The effect of ram pressure on the molecular gas of galaxies: three case studies in the Virgo cluster

1.5 MB·

by  Bumhyun Lee

MNRAS000,1–17(2016) Preprint12January2017 CompiledusingMNRASLATEXstylefilev3.0
The effect of ram pressure on the molecular gas of galaxies: three
case studies in the Virgo cluster
Bumhyun Lee1(cid:63), Aeree Chung1,2,3†, Stephanie Tonnesen4, Jeffrey D. P. Kenney5,
O. Ivy Wong6, B. Vollmer7, Glen R. Petitpas8, Hugh H. Crowl9, Jacqueline van Gorkom10
1DepartmentofAstronomy,YonseiUniversity,50Yonsei-ro,Seodaemun-gu,Seoul03722,Korea
7 2YonseiUniversityObservatory,YonseiUniversity,50Yonsei-ro,Seodaemun-gu,Seoul03722,Korea
1 3JointALMAObservatory,AlonsodeCórdova3107Vitacura,Santiago,Chile
0 4CarnegieObservatories,813SantaBarbaraSt,Pasadena,CA,91101
2 5YaleUniversityAstronomyDepartment,POBox208101,NewHaven,CT06520-8101,USA
6InternationalCentreforRadioAstronomyResearch,TheUniversityofWesternAustraliaM468,35StirlingHighway,Crawley,WA6009,Australia
n 7CDS,ObservatoireastronomiquedeStrasbourg,UniversitédeStrasbourg,CNRS,UMR7550,11ruedel’Université,F-67000Strasbourg,France
a 8Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,USA
J 9DivisionofScienceandMathematics,BenningtonCollege,1CollegeDrive,Bennington,VT05201,USA

0 10DepartmentofAstronomy,ColumbiaUniversity,MailCode5246,550W120thSt,NewYork,NY10027,USA
1

]
A
AcceptedtoMNRAS
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ABSTRACT
p
Wepresent12CO(2–1)dataofthreeVirgospirals–NGC4330,NGC4402andNGC4522
-
o obtained using the Submillimeter Array. These three galaxies show clear evidence of ram
r pressurestrippingduetotheclustermediumasfoundinpreviousHIimagingstudies.Using
t
s high-resolution CO data, we investigate how the properties of the inner molecular gas disc
a change while a galaxy is undergoing HI stripping in the cluster. At given sensitivity limits,
[
wedonotfindanyclearsignsofmoleculargasstripping.However,bothitsmorphologyand

1 kinematicsappeartobequitedisturbedasthoseofHI.Morphologicalpeculiaritiespresentin
v the molecular and atomic gas are closely related with each other, suggesting that molecular
0 gascanbealsoaffectedbystrongICMpressureevenifitisnotstripped.COisfoundtobe
5 modestly enhanced along the upstream sides in these galaxies, which may change the local
7 starformationactivityinthedisc.Indeed,thedistributionofHα emission,atracerofrecent
2
starformation,wellcoincideswiththatofthemoleculargas,revealingenhancementsnearthe
0
localCOpeakoralongtheCOcompression.FUVandHα sharesomepropertiesincommon,
.
1 butFUVisalwaysmoreextendedthanCO/Hα inthethreegalaxies,implyingthatthestar-
0 forming disc is rapidly shrinking as the molecular gas properties have changed. We discuss
7 how ICM pressure affects dense molecular gas and hence star formation properties while
1
diffuseatomicgasisbeingremovedfromagalaxy.
:
v
Keywords: ISM:molecules–galaxies:clusters:intraclustermedium–galaxies:evolution
i
X –galaxies:ISM–galaxies:spiral.
r
a
1 INTRODUCTION observations.Inaddition,Giovanelli&Haynes(1985)havefound
thattheHIcontentiswellcorrelatedwiththelocationofgalaxiesin
Since Gunn & Gott (1972) suggested that a galaxy might lose
asensethatHIismoredeficientatsmallerdistancesfromtheclus-
its interstellar medium (ISM) by interacting with the intracluster
tercentre.ThentheHIsynthesisimagingstudiessuchasWarmels
medium(ICM)intheclusterenvironment,muchevidenceforram
(1988a,b) and Cayatte et al. (1990) have shown that HI-deficient
pressure stripping has been found to date. In early days, Davies
galaxies near the cluster centre have small HI-to-optical extents,
&Lewis(1973)haveshownthatVirgogalaxiesaregenerallymore
truncated within the stellar disc in many cases. In a more recent
deficientinHIcomparedtofieldgalaxiesbasedontheirsingle-dish
high-resolution HI imaging study of ∼50 selected Virgo galaxies
byChungetal.(2009),anumberofgalaxieshavebeenfoundwith
variousscalesofextraplanarHIgasorlongHItails,indicatingthat
(cid:63) E-mail:[email protected]
rampressurestrippingisindeedactingintheclusterenvironment.
† E-mail:[email protected](corresponding)
(cid:13)c 2016TheAuthors
2 B.Leeetal.
AsagalaxylosesitsISM,thestarformationrateisexpectedto
besuppressed,whichagreeswellwiththeobservations.Asagood
example,Koopmann&Kenney(2004a)findthatthemassivestar
formationrateofVirgospiralsislowerthantheirfieldcounterpart
byafactorof2.5onaverage.Koopmann&Kenney(2004b)also
showthatmanyVirgospiralshaveasmallerHα extentcompared
totheirstellardisc,reflectingthattheprocesstruncatingHI discs
mayalsobeactingonthestar-formingdiscandHαemission.
On the other hand, molecular gas is unlikely to be as easily
strippedasatomichydrogen,sinceitismoretightlyboundtothe
galacticcentreandthedensityishigher.Infact,mostpreviousstud-
iesfindthatthemoleculargasmassoftheclusterpopulationisnot
significantlydifferentfromthatoffieldgalaxies(Starketal.1986;
Kenney&Young1989).Inaddition,morerecentstudiesarefinding
clumpydustfeaturesintheupstreamsideofHIgasstrippedgalax-
ies,whicharelikelytobesurvivingdensecloudsthatareunveiled
afterdiffuseatomicgasisremoved(Crowletal.2005;Abramson
&Kenney2014;Kenney,Abramson&Bravo-Alfaro2015).
On the other hand, however, the opposite results also have
beenreported.Rengarajan&Iyengar(1992)findthatH massnor-
2
malizedbythedynamicalmassofgalaxiestendstogetlargerwith
increasing clustercentric distance, which supports that molecular
gascanbealsodeficientintheclusterenvironment.Morerecently,
Bosellietal.(2014)showthatHI-deficientgalaxiesinthecluster
environmenttendtobealsomodestlydeficientinmoleculargas.
However,themoleculargasfractiontotheopticalluminosity
ofspiralgalaxiesmeasuredusing12CO(1−0)rangesquitewidely
forallenvironments(Chung2012).Thisimpliesthatthemolecu-
Figure1.ThelocationsofoursampleareshownontheROSATX-raymap
lar gas fraction alone may not be a good tracer of molecular gas (bluecontours;Böhringeretal.1994).Redellipsesrepresenttheposition
deficiency.Duetothescatterinthisrelationandopposingobser- angleandD25×5ofthesampleintheBband.NGC4569thatwehave
vational results, it is still arguable whether molecular gas can be notobservedusingtheSMAbutisincludedinourdiscussionasagood
affectedbytheICMpressureinasimilarwaytoHIgas,anditis representative of a galaxy at post-peak pressure, is shown in green. The
thenstillpuzzlingwhystarformationappearstobequenchedinHI samplegalaxies,locatedat0.4–1MpcfromM87,adoptingaVirgodistance
strippedgalaxiesifmoleculargas,whichisthemoredirectingre- of 16 Mpc (Yasuda et al. 1997), make a nice sequence of ram pressure
strippingfromearlystage,closetopeakpressureandpost-peakpressurein
dientforstarformation,isnotdeficient.
theirorbit.
No clear evidence for molecular gas stripping yet low star
formationactivitiesinclustergalaxiesmayimplydistinctmolec-
ular gas properties in high-density environments as supported by
some previous observations. For example, Kenney et al. (1990) inside a galaxy under strong ICM pressure need to be probed.
havefoundahighlyasymmetricCOmorphologyinanHIdeficient Hence, using the Submillimeter Array (SMA)1, we have taken
Virgoclustergalaxy.Vollmeretal.(2008)alsohavefoundverype- high-resolution12CO(2−1)imagingdataofasubsampleofVirgo
culiarCOdistributionsinsomeofVirgospiralsundergoingactive galaxies that have lost atomic gas significantly by ICM pressure.
HIstripping.Thesestudiesindicatethatmoleculargascanbepo- Some preliminary results are published in Lee & Chung (2015),
tentiallydisturbedbystrongICMpressurewhetheritisstrippedor andinthiswork,wepresentmorecompleteanalysisonbothmor-
not. phologyandkinematicsofthreeVirgogalaxiesbasedontheSMA
ParticularlyalongthesideexperiencingICMpressure,ithas data.
beensuggestedthatinterstellargasincludingthemolecularphase Thispaperisorganizedasfollows.Weintroducethesamplein
canbepushedupagainstthecentreofagalaxy.ThisleadstoISM Section2.Detailsofobservationsanddatareductionprocedureare
compressionandincreasingH2formation;hence,itlocallytriggers providedinSection3.InSection4,wepresenttheSMAdata,de-
intensivestarformation(Fujita&Nagashima1999;Kronbergeret scribingtheCOmorphologyandkinematics.InSection5,wecom-
al.2008;Merluzzietal.2013;Henderson&Bekki2016).Indeed, paretheCOandotherwavelengthdatatodiscusshowthemolecu-
ISMcompressionwithhighmolecularfractionintheupstreamside largasinthesegalaxieshasbeenaffectedbytheICMpressure.In
isseeninanumberofgalaxiesexperiencingICMpressure(Vollmer Section6,wesummarizetheresultsandconclude.
etal.2012b;Nehlig,Vollmer&Braine2016),whichareoftenac- A distance of 16 Mpc (1 arcsec ∼78 pc) to Virgo cluster is
companied by an enhancement of star formation. Molecular gas adoptedinthiswork(Yasuda,Fukugita&Okamura1997).
enhancementamongtheVirgoclustermembershasbeenalsore-
cently reported by Mok et al. (2016). These observations clearly
showthatmoleculargaspropertiesandhencestarformationactiv-
itywithinastellardisccanbeaffectedbyrampressure. 1 TheSubmillimeterArrayisajointprojectbetweentheSmithsonianAs-
Therefore, in order to get a deeper understanding on how trophysicalObservatoryandtheAcademiaSinicaInstituteofAstronomy
galaxiesbecomepassiveafterHI stripping,notonlythepossibil- and Astrophysics and is funded by the Smithsonian Institution and the
ity of stripping but also the detailed properties of molecular gas AcademiaSinica.
MNRAS000,1–17(2016)
MoleculargaspropertiesofVirgospirals 3
Table1.GeneralInformationofSampleGalaxiesa.
Galaxy NGC4330 NGC4402 NGC4522
Rightascension(J2000) 12h23m17s.0 12h26m07s.6 12h33m39s.7
Declination(J2000) +11◦22(cid:48)03(cid:48)(cid:48).5 +13◦06(cid:48)47(cid:48)(cid:48).4 +09◦10(cid:48)30(cid:48)(cid:48).2
Morphologicaltype Sc Sb SBc
Inclination(◦) 79 82 79
Positionangle(◦) 60 89 35
Vrad(kms−1)b 1565 232 2328
D25(arcmin) 2.29 3.55 3.47
TotalapparentB-bandmagnitude 12.02 12.05 11.86
TotalK-bandluminosity(109L(cid:12),K)c 6.58 21.30 5.64
MHI(108M(cid:12))d 4.45 3.70 3.40
defHId,e 0.80 0.74 0.86
dM87(◦)d 2.1 1.4 3.3
aGeneral information of the sample galaxies from Paturel et al. (2003) (HyperLeda,
http://leda.univ-lyon1.fr/).
bcf.theVirgomean∼1100kms−1(Meietal.2007).
cSkrutskieetal.(2006),cf.Milkyway:8.24×1010 L(cid:12),K (Drimmel&Spergel2001),
M31:1.29×1011L(cid:12),K(Barmbyetal.2006).
dtheVIVAstudy(Chungetal.2009).
edefHI=(cid:104)log∑HI,all(cid:105)−log∑HI,obs, where (cid:104)log∑HI,all(cid:105) is the mean HI surface density
of field galaxies (Haynes & Giovanelli 1984), and log∑HI,obs is the mean HI surface
densityofanobservedgalaxy(Chungetal.2009).Inthiswork,morphologyindependent
deficiencyhasbeenadoptedasChungetal.(2009).
2 SAMPLEGALAXIES mean star formation quenching time-scale, i.e. how long ago a
galaxystoppedformingstars,isnotsignificantlydifferentamong
The sample for the SMA observations has been selected from
our sample by ranging from 100 to 300 Myr (Crowl & Kenney
theVLA(VeryLargeArray)2 ImagingstudyofVirgogalaxiesin
2008; Abramson et al. 2011). Therefore, the detailed CO data of
Atomicgas(VIVA)byChungetal.(2009).TheVIVAisahigh-
thesethreecasesshouldenableustoprobehowmoleculargasprop-
resolution HI imaging study of 53 late-type galaxies that are lo-
ertiesandthusstarformationactivitiesaremodifiedbystrongICM
catedthroughouttheVirgoclusterfromahigh-densitycoreregion
pressure.Inaddition,bycomparingourdatawiththeCOdataof
to low-density outskirts. Among the VIVA sample, we have se-
NGC4569,whichisknowntohavecrossedtheclusterawhileago
lected three galaxies with clear evidence for active HI stripping,
(Fig. 1, and has a star formation quenching time (cid:29) ∼300 Myr;
NGC 4330, NGC 4402 and NGC 4522 (Fig. 1). Although these
Vollmer et al. 2004; Crowl & Kenney 2008), we will probe how
galaxiesarethoughttohavelostasimilarfractionofHIgas,being
moleculargaspropertiesevolvewithtime,duringthefirstinfallto
deficient by a factor of 6∼7 compared to their field counterparts
the cluster and under strong ICM pressure, then after core cross-
(Table1),theyshowdistinctpropertiesintheirHImorphology.
ing.ForNGC4569,whichisnotincludedinourSMAsample,we
NGC 4330 is truncated in HI within one side of the stellar
makeuseoftheCOdatafromtheHERACLESsurvey(Leroyetal.
disc, while it reveals a long HI tail on the opposite side as if the
2009).ThegeneralpropertiesofourSMAsamplearesummarized
HI disc is pushed to the tail side (Chung et al. 2009; Abramson
inTable1.
etal.2011).ThelocationandtheHImorphologysuggestthatthis
galaxyisarecentarrival,enteringthehigh-densityregionforthe
firsttime(Chungetal.2007),andthisgalaxywillreachthepeak
pressureafter100Myr,basedonthesimulationofVollmer(2009).
3 OBSERVATIONSANDDATAREDUCTION
Meanwhile,NGC4402hasbeenexperiencingstrongICMpressure
inthelast∼150−250Myr(Abramson&Kenney2014),currently The SMA is a radio interferometer with eight antenna elements,
crossing the core region (Crowl et al. 2005). Lastly, NGC 4522 6meachindiameter.ItislocatedinMaunaKeaattheelevationof
isfartherawayfromtheclustercentrecomparedtoNGC4402in 4080mabovethesealevel.
projection,butitsHImorphologyisalsosuggestiveofactiveram OurSMAobservationsweredoneinMarch2010andMarch
pressurestrippingasNGC4402.Vollmeretal.(2006)showintheir 2011inthesubcompactconfiguration.Amongeightantennasinto-
simulationsthatithasbeenatleast50Myr,sincethisgalaxyexpe- tal,onlysevenantennaswereavailableforourobservationsinboth
riencedquitestrongICMpressure.Thisislikelyduetoturbulence years. The total bandwidth of 4 GHz is composed of 48 chunks
intheICMthatcouldhavebeencausedbymergingofM49group thatareoverlappedby∼20MHz.Eachchunkwasconfiguredwith
tothemaincluster(Kenneyetal.2004). 128channels,eachofwhichis0.8125MHzwidthor1.1kms−1
In spite of subtle differences in their HI morphologies, the at the rest frequency of 12CO (2−1), 230.538 GHz. 12CO for
eachtargetwasplacedontheuppersidebandsothat13CO(2−1)
(νrest=220.398GHz)andC18O(2−1)(νrest=219.560GHz)fre-
2 TheNationalRadioAstronomyObservatoryisafacilityoftheNational quencieswerecoveredsimultaneously.Thelowersidebandwhere
ScienceFoundationoperatedundercooperativeagreementbyAssociated 13CO was included was separated with the upper sideband by
Universities,Inc. 10GHz.
MNRAS000,1–17(2016)
4 B.Leeetal.
Figure2.TheHIdistributionofNGC4330,NGC4402andNGC4522(fromlefttoright)isshowninbluecontoursoverlaidontheDigitizedSkySurvey2
(DSS2,https://archive.stsci.edu/dss/index.html)redimage.TheredcrossindicatesthestellardisccentreofeachgalaxyestimatedfromSpitzer3.6µmdata
(Saloetal.2015),andthethinredcirclesrepresenttheSMAobservationpoints,eachofwhichcorrespondstothesizeoftheprimarybeamat230GHz
(≈54arcsec).
Table2.Observationparameters.
Galaxy NGC4330 NGC4402 NGC4522
Observationdate 2011Mar02,03 2010Mar20,21,27 2011Feb28,Mar01
Synthesizedbeam(arcsec) 6.35×4.47 7.21×3.89 6.62×3.92
Positionangle(◦) -28.4 73.4 -26.4
Spectralresolution(kms−1) 5.0 5.0 5.0
Integrationtimeperpoint(h) 2.1 5.5 2.6
rmsperchannel(mJybeam−1)
12COJ=2−1 35.2 16.3 32.7
13COJ=2−1a 29.7 13.0 29.9
Bandpasscalibrators 0854+201,1751+096 0854+201 1751+096,3c279
Fluxcalibrators titan,vesta mars,titan mwc349a,vesta
Gaincalibrators 3c273,3c279 3c273,3c279 3c273,3c279
Note.a13CO(2−1)emissionisdetectedonlyinNGC4402.
TheprimarybeamoftheSMAis∼54arcsecat12CO(2−1) 4 RESULTS
rest frequency. Aiming to cover at least half the stellar disc, we
Inthissection,wepresenttheresultsofourSMAobservations.The
mosaicked3–5pointsdependingontheopticalsizeofindividual
12CO(2−1)fluxanditsuncertaintyaremeasuredinJykms−1as
galaxies as shown in Fig. 2. In the case of NGC 4522, one addi-
follows,
tional field in the southwest was included to cover the extrapla-
∼na5r.5HhαpaenrdfieHldIdgeaps.enTdhiengtootnalthinetewgeraatthioenrctoimndeitriaonngse,syefrtowmea∼im2etod SCO=∑FCO×∆V±(cid:16)∑σ2(cid:17)21×∆V, (1)
toachieveauniformsensitivityforeachgalaxy.DetailsoftheSMA where F is the total flux of CO emission in each channel, ∆V
CO
observationsaresummarizedinTable2. is the channel separation of the final cube (5 km s−1), and σ is
thermsofeachchanneloutsideCOemission.Continuousfeatures
above2σ−3σ areconsideredasrealsignalinindividualchannels.
CO linewidths are calculated using the velocities, where the flux
Flux,gainandbandpasscalibrationsweredoneusingtheMIR densitycorrespondsto20percentand50percentofthepeaksin
software(Qi2012).Thecalibratorsusedforourobservationsare the receding and the approaching side, adopting the definition of
listed in Table 2. After the calibration, the data had been anal- Rhee&vanAlbada(1996):
ysedusingtheMIRIAD.Thecontinuumhasbeensubtractedusing W =V20percent−V20percent, (2)
UVLINbyapplyingalinearfittotheuv-datainline-freechannels 20 high low
selected based on the HI emission of each galaxy. The imaging
and mosaicking were done using INVERT. In order to maximize
thesensitivitywhilekeepingoptimalresolution,therobustwasset W50=Vh5i0ghpercent−Vl5o0wpercent. (3)
to0.5(forNGC4402)or1(forNGC4330andNGC4522).The TheCOvelocityisdeterminedfollowingVerheijen(1997):
channelwidthinthefinalcubewasregriddedto5kms−1,which
iscomparabletoHIdata(VIVA;Chungetal.2009).Wehavede- Vsys=0.25(Vl2o0wpercent+Vl5o0wpercent+Vh5i0ghpercent+Vh2i0ghpercent).
tected 12CO (2−1) in all three galaxies, while 13CO (2−1) has
(4)
beendetectedonlyinNGC4402,whichwillbepresentedinasep-
aratepaper(Leeetal.inpreparation). FollowingtherecipefromSolomonetal.(1997),12CO(2−1)
MNRAS000,1–17(2016)
MoleculargaspropertiesofVirgospirals 5
Table3.SMACOpropertiesofsamplegalaxies.
NGC4330 NGC4402 NGC4522
W20(kms−1) 208 255 176
W50(kms−1) 184 226 159
Vsys(kms−1) 1562 246 2326
SCO(Jykms−1) 182.22±8.02 1400.76±11.91 139.43±4.77
MH2(108M(cid:12))a 1.19±0.05 8.83±0.08 0.88±0.03
Note.aTheCO-to-H2 conversionfactorof3.2M(cid:12) pc−2 (Kkms−1)−1 is
adoptedfromStrong&Mattox(1996).
lineluminosityismeasuredasfollows: clump,thevelocitygradientisinverted.Thekinematicalcomplex-
LC(cid:48) O(2−1)=3.25×107SCOνo−b2sD2L(1+z)−3, (5) iotfyaissatelseopcvleelaorclyitysegernadinietnhteinPVthDe(cFeingt.ra3lc∼).1A3dairsctsinecct(c∼o1mkppocn)enist
inKkms−1pc2,whereS isanintegratedtotal12CO(2−1)flux quitenoticeable.Thismayindicateamolecularringorbar.Inad-
CO
inJykms−1,DListheluminositydistanceinMpc,νobsistheob- dition,theCOpeakisofffromthestellardisccentreby∼270pc
servationfrequencyinGHz,andzistheredshift.Adoptingatypi- (Fig.3a),whiletheCOkinematiccentreismoreorlessconsistent
cal 12CO (2−1)/(1−0) ratio of nearby galaxies (R ≈0.8; Leroy withthestellardisccentre(Fig.3c).
21
et al. 2009) and using the conversion relation of normal spirals
(αCO =3.2M(cid:12) pc−2 (Kkms−1)−1;Strong&Mattox1996),the
moleculargasmasscanbecalculatedby 4.2 NGC4402
M = αCO L(cid:48) (2−1), (6) AsshowninFig.4(a),theCOdiscofNGC4402isslightlymore
H2 R21 CO extended in the west (62 arcsec in the west versus 58 arcsec in
whereL(cid:48) (2−1)istheluminosityof12CO(2−1)transition.The the east), but the difference is subtle and not as significant as in
CO HI.Whatmakesthiscaselookhighlyasymmetricisthenorth-west
global CO properties of the sample are summarized in Table 3.
quarterofthedownstreamside(thewestsideofthedisc).Inthis
On the left of Figs 3–5, the CO intensity map, velocity field and
region,amodestCObump(indicatedbyarrowinFig.4a)isfound
position-velocitydiagram(PVD)withglobalandradialprofileare
as in HI (Fig. 4d). Meanwhile, the southern part of the CO disc
presentedin(a),(b)and(c)foreachgalaxy.Theoverlayswithother
looks quite compressed along the enhanced far-ultraviolet (FUV)
wavelengthdatashownontheright-handsideofFigs3–5aredis-
emission(Fig.4e),whichwillbediscussedmoreindetailinSec-
cussedinSection5.
tion5.Thenorthernpartabovethemajoraxisismeasuredtobe91
percentofthatofthesouthinflux.AlthoughtheCOpeakagrees
4.1 NGC4330 wellwiththestellardisccentre,theinnerCOdiscalsoappearsto
beslightlymorestretchedtowardthewest,reflectingtheouterCO
As seen in Figs 3(a)–(c), the CO morphology of NGC 4330 is
disc.
highlyasymmetric,withthesouth-westextentinthedownstream
TheCOkinematicsrevealstheevidencefornon-circularmo-
side being about 75 per cent (40 arcsec versus 53 arcsec) of the
tions as shown in Fig. 4(b). In the inner region, the iso-velocity
northeastbutonly68percentwhentheoutermost4arcsecofthe
curvesarehighlyskewedandnotperfectlyparallelwiththeminor
south-westdisc,whichisbentdowntothesouth(indicatedbyar-
axis. The skewness in velocity must be the result of two distinct
rowinFig.3a)isexcluded(36arcsecversus53arcsec).Thecen-
disccomponentsintheinner∼15arcsec(∼1kpc)beingprojected
tral part, within 30 arcsec in diameter (∼2 kpc at the distance of
ontheskyasseeninitsPVD(Fig.4c).Thevelocitygradientinthe
Virgo),isalsoasymmetricduetothestronglocalpeakinthenorth-
outer part is quite different in the two sides of the disc along the
east from the stellar disc centre. The distinct extents and surface
majoraxis.Inthenorth-westquarter(inthedownstreamside),the
densitiesarealsoclearlyseeninradialprofiles.Intheendofthe
kinematicsofmodestCObumpisslightlydeviatedfromthemain
southwestdiscofthedownstreamside,COisfoundtobeslightly
disc.
bent (Fig. 3a). This bending is also found in many other wave-
ItalsohasadistinctCOcomponentwithsteepvelocitygradi-
lengthssuchasUV,Hα andHI,butallaredifferentinscaleand entintheinner∼15arcsec.Thislikelyindicatesanuclearbaror
angle from one another as further discussed in Section 5. In the
ring,althoughwefavoranuclearbarasthiswouldalsoexplainthe
caseofCO,thisbendingpartisclumpy,almostidentifiedasanin-
skewedisovelocitycontours.InNGC4402,Sofueetal.(2003a,b)
dependentbloborclump(seethearrowinFig.3c).ThisCOclump
alsofindanuclearmoleculardisctracedby12CO(1−0)inthein-
correspondsto2percentofthetotalinflux(∼1.9×106M(cid:12),com- ner∼10arcsec.Wedonotfindanydirectrelevanceofthesenuclear
parabletothatofalargemolecularcloud).Thecentreoftheclump
structureswithrampressurestripping.
isofffromthemid-planeofthemaindisctowardthesouthby∼4
arcsec(∼312pc).
TheCOkinematicsalsoshowspeculiarstructures,especially
4.3 NGC4522
alongthesouth-westconcentrationandtheendofthetailasshown
inFig.3(b).Thevelocitygradientofthesouth-westisquitesteep, While the overall size of 12CO (2−1) disc in the SMA image is
whileitismoreslowlyrisingontheothersidewithinthesmallradii consistentwiththeextentmeasuredbyasingledishforNGC4330
fromthecentre,reaching72kms−1 ontheapproachingsidebut (Vollmeretal.2012a)andNGC4402(Leeetal.inpreparation),
only43kms−1ontherecedingat15arcsecradii.Inthesouth-west ourSMAdataofNGC4522revealonlytheinner∼57percentof
MNRAS000,1–17(2016)
6 B.Leeetal.
Figure3. NGC4330:(a)12CO(2−1)integratedintensitymap(the0thmoment)ingrey-scalewithcontours.Contourlevelsare0.3,1.5,4,8,12,16,20
Jybeam−1kms−1.Thesynthesizedbeamsizeis6.35arcsec×4.47arcsec(blueellipseatthebottomright).Thewhitecrossindicatesthestellardisccentre.
(b)12CO(2−1)velocityfieldmap(the1stmoment).Velocitycontoursaredrawninevery10kms−1from1470kms−1to1670kms−1.Thewhitecross
againindicatesthestellardisccentre.(c)Upperleft:aposition-velocitycutthroughthemajoraxisintegratedalongtheminoraxis.Contourlevelsare0.7,1.4,
2.1,2.8,3.5Jybeam−1.TheCOclumpisindicatedbyblackarrow.Right:theglobalprofileof12CO(2−1).TheCOvelocity(1562kms−1)isindicatedwith
anarrow.Bottom:thegassurfacedensities(HIandH2)alongtheapproachingsideandtherecedingsideontherightandtheleft,respectively.(d)Anoverlay
of12CO(2−1)(bluecontours)onHI(redcontours)andDSS2red(blackcontours).SynthesizedbeamoftheVLAandtheSMAareshowninredandblue
atthebottomleft.TheentireCOdiscislocatedinsidetheopticaldisc.Unlikethestellardisc,however,theCOdiscisfoundtobehighlyasymmetricasthe
atomicgasdisc.(e)12CO(2−1)(bluecontours)isoverlaidonFUVemission(blackcontoursandgrey-scale).FUVtailisalsoextendedandbenttowardthe
south-westasCOandHIgastail.(f)12CO(2−1)(bluecontours)overlaidontheHαemission(grey-scale).Theoverallbendingshapecoincideswellbetween
HαandCO,whileCOisnotextendedasmuchasHα.
MNRAS000,1–17(2016)
MoleculargaspropertiesofVirgospirals 7
Figure4. NGC4402:(a)12CO(2−1)integratedintensitymap(the0thmoment)ingrey-scalewithcontours.Contourlevelsare1,5,10,30,50,70,90
Jybeam−1kms−1.Thesynthesizedbeamsizeis7.21arcsec×3.89arcsec(blueellipseatthebottomright).Thewhitecrossindicatesthestellardisccentre.
AmodestCObumpisindicatedbyblackarrow.(b)12CO(2−1)velocityfieldmap(the1stmoment).Velocitycontoursaredrawninevery20kms−1from
120kms−1to360kms−1.Thewhitecrossagainindicatesthestellardisccentre.(c)Upperleft:aposition-velocitycutthroughthemajoraxisintegratedalong
theminoraxis.Contourlevelsare0.5,1.5,3,5,7,9,11Jybeam−1.Right:theglobalprofileof12CO(2−1).TheCOvelocity(246kms−1)isindicatedwith
anarrow.Bottom:thegassurfacedensities(HIandH2)alongtheapproachingsideandtherecedingsideontherightandtheleft,respectively.(d)Anoverlay
of12CO(2−1)(bluecontours)onHI(redcontours)andDSS2red(blackcontours).12COgasiswellconfinedwithintheopticaldisc,whileHIispushedoff
outsidefromthestellardisc.Theopticaldiscdoesnotlookdisturbed,whilebothmolecularandatomicgascomponentsrevealasymmetryinasimilarsense.
(e)12CO(2−1)(bluecontours)isoverlaidonFUVemission(blackcontoursandgrey-scale).FUVisfoundtobeenhancedalongtheCOcompression.(f)
12CO(2−1)(bluecontours)overlaidontheHαemission(grey-scale).Theoverallextentandmorphologywellcoincidewitheachother.
MNRAS000,1–17(2016)
8 B.Leeetal.
Figure5. NGC4522:(a)12CO(2−1)integratedintensitymap(the0thmoment)ingrey-scalewithcontours.Contourlevelsare0.3,1.5,4,8,12,16,20
Jybeam−1kms−1.Thesynthesizedbeamsizeis6.62arcsec×3.92arcsec(blueellipseatthebottomright).Thewhitecrossindicatesthestellardisccentre.
(b)12CO(2−1)velocityfieldmap(the1stmoment).Velocitycontoursaredrawninevery10kms−1from2250kms−1to2400kms−1.Thewhitecross
againindicatesthestellardisccentre.(c)Upperleft:aposition-velocitycutthroughthemajoraxisintegratedalongtheminoraxis.Contourlevelsare1.5,
2.0,2.5,3.0,3.5,4.0,4.5,5.0Jybeam−1.Right:theglobalprofileof12CO(2−1).TheCOvelocity(2326kms−1)isindicatedwithanarrow.Bottom:the
gassurfacedensities(HIandH2)alongtheapproachingsideandtherecedingsideontheleftandtheright,respectively.(d)Anoverlayof12CO(2−1)(blue
contours)onHI(redcontours)andDSS2red(blackcontours).OurSMAdatarevealonlytheinnerpartofthemoleculargasdisc,missingouttheouterpart
thatisdetectedbytheIRAM(Vollmeretal.2008)duetothelackinpointingsandsensitivity.InSection4and5,wemorefocusonthemorphologyand
kinematicsrevealedbytheSMA.(e)12CO(2−1)(bluecontours)isoverlaidonFUVemission(blackcontoursandgrey-scale).EnhancedFUVemissionis
foundalongthesideonwhichtheICMwindispresumablyacting.(f)12CO(2−1)(bluecontours)overlaidontheHαemission(grey-scale).TheinnerCO
discisslightlybenttowardsouth-eastastheinnerHαdisc.
MNRAS000,1–17(2016)
MoleculargaspropertiesofVirgospirals 9
itssingle-dishmapinsize(Vollmeretal.2008).WhileVollmeret usingLucy’s(1974)iterativedeconvolutionmethod.Thenthestrip
al. (2008) have detected CO along the extraplanar Hα and HI in integralsofapproachingandrecedingsidesaredeprojectedsepa-
both ends of the disc, we did not detect any such features in the ratelytoinfertheface-onHIsurfacedensity(Kregel,vanderKruit
south-west in our SMA data due to the lack of sensitivity and/or &deBlok2004).Thesurfacedensityofbothatomicandmolec-
potentiallyduetothediffusenatureofgasintheoutermolecular ularhydrogenismeasuredinM(cid:12) pc−2.Thecomparisonbetween
disc.Thenorth-eastendwasnotcoveredintheSMAobservations. atomicandmoleculargasradialdistributionsisfoundinthebottom
Therefore, we limit our discussion to only the inner CO disc for on the left-hand column of Figs 3–5. Overlays between CO and
this particular case in this section and we cite the IRAM single- othermultiwavelengthdataarepresentedontheright-handcolumn
dishdatawhenitisneededforcomparisonswithotherwavelength ofFigs3–5andFig.6.
datainSection5. Forthereference,theIRAM12CO(2−1)mapsarealsoover-
AsshowninFig.5,theCOextentiscomparableinbothsides laid in the composite map. The sensitivity of the IRAM cubes is
(27.4arcsecinthenorth-eastversus27.6arcsecinthesouth-west). ∼10mKper5kms−1 channel(thesamechannelwidthasours),
TheinnerCOdiscofNGC4522isfoundtobecurvedintheop- which is comparable to the SMA cubes or slightly better in the
posite way to the outer CO and HI disc, i.e. to the south-east as casesofNGC4330andNGC4522whenthedifferentspatialres-
theinnerHα disc.Wefindtwofeaturesstickingoutfromthein- olutionsaretakenintoaccount(∼11arcsecversus∼5arcsecfor
nerpart,onefromtheendofnorth-eastdiscandanotheralmostto IRAMandSMA,respectively).ForNGC4402,thesensitivityof
thesamedirectionbutconnectedfromthecentreofthemaindisc, SMAdataissomewhatbetterthantheIRAMdatanotonlybecause
moretothesouth.Thenorth-eastblobcoincideswiththemorphol- of the sufficient integration time with the SMA but also because
ogyoftheinnerCOdiscintheIRAMimage(Fig.6c;Vollmeret of the poor weather condition when the IRAM data were taken.
al.2008),whichissmoothlyconnectedtothediscfartherextended For NGC 4330 and NGC 4402, the IRAM and SMA maps are
inthenorth-east,coincidingwithadustloopinthisregion.While generallyingoodagreementinmorphology,consideringthebeam
thedownwardbendingcoincideswiththeinnerspiralstructurein- sizes, while we are missing most of the extraplanar molecular of
cludingthedustfeature(seeFig.A1inAppendixA). NGC4522intheSMAdataduetothelackincoverage,sensitivity,
The CO velocity structure within the main disc generally and/orpotentiallyduetothediffusenatureofmoleculargasinthe
showsthatofaregularlyrotatingdiscasseeninFigs5(b)and(c). extragalacticspaceasfurtherdiscussedinSection5.1.3.
Thevelocitykeepsrisingupto∼10arcseconbothsides,thenal-
mostflattensout,andtowardtheendofthedisc,thevelocityrises
again.Thevelocitygradientalongthenorth-eastbranchisoverall
5.1.1 NGC4330
consistentwiththatofthemaindisconthesameside.However,the
gradientissmallerandthevelocityrisesmoreslowlycomparedto Asymmetry is ubiquitous in a range of wavelengths data except
theotherside. intheredopticalimage(DSS2red).InFig.6(a),weseethatthe
HIistruncatedwithinthestellardiscinthenorth-east,whereHα
and FUV emission reveals an upturn feature (Chung et al. 2009;
5 DISCUSSION Abramsonetal.2011).Ahintofupturnisalsofoundinthenorth-
east end of the upstream of radio continuum at 6 cm and 20 cm
5.1 Comparisonwithotherwavelengthdata
(Chung et al. 2009; Vollmer et al. 2012a). The single-dish 12CO
In this section, we compare 12CO (2−1) properties with other (2−1)data(Vollmeretal.2012a)appeartobesimilartotheradio
wavelengthdataincludingHI (Chungetal.2009),optical(DSS2 morphology,alsowithsomehintofupturn,althoughtheextentof
red), FUV (Gil de Paz et al. 2007), and Hα (NGC 4330 and themoleculargasismuchlessthanthatofHI.InourSMAdata,we
NGC 4569 from Gavazzi et al. 2003; Abramson et al. 2011, aremissingsomeoutermostfeaturesincludingthetipofthenose
NGC 4402 from Crowl et al. 2005, NGC 4522 from Koopmann, inthenorth-east.Instead,theSMAdataclearlyrevealverydetailed
Kenney,&Young2001).COtracesstar-forminggas,andHα and structuresoftheinnerCOdiscsuchasthedifferenceinscaleheight
FUV emission are good indicators of star formation with time- betweenthenorthandthesouthalongthemajoraxis(upto∼0.36
scalesof∼20and100Myr,respectively(Kennicutt1998).Wealso kpc,Fig.3a)whichisnotasclearasthisintheprevioussingle-dish
comparetheradialgassurfacedensitybetweenatomicandmolec- data.
ularhydrogen.TheHIcolumndensityiscalculatedusing, Thesouth-westside,i.e.thedownstreamofthegalaxywhere
anHIgastailispresent,isbenttowardthesouth.Onthisside,we
N(HI)=1.82×1018×∑T (cid:52)V (7)
B also find FUV, Hα and radio continuum tail but the extents and
incm−2,where∑TB(cid:52)V istheintegratedintensity(brightnesstem- bending angles are all different in various wavelengths (Fig 6a).
perature)inKkms−1(Walteretal.2008).TheH surfacedensity The distribution becomes patchy in the outer CO disc as seen in
2
isestimatedbythefollowingrelation: bothIRAMsingle-dishdata(Vollmeretal.2012a)aswellasour
SMAdata.Intriguingly,theCOblobtowardtheendislocatedin
N(H2)= αRCO I(CO) (8) almostthesamegalacticradiusasoneofthedistinctHαblobsand
21 offfromthemid-planeinthesamedirectionasthetailsoftheother
inM(cid:12)pc−2,whereI(CO)is12CO(2−1)integratedintensity,and wavelengths(Fig6a).
αCO is the same as in equation (6). Considering the high inclina- Asclearlyseen,themoleculardisc,whichextendstoonlyhalf
R21
tionofallthreegalaxiesinoursample,weutilizethestripintegral thestellardiscorless,showsmanypropertiesincommonwiththe
methodinsteadofellipsefittingtoderiveradialsurfacedensitybe- morphologiesintheotherwavelengths,reflectingtheimpactofthe
cause ellipse fitting is usually inappropriate for edge-on galaxies ICMpressure.Althoughtherearesubtledifferences,thissuggests
whenthespatialresolutionislimited(Warmels1988a;Rhee&van thatthemoleculargasinthisgalaxyhasbeenalsoaffectedinsim-
Albada1996;Swaters1999).Thismethodcalculatesstripintegrals ilar ways by the same mechanism that is responsible for the pe-
byintegratingHIcolumndensityperpendicularlytothemajoraxis, culiaritiesfoundintheotherwavelengths.ThisindicatesthatICM
MNRAS000,1–17(2016)
10 B.Leeetal.
Figure6.AcompositemapofFUV(blue;GildePazetal.2007),Hα (NGC4330andNGC4569fromGavazzietal.2003,NGC4402fromCrowletal.
2005,NGC4522fromKoopmannetal.2001),optical(black;DSS2red),HI(green;Chungetal.2009)and12CO(2−1)(whitecontoursfromourSMA
data)ofNGC4330(redshifted),NGC4402(blueshifted),NGC4522(redshifted),andNGC4569(blueshifted).TheIRAM12CO(2−1)dataareshownin
yellowcontours.Contourlevelsare3.7,7.5Jybeam−1kms−1forNGC4330(Vollmeretal.2012a);5,20Jybeam−1kms−1forNGC4402(Leeetal.in
preparation);1.7,3.5,5.3Jybeam−1kms−1forNGC4522(Vollmeretal.2008)and14,71,106,214,498,712Jybeam−1kms−1forNGC4569(Leroyet
al.2009).TheICMwinddirectiondeducedfromHImorphologyisshownwithblackarrows(Vollmeretal.2004;Crowletal.2005;Abramsonetal.2011;
Abramson&Kenney2014).NGC4330,NGC4402andNGC4522areatrelativelyearlytoactiveHIstrippingstageduetotheICM,whileNGC4569is
thoughttobeapost-peakpressurecasewheresomeHIgasisfallingbackonthediscaftercorecrossing.
pressurecanchangemoleculargaspropertieswellinsidethestellar in the west (indicated by arrow in Fig. 4a), although the scale is
disc. distinct because the HI bump is visible in extraplanar gas. HI is
truncatedwithinthemid-planeofthestellardisconbothsides,with
asharpcut-offintheeasttosouthalongtheupstream.Meanwhile,
the northern side is more extended with a short tail pointing the
5.1.2 NGC4402
north-westinthedownstreamside.Theextentofthemoleculargas
TheCOemissionshowsanumberofpropertiesincommonwith is not significantly different in the receding and the approaching
theHI gas(Fig.4d).Itissomewhatextendedinthewest,i.e.the side,yetitsdistributionisquitedistinct.Thesedifferencesbetween
samesidewheretheHIismoreextended.Bothphasesshowabump thetwosidesoftheCOdisccanbealsoclearlyseenintheradial
MNRAS000,1–17(2016)