Molecular Gas in NUclei of GAlaxies (NUGA) XIV. The barred LINER/Seyfert 2 galaxy NGC 3627

1.5 MB·English

by  V. Casasola

Astronomy&Astrophysicsmanuscriptno.ngc3627˙astro-ph January14,2011
(DOI:willbeinsertedbyhandlater)
Molecular Gas in NUclei of GAlaxies (NUGA)
⋆
XIV. The barred LINER/Seyfert 2 galaxy NGC3627
V.Casasola1,2,L.K.Hunt1,F.Combes3,S.Garc´ıa-Burillo4,andR.Neri5
1 INAF-OsservatorioAstrofisicodiArcetri,LargoE.Fermi,5,50125Firenze,Italy
2 INAF-IstitutodiRadioastronomia,viaGobetti101,40129Bologna,Italy
3 ObservatoiredeParis,LERMA,61Av.del’Observatoire,F-75014,Paris,France
1
4 ObservatorioAstrono´micoNacional(OAN)-ObservatoriodeMadrid,C/AlfonsoXII,3,28014Madrid,Spain
1
5 IRAM-InstitutdeRadioAstronomieMillime´trique,300RuedelaPiscine,38406-St.Mt.d‘He`res,France
0
2
Received;accepted
n
a
J Abstract. Wepresent12CO(1–0)and12CO(2–1)mapsoftheinteractingbarredLINER/Seyfert2galaxyNGC3627obtained
withtheIRAMinterferometeratresolutionsof2′.′1×1′.′3and0′.′9×0′.′6,respectively.Wealsopresentsingle-dishIRAM30m
3
1 12CO(1–0) and 12CO(2–1) observations used to compute short spacings and complete interferometric measurements. These
observationsarecomplementedbyIRAM30mmeasurementsofHCN(1–0)emissiondetectedinthecenterofNGC3627.The
] moleculargasemissionshowsanuclearpeak,anelongatedbar-likestructureof∼18′′ (∼900pc)diameterinboth12COmaps
O and,in12CO(1–0),atwo-armspiralfeaturefromr∼9′′(∼450pc)tor∼16′′(∼800pc).Theinner∼18′′bar-likestructure,with
C anorth/south orientation(PA=14◦),formstwopeaks attheextremes ofthiselongated emissionregion. Thekinematicsof
. theinnermoleculargasshowssignaturesofnon-circularmotionsassociatedbothwiththe18′′bar-likestructureandthespiral
h featuredetectedbeyondit.The1.6µmH-band2MASSimageofNGC3627showsastellarbarwithaPA=−21◦,differentfrom
p
thePA(=14◦)ofthe12CObar-likestructure,indicatingthatthegasisleadingthestellarbar.Thefar-infraredSpitzer-MIPS70
-
o and160µmimagesofNGC3627showthatthedustemissionisintensifiedatthenucleusandattheansaeattheendsofthe
r bar,coincidingwiththe12COpeaks.TheGALEXfar-ultraviolet(FUV)morphologyofNGC3627displaysaninnerelongated
st (north/south)ringdelimitingaholearoundthenucleus,andthe12CObar-likestructureiscontainedintheholeobservedinthe
a FUV.ThetorquescomputedwiththeHST-NICMOSF160WimageandourPdBImapsarenegativedowntotheresolution
[ limitofourimages,∼60pcin12CO(2–1).Ifthebarendsat∼3kpc,coincidentwithcorotation(CR),thetorquesarenegative

1 betweentheCRofthebarandthenucleus,downtotheresolutionlimitofourobservations.Thisscenarioiscompatiblewith
v arecently-formedrapidlyrotatingbarwhichhashadinsufficienttimetoslowdownbecauseofsecularevolution,andthushas
6 notyetformedaninnerLindbladresonance(ILR).ThepresenceofmoleculargasinsidetheCRoftheprimarybar,wherewe
2 expect that the ILRwillform, makes NGC3627 a potential smoking gun of inner gas inflow. Thegasisfueling thecentral
6 region,andinasecondstepcouldfueldirectlytheactivenucleus.
2
.
1 Keywords.galaxies:individual:NGC3627–galaxies:spiral–galaxies:active–galaxies:nuclei–galaxies:ISM–galaxies:
0 kinematicsanddynamics
1
1
:
v
i
X 1. Introduction the galaxies of our sample, and to study the different mecha-
r nismsforgasfuelingofLLAGN.
a TheNucleiofGalaxies(NUGA)project(Garc´ıa-Burilloetal.
NUGA galaxies analyzed so far show that there is
2003) isanIRAM Plateau deBureInterferometer(PdBI)and
no unique circumnuclear molecular gas feature linked with
30msingle-dishsurveyofnearbylow-luminosityactivegalac-
nuclear activity, but rather a variety of molecular gas
tic nuclei (LLAGN). The aim is to map, at high resolution
morphologies which characterize the inner kpc of active
(∼0′.′5-2′′) and high sensitivity (∼2-4mJybeam−1), the distri-
galaxies. We have found one- and two-armed instabili-
bution and dynamicsof the moleculargas in the inner kpc of
ties (Garc´ıa-Burilloetal. 2003), well-ordered rings and nu-
clear spirals (Combesetal. 2004; Casasolaetal. 2008a), cir-
Sendoffprintrequeststo:[email protected] cumnuclear asymmetries (Kripsetal. 2005), large-scale bars
⋆ Based on observations carried out with the IRAM Plateau de (Booneetal. 2007; Huntetal. 2008), and smooth disks
BureInterferometer.IRAMissupportedbytheINSU/CNRS(France), (Casasolaetal. 2010). Among these morphologies, analyz-
MPG(Germany),andIGN(Spain). ing the torques exerted by the stellar gravitational potential
2 Casasolaetal.:NUGAXIV:NGC3627
on the molecular gas shows that only four NUGA galaxies: Table1.FundamentalparametersforNGC3627.
NGC6574 (Lindt-Kriegetal. 2008), NGC2782 (Huntetal.
2008), NGC3147 (Casasolaetal. 2008a), and NGC4579 Parameter Valueb Referencec
(Garc´ıa-Burilloetal. 2009) show evidence for gas inflow. α a 11h20m15.02s (1)
J2000
Thesegalaxieshaveseveralfeaturesincommon:(1)alargecir- δJ2000a 12◦59′29′.′50 (1)
cumnuclearmassconcentration(i.e.,adominantstellarbulge); Vhel 744kms−1 (1)
(2) a high circumnuclear molecular gas fraction (>∼10%); and RC3Type SAB(s)b (2)
NuclearActivity LINER/Seyfert2 (3)
(3) kinematically decoupledbars with overlappingdynamical
Inclination 61◦.3 (1)
resonances.Thelargeamountofgasaroundthenucleus,com-
PositionAngle 178◦±1◦ (1)
binedwithdynamicalfeaturesthatenablethegastopenetrate
Distance 10.2Mpc(1′′=49pc) (2)
theinnerLindbladResonance(ILR),seemtobenecessary(and L 4.2×1010L (4)
B ⊙
perhaps sufficient) ingredients for inducing gas inflow in cir- M 8.1×108M (5)
HI ⊙
cumnuclearscales. M 4.1×109M (6)
H2 ⊙
The existence of different nuclear molecular morpholo- Mdust(60and100µm) 4.5×106M⊙ (4)
gies can be sought in the variety of timescales characteriz- LFIR 1.2×1010L⊙ (7)
ing nuclear activity. Strong fueling only lasts for a time of
tfuel ∼ 0.002×tH, where tH ∼ 1.4×1010yr is the age of the a (αJ2000,δJ2000)isthephasetrackingcenterofour12COinterfero-
metricobservations,assumedcoincidentwiththedynamicalcen-
Universe (Heckmanetal. 2004). Thus, the total time during
terofNGC3627(seeSect.4.1).
which strong fueling can occur is around t ∼ 3 × 107yr;
fuel b Luminosity and mass values extracted from the literature have
if there are N fueling events per black hole per Hubble time,
beenscaledtothedistanceofD=10.2Mpc.
eacheventwouldhaveadurationoft ∼3×107/Nyr.This
event c (1) This paper; (2) NASA/IPAC Extragalactic Database (NED,
impliesthatthestrongaccretionphaseisafraction≃0.3/N of
http://nedwww.ipac.caltech.edu/); (3) Hoetal. (1997); (4)
thecharacteristicgalaxydynamicaltime(∼ 108yr).Although Casasolaetal. (2004); (5) Haanetal. (2008); (6) Kunoetal.
large-scalebarscanproducegasinflow(e.g.,Combes&Gerin (2007);(7)IRASCatalog.
1985;Sakamotoetal.1999)andinsomecasesalsodrivepow-
erfulstarbursts (e.g., Knapenetal. 2002; Jogeeetal. 2005), a
correlation between large-scale bars and nuclear activity has
notyetbeenverified(e.g.,Mulchaey&Regan1997).Thislack
of correlation is probably related to the different timescales aLINER/Seyfert2typenuclearactivity(Hoetal.1997).With
for bar-induced gas inflow (&300 Myr, Jogeeetal. 2005),
NGC3623andNGC3628,itformsthewell-knownLeoTriplet
AGN duty cycles (∼107yr), and intermittent active accre-
(M66 Group, VV308). Since the discovery of a long plume
tion every ∼108yr (Ferrareseetal. 2001; Mareckietal. 2003; in Hi extending about 50′ to the east of NGC 3628 (Zwicky
Janiuketal.2004;Hopkins&Hernquist2006;King&Pringle
1956; Haynesetal. 1979), evidence of past interactions be-
2007). The comparison of these different timescales suggests
tweenNGC3627andNGC3628(thetwolargestspiralsinthe
thatmostAGNareinanintermediatephasebetweenactiveac-
group),the Leo Triplethas been extensivelystudied from the
cretionepisodesmakingthedetectionofgalaxieswithnuclear
radio to the optical, and in X-ray bandpasses. Optical broad-
accretionsomewhatdifficult.
bandimagesofNGC3627revealapronouncedandasymmet-
Gravitational torques act on timescales of ∼106−7yr and ricspiralpatternwithheavydustlanes,indicatingstrongden-
are the most efficient mechanism in driving the gas from sity wave action (Ptaketal. 2006). While the western arm is
large spatial scales (some tens of kpc) to intermediate spatial accompaniedby weak traces of star formation(SF) visible in
scales(afewhundredsofpc).Dynamicalfrictionandviscous Hα,theeasternarmcontainsastar-formingsegmentinitsinner
torquesareofteninvoked,inadditiontogravitationaltorques, part(Smithetal.1994).NGC3627alsopossessesX-rayprop-
as possible mechanisms of AGN fueling. However, dynami- ertiesofagalaxywitharecentstarburst(Dahlemetal.1996).
cal friction of giant molecular clouds in the stellar bulge of Boththeradiocontinuum(2.8cmand20cm)andthe12CO(1–
a galaxy tends to be a slow, inefficient process which, to first 0) emissions show a nuclear peak, extend along the leading
approximation, can be neglected relative to gravity torques edges of the bar forming two broad maxima at the bar ends,
(Garc´ıa-Burilloetal. 2005). Viscous torques can be more ef- andthenthespiralarmstrailofffromthebarends(Haanetal.
fective,andarefavoredinthepresenceoflargedensitygradi- 2008;Paladinoetal.2008;Haanetal.2009).Onthecontrary,
entsandhighgalacticshear(seeGarc´ıa-Burilloetal.2005,for the Hi emission exhibits a spiral morphology without signa-
details).Nevertheless,theyarerelativelyinefficientwhenthere turesofabarintheatomicgas(Haanetal.2008;Walteretal.
arestrong(positive)gravitytorques. 2008;Haanetal.2009).
This paper is dedicated to the galaxy NGC3627, the ThemostrecentHimassdeterminationforNGC3627has
eleventh object of the core NUGA sample studied on a case- been obtained by Haanetal. (2008), M = 8.1×108M (re-
HI ⊙
by-case basis. NGC3627 (Messier 66, D = 10.2Mpc, H = portedto our adopteddistance of D = 10.2Mpc), on average
0
73kms−1 Mpc−1)isaninteracting(e.g.,Casasolaetal.2004) lessthanthetypicalvalueexpectedforinteractinggalaxiesof
andbarredgalaxyclassifiedasSAB(s)bshowingsignaturesof thesameHubbletype(Casasolaetal.2004).TheH masscon-
2
Casasolaetal.:NUGAXIV:NGC3627 3
Table2.12CO(1–0)fluxvalues,bothobtainedbyourobservationsandextractedfromtheliterature,forNGC3627.
Reference Telescope Diameter PrimarybeamorFOVa Beam Flux
[m] [′′] [′′×′′] [Jykms−1]
Youngetal.(1995) FCRAO 14 45 786
Thispaper PdBI+30m 42 2.1×1.3 668
Thispaper PdBI+30m 22b 2.1×1.3 359
Thispaper PdBI 22b 2.0×1.3 251
Thispaper 30m 30 22(centralposition) 343c
Helferetal.(2003) NRAO 12 55(inner50′′×50′′) 1100–1200
Thispaper 30m 30 22(inner50′′×50′′) 1097d
a Primary beam is considered for single-dish observations, while field-of-view (FOV) for interferometric or combined
(interferometric+single-dish)ones.
b Thephotometryhasbeenperformedwithin22′′,the12CO(1–0)primarybeamforthe30mtelescope.
c The12CO(1–0)recoveredfluxforthecentralposition(0′′,0′′).
d The12CO(1–0)recoveredfluxforinner∼50′′×50′′,5×5mappingwith7′′spacing(seeSect.2.2).
tentestimatedbyKunoetal.(2007)is4.1×109M (scaledto 2. Observations
⊙
ourdistanceofD=10.2MpcforNGC3627).
2.1.Interferometricobservations
TheseH andHimassvaluesgiveaH /Himassratioof5.1,
2 2
We observedNGC3627with the IRAM PdBI (6 antennas)in
highcomparedtotheaverageratioexpectedforgalaxiessim-
theABCDconfigurationofthearraybetween2003September
ilartoNGC3627,M /M = 0.9(Casasolaetal.2004).The
highH /HimassratioHi2nNHGIC3627isprobablyduetothetidal and 2004 February in the 12CO(1–0) [115GHz] and the
2 12CO(2–1) [230GHz] line. The PdBI receiver characteristics,
interaction with NGC3628, since this galaxy has “captured”
muchoftheHiinNGC3627(Zhangetal.1993). the observing procedures, and the image reconstruction are
similar to those described in Garc´ıa-Burilloetal. (2003). The
Other molecular transitions have been detected in quasar3C454.3was usedforbandpasscalibration,3C273for
NGC3627, including HCN(1–0), HCN(2–1), HCN(3–2), flux calibration, and 1546+027 for phase and amplitude cali-
HCO+(1–0),andHCO+(3–2),suggestingthepresenceofhigh brations.
densitygas(Gao&Solomon2004;Kripsetal. 2008).We list Data cubes with 512 × 512 pixels (0′.′27pixel−1 for
inTable1themainobservationalparametersofNGC3627. 12CO(1–0)and0′.′13pixel−1for12CO(2–1))werecreatedover
avelocityintervalof-242.5kms−1to+242.5kms−1inbinsof
The structure of this paper is as follows. In Sect. 2, we 5kms−1.Theimagespresentedherewerereconstructedusing
describe our new observations of NGC3627 and the litera- the standard IRAM/GILDAS2 software (Guilloteau&Lucas
ture datawith which we comparethem. InSects. 3 and4, we 2000) and restored with Gaussian beams of dimensions 2′.′0
present the observational results, both single-dish and inter- × 1′.′3 (PA = 23◦) at 115GHz and 0′.′9 × 0′.′6 (PA = 28◦)
ferometric,describingmorphology,excitation conditions,and
at 230GHz. We used natural and uniform weightings to gen-
kinematicsofthemoleculargasintheinnerkpcofNGC3627. erate 12CO(1–0) and 12CO(2–1) maps, respectively. This al-
Comparisonsbetween12COobservationsandthoseobtainedat lows to maximize the flux recovered in 12CO(1–0) and opti-
otherwavelengthsaregiveninSect.5.InSect.6,wedescribe mizethespatialresolutionin12CO(2–1).Inthecleanedmaps,
thecomputationofthegravitytorquesderivedfromthestellar the rms levels are 3.7mJybeam−1 and 6.7mJybeam−1 for
potentialin the inner regionof NGC3627,and in Sect. 7, we the 12CO(1–0) and 12CO(2–1) lines, respectively at a veloc-
giveandynamicalinterpretationoftheresults.Finally,Sect.8 ity resolution of 5kms−1. At a level of 3σ no 3mm (1mm)
summarizesourmainresults.
continuum was detected toward NGC3627 down to an rms
noise level of 0.34mJybeam−1 (0.48mJybeam−1). The con-
We assume a distance to NGC3627 of D = 10.2Mpc,
version factors between intensity and brightness temperature
(HyperLeda DataBase1) and a Hubble constant H =
0 are 34K(Jybeam−1)−1 at 115GHz and 41K(Jybeam−1)−1 at
73kms−1Mpc−1. This distance means that 1′′ correspondsto
230GHz. All velocities are referred to the systemic veloc-
49pc.
ity V = 744 kms−1 and (∆α,∆δ) offsets are relative to
sys,hel
the phase tracking center of our observations (11h20m15.02s,
12◦59′29.50′′) [see later Sect. 4.1]. All maps are centered on
1 Patureletal.(2003),http://leda.univ-lyon1.fr 2 http://www.iram.fr/IRAMFR/GILDAS/
4 Casasolaetal.:NUGAXIV:NGC3627
Fig.1. SpectramapsofNGC3627madewiththeIRAM30mwith7′′spacingin12CO(1–0)[top]and12CO(2–1)[bottom].The
positionsarearcsecoffsetsrelativetothephasetrackingcenterofourinterferometricobservations(seeTable1).Eachspectrum
hasavelocityscalefrom−300to300kms−1,andabeam-averagedradiationtemperaturescale(T )from−0.10to0.48Kfor
mb
12CO(1–0)andfrom−0.25to0.70Kfor12CO(2–1).
this position (see Table 1) and are not corrected for primary 2.2.Single-dishobservations
beamattenuation.
We performed IRAM 30m telescope observations of
NGC3627 on July 16-19, 2002, in a 5 × 5 raster pattern
with7′′ spacing.Byusing4SISreceivers,wesimultaneously
observed the frequencies of the 12CO(1–0) [115GHz], the
Casasolaetal.:NUGAXIV:NGC3627 5
12CO(2–1)[230GHz],andtheHCN(1–0)[89GHz]lines.The
12CO(2–1) line has been observed in dual-polarization. The
half power beam widths (HPBW) are 22′′, 12′′, and 29′′ for
12CO(1–0), 12CO(2–1), and HCN(1–0) lines, respectively.
Typical system temperatures were ∼110-145K at 115GHz,
∼320-750K at 230GHz, and ∼110-145K at 89GHz. For the
single-dish data reduction, the Continuum and Line Analysis
Single-dish Software (CLASS2) was used. Throughout the
paper we express the line intensity scale in units of the
beam-averaged radiation temperature (T ). T is related
mb mb
to the equivalent antenna temperature reported above the
atmosphere (T∗) by η =T∗/T , where η is the telescope
A A mb
main-beam efficiency. At 115GHz η = 0.79, at 230GHz η =
0.54,andat89GHzη=0.82.Allobservationswereperformed
in“wobbler-switching”mode,withaminimumphasetimefor Fig.2. HCN(1–0) spectrum toward the center of NGC3627,
spectral line observations of 2s and a maximum beam throw averaged over the 25-point map made with the IRAM 30m
of240′′.Thepointingaccuracywas∼3′′ rms.Thesingle-dish with7′′ spacing.Thespectrumhasavelocityscalefrom−850
mapspresentedinthispaperarecenteredonthephasetracking to850kms−1andabeam-averagedradiationtemperaturescale
centerofourinterferometricobservations(seeTable1). (Tmb)from−0.003to0.010K.
2.3.Shortspacingcorrection
fluxes we obtained with interferometric observations, single-
Aninterferometerislimitedbytheminimumspacingofitsan-
dish, and combined measurements (PdBI+30m) are in good
tennas. Because two antennas can not be placed closer than
mutual agreement with each other and with data present in
someminimumdistance(D ),signalsonspatialscaleslarger
min
literature. Our 30m observations give a value S =
thansomesize(∝λ/D )willbeattenuated.Thiseffect,called CO(1−0)
min 343Jykms−1 for the central position, consistent with flux
the “missing flux” problem, is resolved by using single-dish
valuefoundwithPdBI+30mdatawithinthe12CO(1–0)30m–
observationstocomputeshortspacingsandcompletetheinter-
HPBW (22′′). The whole region covered with 30m observa-
ferometricmeasurements.
tions (∼50′′×50′′) gives a flux of 1097Jy km s−1, in agree-
By combining 30m and PdBI data, we found the best
ment with the BIMA SONG survey3 (NRAO 12m) measure-
compromise between good angular resolution and complete
ments(Helferetal.2003,seeFig.50,∼1100–1200Jykms−1).
restoration of the missing extended flux by varying the rel-
Moreover,within the 42′′primary beam field of the PdBI, we
ative weights of 30m and PdBI observations. The combined
recovered∼85%ofthefluxdetectedbyYoungetal.(1995)for
PdBI+30m maps have angular resolutions of 2′.′1 × 1′.′3 at
the central position with the FCRAO (786Jykms−1), a good
PA = 23◦ for the 12CO(1–0) and 0′.′9 × 0′.′6 at PA = 30◦ for
agreement considering the uncertainties in the amplitude cal-
the 12CO(2–1). In the combined maps, the rms uncertainty σ
ibration and the non-correctionby the primary beam attenua-
in5kms−1 widthvelocitychannelsis3.6mJybeam−1 and6.5
tion.
mJybeam−1 for the 12CO(1–0) and 12CO(2–1) lines, respec-
tively.Forthesemaps,theconversionfactorsbetweenintensity
andbrightnesstemperatureare32K(Jybeam−1)−1 at115GHz 2.4.OtherimagesofNGC3627
and 41K(Jybeam−1)−1 at 230GHz. All interferometric fig-
We also acquired the large-scale 12CO(1–0) emission image
ures presented in this paper are realized with short-spacing-
available thanks to the BIMA SONG survey performed with
correcteddata.
the 10-element BIMA millimeter interferometer (Welchetal.
Within22′′,the12CO(1–0)HPBWforthe30mtelescope,
1996)atHatCreek,California.Thisimagewasfirstpublished
the map including only PdBI observations recovers a flux
by Reganetal. (2001) and Helferetal. (2003), and covers a
S = 251 Jy km s−1, 70% of the total flux measured
CO(1−0) field of 350′′×410′′(centered on the galaxy) with a pixel size
with the merged PdBI+30m map, S = 359 Jy km s−1.
CO(1−0) of1′′,andabeamof6′.′6×5′.′5.
Table 2 reports both 12CO(1–0) flux values determined with
Several infrared (IR) images are included in our anal-
our observations (single-dish, interferometric, and combined
ysis: the Spitzer-IRAC 3.6µm image (to trace the stellar
PdBI+30m) andthose presentin literature. In this table, Col.
component), the Spitzer-IRAC 8µm image (to visualize the
(1)indicatesthe reference,Cols. (2)and (3)are the telescope
Polycyclic Aromatic Hydrocarbons [PAH] features), and the
(single-dishor interferometer)and the diameterof the single-
Spitzer-MIPS70 and 160µm images (to study the dust emis-
dishtelescoperespectively,Col.(4)istheprimarybeamofthe
sion and resolve the SF regions). These IR images are avail-
instrument or the diameter used for the performed photome-
try,Col.(5)isthebeamininterferometricmeasurements,and 3 Berkley-Illinois-Maryland Association Survey of Nearby
Col.(6)givesthemeasuredflux.Table2showsthat12CO(1–0) Galaxies.
6 Casasolaetal.:NUGAXIV:NGC3627
Fig.3. Leftpanel:12CO(1–0)integratedspectrumandgaussianfit(red)intheinner∼2′′ofNGC3627forPdBI+30mcombined
data.Thegaussianfitshowsthattheheliocentricsystematicvelocityisredshiftedby16kms−1 withrespecttotheheliocentric
velocity of the center (0 kms−1). Right panel: Same for 12CO(2–1). The gaussian fit shows that the heliocentric systematic
velocityisredshiftedby18kms−1.
Fig.4.Leftpanel:12CO(1–0)integratedintensitycontoursobservedwiththeIRAMPdBI+30mtowardthecenterofNGC3627.
Thewhitestarmarksthecoordinatesofthedynamicalcenterofthegalaxycoincidentwithourphasetrackingcenter(seeTable
1),withoffsetsinarcseconds.Themap,derivedwith2σclipping,hasnotbeencorrectedforprimarybeamattenuation.Therms
noiselevelisσ=0.16Jybeam−1kms−1andcontourlevelsrunfrom3σto33σwith6σspacingandfrom39σtothemaximum
with18σspacing.Inthismapthe±200kms−1velocityrangeisused.Thebeamof2′.′1×1′.′3isplottedinthelowerleft.Right
panel:Samefor12CO(2–1).Thermsnoiselevelisσ = 0.30Jybeam−1kms−1 andcontourlevelsrunfrom3σto39σwith6σ
spacingandfrom45σtothemaximumwith18σspacing.Thebeamof0′.′9×0′.′6isplottedatlowerleft.
ablethankstotheproject“SINGS:TheSpitzerInfraredNearby We also use twonear-infrared(NIR)images H (1.65µm):
Galaxies Survey” (Kennicuttetal. 2003). The IRAC images the first was taken from the Two Micron All Sky Survey
cover a sky area of ∼1600′′×1890′′and ∼1220′′×1420′′at (2MASS)andcoversaFOVof∼12′×12′,witharesolutionof
3.6µm and 8µm respectively, both with a pixel size of 0′.′75, 2′.′5. Thesecond1.6µm H-bandimage ofNGC3627is avail-
andspatialresolutionsof∼1-2′′.TheMIPS70µmimagecov- able thanks to the F160W filter on the Near-Infrared Camera
ers ∼1940′′×3645′′with a pixel size of 4′.′5, and the MIPS and Multi-Object Spectrometer (NICMOS, camera 3 [NIC3])
160µmimage∼2025′′×3460′′withapixelsizeof9′′. onboardtheHubbleSpaceTelescope(HST).Thisimagecov-
Casasolaetal.:NUGAXIV:NGC3627 7
lowsustoderivetheH masswithintheobservedregionas:
2
M [M ] = 8.653×103D2[Mpc]S [Jykms−1] (1)
H2 ⊙ CO(1−0)
We derive an H mass of M ∼9.9×108M within the in-
2 H2 ⊙
ner ∼50′′×50′′, and taking into account the mass of helium,
the total molecular mass is M = M = 1.36 ×
mol H2+He
M ∼1.3×109M .
H2 ⊙
The HCN(1–0) line has been observed for inner 25 po-
sitions with 7′′ spacing, covering the central ∼56′′ (∼2.7kpc
in diameter). The HCN(1–0) average spectrum over the 5×5
grid is displayed in Fig. 2 and shows a peak at T ∼0.009K.
mb
The HCN(1–0) velocity integrated intensity of the central
position (0′′, 0′′) is I = 3.1±0.3Kkms−1 with ∆v
HCN(1−0)
= 237±32kms−1, consistent with the results obtained by
Kripsetal. (2008) for the same position observed with the
same instrument (I = 2.7±0.2Kkms−1 with ∆v =
HCN(1−0)
290±30kms−1).TheCO(1–0)/HCN(1–0)ratioaveragedonthe
center of galaxy is roughly 10, a value intermediate between
Fig.5. Color scale of the CO(2–1)/CO(1–0) ratio map and
the ratios found in spatially resolved molecular disks around
12CO(1–0)intensitymapcontoursasinFig.4(leftpanel).
AGN, such as NGC6951 (Kripsetal. 2007) and NGC1068
(Kripsetal. 2008), and those foundin pure starburst galaxies
suchasM82(Kripsetal.2008).
ersaFOVof51′′×51′′,hasaresolutionof0′.′2,andisnotex-
4. Interferometricresults
actly centered on the galaxy but offset from our phase track-
ingcenter7′′ towardwestand7′.′4towardsouth.Itispartofa 4.1.Dynamicalcenter
surveyof94nearbygalaxiesfromtheRevisedShapleyAmes
Thephasetrackingcenterofourobservations(seeTable1)co-
Catalog(Bo¨keretal.1999).
incides almost exactly with the nuclear radio source detected
Finally,wealsouseafar-ultraviolet(FUV)imagefromthe
at 15GHz (VLA/2cm) by Nagaretal. (2000) [11h20m15.01s,
GALEX satellite, whose band is centered at λeff = 1516 Å. 12◦59′29′.′76] and at 8.4GHz (VLA/3.6cm) by Filhoetal.
This image has been already used and studied in the context
(2000) [11h20m15.0s, 12◦59′30′′]. Thus, in the following, we
oftheGALEXNearbyGalaxiesSurvey(NGS,GildePazetal.
assume that our observations are centered on the dynamical
2007). The image covers a square region on the sky of size
centerofNGC3627.
∼5760′′×5760′′, i.e., much larger than the extent of the opti-
Thespectralcorrelatorswerecenteredat114.992GHzfor
cal disk of NGC3627, with 1′.′5 pixels. As the image was re-
12CO(1–0)and229.979GHzfor12CO(2–1),correspondingto
duced with the GALEX data pipeline, it is already expressed
V =727kms−1.SinceforNGC3627thedifferencebetween
LSR
in intensityunitsandskysubtracted.The totalFUV calibrated
LSR and heliocentric velocity is ∼0 kms−1, our observations
magnitudeis16.34±0.02,correspondingtoaFUVfluxdensity were centered on V = V (PdBI) = 727 kms−1. In the in-
of1057±19µJy. LSR hel
ner ∼2′′ of NGC3627 the velocity centroid is 16 kms−1 red-
shifted with respect to the heliocentric velocity of the center
ofour12CO(1–0)observations(Fig.3,leftpanel).Similarlyto
3. Single-dishresults
12CO(1–0),for12CO(2–1)wefindthatthevelocitycentroidis
TheobservationsperformedwiththeAandBreceiversofthe 18 kms−1 redshifted with respect to the heliocentric velocity
IRAM 30m telescope in the two 12CO lines covered the in- (Fig.3,rightpanel).Assuminganintermediatevaluebetween
ner∼50′′,correspondingtothecentral∼2.5kpc(indiameter) thesystemicheliocentricvelocitydeterminedforthe12CO(1–
of the galaxy (Fig. 1). The observed positions show that the 0)andthatforthe12CO(2–1),weestimateV =744kms−1.
sys,hel
centralregionofNGC3627hostsextendedmolecularemission This estimation of the systemic heliocentric velocity is
bothin12CO(1–0)and12CO(2–1).ThemaximumdetectedT 24 kms−1 redshifted with respect to the systemic heliocen-
mb
is0.4Kin12CO(1–0)attheoffsetposition(0′′,-7′′),and0.6K tric velocity determined from Hi observations (720 kms−1,
in12CO(2–1)attheoffsetposition(0′′,7′′). HyperLeda Database; Haanetal. 2008). In interacting galax-
We estimate a flux of 1097 Jykms−1 within the in- iesandinthosewithalopsidedHimorphology,adiscrepancy
ner ∼50′′×50′′(see Table 2 and Sect. 2.3), in good agree- betweensystemicvelocityderivedfrom12COandHiobserva-
ment with previous single-dish flux determinations (e.g., tionsisnotunusual.NGC4579(Garc´ıa-Burilloetal.2009)and
Helferetal. 2003). Assuming a H -CO conversion factor NGC5953 (Casasolaetal. 2010) exhibit differences of ∼50
2
of X = N(H )/I = 2.2 × 1020 cm−2 (K km s−1)−1 kms−1 between12COandHivelocities,perhapsduetothein-
2 CO
(Solomon&Barrett 1991), the 12CO(1–0) integrated flux al- teractionhistoryofthegalaxyandthedifferenteffectoftheram
8 Casasolaetal.:NUGAXIV:NGC3627
Fig.6.12CO(1–0)velocitychannelmapsobservedwiththeIRAMPdBI+30minthenucleusofNGC3627,withaspatialresolu-
tion of 2′.′1 × 1′.′3 (HPBW). The maps are centered on the phase tracking center of our observations(α = 11h20m15.02s,
J2000
δ = 12◦59′29′.′50) assumed to be coincident with the dynamical center of the galaxy. Velocity channels range from
J2000
∆V = −200kms−1 to +200kms−1 in steps of 5kms−1 relative to V = 744kms−1 (see Sect. 4.1). The contoursrun from
sys,hel
−40mJybeam−1to260mJybeam−1withspacingsof60mJybeam−1.
pressureontheatomicandmoleculargas(Garc´ıa-Burilloetal. 4.2.COdistributionandmass
2009). In NGC3627, the role of interaction history and the
ram-pressure, although not negligible, could have shifted the
Figure4showsthe12CO(1–0)and12CO(2–1)integratedinten-
Hi barycenter with respect to the molecular one less strongly
sitydistributionsintheinner∼40′′(∼2kpc)ofNGC3627.The
thaninNGC4579andNGC5953.
12CO(1–0) emission (Fig. 4, left panel) exhibits a peak at the
nucleus, extendsalong a bar-like structure of ∼18′′ (∼900pc)
diameterwitha north/southorientation(see laterSect.4.4for
the discussion on the PA) and two peaks at its extremes, at
r∼5-6′′ (∼270pc), with the southern one more evident. The
Casasolaetal.:NUGAXIV:NGC3627 9
Fig.7.SameasFig.6butforthe12CO(2–1)line,withaspatialresolutionof0′.′9×0′.′6.Thecontoursrunfrom−50mJybeam−1
to350mJybeam−1withspacingsof50mJybeam−1.
12CO(1–0) morphology also shows a two-arm spiral feature observedtheinner∼2kpcofthegalaxy,thegoodPdBIresolu-
fromr∼9′′(∼450pc)tor∼16′′(∼800pc),withtwopeaksover tionallowsustoinvestigatethenuclearmoleculargasdistribu-
thesespiralarmsatr∼12-14′′(∼650pc).Asimilarandmorere- tioninNGC3627moreindetailthaninBIMASONGsurvey
solveddistributionisfoundin12CO(2–1)[Fig.4,rightpanel]. (typicalresolutionof∼6′′)andwiththe45mNRAOtelescope
Like the nuclear peak, the two peaks at the ends of the inner (FWHM∼15′′). The 12CO distribution is completely different
∼18′′ bar-like structure at r∼5-6′′, are more evident than in from the ringed Hi morphology which exhibits an inner hole
12CO(1–0). where instead the molecular gas is located (Haanetal. 2008;
Walteretal.2008).
The12COdistributionfoundhereagreeswellwithprevious
moleculargasmaps,suchasthatgivenbyReganetal.(2001) ApplyingEq.(1)tocombinedPdBI+30mdata,wederived
andHelferetal.(2003)inthecontextoftheBIMASONGsur- atotalH massofM ∼6.0×108M (S =668Jykms−1,see
2 H2 ⊙ CO
veyandthatobtainedbyKunoetal.(2007)withthe45mtele- Table2)withinthe42′′primarybeamfieldofthePdBI.Taking
scopeoftheNobeyamaRadioObservatory.Althoughweonly into account the mass of helium, the total molecular mass is
10 Casasolaetal.:NUGAXIV:NGC3627
Fig.8. Left panel: Overlay of the integrated12CO(1–0)emission, same as Fig. 4 (left panel), with CO mean-velocityfield in
contoursspanningtherange-180to180kms−1instepsof10kms−1.Thewhitestarindicatesthedynamicalcenterofthegalaxy.
ThevelocitiesarereferredtoV =744kms−1,solid(red)linesareusedforpositivevelocities,anddashed(blue)linesfor
sys,hel
negativevelocities.Thedashedlineindicatesthepositionangleofthemajoraxisofthewholeobservedregion(PA=178◦±1◦),
whilethedot-dashedlinetracesthepositionangleofthemajoraxisofthebar-likestructure(PA=14◦±2◦).Thecontinuumline
indicatesthepositionangleoftheprimarystellarbaridentifiedwiththeNIRH-band2MASSimage(PA=−21◦)[seelaterthe
leftpanelofFig.16andSect.5.2].Rightpanel:Samefor12CO(2–1).Thedashedlineindicatesthepositionangleofthemajor
axisofthewhole12CO(1–0)observedregion(PA=178◦±1◦,seeleftpanel),whilethedot-dashedlinetracesthepositionangle
of the major axis of the 12CO(2–1)bar-like structure (PA = 15◦ ± 2◦). The continuumline indicates the position angle of the
primarystellarbaridentifiedwiththeNIRH-band2MASSimage(PA=−21◦)[seelaterFig.16andSect.5.2].
M ∼8.2×108M . This is roughly 63% of the molecular gas A higher excitation of the molecular gas in the nucleus, sug-
mol ⊙
masswithina50′′diameter(seeSect.3).The∼18′′12CO(1–0) gestedbyahigherR lineratio,isconsistentwiththeHCN(1–
21
bar-likestructurecontributesanH massofM ∼2.1×108M , 0)emissioninthesameregion(seeSect.3).
2 H2 ⊙
roughly one-third of the H mass computed within 42′′, al-
2
though the feature occupies an area of only ∼5% of the 42′′
4.4.COKinematics
beam. NGC3627, compared to other NUGA galaxies, is not
particularly massive in molecular gas, especially with respect
Figures 6 and 7 show the velocity-channel maps of 12CO(1–
to the extraordinary case of NGC1961 with an H mass of
2 0)and12CO(2–1)emission,respectively,in thecentralregion
∼1.8×1010M (Combesetal.2009).
⊙ ofNGC3627.Theinner12COemissionofthegalaxyexhibits
signaturesofnon-circularmotionsbothatnegativeandpositive
velocities.Thesenon-circularcomponentsareassociatedboth
4.3.CO(2–1)/CO(1–0)lineratio
with the 18′′ bar-like structure and the spiral feature detected
beyond the bar-like structure and will be discussed in detail
Informationaboutthelocalexcitationconditionsofthemolec-
ular gas can be inferred from the line ratio R =I /I . This later,inSect.4.5,whereweanalyzetherotationcurvederived
21 21 10
ratioisobtainedbycomparingthe12COmapsofthetwotran- withour12COdata.
sitions, at the same resolution and with the same spatial fre- 12CO(1–0)isovelocitycontours(first-momentmap)aresu-
quency sampling. Figure 5 shows R ratio with 12CO(1–0) perposedonthe12CO(1–0)integratedintensityinFigure8(left
21
contours as in Fig. 4 (left panel). In the observed region, the panel). The white star indicates the dynamical center of the
line ratio ranges from 0.25 to 1 but the bulk of the emission galaxy, assumed coincident with the phase tracking center of
hasaratiobetween0.4and0.7.TheseR lineratiovaluesare ourobservations,andthevelocitiesarerelativetothesystemic
21
consistentwith R = 0.6obtainedby Kripsetal. (2008), and heliocentricvelocity,V =744kms−1 (seeSect.4.1).The
21 sys,hel
more in general with optically thick emission in spiral disks dashed line traces the position angle of the major axis of the
(e.g.,Braine&Combes1992;Garc´ıa-Burilloetal.1993).The observedregion,PA=178◦±1◦(almostvertical),obtainedby
R peaksof∼1arereachedinthecenterofNGC3627andat maximizing the symmetry in the position velocity diagrams.
21
the southern extreme of the elongated 12CO emission region. Thispositionangleisclosetothatoftheentiregalaxy,asgiven