Molecular Gas in NUclei of GAlaxies (NUGA): VI. Detection of a molecular gas disk/torus via HCN in the Seyfert2 galaxy NGC6951?

0.27 MB·English

by  M. Krips

Astronomy&Astrophysicsmanuscriptno.n6951-dec06-rev (cid:13)c ESO2008
February5,2008
LE
Molecular Gas in NUclei of GAlaxies (NUGA):
VI. Detection of a molecular gas disk/torus via HCN in the Seyfert 2
⋆
galaxy NGC 6951?
M.Krips1,R.Neri2,S.Garc´ıa-Burillo3,F.Combes4,E.Schinnerer5,A.J.Baker6,A.Eckart7,F.Boone5,L.Hunt8,S.
Leon9,andL.J.Tacconi10
7
0
0 1 Harvard-Smithsonian Center for Astrophysics, SMA project, 645 N A‘ohoku Pl., Hilo, HI,96720, USA; e-mail:
2 [email protected]
2 InstitutdeRadioAstronomieMillime´trique(IRAM),SaintMartind’He`res,F-38406,France
n
3 ObservatorioAstrono´micoNacional(OAN)-ObservatoriodeMadrid,C/AlfonsoXII3,28014Madrid,Spain
a
4 ObservatoiredeParis,LERMA,61Av.del’Observatoire,75014Paris,France
J
5 Max-Planck-Institutfu¨rAstronomie,Ko¨nigstuhl17,69117Heidelberg,Germany
3 6 DepartmentofPhysicsandAstronomy,Rutgers,theStateUniversityofNJ,136FrelinghuysenRd.,Piscataway,NJ08854-8019,
1 USA
7 Universita¨tzuKo¨ln,I.PhysikalischesInstitut,Zu¨lpicherStr.77,50937Ko¨ln,Germany
1 8 INAF-IstitutodiRadioastronomia/SezioneFirenzeLargoE.Fermi5,50125Firenze,Italy
v 9 IRAM,AvenidaDivinaPastora,7,Nu´cleoCentral,18012Granada,Spain
3 10 Max-Planck-Institutfu¨rextraterrestrischePhysik,Postfach1312,85741Garching,Germany
0
4
1 ABSTRACT
0
7 Context.SeveralstudiesofnearbyactivegalaxiesindicatesignificantlyhigherHCN-to-COintensityratiosinAGN(e.g.,NGC1068)
0 thaninstarburst(e.g.,M82)environments.HCNenhancementcanbecausedbymanydifferenteffects,suchashighergasdensities
/ and/ortemperatures,UV/X-rayradiation,andnon-collisionalexcitation.Asactivegalaxiesoftenexhibitintensecircumnuclearstar
h
formation,highangular resolution/high sensitivityobservationsareofparamount importancetodisentanglingtheinfluenceof star
p
formationfromthatofnuclearactivityonthechemistryofthesurroundingmoleculargas.ThetightrelationofHCNenhancement
-
o andnuclearactivitymayqualifyHCNasanidealtracerofmoleculargasclosetotheAGN,providingcomplementaryandadditional
r informationtothatgainedviaCO.
t Aims.NGC6951 housesnuclear andstarburstactivity,makingitanidealtestbedinwhichtostudytheeffectsofdifferentexcita-
s
a tionconditions on themolecular gas. Previous lower angular resolution/sensitivity observations of HCN(1–0) carriedout withthe
: NobeyamaMillimeterarraybyKohnoetal.(1999a)ledtothedetectionofthestarburstringbutnocentralemissionhasbeenfound.
v OuraimwastosearchfornuclearHCNemissionand,ifsuccessful,fordifferencesofthegaspropertiesofthestarburstringandthe
i
X nucleus.
Methods.WeusedthenewA,B,CandDconfigurationsoftheIRAMPdBIarraytoobserveHCN(1–0)inNGC6951athighangular
r resolution(1′′≡96pc)andsensitivity.
a
Results.Wedetectverycompact(≤50pc)HCNemissioninthenucleusofNGC6951,supportingprevioushintsofnucleargasstruc-
ture.OurobservationsalsorevealHCNemissioninthestarburstringandresolveitintoseveralpeaks,leadingtoahighercoincidence
betweentheHCNandCOdistributionsthanpreviouslyreportedbyKohnoetal.(1999a).
Conclusions.WefindasignificantlyhigherHCN-to-COintensityratio(≥0.4)inthenucleusthaninthestarburstring(0.02-0.05).As
forNGC1068,thismightresultfromahigherHCNabundanceinthecentreduetoanX-raydominatedgaschemistry,butahigher
gasdensity/temperatureoradditionalnon-collisionalexcitationofHCNcannotbeentirelyruledout,basedontheseobservations.The
compactHCNemissionisassociatedwithrotatinggasinacircumnucleardisk/torus.
Keywords.Galaxies:individual(NGC6951)–Galaxies:active–Galaxies:nuclei–Galaxies:Seyfert–Galaxies:starburst
1. Introduction gions.WhilesurprisinglylargeR (upto1ormore)have
HCN/CO
beenobservedinAGN(e.g.,NGC1068,NGC1097,NGC5194;
Overthe pastdecade,severalstudies(e.g.,Tacconiet al. 1994,
Kohno et al. 1999a, Tacconi et al. 1994, Usero et al. 2004),
Sternberget al. 1994,Kohnoetal. 1999a,b,2000,2001,2005)
much smaller R (<0.3)are detected in pure SB or com-
have shown that the HCN-to-CO intensity ratio (≡R ) HCN/CO
HCN/CO posite (AGN+SB) galaxies (e.g., Arp220, M82, NGC 6951;
canbesignificantlyhigherinactivegalacticnuclei(AGN;e.g.,
Gao&Solomon2004a,b,Nguyen-Q-Rieuetal.1992).Inactive
NGC 1068) than in starburst (SB; e.g., M82) or quiescent re-
galaxies have even lower ratios of R <0.1. Many differ-
HCN/CO
ent effects can contribute to increased R in active envi-
HCN/CO
Sendoffprintrequeststo:M.Krips ronments including higher gas opacities/densities and/or tem-
⋆ BasedonobservationscarriedoutwiththeIRAMPlateaudeBure peratures,non-standardmolecularabundancescausedbystrong
Interferometer (PdBI). IRAM is supported by INSU/CNRS (France), UV/X-ray radiation fields or additional non-collisional excita-
MPG(Germany)andIGN(Spain).
2 Kripsetal.:HCN(1–0)emissioninNGC6951
Fig.1.IntegratedHCN(1–0)emission(blackcontours)overlaidonCO(2–1)(colorscale;Schinnereretal.,inprep.,Garc´ıa-Burillo
et al. 2005) in natural (left; 1a) and uniform weighting (right;1b); the CO and HCN emission have been both integrated from
−200kms−1to+200kms−1.Weusedauv-taperforCOtomatchtheangularresolutionofourHCNdatawhichisbyafactorof∼2
lower,andobtainidenticalbeamsizes.Blackcontoursrunfrom3σto23σ(right:7σ)instepsof1σ=0.06Jybeam−1kms−1 (right:
1σ=0.10Jybeam−1 kms−1);CO(2–1):3σ=1.0Jybeam−1 kms−1(right:3σ=1.3Jybeam−1 kms−1).Theblackline(left)indicates
themajoraxisofthebar(PA=100◦)andthegreylinethemajoraxisofthegalaxy(PA=130◦).Theblackline(right)representsthe
mostextremecentralvelocitygradients(PA=(160±20)◦).The(0,0)positionisatα =20h37m14.123s andδ =66◦06′20.09′′
J2000 J2000
whichisslightlydifferentfromthephasecentreoftheobservations.
tion such as IR pumping through UV/X-ray heated dust. Gao Component peakflux FWHM I I a
HCN(1−0) CO(2−1)
& Solomon(2004a)have ruledoutthe latter scenario for large (mJybeam−1) (kms−1) (Jybeam−1kms−1)
scale HCN emission and the lack of any clear correlation be- N 11±2.0 150±6 1.8±0.2 60±6
tween the hard X-ray and MIR luminosity in AGN (Lutz et al. W 10±1.0 100±8 1.1±0.1 22±2
E 12±1.0 70±5 1.0±0.1 23±2
2004) reduces the significances of IR pumping also at small
S 13±1.0 60±4 0.9±0.1 38±4
scales. However,Useroetal.(2004)presentstrongevidencein
Cb 4.8±0.5 170±20 0.9±0.1 2.4±0.2
the case of NGC 1068 that the nuclear gas chemistry is dom-
Totalfluxc 36.0±4.0d 320±14 12.0±1.0e 470±50e
inated by X-ray radiation from the AGN yielding significantly
differentmolecularabundancesthaninSBorquiescentenviron- Table1.HCN(1–0)lineparameters,obtainedatthevariouspeak
ments (Lepp & Dalgarno 1996; Maloney et al. 1996). Recent (i.e.,notspatiallyintegrated)byfittingaGaussianprofiletothe
IRAM30mobservationsofseveralHCNtransitionsinasample (naturallyweighted)data.Errorsincludeuncertaintiesofthefit
of12nearbyactivegalaxiesalsoseemtosupportasignificantly and calibration. a from Schinnerer et al., in prep. b from uni-
higherHCNabundanceinAGNthaninSBorinactiveenviron- formlyweightedmapstoavoidcontaminationbytheringemis-
ments,ratherthanapuredensity/temperatureornon-collisional sion.c spatiallyintegratedovertheentireareaof±9′′;d inmJy;
excitation effect (Krips et al., in prep.). If true, this has a se- e inJykms−1.
vere impact on the interpretation of R as a measure of
HCN/CO
thedensetototalmoleculargasmassfractioninactivegalaxies
(e.g., Gao & Solomon et al. 2004a,b) as discussed in Gracia´- recentlyhave high angularresolution/highsensitivity PdBI ob-
Carpio et al. (2006). The study of nearby active galaxies also servations revealed faint CO(2–1) emission in the central 0.5′′
revealsthelimitationsofRHCN/CO asauniquediagnosticindis- (Garc´ıa-Burilloetal.2005;Schinnereretal.,inprep.).Thelatter
tant sources whose starburst and AGN components cannot be observationsarepartofthePdBINU(cleiof)GA(laxies)project
separated. (e.g., Garc´ıa-Burillo et al. 2003). We observed NGC 6951 in
NGC6951isanactivegalaxyofHubbletypeSAB(rs)bcat HCN(1–0)tosearchfornuclearemissionandassessdifferences
a distance of 24 Mpc (Tully 1988);its active nucleusis classi- betweentheSBringandtheAGN;theresultsoftheseobserva-
fiedasatransitionobjectbetweenaLINERandatype2Seyfert tionsarepresentedhere.
(Pe´rez et al. 2000).In addition to its AGN, NGC6951 also ex-
hibitsa pronouncedSB ringat a radiusof5′′ (≡480pc) in Hα
2. Observations
(Marquez&Moles1993,Wozniaketal.1995,Rozasetal.1996;
Gonzalez-Delgado& Perez 1997, Perez et al. 2000) and radio NGC 6951 has been observed at 3mm and 1mm using all six
emission(Vilaetal.1990,Saikiaetal.1994&2002).StrongCO antennaeof the IRAM PdBI in the new A, B, C and D config-
and HCN emission is associated with the SB ring (e.g.,Kohno urations (see http://www.iram.frfor telescope parameters) dur-
et al. 1999a,Garc´ıa-Burillo et al. 2005) while almost no emis- ing January, February (A+B), April and May (C+D) 2006 .
sion had been hitherto foundin the centre of NGC 6951.Only The phase reference centre was set to α =20h37m14.470s
J2000
Kripsetal.:HCN(1–0)emissioninNGC6951 3
CO, split up into several sub-maxima (N,W,E,S; Fig. 1) at the
higherangularresolution(<3′′)ofthePdBI.TheHCNpeaksN
andSareconsistentinpositionwiththoseinCO(Fig.1;Garc´ıa-
Burilloetal.2005).AdditionalHCNpeaksarefoundtothewest
(W)andeast(E)(seeFig.1).TheW-to-NandE-to-Speakratios
are higher in HCN (∼0.6-1.0) than in CO (∼0.4-0.6; Table 1)
explaining why the positions of the merged peaks in Kohno et
al. (1999a) do not agree between HCN and CO. The HCN-to-
CO ratios (≡R =I /I ; with I≡velocity inte-
HCN/CO HCN(1−0) CO(1−0)
gratedintensity)thusalsodifferbetweentheS/NandW/Epeaks.
WhileR isfoundtoberoughly∼0.03atSandN(assum-
HCN/CO
ing I /I =1; Table 1), we estimate R ≃ 0.05
CO(2−1) CO(1−0) HCN/CO
at E and W. This might indicate a variation of the gas den-
sity/temperaturealongthering.TheHCNkinematicsinthering
agreewellwiththoseseeninCOandarehencenotfurtherdis-
cussedinthisletter.
3.2.Centralemission
HCN(1–0) emission is detected in the central ∼1′′ (≃100 pc;
≡C) because of the higher sensitivity and angular resolution
Fig.2. Iso-velocity map of HCN(1–0) (solid red & dashed of our data compared to the one obtained by Kohno et al.
blue lines) overlaidto the integratedHCN(1–0)emission (grey (1999a), confirming previous hints found in CO(2–1) (Garc´ıa-
scale & grey contours (same as in Fig. 1)). Contours run from Burillo et al. 2005; Schinnerer et al., in prep.). The nuclear
vLSR=1349 kms−1 (dashed blue) to 1479 kms−1 (solid red) HCN component C appears to be compact and unresolved in
in steps of 10 kms−1aroundthe dynamicalcenter (∼10 kms−1 the PdBI beam. Assuming the “HCN conversion” factor of
offfromthesystemicvelocity).TheblacklinesindicatethePA X (=M (H )/L )=20+30 M (K kms−1 pc2)−1 from
(=160±20◦)ofthevelocitygradient. SoHlCoNmon eHtCNal. (2199H2C)N, we fi−n10d a c⊙entral dense gas mass of
M (=M(H +He))≈(2-10)·107M which is a factor of ∼3-17
gas 2 ⊙
and δ =66◦06′19.70′′. The 3mm receivers were tuned to higher than the mass of ∼6·106M⊙ derived from the CO(2–
J2000 1) line (assuming I /I =1 (Table 1; see also Garc´ıa-
the frequency of HCN(1–0) shifted to the LSR velocity of CO(2−1) CO(1−0)
v =1424kms−1, while the 1mm receivers were set to the Burilloetal.2005).Thislargediscrepancycannotbesolelydue
LSR to the large uncertainties of the conversion factors but might
13CO(2–1) line. The 1mm data will be discussed in a separate
further indicate non-standard gas conditions in the AGN. The
paper(Kripsetal.,inprep.).Upperandlowersidebandsateach
HCN-to-CO ratio of the velocity integrated line flux amounts
frequency had 580 MHz bandwidth and 1.25 MHz resolution.
to ≥0.4 (Table 1; I /I =1). This is a factor of ∼4-8
Weatherconditionsweregoodthroughouttheobservationswith CO(2−1) CO(1−0)
largerthaninthestarburstringandthevalueof0.09forthenu-
awatercolumnvaryingbetween4mmand10mmandSSBsys-
cleuspublishedbyKohnoet al. (1999a)indicatingthat the nu-
temtemperaturesof100-150K(A+B)and100-200K(C+D)at
clearHCNissignificantlyenhancedinNGC6951.Thus,unlike
3mm.3C273,3C454.3,2200+420and1749+096wereusedas
Kohnoet al. (1999a),we concludethatthe nucleargas proper-
bandpasscalibratorswhile1928+738and2037+511wereused
tiesinNGC6951arenotthatdifferentfromthoseintheSeyfert
asgaincalibrators.Fluxeshave beencalibratedwith MWC349
galaxiesNGC1068orM51,bothofwhichhavesimilarlyhigh
andcheckedon3C273,1928+738and2037+511withtheflux
central HCN-to-CO ratios. The molecular gas chemistry in the
monitoringprogramoftheIRAMPdBI,resultinginanaccuracy
AGN influencedregionsof NGC 1068and NGC 5194is dom-
of ∼10% at 3mm. The data were calibrated, mapped and ana-
inated by X-ray radiation (NGC 1068: e.g., Usero et al. 2004;
lyzed using the standard IRAM GILDAS programs CLIC and
M51:e.g.,Matsushitaetal.1998)suggestingasimilarscenario
MAPPING.Usinguniformweighting,thesynthesizedbeamsize
is1.25′′×0.87′′ atapositionangle(PA)of72◦ at3mm;natural inNGC6951.Thus,thecentraldensegasmassderivedviaHCN
weightingresultsin2.78′′×2.27′′ataPAof112◦.Wereachan hastobeconsideredanupperlimit.
rmsnoiseof∼0.8mJybeam−1 (∼1.2mJybeam−1) ata spectral ThecentralHCN emission showsthe steepestvelocitygra-
resolution of 5 MHz (≡17kms−1) at 3.4 mm for natural (uni- dient at PA≈160◦ with a velocity range of ±70 kms−1 around
form)weighting. the dynamical center over a radius of ∼0.5′′ (Fig. 2), eventu-
ally indicating a gas rotation in a circumnuclear disk or torus.
Even if accounting for the uncertainties of the PA, estimated
3. Results&Discussion to be ±20◦, the central kinematic axis is significantly different
from the major axis of the galaxy (PA=135◦) and of the bar
3.1.Starburstring
(PA=100◦). This non-alignmentmightbe caused by a different
HCN emissionisclearlydetectedalongthe starburstring,con- inclination of the central gas disk/torus than of the galaxy, or,
sistent with Kohno et al. (1999a).The total flux observed with alternatively,bynon-circularvelocitiesorawarp.However,the
thePdBIislowerbyafactorof∼2-3thanthefluxobtainedwith central velocity gradient allows us to give a crude estimate of
the IRAM 30m telescope (Krips et al., in prep.), i.e., some of theencloseddynamicalmass.Assumingaradiusof∼50pcand
the HCN emission is resolved out by the PdBI. The two main avelocityrangeof±70kms−1,wefindM =5·107·(sini)−2M ,
dyn ⊙
maxima,whichare seenin HCN byKohnoetal. (1999a)atan withi≡inclinationofthedisk.Thisissimilartothegasmassof
angularresolutionof∼4.5′′ andpeakatdifferentpositionsthan 0.6-8·107M , suggestingthatthe disk is seen closerto face-on
⊙
4 Kripsetal.:HCN(1–0)emissioninNGC6951
thantoedge-on.Assumingfurthertheinclinationofthegalaxy Kohno, K.; Matsushita, S.; Vila-Vilar, B.; Okumura, S. K.; Shibatsuka, T.;
(≈40◦;Garc´ıa-Burilloetal.2005)asroughestimatefortheone Okiura,M.;Ishizuki,S.;Kawabe,R.,TheCentralKiloparsecofStarbursts
of the central gas disk, the enclosed dynamicalmass increases andAGN:TheLaPalmaConnection,ASPConferenceProc.Vol.249.ed.
byJ.H.Knapen,J.E.Beckman,I.Shlosman,andT.J.Mahoney.ASP,2001,
to ∼2·108M . This is of the order of the nuclear black hole
⊙ p.672.
massof∼2·108M⊙,estimatedviathestellarvelocitydispersion Kohno,K.,Theevolutionofstarbursts:The331stW.andE.HeraeusSeminar,
ofthebulge(σ =232kms−1fromHo&Ulvestad2001;seealso AIPConferenceProceedings,2005,Volume783,p.203
s
Gebhardtet al. 2000).However,within a radiusof 50 pc, stars Lepp,S.;Dalgarno,A.,1996,A&A,306,21
Lutz,D.;Maiolino,R.;Spoon,H.W.W.;Moorwood,A.F.M.,2004,A&A,418,
mightstillcontributeasignificantfractiontothedynamicalmass
465L
aswellpointingtowardsanevenlowerinclinationofthenuclear
Maloney,P.R.;Hollenbach,D.J.;Tielens,A.G.,1996,ApJ,466,561
diskthanassumed. Pe´rez, E.; Ma´rquez, I.; Marrero, I.; Durret, F.; Gonza´lez Delgado, R. M.;
Masegosa,J.;Maza,J.;Moles,M.,2000,A&A,353,893
Ma´rquezI.,&MolesM.,1993,AJ,105,2090
4. Conclusions Matsushita,S.;Kohno,K.;Vila-Vilaro,B.;Tosaki,T.;Kawabe,R.,1998,ApJ,
495,267
We have presented high angular resolution/sensitivity observa- Nguyen,Q.-R.;Jackson,J.M.;Henkel,Ch.;Truong,B.;Mauersberger,R.,1992,
tionsoftheHCN(1–0)emissioninNGC6951inthisletterwhich ApJ,399,521
RozasM.,BeckmanJ.E.,KnapenJ.H.,1996,A&A307,735
exploit the new ABCD configurationsat the IRAM PdBI. The
SaikiaD.J.,PedlarA.,UngerS.W.,etal.,1994,MNRAS,270,46
mainresultscanbesummarizedasfollows: Saikia,D.J.;Phookun,B.;Pedlar,A.;Kohno,K.,2002,A&A,383,98
Solomon,P.M.;Downes,D.;Radford,S.J.E.,1992,ApJ,387,55
1. The two main HCN peaks in the ring seen by Kohno et Sternberg,A.;Genzel,R.;Tacconi,L.,1994,ApJ,436,131
al.(1999a)splitupintoseveralsub-maximawhichcoincide Tacconi, L.J.;Genzel, R.;Blietz, M.;Cameron, M.;Harris,A.I.;Madden, S.,
1994,ApJ,426,77
withthosefoundinCO(2–1)byGarc´ıa-Burilloetal.(2005)
Tacconi,L.J.;Blietz,M.;Cameron,M.;Downes,D.;Genzel,R.;Krabbe,A.;
andSchinnereretal.(inprep.).However,theratiosofpeaks
Sternberg,A.;Tacconi-Garman,L.E.;Weitzel,L.,1996,VA,40,23
withintheringdifferbetweenHCNandCO,probablyindi- Tielens,A.G.G.M.,&Hollenbach,D.,1985,ApJ,291,722
catingdifferentgasdensities/temperaturesalongthering. Tully, R.B., 1988, Nearby Galaxies Catalogue (Cambridge University Press,
2. We clearly detect compact HCN(1–0) emission in the nu- Cambridge)
Usero, A.; Garc´ıa-Burillo, S.; Fuente, A.; Mart´ın-Pintado, J.; Rodr´ıguez-
cleus of NGC 6951 tracing a dense molecular gas mass in
Ferna´ndez,N.J.,2004,A&A,419,897
the range of ∼0.6-6×107M⊙. A position velocity cut taken VilaM.B.,PedlarA.,DaviesR.D.,etal.,1990,MNRAS242,379
alongPA=(160±20)◦,coincidingneitherwiththegalaxynor WozniakH.,FriedliD.,MartinetL.,etal.,1995,A&AS,111,115
the bar major axis, yields the steepest velocity gradient in
the nucleus. The gradientand the compactnessof this cen-
tral HCN emission suggest that it might arise in a rotating
circumnucleargasdiskortoruswitharadiusof.50pc.
3. In contrast to Kohno et al. (1999a), we find significantly
different HCN-to-CO line ratios in the starburst ring and
the nuclear emission. While the starburst ring shows typi-
cal R =0.02-0.05, HCN is significantly enhanced in
HCN/CO
the centre with R ≥0.4. Either the molecular gas
HCN/CO
in the nucleus is denser and/or hotter than in the starburst
ring, increasing R , or the gas chemistry in the nu-
HCN/CO
cleus of NGC 6951 is dominated by X-ray radiation from
the AGN, producinga higher nuclear HCN abundance and
thusahigherR similartothecenterofNGC1068.
HCN/CO
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