The Atlas3D project - XXVI. HI discs in real and simulated fast and slow rotators

5.2 MB·

by  Paolo Serra

Mon.Not.R.Astron.Soc.000,1–18(2013) Printed20June2014 (MNLATEXstylefilev2.2)
The ATLAS3D project – XXVI. H I discs in real and simulated fast
and slow rotators
Paolo Serra,1,2(cid:63) Ludwig Oser,3,4 Davor Krajnovic´,5 Thorsten Naab,3 Tom Oosterloo,1,6
Raffaella Morganti,1,6 Michele Cappellari,7 Eric Emsellem,8,9 Lisa M. Young,10
Leo Blitz,11 Timothy A. Davis,8 Pierre-Alain Duc,12 Michaela Hirschmann,13
Anne-Marie Weijmans,14,15† Katherine Alatalo,11,16 Estelle Bayet,7 Maxime Bois,8,9
4 Fre´de´ric Bournaud,12 Martin Bureau,7 Roger L. Davies,7 P. T. de Zeeuw,8,17 Sadegh
1
0 Khochfar,18,19 Harald Kuntschner,8 Pierre-Yves Lablanche,8,9 Richard M. McDermid,20
2
Marc Sarzi,21 and Nicholas Scott22,23
n
a 1NetherlandsInstituteforRadioAstronomy(ASTRON),Postbus2,7990AADwingeloo,TheNetherlands
J 2CSIROAstronomyandSpaceScience,AustraliaTelescopeNationalFacility,POBox76,Epping,NSW1710,Australia

4 3Max-Planck-Institutfu¨rAstrophysik,Karl-Schwarzschild-Str.1,85741Garching,Germany
1 4DepartmentofAstronomy,ColumbiaUniversity,NewYork,NY10027,USA
5Leibniz-Institutfu¨rAstrophysikPotsdam(AIP),AnderSternwarte16,D-14482Potsdam,Germany
] 6KapteynAstronomicalInstitute,UniversityofGroningen,Postbus800,9700AVGroningen,TheNetherlands
A 7Sub-Dept.ofAstrophysics,Dept.ofPhysics,UniversityofOxford,DenysWilkinsonBuilding,KebleRoad,Oxford,OX13RH,UK
G 8EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,85748Garching,Germany
9Universite´ Lyon1,ObservatoiredeLyon,CentredeRechercheAstrophysiquedeLyonandEcoleNormaleSupe´rieuredeLyon,9avenueCharlesAndre´,
.
h F-69230Saint-GenisLaval,France
p 10PhysicsDepartment,NewMexicoInstituteofMiningandTechnology,Socorro,NM87801,USA
- 11DepartmentofAstronomy,CampbellHall,UniversityofCalifornia,Berkeley,CA94720,USA
o 12LaboratoireAIMParis-Saclay,CEA/IRFU/SAp–CNRS–Universite´ParisDiderot,91191Gif-sur-YvetteCedex,France
r 13INAF-AstronomicalObservatoryofTrieste,viaG.B.Tiepolo11,I-34143Trieste,Italy
t
s 14DunlapInstituteforAstronomy&Astrophysics,UniversityofToronto,50St.GeorgeStreet,Toronto,ONM5S3H4,Canada
a 15SchoolofPhysicsandAstronomy,UniversityofStAndrews,NorthHaugh,StAndrewsKY169SS,UK
[ 16InfraredProcessingandAnalysisCenter,CaliforniaInstituteofTechnology,Pasadena,California91125,USA

17SterrewachtLeiden,LeidenUniversity,Postbus9513,2300RALeiden,theNetherlands
1
18Max-PlanckInstitutfu¨rextraterrestrischePhysik,POBox1312,D-85478Garching,Germany
v
19InstituteforAstronomy,UniversityofEdinburgh,RoyalObservatory,BlackfordHill,EdinburghEH93HJ,UK
0
20GeminiObservatory,NorthernOperationsCentre,670N.A‘ohokuPlace,Hilo,HI96720,USA
8
21CentreforAstrophysicsResearch,UniversityofHertfordshire,Hatfield,HertsAL19AB,UK
1
22SydneyInstituteforAstronomy(SIfA),SchoolofPhysics,TheUniversityofSydney,NSW2006,Australia
3
23ARCCentreofExcellenceforAll-skyAstrophysics(CAASTRO),44RosehillStreet,Redfern,NSW2016,Australia
.
1
0
4 Accepted....Received...;inoriginalform...
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c 2013RAS
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2 PaoloSerraetal.
ABSTRACT
Onequarterofallearly-typegalaxies(ETGs)outsideVirgohostadisc/ringof HI withsize
fromafewtotensofkpcandmassupto∼109M(cid:12).HereweinvestigatewhetherthisHIisre-
latedtothepresenceofastellardiscwithinthehostmakinguseoftheclassificationofETGs
infastandslowrotators(FR/SR).Wefindalargediversityof HI massesandmorphologies
within both families. Surprisingly, SRs are detected as often, host as much HI and have a
similar rate of HI discs/rings as FRs. Accretion of HI is therefore not always linked to the
growthofaninnerstellardisc.TheweakrelationbetweenHIandstellardiscisconfirmedby
theirfrequentkinematicalmisalignmentinFRs,includingcasesofpolarandcounterrotating
gas.InSRstheHIisusuallypolar.ThiscomplexpicturehighlightsadiversityofETGforma-
tionhistorieswhichmaybelostintherelativesimplicityoftheirinnerstructureandemerges
whenstudyingtheirouterregions.
We find that ΛCDM hydrodynamical simulations have difficulties reproducing the HI
propertiesofETGs.Thegasdiscsformedinsimulationsareeithertoomassiveortoosmall
dependingonthestarformationfeedbackimplementation.Kinematicalmisalignmentsmatch
theobservationsonlyqualitatively.ThemainpointofconflictisthatnearlyallsimulatedFRs
andalargefractionofallsimulatedSRshostcorotatingHI.ThisestablishestheHIproperties
ofETGsasanovelchallengetosimulations.
Keywords: ISM:kinematicsanddynamics–galaxies:ellipticalandlenticular,cD–galaxies:
evolution–galaxies:formation–galaxies:kinematicsanddynamics.
1 INTRODUCTION withtypicalradius∼ 1kpc,justafractionofthegalaxyeffective
radius(Alataloetal.2013;Davisetal.2013).
Theobservationofbothneutralhydrogen(HI)andmoleculargas
(H ,tracedbyCO)hasshownthatearly-typegalaxies(E/S0s,here- Taken together, these studies establish that about half of all
2
afterETGs),whileonaveragegas-poorerthanspirals,caninsome ETGshostbetween∼107and∼109M(cid:12)ofcoldgas(atomicand/or
caseshostasignificantmassofcoldgas(forearlyHIstudiessee, molecular)usuallysettledwithinthegalaxypotential.Verysensi-
e.g., Gallagher, Faber & Balick 1975; Knapp et al. 1977; Bieg- tive observations are required to reach such a detection rate (for
ing&Biermann1977;Knapp,Turner&Cunniffe1985;Wardle& example, this is significantly higher than in the recent surveys of
Knapp1986;forCOseeWiklind&Henkel1989;Leesetal.1991). massivegalaxiesbyCatinellaetal.2010andSaintongeetal.2011)
Whathasremainedunclearuntilrecentlyiswhethergas-richETGs butwhentheseareperformedwefindthatETGswithasignificant
are peculiar, rare objects or represent a significant fraction of the mass of cold gas are not peculiar, rare systems. The obvious im-
overallpopulation(e.g.,vanGorkom&Schiminovich1997).This plicationisthatinordertounderstandtheassemblyofETGsasa
question has now been answered by large HI and H2 surveys of familyweneedtounderstandhowtheygetorretaintheirgasand
morphologically-selectedETGscarriedoutaspartoftheATLAS3D whatimpactthishasonthehost,onitsstarformationhistoryand
project(Cappellarietal.2011a,hereafterPaperI). onitsstructure.Herewefocusonthelatteraspect.
InSerraetal.(2012,hereafterPaperXIII)weshowthat∼40 InmostpreviousstudiesofthiskindETGsaresimplydivided
per cent of all ETGs outside the Virgo cluster and ∼ 10 per cent intoellipticalsandlenticulars.Whenthisisdone,moleculargasis
insideVirgohost HI downtoamassof∼ 107 M(cid:12) andacolumn almost exclusively found in lenticulars (Lees et al. 1991, Welch,
densityof∼ 3×1019 cm−2,placingrecent,previousresults(Mor- Sage & Young 2010, Paper IV), consistent with the fact that CO
gantietal.2006;diSeregoAlighierietal.2007;Grossietal.2009; is invariably associated with dust lanes. Furthermore, HI seems
Oosterlooetal.2010)onastrongstatisticalbasis.Wefindthatin to be more abundant in lenticular than in elliptical objects at a
∼2/3ofalldetections(and1/4ofallETGsoutsideVirgo)theHI typical M(HI)sensitivityof∼ 109 M(cid:12) (Roberts1975;Wardle&
isdistributedinalow-column-densitydiscorringwithsizefroma Knapp 1986; Bregman, Hogg & Roberts 1992; Sadler 2001) al-
fewtotensofkpc,andmassfrom∼107tomorethan109M (see thoughmoresensitiveobservationsofsmallersamplesdonotre-
(cid:12)
also van Driel & van Woerden 1991; Oosterloo et al. 2007). The vealanysignificantdifference(Grossietal.2009;Oosterlooetal.
remaining detected ETGs host HI on an unsettled configuration 2010).Onecomplicationisthatthedistinctionbetweenellipticals
whichusuallyrevealsrecentdynamicalinteractionwithinagroup. andlenticularscanbeambiguousandisaffectedbyprojectionef-
Concerningmoleculargas,Youngetal.(2011,hereafterPa- fects (Jorgensen & Franx 1994). A more robust and physically
perIV)detectCOin22percentofallETGsindependentofgalaxy meaningfulapproachistoclassifyETGsonthebasisoftheirin-
luminosity or environment, with M(H ) between ∼ 107 M (the ternalkinematics(e.g.,Daviesetal.1983;Cappellarietal.2007;
2 (cid:12)
typicalsensitivityoftheirdata)and∼109M (forrecent,previous Emsellemetal.2007).
(cid:12)
surveys of smaller samples see Combes, Young & Bureau 2007; InthiscontextakeyresultoftheATLAS3Dprojectisthatthe
Sage, Welch & Young 2007). Follow-up interferometric observa- vast majority of all ETGs (∼ 85 per cent) are structurally simi-
tions show that this gas is mostly distributed on discs and rings
lar to spirals. They rotate like discs (Krajnovic´ et al. 2011, here-
afterPaperII),canbedescribedasafamilyofoblaterotatorson
theλ -(cid:15) diagram(Emsellemetal.2011,hereafterPaperIII),and
R
(cid:63) E-mail:[email protected] coverthesamerangeofintrinsicflatteningandbulge-to-discratio
† DunlapFellow ofSatoScspirals(Cappellarietal.2011b,2013b,hereafterPaper
c 2013RAS,MNRAS000,1–18
(cid:13)
HI discsinfastandslowrotators 3
VIIandPaperXX,respectively;Krajnovic´etal.2013a;Weijmans a function of their structure within the context of the ATLAS3D
et al., submitted; see also Laurikainen et al. 2011; Kormendy & project.Thefewpreviousattemptsinthisdirectionrevealnoclear
Bender2012).WecalltheseETGsfastrotators(FRs).Inpractice, relation between HI and ETG structure but are based on signif-
FRsincludenearlyallclassicallenticularsplusalargenumberof icantly smaller samples (Morganti et al. 2006; Oosterloo et al.
galaxiesmorphologicallymisclassifiedasellipticalsbecauseofun- 2010).Hereweaimatclarifyingthesituationbystudyingthelarge
favourable viewing angle (Paper III). The evolutionary history of ATLAS3D HIsample.WedescribethesampleandtheHIdatain
most FRs must include the growth and survival of a stellar disc Sec.2,discussHIdetectionrate,massandmorphologyasafunc-
following accretion of cold gas from the surrounding medium as tionofgalaxystructureinSec.3,analysethekinematicalmisalign-
well as gas-rich minor and major mergers (Khochfar et al. 2011, mentbetween HI andstarsinSec.4,performforthefirsttimea
hereafterPaperVIII;Naabetal.2013,hereafterPaperXXV). comparisontothepredictionsofΛCDMhydrodynamicalsimula-
Theremaining∼ 15percentofallETGsdonotexhibitthe tionsinSec.5,andsummarizetheresultsinSec.6.
disc-like stellar rotation observed in FRs and occupy a region of
theλ -(cid:15) diagramcorrespondingtointrinsicallylowerspecifican-
R
gularmomentumandflattening(PaperIII).Wecallthesegalaxies
slowrotators(SRs).WefindthatSRsformaheterogeneousfamily 2 SAMPLEANDHISURVEY
includingsystemswithnorotationatall,non-disc-likerotationand
kinematically distinct cores (KDCs). Massive SRs are thought to The ATLAS3D HI sample and the radio observations which this
growvia(asmallnumberof)majormergersatz>1,whichlower study is based on are described in detail in Paper XIII. Here we
the system angular momentum and may create a KDC, followed provideashortsummary.WestudyasubsetofthefullATLAS3D
bymanyminormergerswithlowgasfraction,whichpreservethe sampleofETGs(PaperI;distance < 42Mpc, MK < −21.5).This
KDCandmakethestellarbodyrounder(Naab,Johansson&Os- subset includes all galaxies with declination δ (cid:62) 10◦ except the
triker2009;Boisetal.2011,hereafterPaperVI;PaperVIII;Paper fourobjectsclosesttoVirgoA,resultinginavolume-limitedsam-
XXV).Thissequenceofeventsismostlikelytooccurinadense pleof166ETGs(127outsidetheVirgocluster).WeobtainWester-
environment,inagreementwiththeincreasedfractionofSRsinthe borkSynthesisRadioTelescopeHIdataformostofthemeitheras
centreoftheVirgocluster(PaperVII). partoftheATLAS3D projectorfrompreviousprojects(Morganti
WithinthispictureonemaynaivelythinkthatSRsshouldlack etal.2006;Jo´zsaetal.2009;Oosterlooetal.2010).Wemakeuse
a significant cold-gas phase, unlike FRs. Indeed, Paper IV shows ofVeryLargeArraydatafor1galaxywithHI(Chungetal.2009)
thattheCOdetectionrateismuchlowerinSRsthaninFRs.How- andofArecibospectrafor20galaxies(allundetected;Giovanelli
ever,thesituationisnotnecessarilysosimple.Forexample,thede- etal.2005).
tectionofkinematically-misalignedionizedgas(Davisetal.2011, AsmentionedinSec.1,ourWesterborkobservationsallowus
hereafterPaperX),colddust(Smithetal.2012)andstellarnuclear todetectHIdowntoacolumndensityofafewtimes1019cm−2and
cusps(Krajnovic´ etal.2013b,hereafterPaperXXIII)inSRssug- amassof∼107M(cid:12).WedetectHIassociatedwiththetargetETG
geststhattheytoocanaccretesomegasduringtheirlife;andsim- in53galaxies(49outsideVirgo).InPaperXIIIweclassifydetected
ulationsshowthatatleastsomeSRsmayhaveformedingas-rich ETGsinthefollowingclassesonthebasisoftheHImorphology:
mergers (Paper VI; Paper XXV). Furthermore, the mere fact that D=largediscs(14objects);R=largerings1(10);d=smalldiscs
FRs are found at basically all environment densities (Paper VII) (10);u=unsettleddistributions(14);c=gasclouds(5).Gasdistri-
impliesthattheremustbealargevarietyofevolutionaryhistories butionscharacterizedbyorderedrotation(D,Randdclasses)are
leadingtotheirformation,andthereforepotentiallyalargevariety thereforethemostcommononesamongHI-richETGs,amounting
ofresultingcold-gasproperties. to2/3ofalldetectionsand1/4ofallETGsoutsideVirgo.Asdis-
All that is required to mark the visual difference between a cussedinPaperXIII,theaboveHIclassesareawaytosimplifythe
FRandaspiralatz = 0isthattheformershouldhavelowerstar- observedcontinuumofgasmorphologies,withanumberofobjects
beingintermediate(orintransition)betweenclasses.Wemakeuse
formation-rate surface density on the stellar disc. In other words,
they should be quenched owing to a lower M /M and/or star- oftheHIclassesD,R,d,uandcintherestofthispaper.
gas (cid:63)
formationefficiencywithinthestellarbody.Observationsindicate InPaperXIIIweshowHIimagesofalldetections.Eachim-
ageisobtainedaftercreatingamaskincludingallemissioninthe
that such quenching may be related to the growth of the stellar
bulge: the cold gas content of the stellar body decreases as the corresponding HI cube. The images shown in Paper XIII are the
zero-thmomentofthemaskedcubes.Inthepresentpaperweshow
bulge becomes more dominant (Catinella et al. 2010; Saintonge
et al. 2011; Paper XX) and, additionally, the bulge may stabilize andanalyseforthefirsttimetheHIvelocityfieldsobtainedasthe
firstmomentofthesamemaskedcubes.
theremaininggasagainststarformation(Kawata,Cen&Ho2007;
Martigetal.2009;Saintongeetal.2012;Martigetal.2013).Yetit
isstillpossibleforaFRtohostalargemassofcoldgaswellout-
sidethestellarbodyandatlowsurfacedensity(e.g.,Morgantietal.
2006;Oosterlooetal.2007;PaperXIII).Suchgasdistributionsare 3 RELATIONBETWEENHIANDETGINNER
indicativethattherearemanywaysofmakingaFRandthatthisin- STRUCTURE
formation,lostintheapparenthomogeneityoftheirshapeandstel-
InthissectionweinvestigatewhethertheHIdetectionrate,mass
larkinematics,mayemergewhenobservingtheircold-gasphase.
andmorphologyofETGsarerelatedtotheirstellarkinematicsas
Furthermore,thesetenuousgassystemsareonlyweaklyboundto
thehostandhenceareeasilyperturbed.Theiractualmorphology
isthereforeasensitiveprobeofthepastdynamicalhistoryofthe
host, and indeed in Paper XIII we show that the HI morphology 1 Infact,inPaperXIIIwegroupDsandRsinasingleclass.Intable1of
variessystematicallywiththedensityoftheenvironment. thatpaperweindicategalaxieswheretheHIisdistributedonaring.Here
In this spirit we study here the HI properties of ETGs as weusethattableasabasisforthedivisionintoDandRobjects.
c 2013RAS,MNRAS000,1–18
(cid:13)
4 PaoloSerraetal.
Table1.NumberofgalaxiesinthevariousHImorphologicalclasses(Pa- Table2.NumberofgalaxiesinthevariousHImorphologicalclasses(Pa-
perXIII)asafunctionofstellarkinematicsclass(PaperIII)andSR/FR perXIII)asafunctionofstellarkinematicsclass(PaperIII)andSR/FR
classification(PaperII)forthefullATLAS3DHIsample. classification(PaperII)fornon-VirgoETGsintheATLAS3DHIsample.
Full Stellarkinematics Outside Stellarkinematics
Sample All a(cid:63) b(cid:63) c(cid:63) d(cid:63) e(cid:63) SR FR Virgo All a(cid:63) b(cid:63) c(cid:63) d(cid:63) e(cid:63) SR FR
All 166 4 8 14 4 136 22 144 All 127 2 7 11 1 106 16 111
Undet. 113 3 5 7 4 94 13 100 Undet. 78 1 4 5 1 67 8 70
Det. 53 1 3 7 0 42 9 44 Det. 49 1 3 6 0 39 8 41
HI D 14 0 1 1 0 12 2 12 HI D 14 0 1 1 0 12 2 12
R 10 0 0 2 0 8 2 8 R 9 0 0 2 0 7 2 7
d 10 0 0 0 0 10 0 10 d 9 0 0 0 0 9 0 9
u 14 1 2 2 0 9 3 11 u 12 1 2 1 0 8 2 10
c 5 0 0 2 0 3 2 3 c 5 0 0 2 0 3 2 3
wellaspositionontheλ -(cid:15)diagram2andmass-sizeplane.Inpar- formation, and gas-poor evolutionary paths are not the only way
R
ticular,wemakeuseofthekinematicalclassificationpresentedin tomakeagalaxywiththisinternalstructure.Asimilarconclusion
Paper II. In that paper ETGs are divided in the following classes isreachedbyPaperXbasedonthemisalignmentbetweenionized
basedonaquantitativeanalysisoftheirstellarvelocityfield3:a = gasandstellarkinematicsinthesegalaxieswithin∼1R ,bySmith
(cid:63) e
norotation;b =somerotationbutwithavelocityfieldnotconsis- etal.(2012)basedonthefrequentdetectionofcolddustinSRs,by
(cid:63)
tentwiththatofarotatingdisc;c =KDC;d =twocounterrotat- Paper XXIII based on the existence of SRs with a cuspy nuclear
(cid:63) (cid:63)
ingstellardiscs;e =velocityfieldconsistentwiththatofarotating light profile, and by both idealized and ΛCDM hydrodynamical
(cid:63)
disc.Theseclassesareimportanttounderstandtheclassificationof simulations(PaperVI;PaperXXV).Additionalobservationssuch
ETGsintoFRsandSRsdiscussedinSec.1.InPaperIIIwedefine asverydeepopticalimagingmayprovidefurthercluesaboutthe
√
FRs andSRs asgalaxies above andbelow theline λ = 0.31 (cid:15) formationofSRsinpresenceofacoldgascomponent(Ducetal.
R
on the λ -(cid:15) diagram, respectively. Fast rotators and e ETGs are 2011).
R (cid:63)
essentiallythesamefamilyandrepresent∼ 85percentofallob- These qualitative results are illustrated in more quantitative
jects.Ontheotherhand,theSRfamilyisveryheterogeneousand termsinTable1forthefullsampleandinTable2fornon-Virgo
includesgalaxiesinallothergroups(a tod ). galaxies only. Within the limited statistics available for SRs (and
(cid:63) (cid:63)
forclassesa(cid:63) tod(cid:63)),theirHIdetectionrateisthesameasthatof
FRs (and e(cid:63) objects). Namely, we detect HI in 41±14 per cent
3.1 Fastversusslowrotators ofallSRsand31±5percentofallFRs,whileoutsideVirgothe
Figure1showsallgalaxiesontheλ -(cid:15) diagram.Inthisfigurewe detectionratebecomes50±18percentforSRsand37±6percent
R
indicategalaxieswithdifferentstellarkinematicsandHImorphol- forFRs(errorbarsassumeabinomialdistribution).Thefractionof
ogyusingdifferentcoloursandmarkers,respectively.Tofirstorder galaxieshostingarotatingHIdistribution(HIclassesD,Randd)
Fig.1confirmstheresultsofMorgantietal.(2006)andOosterloo is18±9percentforSRsand21±4percentforFRsoverthefull
etal.(2010):therelationbetweenHIpropertiesandgalaxystruc- sample,whileoutsideVirgoitbecomes25±13percentforSRs
tureisnotasimpleone. and25±5percentforFRs.
WedetectHIingalaxiesatalllocationsontheλR-(cid:15) diagram The above paragraph describes the HI properties (detection
where there are ETGs, both FRs and SRs. Many of the HI discs rateandmorphology)asafunctionofstellarkinematics.Acom-
andrings(D,Randd)arehostedbyflat,nearlyedge-onFRs(top- plementaryapproachistoexaminethestellarkinematicsasafunc-
right of the diagram) and their lower-inclination counterparts (at tionofHIproperties,butalsointhiscaseTables1and2showthat
lowervaluesofλ and(cid:15)).Thisisreasonableasthesegalaxieshave thereisnocleartrend.WithineachHIclass(includingthatformed
R
apronouncedstellardisccomponentandthereforemusthaveex- byallundetectedETGs)thefractionofSRsandFRsisconsistent
periencedaconsiderablelevelofdissipationduringtheirassembly. withtheglobalvaluesof∼15and∼85percent,respectively.The
TheircurrentHIdiscs/ringsmaybearemnantofthatprocess.The sameistrueconsideringallgalaxieswithasettledHI√distribution
moredetailedanalysisofSec.4showsthatthisisindeedthecase together(D,Randd).WealsocomparetheλR/(0.31 (cid:15))distribu-
forabouthalfofallgas-richFRs,butalsothatintheremaininghalf tionofD+R+dETGstothatofundetected+cobjects4andfind
thegasaccretionepisodetracedbyHIdoesnotseemtoberelated thattheyareindistinguishable–accordingtoatwo-sampleKStest
totheformationofthecurrentstellardisc. theprobabilitythatthetwosamplesarenotdrawnfromthesame
WhatmightbemoresurprisingistofindHIalsoamongSRs parentdistributionis78percent,insufficienttoclaimasignificant
(a ,b andc objectsonly)andthatinsomeofthemthedetected difference.
(cid:63) (cid:63) (cid:63)
HIisdistributedonalargediscorring.ThisindicatesthatSRstoo Theonlypossibleexceptiontothislackoftrendsisthatsmall
canexperiencegasaccretionanddissipativeprocessesduringtheir HI discs(d)includenoSRs.Thisisconsistentwiththefactthat
HIinthesegalaxiesseemstobestronglylinkedtothepresenceof
moleculargasandstarformationwithinastellardisc(PaperXIII).
2 WeadopttheλRand(cid:15)measurementsobtainedwithina1Reaperturein
PaperIII.Notethatnotallgalaxieshavetheirstellarkinematicsmeasured
outto1Re,asdiscussedinthatpaper. 4 InthiscontextcETGscanbegroupedtogetherwithnon-detectionssince
3 Note the addition of a (cid:63) subscript to the kinematical classes name to M(HI)isverylowandthedetectedgasdoesnotappearobviouslyassoci-
avoidconfusionwiththeHIclassesdefinedinSec.2. atedwiththehost(PaperXIII).
c 2013RAS,MNRAS000,1–18
(cid:13)
HI discsinfastandslowrotators 5
0.8
e
?
regular
0.7 N4036 N2685 rotation
N3838
N3626
N2764 N4710
U09519
N4111
0.6 N5422
U06176 N3489
N5103 d
N5582 N7280 N4026 2σ?
N3945
N4150
0.5 N2824
N6798
N3941
N3998
c
λRe0.4 N0680 N1023 N2594 K?DC
N2859
N3032
N3499 N5866
N4262 N3182
0.3
N4203 N3619 N7465 N4694
U05408 b
N2768 ?
complex
Hiclass rotation
0.2 N3193
N4278 undet.
N5173
N3073 D
U03960 R
0.1 N3457 N5631 a?
N3414 d no
N5198 N4406 N3522 u rotation
N5557
c
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
(cid:15)
Re
Figure1.HIontheλR-(cid:15)diagram.ThecolourschemerepresentsthekinematicalclassificationofETGspresentedinPaperII.Themarkersshaperepresentthe
HImorphologyasinPaperXIIIwiththesmallmodificationdescribedinSec.2.Themagentalinerepresentstheedge-onviewofanisotropic,oblatemodels
withβ=0.65×(cid:15)(Cappellarietal.2007),andtheblackdashedlinesrepresentthemodelswith(cid:15)=0.75and0.85viewedunderallpossibleinclinationsfrom
√
edge-on(topright)toface-on(bottomleft).ThegreenlinerepresentstheempiricalseparationbetweenFRsandSRsλR=0.31 (cid:15)proposedinPaperIII.We
indicatethenameofallHIdetectionsexceptthatofcgalaxies.
However,thenumberofdsislowandonly∼ 1SRwouldbeex- rangeofM(HI)/M(cid:63).TheyseemnottoreachthelargeM(HI)/M(cid:63)
pectedamongthembasedontheglobalSRfraction.Betterstatis- valuesallowedforFRsbuttheoveralllownumberofSRsmakes
ticswouldthereforebeneededtoconfirmthisresult. thisdifferencestatisticallyinsignificant.Forexample,thefraction
AnotherimportantquantitytoanalyseistheHImas√s.Figure ofgalaxieswithM(HI)/M(cid:63)(cid:62)0.002is23±10percentwithinthe
2showstheM(HI)/M(cid:63)ratioplottedagainsttheλR/(0.31 (cid:15))ratio. SRfamilyand24±4percentforFRs.Thesefractionsremaincom-
HereandintherestofthispaperweadoptforM thevaluederived parabletooneanotherifweconsiderETGsoutsideVirgoonly,and
(cid:63)
byCappellarietal.(2013a,hereafterPaperXV)basedondynam- these(andthefollowing)resultsdonotchangeifweadoptadiffer-
ical modelling – MJAM in that paper5. One galaxy, PGC 071531, entM(HI)/M(cid:63)threshold.Conversely,wedefinetwoETGsubsam-
has no reliable M(cid:63) measurement and is excluded from this anal- pleswithM(HI)/M(cid:63)aboveandbelow0.002,including39and126
ysis.Figure2showsthat HI-detectedSRsarefoundoverawide galaxiesrespectively.Wefindthattheycontainasimilarfractionof
SRsconsistentwiththeglobalvalueof∼15percent,bothconsid-
eringallgalaxiesornon-Virgogalaxiesonly.Finally,accordingto
√
5 The results discussed in this section do not change if we normalize atwo-sampleKStesttheprobabilitythattheλR/(0.31 (cid:15))distri-
M(HI)to LK.Thisfollowsfromthefactthatthedistributionof M(cid:63)/LK butionsofthetwosubsamplesarenotdrawnfromthesameparent
ratioisverynarrowforETGs,peakingat1.2M(cid:12)/L(cid:12)andwithanrmsof0.4 distributionisjust4percent.
M(cid:12)/L(cid:12).Thisscatterisinsufficienttosignificantlyalteraplotliketheone
showedinFig.2,wheregalaxiesspreadover∼ 3ordersofmagnitudein
M(HI)/M(cid:63).
c 2013RAS,MNRAS000,1–18
(cid:13)
6 PaoloSerraetal.
N7465
e
?
regular
rotation
U09519
1 N2685
− N5173
N5582
N3073 U05408 N6798 N2764 U06176 d
N4203 ?
NN21509243 N3626 N0680 2σ
N4694 N5103 N3941
N4262
N3522 N3457
N4111
M?−2 N5631 N3619 N4026 N3838 N3032
/ N3945
)
Hı N4278 c?
(
M N7280 N3998 N2859 KDC
U03960
0
g1 N5198 N4036
lo N5557 N3414 N3193 N2824
3
− N5422 N3499 b
?
N3489 complex
rotation
N4406 Hıclass
N4150
N2768 N3182 undet.
D
−4 N5866 N4710 R a?
d no
u rotation
c
0 1 2 3 4 5
λRe/(0.31√(cid:15)Re)
√
Figure2.M(HI)/M(cid:63) plottedagainsttheratioλR/(0.31 (cid:15)),suchthatSRsandFRsarefoundtotheleftandrightofthegreen,verticalline,respectively.
ColoursandmarkersareasinFig.1.
3.2 HIpropertieswithintheFRfamily HIdiscorring(D,Randd)isstatisticallyundistinguishablefrom
that of galaxies with no HI (undetected and c). The two-sample
ConsideringnowFRsalone,wefindnosystematicvariationofHI KS-test probability that the two distributions are not drawn from
properties as a function of position on the λ -(cid:15) diagram. Specif- thesameparentdistributionis38percent.
R
ically, we compare intrinsically flatter FRs to rounder ones. We Wealsoinvestigatepossiblevariationsof M(HI)/M(cid:63) within
dothisbyconsideringe(cid:63) galaxiesonlyandsplittingthemintwo theFRfamily.Wefindthat19±6percentofallflatgalaxieshave
groupsaboveandbelowthelineof0.75intrinsicellipticity,respec- M(HI)/M(cid:63) (cid:62) 0.002,comparabletothe26±5percentfoundfor
tively(seeFig.1;wecommentbelowonalternativewaysofdefin- round objects. Conversely, we define an HI-rich and an HI-poor
ingflatandroundgalaxies).Thisresultsin47‘flat’and89‘round’ subsampleofe(cid:63)ETGsadoptingthesameM(HI)/M(cid:63)separationas
e(cid:63) ETGs(39and67outsideVirgo,respectively).WedetectHIin above.Thetwosamplesinclude32and103galaxies,respectively.
32±8percentofallflatobjectsand30±6percentofallround Thefractionofflatobjectsinthetwogroupsiscomparable(28±9
ones.ThedetectionrateofrotatingHIdistributions(D,Randdob- and 37±6 per cent, respectively). Finally, a two-sample KS test
jects)is26±7and20±5percent,respectively,withbothflatand gives a probability of just 10 per cent that the λR/λR,(cid:15)=0.75 distri-
roundETGsexhibitingamixofD,Randdobjects.Nosignificant butionsofthetwosubsamplesarenotdrawnfromthesameparent
differenceemergesifweconsidernon-Virgogalaxiesonly. distribution.
Conversely, the fraction of flat and round e ETGs does not ThedefinitionofflatandroundFRsadoptedaboverelieson
(cid:63)
vary significantly as a function of HI class (also when grouping asimplede-projectionofgalaxies’shapealongmodellinesonthe
togetherallD,Randdsystems).Withintheerrorbars,wealways λ -(cid:15) diagram.Amoreaccurateestimateoftheintrinsicellipticity
R
find∼35percentofflatobjectsand∼65percentofroundgalax- ofETGsisderivedinPaperXXbasedonthemodellingpresented
ies. Furthermore, the λR/λR,(cid:15)=0.75 distribution of galaxies with an inPaperXVandontheellipticitymeasurementatlargeradiusre-
c 2013RAS,MNRAS000,1–18
(cid:13)
HI discsinfastandslowrotators 7
0.5 relativesimplicityoftheirinnershapeandkinematics.OurHIsur-
− veydemonstratesthatthereareverydiverseevolutionarypathsfor
1.0 1.0 FRs,whichincludetheformationofbothgas-poorobjects(unde-
− tectedinHIorwithverylowM(HI)/M(cid:63))aswellasgalaxieswith
1.5 aconsiderablemassofHIspreadoveralargearea(andtherefore
0.8 −
withlowmeancolumndensity;seePaperXIII),butthatthisdiver-
c) 2.0M? sityisnotreflectedinthestructureofETGswithintheFRfamily.
majR(kpe 00..46 −−2.5M(Hı)/ Fliufertahnedrmthoarte,thweeyfidnodsothaattaSRsismtioloarcraanteaacscrFeRtesc.oldgasduringtheir
log10 0.2 Hıclass −3.0log10 entinItteirsniamlsptorurtcatnutretothraenalFisResthdaetsSpiRteshhaavvienagfsuimndilaamreHntIapllryopdeifrftieers-
(in a statistical sense). This means that the accretion of cold gas
undet.
3.5
D − has not altered their structure, which remains characterized by a
0.0 R relativelyroundshapeandlowspecificangularmomentumwithin
d 4.0
u − ∼1Re.Inotherwords,gasaccretionhasnotresultedinthegrowth
0.2 c ofasignificantstellardisccomponentinthecentralgalaxyregion.
− 4.5
9.5 10.0 l1o0g.5 M (M11.0) 11.5 12.0 − The same might be true for FRs: the HI discs and rings in FRs
10 ?
(cid:12) donotneedtobecausallyrelatedtothestellardiscsrevealedby
integral-fieldspectroscopyinside∼1R ,althoughtheymightwell
Figure3.HIpropertiesonthemass-sizeplane.DifferentHImorphologies e
areindicatedasinpreviousfigures(seelegendonthebottomright)while beinatleastsomecases.Toinvestigatethisaspectinmoredetail
markersarecolour-codedaccordingtotheM(HI)/M(cid:63)value.Thevelocity wenowturntothestudyofthekinematicalmisalignmentbetween
dispersionisconstantalongthebluedashedlinesforgalaxiesfollowingthe HIandstarsintheseobjects.
virialrelation(σe =50,100,200and300kms−1fromtop-lefttobottom-
right,respectively;seeCappellarietal.2006).Theredsolidlineindicates
theZoneofExclusiondiscussedinPaperXX.
4 KINEMATICALMISALIGNMENTBETWEENHIAND
STARS
portedinPaperII.However,wehaveverifiedthatusingthesein-
trinsicellipticityvaluesdoesnotchangeanyoftheaboveconclu- In order to study the kinematical misalignment between HI and
starswedefineasamplemadeofallgalaxiesforwhichwecanre-
sions.FlatandroundFRskeephavingequalHIdetectionratesas
wellasequalfractionsofHIdiscsandofgalaxieswithM(HI)/M(cid:63) liablymeasurethepositionangle(PA)oftheHIkinematicalmajor
(cid:62) 0.002.Similarly,thedistributionofFRintrinsicellipticitydoes axis,PA(HI).ThisincludesallgalaxiesintheD,Randd classes
notchangeasafunctionofHImassormorphology. plustwoobjectshosting HI onanoverallunsettledconfiguration
(u)butwheresomeofthegasshowsorderedrotation–NGC1023
and NGC 4026. The total sample studied in this section includes
therefore36ETGs.
3.3 Mass-sizeplane
Although the λ -(cid:15) diagram is very useful to separate SRs from
R
FRs,Paper XXshowsthat anumberof keyETGproperties vary 4.1 Polar,corotating,counterrotatingandwarpedHI
in a much clearer way on the mass-size plane instead. In par-
ticular, properties linked to the star formation history of galaxies Figure 4 shows HI images (from Paper XIII), HI velocity fields
(shown for the first time here) and stellar velocity fields (already
suchastheirmass-to-lightratio,opticalcolour,Hβabsorptionand
showninPaperII)ofallD,Randuobjectsinthissample(26ob-
M(H )/M ratio correlate with bulge-to-disc ratio (traced by ve-
2 (cid:63)
jectsintotal;the10galaxiesclassifiedasdareanalysedseparately
locitydispersion)onthisplane.We thereforeinvestigatewhether
theETGHIpropertiestooexhibitanysystematicvariationonthe below). The figure reveals a diversity of HI-stars misalignments,
includingcorotating,polarandcounterrotatinggasdistributions.In
mass-sizeplane.
Figure 3 shows all galaxies on the mass size plane of Paper anumberofcasesthestrongvariationofPA(HI)withradiusindi-
catesthewarpingofthegasdisc.
XX.Thisfiguredoesnotshowanycleartrendotherthanthede-
creaseofthetypical M(HI)valueandtheprevalenceofdisturbed Morequantitatively,wederivePA(HI)asafunctionofradius
HI morphologies at high galaxy mass, already reported in Paper from the HI velocity field using the KINEMETRY software (Kra-
jnovic´ et al. 2006). In order to maximize the radius out to which
XIII.AswithFig.2,themainobservationalresultisaverylarge
scatter of HI properties, reinforcing our conclusion that HI does PA(HI)canbemeasuredwelowerthedefaultminimumcoverage
of an ellipse with data points to 30 per cent. As we are only in-
notappeartobelinkedinanyclearwaytotheinnergalaxystruc-
terestedinthePAvaluesthisisanacceptablechoice.Wecompare
ture.
PA(HI)tothePAofthestellarkinematicalmajoraxis,PA(stars),
whichisgiveninPaperII.Beforeproceedingwehighlighttwoas-
pectswhichshouldbekeptinmindinordertointerprettheresults
3.4 Diversityofgasaccretionhistories
ofthisanalysiscorrectly.
ThelackofarelationbetweenETGHIproperties(detectionrate, First, while PA(HI) is a function of radius and can be mea-
massandmorphology)andETGstructuredemonstratestheimpor- sured sometimes out to tens of R , PA(stars) is an average value
e
tanceofcoldgasasanindependentprobeoftheevolutionofthese measured over an aperture of typically ∼ 1 R (see Paper II for
e
objects.Neutralhydrogenobservationsgiveuscluesaboutthedi- a detailed discussion). Therefore, we are not comparing the HI
versityofETGassemblyhistorieswhichareotherwiselostinthe kinematics to the stellar kinematics at the same radius, but the
c 2013RAS,MNRAS000,1–18
(cid:13)
8 PaoloSerraetal.
NGC1023(u) NGC2594(R) NGC2685(D) NGC2764(D) NGC2859(R) NGC3414(R) NGC3522(R)
1.7×1019cm−2 2.6×1019cm−2 1.1×1019cm−2 2.3×1019cm−2 2.9×1019cm−2 4.1(1.4)×1019cm−2 2.2(2.3)×1019cm−2
210 150 180 150 180 130 130
± ± ± ± ± ± ±
210 150 180 150 180 130 130
± ± ± ± ± ± ±
NGC3619(D) NGC3626(D) NGC3838(R) NGC3941(R) NGC3945(R) NGC3998(D) NGC4026(u)
3.7×1019cm−2 2.0×1019cm−2 3.7×1019cm−2 2.8×1019cm−2 3.2×1019cm−2 2.7×1019cm−2 2.6×1019cm−2
150 210 170 170 260 240 200
± ± ± ± ± ± ±
150 210 170 170 260 240 200
± ± ± ± ± ± ±
-V -V/2 0 +V/2 +V
Figure4.Fromtoptobottom(threepanelspergalaxy),HIimages(100×100kpc2),HIvelocityfields(50×50kpc2)andstellarvelocityfields(10×10kpc2)
ofallDandRETGs,plusthetwou’sshowingsomeorderedrotation(NGC1023andNGC4026).Thegreyscale-barrepresents5kpcinallfigures.Theblack
ellipsesoverlaidonallvelocityfieldshavesemi-majoraxisequalto3ReandellipticitytakenfromPaperII.TheblackarrowoverlaidontheHIvelocityfields
indicatesthePAofthestellarkinematicalmajoraxis(recedingside)fromPaperIIandscaleswiththesizeoftheellipse.Inafewcasesthestellarkinematical
PAcouldnotbedeterminedreliablyandnoarrowisshown(seethetext).Forallvelocityfieldsthevelocityrangeisindicatedonthetop-right.Thisiskept
fixedforthestellarandHIvelocityfieldofasamegalaxyinordertoenableacomparisonoftherelativeline-of-sightrotationamplitude.Thecolourscheme
ofthevelocityfieldsisrepresentedbythehorizontalcolourbaratthebottomofthefigure.
HI kinematics (out to large radius) to the inner stellar kinemat- terpretation may be ambiguous in SRs, where the stellar kine-
ics.Acomparisontothestellarkinematicsatlargerradiuswould matics is more complex or characterized by very low-level rota-
requiredeeperopticalspectroscopyoveralargerfield(e.g.,Proctor tion.TheSRsforwhichwecanmeasurePA(HI)areNGC3414,
etal.2009;Weijmansetal.2009;Arnoldetal.2011,2013)orthe NGC 3522, NGC 5631 (all KDCs) and UGC 03960 (b ). In the
(cid:63)
studyofplanetarynebulae(e.g.,Romanowskyetal.2003;Coccato caseofNGC3414PA(stars)reflectsthekinematicsoftheregion
etal.2009;Cortesietal.2013)orglobularclusterkinematics(e.g., outside(andcounterrotatingrelativeto)theKDC.ToobtainthePA
Karthaetal.2013;Potaetal.2013).Acomparisonbetweengasand oftheKDConewouldhavetosubtract180◦fromthevaluegiven
stellarkinematicsatacommon,smallradiusispresentedinPaper inPaperII.Onthecontrary,inNGC5631PA(stars)representsthe
Xusingionizedandmoleculargasdata. inner(i.e.,theKDC’s)stellarrotation.InNGC3522theerroron
PA(stars)isverylargebecauseoftheoveralllow-levelrotation.In
Secondly,whilePA(stars)isawelldefinedquantityforFRs, this case we calculate the misalignment relative to the photomet-
where it is typically constant within the observed field, its in-
c 2013RAS,MNRAS000,1–18
(cid:13)
HI discsinfastandslowrotators 9
NGC4036(R) NGC4203(D) NGC4262(R) NGC4278(D) NGC5103(D) NGC5173(D) NGC5582(R)
3.3×1019cm−2 2.9×1019cm−2 2.5×1019cm−2 1.1(0.9)×1019cm−2 3.6×1019cm−2 3.2×1019cm−2 2.0×1019cm−2
260 140 180 220 120 70 210
± ± ± ± ± ± ±
260 140 180 220 120 70 210
± ± ± ± ± ± ±
NGC5631(D) NGC6798(D) UGC03960(D) UGC06176(D) UGC09519(D)
3.5×1019cm−2 3.6×1019cm−2 2.6×1019cm−2 2.4×1019cm−2 2.8×1019cm−2
170 190 110 140 110
± ± ± ± ±
170 190 110 140 110
± ± ± ± ±
-V -V/2 0 +V/2 +V
Figure4.Continued
ric major axis, which coincides with the (uncertain) kinematical andUGC09519).Inmostoftheremainingsystemsthemisalign-
one. Similarly, PA(stars) is unconstrained in UGC 03960 and we mentanglevariessignificantlywithradius,indicatingthepresence
adoptthephotometricPAasareference.Notethatusingthepho- of a warp. In the two most extreme cases the HI changes from
tometricratherthankinematicalPAwouldmakenodifferencefor polar to corotating (NGC 2685) and from counter- to corotating
NGC3414,whileNGC5631isnearlyroundontheskyandkine- (NGC5173)withincreasingradius.
maticsistheonlypossiblechoice. The situation is somewhat different for ds. The HI in these
InFig.A1weshowPA(HI)−PA(stars)asafunctionofra- objects is typically very faint and extends over only a couple of
dius for all 26 galaxies in this subsample. This figure and Fig. Westerbork beams. For this reason instead of using KINEMETRY
4 show that ∼ 30 per cent of the systems host corotating HI we assume that PA(HI) does not vary with radius and determine
(NGC 2859, NGC 3838, NGC 3945, NGC 4026, NGC 4036, itsvaluebydirectanalysisoftheHIcubes.Position-velocitydia-
NGC 4203, NGC 5582, UGC 06176), while counterrotating and gramsalongtheadoptedHIkinematicalmajoraxesarepublished
polarHIdistributionsaccounteachfor∼ 10percentofthesam- inPaperXIII(theirFig.5).Wefindthatmost(8/10)ofthedsare
ple (NGC 3626, NGC 3941 and NGC 6798 in the first group, corotating.Theexceptionsareonecounterrotator(NGC3032)and
NGC2594,NGC3998andNGC4262inthesecond).Notehow- onepolarsystem(NGC3499),asdiscussedinthatpaper.
everthattheobservedmisalignmentofpolarsystemsdependson Consideringall36galaxiestogetherwefindthereforethatin
viewingangleanditislikelythatthe HI isonapolarconfigura- ∼45percentofallETGswithanHIdiscorringthegas(whichcan
tioninalargerfractionofallETGs(e.g.,NGC3619,NGC5103 spreadouttotensofR )iscorotatingwiththestarsinside∼1R .
e e
c 2013RAS,MNRAS000,1–18
(cid:13)
10 PaoloSerraetal.
ThesegalaxiesareallFRswherethegasaccretionepisodetraced alreadyemphasizedabove,theHIdiscsandringsdetectedinFRs
bytheHImightberelatedtotheformationofthestellardisc.They arenotalwaysassociatedwiththeinnerstellardisc,notevenforthe
areinmanyrespectssimilartospiralgalaxiesastheyhostcoldgas flattestandfastestrotators(seeforexampleNGC3626,NGC5103
and stars corotating on the same disc – the only difference being andUGC09519).InmanyFRsthelargescaleHIkinematicsisob-
theabsenceofbright,star-formingspiralarmsowingtothelowHI viouslynotacontinuationoftheinnerstellarkinematicstolarger
columndensity(PaperXIII). radius.Onthecontrary,counterrotatingandpolarHIdistributions
The remaining 55 per cent of this sample is composed of ∼ accountforexactly1/4ofallFRsanalysedinthissection,while
10percentofcounterrotatingHIsystems,∼ 10percentofpolar another1/4consistsofwarpeddiscs.ThedetectedHIistherefore
objects, and ∼ 35 per cent of ETGs with a complex, warped HI alignedwiththeinnerstellardisconlyinhalfofthecases.Neutral
distribution,someofwhichmayinfactbepolar.Thelinkbetween hydrogenobservationsshowthereforethatFRs,whilebeingady-
HIdiscandstellardiscintheseobjectsislessobvious. namicallysimpleandhomogeneousfamilyinside∼1Re,aremuch
Theseresultsshowthattheexistenceofmultiplekinematical morecomplexanddiverseintheirouterregions,inagreementwith
sub-components in ETGs, well documented in their inner region resultsobtainedusingotherkinematicaltracersatlargeradius(e.g.,
for the stellar and ionized gas phases, holds also on much larger Coccatoetal.2009;Potaetal.2013).
scales.FollowingourresultsonthefractionofETGswithakpc- TheotherclearresultisthatinnoneofthefourSRsthe HI
scaleKDC(∼7percentincludingcounterrotatingcases;PaperII) kinematics is aligned with the stellar kinematics or morphology.
andmisalignedionized-gaskinematics(∼36percent;PaperX;see Infact,allthesesystemsexhibita(closeto)polarHIdistribution,
alsoBureau&Chung2006)wearenowfindingmisalignedHIdis- although PA(HI) typically varies with radius (Fig. A1). We also
tributionsonscalesfromafewtotensofkpcin∼12percentofall note that the HI morphology of these systems, although disc- or
ETGs(∼ 15percentoutsidetheVirgocluster;forpreviousstud- ring-like,appearsdisturbed(Fig.4),withsomecasesintermediate
iesofsmallersamplesofETGswithmisalignedHIsee,e.g.,van betweenaD/Randauclassification.Thelackofveryregular,set-
Gorkom, Schechter & Kristian 1987; Bertola, Buson & Zeilinger tledHIdiscs/ringsinSRsandtheprevalenceofpolarsystemsmay
1992;Bettonietal.2001). betheonlydifferencefromtheFRfamily(HI-wise),althougheven
Whetherornotassociatedwiththepresenceofastellardisc, amongFRsmanygasdiscsandringsappeardisturbedornotper-
theHIdetectedinETGsprovidesimportantcluesontheoriginof fectlysettled(e.g.,NGC2764,NGC3998,NGC4203,NGC4278,
the cold interstellar medium detected in the inner region of these NGC5103).Alargersampleof HI-richSRswouldbeneededto
objectsintheformofionizedandmoleculargas.Thisisclearin confirmthisindication.
Fig. A1, where we show at R = 0 the data point corresponding Theaboveresults,togetherwiththeestablishedweakrelation
totheionizedgaskinematics(whendetected),whichwasalready betweenM(HI)andETGluminosity(e.g.,Knapp,Turner&Cun-
foundtobeinvariablyalignedwiththeCOkinematics(PaperX). niffe1985;Wardle&Knapp1986;PaperXIII),confirmthesevere
The figure confirms previous claims that HI, H2 and ionized gas disconnect between ETG HI properties and their stellar compo-
sharethesamekinematicsintheregionwheretheyoverlap(Mor- nent.Theyshow,asarguedinSec.3.4,thatHIprovidescluesabout
ganti et al. 2006; Paper X). The few exceptions are NGC 1023, the complex evolution of ETGs which would go unnoticed using
NGC3414,NGC4278,NGC5103andNGC5631.Thefirsthosts only other observables. Confirming this complexity, we also find
an unsettled gas distribution with very complex gas kinematics, thattheHIkinematicalmisalignmentdoesnotdependinanyclear
and it is unlikely that the central value of PA(HI) derived from wayonETGstellarmass,environmentdensity(quantifiedinPaper
thevelocityfieldisrepresentativeoftheactualinnergasmotion. VII)orHImass.ForexampleamongtheveryHI-richgalaxiesin
Furthermore,Morgantietal.(2006)showthatinbothNGC3414 poorenvironmentswefindNGC5582(corotatingR),NGC6798
andNGC4278theionizedgasvelocityfieldtwistsinsuchaway (counterrotating D)andUGC09519(warped D).Low-mass,iso-
thattheouterionizedgaskinematicsisalignedwiththeinnerHI latedgalaxiesincludeNGC2594(polarR),UGC06176(aligned
kinematics.ThesameoccursinNGC 5631.Therefore,thesethree D)andagainUGC09519.AmongthemassiveETGswefindrela-
galaxies conform to the general rule too. We do not have a good tivelysmall,alignedHIdiscs/ringsasinNGC2859andNGC4036
explanation for the misalignment between HI and ionized gas in butalso(closeto)polarHIasinNGC3414andNGC5631,andthe
NGC5103,whichrepresentstheonlygenuineexceptiontotherule. warpedDNGC4278.Theonlyexceptionisthatthelowest-M(HI)
NotshowninFig.A1,d galaxiescontributetostrengtheningthis systems are all ds (Paper XIII) and, as illustrated above, mostly
result.The HI isalwayscorotatingwiththeCOandionizedgas, kinematicallyalignedtothestellarrotation.Thereisofcoursesome
includingthetwocaseswheretheHIismisalignedrelativetothe dependenceofHIpropertiesonETGmassandenvironmentacross
stars(NGC3032andNGC3499). the entire ATLAS3D sample. This is discussed in detail in Paper
XIII,whereweshowthatunsettledHIdistributionsbecomemore
common,andthetypicalHImassdecreases,asenvironmentden-
4.2 Misalignmentinfastandslowrotators
sityandgalaxymassincrease.Herewefindthatthereisnoaddi-
Figure5summarizesourfindingsby√showingallHImisalignments tionaltrendwithenvironmentandstellarmasswhenstudyingthe
on the M(HI)/M(cid:63) versus λR/(0.31 (cid:15)) plane of Fig. 2. For each kinematicalmisalignmentofsettledHIdiscs.
galaxy we adopt here the median misalignment value measured EversincetheworkofKnapp,Turner&Cunniffe(1985)this
fromtheradialprofileshowninFig.A1.Themedianvalueisafair decoupling between stars and HI in ETGs has been attributed to
representation of the overall misalignment for all galaxies except the external origin of the HI. The main point of this interpreta-
theheavilywarpedNGC2685andNGC5173,whichwehighlight tionisthatmostofthepresent-daygasmustbeaccretedoncethe
in the figure. On the one hand Fig. 5 shows a significant popu- formationofthe(innerregionofthe)hostETGisbasicallycom-
lationofFRswhere HI andstarsarekinematicallyaligned(dark plete.Theaccretedgascouldcomefromtheintergalacticmedium
bluesymbols,withthejustmentionedexceptionofNGC2685and orbebroughtinbysmallgas-richsatelliteswhichindividuallydo
NGC5173).Inthesegalaxiesgasandstarsarejusttwodifferent notcontributesignificantlytothestellarmassoftheETGbutmay
but coupled components of the same disc. On the other hand, as bringasignificantadditiontoitsinterstellarmedium.Infact,even
c 2013RAS,MNRAS000,1–18
(cid:13)