-K blackbody model is adopted instead. This was
considered in detail by Simpson et al. (1996), although
RamosAlmeidaet al. (2006) usean indexof−2.0 through-
out their simulations.
In order to compare results with physical models and
extract physical information from the line ratios, we have
used the cloudy program (Ferland et al. 1998), version
c06.02b.Modelshavebeencalculatedforionizationparame-
tersrangingfrom10−3.5 to10−1.5 instepsof100.5 andelec-
tron densities between 103.5 and 106, also in steps of 100.5.
For each combination of ionization parameter and density,
Figure 7. Velocity profile of Mkn 78. Three lines are shown,
Hei(circles),Paβ (crosses),[Feii](squares). Linequantities have two different metallicities (0.4Z⊙ and Z⊙) and two values
been measured by Gaussian fits to successive rows of the spec- ofthespectralindexofthepower-lawphotoionizingcontin-
tralimage,andtheamplitudewithineachsequenceoflinefitsis uum(−1and−2)havebeenused.Grainswereaddedtothe
proportional to the size of the symbol. Each successive line has simulation and were adjusted tobe similar toOrion grains.
been offset by200kms−1 forclarity. Theradiomap(top panel) Thediagnosticdiagramof[Feii]λ1.257/Paβ vs.[Oi]/Hα
ismadeusingthedataofWhittle&Wilson(2004); itshowsthe is shown for Mkn 34 in Figure 8. This is the same diagram
centralpartoftheradioemission,rotatedtotheaxisofthespec- as presented in Figure 3 of Mouri et al. (2000). The trends
trographslit.Superposedonthisradiomapisahigherresolution
in line ratio with ionization parameter, hydrogen density
(0′.′075)uniformlyweightedradioimageofthecoreandinnerjet.
and slope of the ionizing continuum agree with previous
This inner image uses the same contouring scheme as the outer
image with a lowest contour value contour of 0.1mJybeam−1. determinations. In particular, an increase in ionization pa-
Theapparenthighamplitudeofthe[Feii]lineat0′.′7istheresult rameter causes decreases in both the [Feii]λ1.257/Paβ and
of a noisy fit which is unlikely to be representative of the true the [Oi]/Hα ratio of about the same magnitude, provided
flux;thesameistrueofthePaβ lineat0′.′45.Thebottom panel that the slope of the ionizing continuum, is relatively steep
shows the line ratios compared to Hei of [Feii] (solid line), and (< −1.5); in practice, relatively steep continua are needed
Paβ (dashedline). to fit thesedata.
We have already noted that the ratio between
[Feii]λ1.257andthehydrogenrecombination linesincreases
hydrogen, ionization parameter, ionizing power-law slope, intheobjectsstudiedasonemovesfromthenuclearregions
metal abundance and presence or absence of dust grains totheareasassociatedwithradiolobeemission.Inprinciple,
(Simpson et al. 1996; Ramos Almeida et al. 2006). Others if the sole mechanism operating is ionization by hard-UV
haveusedthemappingscodeormodificationstoit(Suther- radiation from an active nucleus, this change in line ratio
land et al. 1993; Mouri et al. 2000). Of particular interest could be caused by a decrease in ionization parameter re-
is the ratio of [Feii] to the other lines available in the near- sulting from a geometrical dilution in the ionizing photon
Infra-red spectra of Seyferts 7
Figure 9. (Bottom panel) Area around the [Pii] line in the six
pixels(0′.′7)nearthesoutheastern radiolobeofMkn34withthe
strongest [Feii] emission. The predicted [Pii] emission, deduced
Figure 8. Diagnostic diagram of [Feii]λ1.257/Paβ vs. [Oi]/Hα from scaling the [Feii] emission by the known wavelength ratio
and by scaling in intensity by factors of 4 and 20, is shown by
compared to the observations of Mkn 34. Observational values
(circles)within2′′eithersideofthenucleusareshown,andpixels dottedlines.Thewavelengthscaleisthatoftheobservedframein
<0′.′4 from the nucleus are shown as solid blobs surrounded by Mkn34. (Top panel) Combined extranuclear emission from near
acircle.Modeltracks fromthecloudyprogramarealsoshown, tothesoutheasternradiolobeofMkn34,asabove,andtheregion
of0′.′6aroundthesouthwesternradiolobeofNGC5929.Bothof
forvariousvaluesoftheionizingcontinuum power-lawslope(p),
hydrogendensity(n,inlog10cm−3).Alltracksarecalculatedfor these regions have enhanced [Feii] emission. The spectra have
beencoaddedandcorrectedtothesamewavelengthscalebyuse
fZro=m0.41Z.⊙5.tWo ith3i.n5,ewacihthtrtahcek,htihgeheiornvizaalutieosnopfa[rFaemiie]λte1r.2U57v/aPraieβs ofthe[Feii]line.Thepredictedpositionofthe[Pii]lineisshown
− − ineachcase;thedottedlinesshowthe[Pii]lineasitwouldappear
occurringatlowerU.
ifitwerefactors of 4and20 weaker than [Feii]. Thewavelength
scaleisthatoftheobservedframeofNGC5929.
flux or an increase in the electron density associated with
compressionbytheradiojet.Thisisindeedtheconclusionof The[Feii]lineinthisregionhasbeenfittedwithaGaussian,
Falckeetal.(1998)whostudythevariationofthe[Oiii]/Hα and is shown shifted to the [Pii] wavelength and divided in
ratio and find that it decreases away from the nucleus. In
intensity by factors of 4 and 20. There is a spike at exactly
thiscase, however, onewould also expectthe[Oi]/Hαratio
the expected wavelength, although it is extremely close to
to increase, and this does not seem to be the case (Figure
the noise level. If this is real, it clearly indicates that the
8). In fact, this ratio is actually smaller in the southeast-
mechanism operating in this extended region is photoion-
ernlobe ofMkn34, wherethestrongest interaction may be
ization, with little or no shock contribution. Unfortunately
taking place, than in the nucleus of Mkn 34. Since the [Oi]
the signal-to-noise in this spectrum is just too low to make
and [Feii] lines are produced in the same type of partially
a definitestatement.
ionized zone and in similar physical conditions, the lack of
However,repeatingthisexerciseintheextendedregion
correlation between their ratios to hydrogen recombination
of NGC5929 associated with the southwestern radio lobe
lines is puzzling. (Figure 5) and in which the [Feii] is also enhanced over its
The same, however, does not appear to be the case in
value in the nuclear region, reveals a similar very marginal
Mkn 78. Whittle et al. (2005)’s extensive study of the ex- [Pii]detectionintheextendedemission.Addingthetwoto-
tended optical emission lines in this object suggest that all
gether,withtheappropriatescalinginwavelength(Figure9)
lines, including [Oi], follow standard tracks from photoion- givesamoresecuredetectionoftheextended[Pii]atalevel
ization diagnostics, with a variation of about a factor of 10 of about 0.2 times the strength of the extended [Feii] line.
in U; this is sufficient to produce a difference of about a
This suggests, somewhat surprisingly, and despite theasso-
factor of 2 in [Feii]λ1.257/Paβ.
ciation with the radio jet, that the dominant mechanism of
Images of some optical emission lines in the extended
lineexcitationintheseregionsisstillphotoionizationrather
regions of NGC 5929 are presented by Bower et al. (1994)
than shock excitation.
anddiscussedfurtherbySuetal.(1996);however,wedonot
have enough information on this object to present detailed
diagnostic diagrams comparable with previouswork.
5 CONCLUSIONS
4.2.2 Tentative detections of extended [Pii] A number of near-IR studies of Seyfert galaxies with ex-
tended optical emission have now been carried out, in par-
A definitive test would be the detection of the [Pii] line in ticular Mkn78 (Ramos Almeida et al. 2006 and this work),
theextendedemission lineregion.InFigure9anextraction Mkn34andNGC1068(Olivaetal.2001).Thepicturewhich
isshownof0′.′7oftheslitintheregionwherethe[Feii]lineis emerges with reasonable consistency is that the nuclear re-
strongest,justbehindthesoutheasternradiolobeofMkn34. gionsarealmost certainly dominatedbyphotoionization by
8 N. Jackson and R. J. Beswick
UVphotons,consistent with amechanism involvingtheac- HorneK., 1986. PASP98, 609
tive nucleus. This is indicated both by the ability of pho- Mouri H., Kawara K., Taniguchi Y., 2000, ApJ, 528, 186.
toionization toreproduceobservedemission lineratios, and Oliva E., Marconi A., Maiolino R., Testi L., Mannucci F.,
specifically by the inconsistency of the [Feii]/[Pii] diagnos- Ghinassi F., Licandro J., Origlia, L., Baffa, C., Checcucci,
tic with the enhanced ratios expected from shock models. A., 2001, A&A,369L, 5.
In the extended emission regions which coincide with radio Pedlar A., Harrison B., Unger S.W., Graham D.A., Preuss
jets,thereisaconsistentpatternofchangesinemissionline E., Saikia D.J., Yates G.J. 1988, LNP, 307, 310.
ratio consistent with compression of the gas by the bow- Perryman M.A.C., 1997. The Hipparcos and Tycho cata-
shock associated with radio emission. Tentative detections logues.Astrometricandphotometricstarcataloguesderived
of [Pii] in these regions, however, suggest that photoioniza- fromtheESAHipparcosSpaceAstrometryMission,Noord-
tionisstillthedominantmechanism.Thefactthatthepeak wijk, Netherlands, ESA Publications Division, 1997, ESA
line emission is slightly behind the head of the radio jet in SPSeries vol. 1200.
Mkn34 may suggest that the gas has been compressed by PressW.H.,TeukolskyS.A.,VetterlingW.T.,FlanneryB.P.,
the shocks but ionized by theactive nucleus after the [Feii] 1992. NumericalRecipesinC,CambridgeUniversityPress,
hasbeenreturnedtothegas phase,ratherthanbyUVand Cambridge.
X-ray photons from the shocks themselves. In this case the RamosAlmeidaC.,P´erez-GarciaA.M.,Acosta-PulidoJ.A.,
factthatNGC5929’speakof[Feii]iscoincidentwiththera- Rodr´ıguez-Espinosa J.M., Barrena R., Manchado A., 2006.
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large (8-m class) telescopes. Rodr´ıgues-Ardila A., Pastoriza M.G., Viegas S., Sigut
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Rodr´ıgues-Ardila A., Riffel R., Pastoriza M.G., 2005, MN-
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ACKNOWLEDGMENTS
Simpson, C. Forbes, D.A., Baker, A.C., Ward, M.J. 1996,
Based on observations obtained at the Gemini Observa- MNRAS,283, 777.
tory, which is operated by the Association of Universities SuB.M.,MuxlowT.W.B.,PedlarA.,HollowayA.J.,Steffen
forResearchinAstronomy,Inc.,underacooperativeagree- W., KukulaM.J., Mutel R.L. 1996, MNRAS,279, 1111.
ment with the NSF on behalf of the Gemini partnership: SutherlandR.S.,BicknellG.V.,DopitaM.A.2003,ApJ,591,
theNationalScienceFoundation(UnitedStates),theParti- 238.
clePhysicsandAstronomyResearchCouncil(UnitedKing- Ulvestad J.S., Wilson A.S. 1984, ApJ, 278, 544.
dom),theNationalResearchCouncil(Canada),CONICYT WhittleM., Wilson A.S.2004, AJ, 127, 606.
(Chile),theAustralianResearchCouncil(Australia),CNPq Whittle M., Pedlar A., Meurs E.J.A., Unger S.W., Axon
(Brazil) and CONICET (Argentina). RJB acknowledges fi- D.J., Ward M.J. 1988, ApJ, 326, 125.
nancial support from the European Commission’s I3 Pro- WhittleM.,SilvermanJ.D.,RosarioD.J.,WilsonA.S.,Nel-
gramme “RADIONET” under contract No. 505818. The son C.H., 2004, I.A.U. Symp. 222 “The interplay among
VLA is operated by the National Radio Astronomy Obser- black holes, stars and ISM in galactic nuclei”, Gramado,
vatory which is a facility of the National Science Founda- Brazil. March 2004, Eds. T. Storchi-Bergmann, L. C. Ho
tion operated under cooperative agreement by Associated and H. R.Schmitt.p299-302
Universities, Inc. We thank the observers, I. Song and S. WhittleM.,RosarioD.J.,SilvermanJ.D.,NelsonC.H.,Wil-
Knights and the operators, B. Walls and G. Trancho, for son A.S.2005, AJ, 129, 104.
carrying out theobservations.
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