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The G alaxy D isk in a Cosm ologicalContext

Proceedings IAU Sym posium N o. 254,2008

J.Andersen,J.Bland-H awthorn & B.N ordstr�om ,eds.

c 2008 InternationalA stronom icalU nion

D O I:00.0000/X 000000000000000X

M easuring O uter D isk W arps w ith O ptical

Spectroscopy

D anielC hristlein1and Joss B land-H aw thorn

2

1M ax-Planck-Institutf�urAstrophysik,

K arl-Schwarzschild-Str.1,85748 G arching,G erm any

em ail:[email protected] ofAstronom y,SchoolofPhysics

University ofSydney,NSW 2006,Australia

em ail:[email protected]

A bstract. W arps in the outer gaseous disks of galaxies are a ubiquitous phenom enon,but

it is unclear what generates them .O ne theory is that warps are generated internally through

spontaneousbendinginstabilities.O thertheoriessuggestthatthey resultfrom theinteraction of

theouterdisk with accreting extragalactic m aterial.In thiscase,we expectto �nd caseswhere

the circular velocity ofthe warp gas is poorly correlated with the rotationalvelocity ofthe

galaxy disk atthesam eradius.O pticalspectroscopy presentsitselfasan interesting alternative

to 21-cm observations for testing this prediction,because (i) separating the kinem atics ofthe

warp from thoseofthedisk requiresa spatialresolution thatishigherthan whatisachieved at

21 cm atlow HIcolum n density;(ii)opticalspectroscopy also providesim portantinform ation

on star form ation rates,gas excitation,and chem icalabundances,which provide clues to the

origin ofthegasin warps.W epresenthereprelim inary resultsofastudy ofthekinem aticsofgas

in the outer-disk warpsofseven edge-on galaxies,using m ulti-hourVLT/FO RS2 spectroscopy.

K eyw ords.galaxies:evolution,galaxies:form ation,galaxies:kinem aticsand dynam ics,galax-

ies:structure

1. Introduction

W arps in the outer disks ofgalaxies are a ubiquitous phenom enon.They are seen

both in the distribution ofstars (Sanchez-Saavedra et al.1990;Cox et al.1996) and

neutralhydrogen (e.g.,Sancisi1976;Bosm a 1981),and surveys estim ate that possibly

50% or m ore ofallgalaxies show evidence for warps beyond the isophotalradius R 25

(Briggs1990).(which typically contains� 90% ofthetotallight).Usually,thesem anifest

them selvesin theform oftheouterdiskbendingawayfrom theplanede� ned by theinner

disk,de� ning either the shape ofa bowl,or,m ore com m only,an integralsign as seen

edge-on.Thissuggeststhatspeci� c angularm om entum ofm aterialin the warpsisnot

aligned with thatofthe innerdisk.

Num eroussuggestionshavebeen m adeovertheyearsfortheresponsiblem echanism s.

Am ong the earliest such proposals were internalbending m odes in the disk (Lynden-

Bell1965),butsuch m odesweresoon recognized to bepersistentonly in a disk with an

unrealisticallysharp m asstruncation (Hunter& Toom re1969).Revaz& Pfenniger(2004)

haverevived thisdiscussion by identi� ying short-lived bending instabilitiesasa possible

cause.O therproposed m echanism sfocuson theinteraction ofthebaryonicdisk with its

environm ent:A bending oftheouterbaryonicdisk m ay beintroduced by am isalignm ent

between the angular m om entum ofthe inner baryonic disk and the hypotheticalnon-

spherical,dark m atter halo that it is em bedded in (Toom re 1983;Dekel& Shlosm an

1983;K uijken 1991;Sparke & Casertano 1998),creating a gravitationaltorque on the

119

http://arxiv.org/abs/0808.3878v1

120 DanielChristlein & JossBland-Hawthorn

disk.However,it has been argued that the inner halo would realign with the baronic

disk overtim e,and the warp would dissipate (Nelson & Trem aine 1999;Binney,Jiang

& Dutta 1998;Dubinski& K uijken 1995;New et al.1998).O striker& Binney (1989),

Jiang& Binney (1999),Shen & Sellwood (2006)considertheim pactofongoingaccretion

onto an outerdark m atterhalo and arguethat,sincetheangularm om entum ofinfalling

m aterialwillin generalnotbe aligned with the presentgalaxy disk,thiswillcreate an

ongoing torqueon theoutergalaxy disk.Binney (1992)hasfurtherm oresuggested that,

ifinfalling m ateriallosesangularm om entum to thehalo,itm ightpenetrateasfarasthe

outeredgeofthe baryonicdisk itself.

Itisthishypothesisthatwewish to testwith the presentwork.How galaxiesacquire

gasisoneofthekey questionsin ourunderstanding ofhow they evolve,and determ ining

whetherwarpsareindeed signaturesofsuch processesthereforeisofgreatim portance.

How can the direct accretion hypothesis be tested? The speci� c angularm om entum

vectorofaccreting m aterialwill,in general,neitherbe aligned exactly with thatofthe

innerdisk,norhave the sam e size.Ifsuch infalling m aterialisindeed in directcontact

and exchanging angular m om entum with gas in the outerm ost baryonic disk,then it

willintroduce kinem atic anom alies,i.e.,a deviation from disk-like rotation,such as a

lag orexcessin the circularvelocity.M easuring the circularvelocity ofgasin the warps

thereforebecom esan im portantobservationaldiscrim inator.

O ur project has m easured line-of-sight velocities ofgas in the outer disks ofseven

galaxies,the m ajority ofwhich display clearsignsofopticalwarps.O urm easurem ents

were obtained via opticalspectroscopy ofthe H� line.Although the tradititionalway

ofobserving gas in the outer disk is via the 21-cm line ofneutralhydrogen,optical

spectroscopy hasproven a surprisingly successfulalternativeforstudying theouterdisk

(Bland-Hawthorn,Freem an & Q uinn 1997;Christlein & Zaritsky 2008),fora num berof

reasons:1) M ost im portantly,the spatialresolution | an order ofm agnitude better

than even the highest-quality interferom etric HIm aps | allows us to clearly separate

gasin the warpsfrom gasin the plane ofthe galaxy.This,in turn,allowsusto access

galaxieswith sm allerangulardiam etersatlargerredshifts,greatly increasingthenum ber

ofsuitable targets.2)Sporadic localstarform ation orillum ination ofgasin the warps

by escaping UV ux from the innerstar-form ing disk guarantee low-levelH� ux from

the outerdisk farbeyond whatis usually perceived asthe starform ation threshold.It

hasbeen dem onstrated (Bland-Hawthorn,Freem an & Q uinn 1997;Christlein & Zaritsky

2008)thatm ulti-houropticalspectroscopy ofsuch low-levelem ission can,in som ecases,

probe outer galaxy disks to sim ilar extents as 21-cm .3) O pticalspectroscopy yields a

plethora ofancillary data,particularly stellarcontinuum and m etallines.In determ ining

theorigin ofgasin theouterdisksand warps,m etallicity indicatorsm ay providevaluable

additionalinsight(although an analysisexceedsthescopeoftheseproceedings).O uraim

is to a) m easure the rotationalvelocity ofgas in the warps and b) determ ine whether

there are kinem atic anom aliesin thisrotationalvelocity,i.e.,sudden breaksorupturns

thatare notconsistentwith an extrapolation ofthe rotation curve from the innerdisk,

which m ay be associated with the onsetofthe warp.

2. O bservations

O ur sam ple consists ofseven galaxies,ofwhich six were taken from the catalog of

optically warped galaxies by Sanchez-Saavedra et al.(2004);one object was targetted

blindly withoutpriorknowledgeofa warp.Allare nearly edge-on,with redshiftsin the

range ofseveralthousand km /s,and their angular size is well-m atched to the � eld of

view.

M easuring W arps with Spectroscopy 121

Figure 1.ESO 184-G 063.Forexplanation,see

text.H� ism oreextended in theo�setspectrum

than along them ajoraxis,butnokinem aticdis-

ruption isassociated with the warp.

Figure 2.UG CA 23.Forexplanation,seetext.

H� ism oreextended in theo�setspectrum than

along the m ajor axis,butno kinem atic disrup-

tion isassociated with the warp.

O urobservationswerecarried outoverthreenightsin Septem ber2007,using FO RS2

on the VLT-UT1.The typicalobserving strategy wasto observe fora totalofone hour

with a long slitof0.500 width aligned along them ajoraxis(to obtain a referenceforthe

rotationalvelocity and angularextentgasin theplaneofthegalaxy),then fortwo hours

with a position angle o� set from the m ajor axis PA by a few degrees.The purpose of

the latterobservation wasto assurethatH� em ission em anating from the warpswould

be observed,ratherthan from the plane ofthe galaxy.Sense and sizeofthiso� setwere

determ ined on a case-by-casebasisafterinspecting platesfrom theDigitzed Sky Survey

aswellasourown acquisition im ages,and typically chosen so thatthe slitwould pass

within orjustbeyond theouterm ostcontoursofstellarcontinuum lightattheend ofthe

warp.

3. R esults

� ESO 184-G 063 (Fig.1)isa sm all(M = � 18:5)Sb-typegalaxy atcz= 3207km /s,

and has a strong integral-sign warp,which is distinctly stronger on one side than the

other.

This� gure,aswellasallsubsequentones,showstherotation curvealong them ajoraxis

(top panel)and in the o� setposition (bottom panel;m ajoraxisrotation curve plotted

with sm alldots/dashed linesforreference).The radialextentis2 R 25 on both sidesof

the nucleus;the acquisition im ageisplotted to the sam escale.

Alongthem ajoraxis,ourrotation curveextendsto1.4R 25,which in itselfisrem arkable,

given thatnorm alH� rotation curveswith conventionalexposuretim esand sm allerin-

strum entstypically reach 0:7R 25,and rarely asfarasR 25.O uro� -axisspectrum extends

outto 1.6 R 25,clearly intersecting the tip ofthe opticalwarp.Both the m ajoraxisand

the o� setrotation curvesare consistent,and there is no sign ofa sudden break in the

rotationalvelocity coincidentwith the onsetofthe warp.

� U G C A 23 (Fig.2)isan M = � 19 Sd-type galaxy atcz= 3864 km /s.Acquisitionim ages show lum py em ission and slightly bent isophotes on one side.The m ajor axis

rotationcurveextendsto� R 25 in thiscase.Theo� setspectrum sam plesthewarpfeature


Figure 3. M CG -01-10-035. H� is m ore ex-

tended in theo�setspectrum than alongthem a-

joraxis.A break in the rotation curve isfound

along the m ajoraxis,coinciding with the onset

ofa tidal-tail-like warp feature.

Figure 4.UG C 12423.See textfor

explanation.

and showsem ission asfaras1.2 R 25.Thetwo rotation curvesareindistinguishable;the

kinem aticsofthe extraplanarfeature aredisk-like.

� M C G -01-10-035 (Fig.3) is a sm all(M = � 18:3)Sc-type galaxy at cz = 3788

km /s.O ur acquisition im ages reveala very extended tailoflow surface brightness ex-

tending as far as 2R 25 and then curving back.Its appearance is very suggestive ofa

tidalfeature.W hiletheothersideofthegalaxy doesnotexhibitan equivalently striking

feature,the outerisophotesappearbenton both sides.

Them ajoraxisspectrum sam plesfarinto thislow-surfacebrightnesstailand extendsto

1.6 R 25 on thatside,and 1:4R 25 on the other.Rem arkably,atthe onsetofthisfeature,

around R 25,weseeasudden break in therotation curvealongthem ajoraxisin thesense

ofa jum p to lowerrotationalvelocitiesby � 60� 70 km /s.O n the otherside,no such

strong break is discernible,but there is H� em ission at velocities distinctly below the

extrapolated rotation velocity,going asfardown asthe system ic velocity ofthe galaxy

itself.

O ur o� set spectrum intercepts the tailat its largest extent.W hile we do not see a

continuousH� signalbeyond R 25,there are individualH�-brightregionsextending as

faras2R 25 on both sidesofthegalaxy.Therotation curve� tted to thisspectrum shows

no breaksin the velocity;however,atlarge radii,its line-of-sightvelocity is consistent

with the lower-velocity gasin the tidalfeature.

� U G C 12423 (Fig.4) is an Sc-type galaxy at cz = 4839 km /s with an absolute

m agnitude M = � 19:4.It shows a strong and asym m etricalopticalwarp.The m ajor-

axis rotation curve extends to 1.4 R 25 on the side with the stronger warp,which is

even fartherthan the extentofthe warped disk.Atthe outerm ostpoint,the rotational

velocity is stillconsistent with the extrapolated rotation curve.A possible dip in the

rotation velocity atthe position ofthe warp featuredoesnotappearsigni� cant.

The o� set spectrum is,in this case,m uch less extended;it reaches R25 on the side of

the strongerwarp,and only 0.7 R 25 on the otherside,with the exception ofone sm all

H�-em itting region at1.4 R 25.There are no signsfor any kinem atic anom aliesin this

rotation curve.

� N G C 259 (Fig.5) is am ong the brighter galaxiesin this sam ple (M = � 20),an


Figure 5.NG C 259.See textforexplanation. Figure 6.ESO 473-G 025.See textfor

explanation.

Sbc-typeatcz= 4045km /s.Itisalsotheleastinclined am ongourtargets.Theacquisition

im agesshow thatthesupposed warp featureisin factm oresuggestiveofa continuation

ofa spiralarm .The m ajor axis spectrum sam ples its kinem atics out to 1.2 R 25,but

showsno kinem aticdisturbance.The o� setspectrum only extendsto � R25.

� ESO 473-G 025 (Fig.6),the brightest object in our sam ple (M = � 20:3),is anSb-typegalaxy atcz= 7110km /s.Acquisition im agesshow whatisatm osta very subtle

warp.The m ajor axis spectrum extends a little beyond R 25.The kink visible in the

rotation curvein Fig.6 correspondsto a region wheretwo com ponentsoftheH� lineare

visible atthe sam e position in projection along the slit.However,there are no signsof

kinem aticanom aliestowardstheedgeofthedisk.Theo� setrotation curveism uch less

extended.A single em ission region around 1.4 R 25 coincideswith whatappearsto be a

sm allsatellite galaxy on the acquisition im age.

� ESO 340-G 026 (Fig.7)isan Sc-type galaxy atcz= 5483 km /swith M = � 19:5.Therearelow-surface-brightnesstailsattheedgeofthedisk,butthey show nosigni� cant

warp.W e exposed in two di� erent o� set positions,but obtained su� cient data for an

analysisin only onecase.Them ajoraxisspectrum ,in any case,extendsto 1.3 R 25 and

thus farenough to sam ple the kinem aticsofthe outlying features.The rotation curve,

again,showsno signsofkinem aticanom alies.

4. C onclusions

In an e� ortto m easurethekinem aticsofgasin thewarpsin theouterdisksofgalaxies

with high spatialresolution,wehavetargetted seven edge-on galaxies,m ostofthem with

known opticalwarps,using m ulti-hourlong slitspectroscopy.

� In at least three ofseven cases,we have observed m ore extended H� em ission if

the slit wasaligned along the warp ratherthan along the m ajoraxis.This showsthat

we have been successfulatdetecting gasin the warps,and thatwe have thusattained

ourobservationalgoal.Counting thesingleem ission region detected in UG C 12423,this

ratio isfouroutofseven.

� In threeofthesefourcases,thereareno signsofkinem aticanom aliesin therotation

curveassociated with the onsetofthe warp.

� In one ofthese fourcases,there isa sharp break in the rotationalvelocity roughly


Figure 7.ESO 340-G 026.See textfor

explanation.

at the onset ofthe warp.In this case,the opticalappearance ofthe \warp" is highly

suggestiveofa tidalfeature.However,thebreak occursin them ajoraxisrotation curve.

� In the rem aining objects,the outer-disk features are sam pled by the m ajor-axis

spectrum ,butthereisno evidenceforkinem aticanom aliesin these regions.

Based on a sm alldata set (four kinem atic detections ofextraplanar outer disk gas

likely to be associated with a warp),we � nd evidence for kinem atic anom aliesonly in

the one case whose opticalappearance strongly suggestsa tidalfeature.In othercases,

the kinem atics ofthe presum ed warp gas are consistent with the extrapolation ofthe

innerdisk rotation curve.Thissam pleisthereforenotin supportofthehypothesisthat

the outer,warped disks are in directcontactand exchanging angularm om entum with

m aterialbeing newly accreted;however,thepossibility cannotberuled outyetthatthe

realpointofcontactbetween the galaxy disk and accreted m aterialliesatlargerradii,

and thatthegasthatweseeattheonsetoftheopticalwarp hasalready settled intodisk-

likekinem atics.W eproposeto explorethreeavenuesforfurtherinvestigation:(i)G asat

largerradii,substantially beyond theopticalwarps,should be targetted;ideally,targets

should thusbeselected on thebasisofHIm apswhereavailable.(ii)Thesam pleshould be

increased in size from the presentthree cleardetections.(iii)Additionalindicatorsthat

m ightconstrain theorigin ofthewarp gas,in particular,m etallicity,should be included

in the analysis.

Based on observationsm adewith ESO TelescopesattheParanalO bservatoriesunder

program m eID < 079.B-0426> .

R eferences

Binney,J.1992,ARA& A ),30,51

Binney,J.,Jiang,I.-G .,D utta,S.1998,M NRAS 297,1237

Bland-Hawthorn,J.,Freem an,K .C.,Q uinn,P.J.1997,ApJ ),490,143

Bosm a,A.1981,AJ,86,1791

Briggs,F.H.1990,ApJ,352,15

Christlein,D .,Zaritsky,D .2008,ApJ ),680,1053

Cox,A.L.,Sparke,L.S.,van M oorsel,G .,Shaw,M .1996,AJ,111,1505

D ekel,A.,Shlosm an,I,1983,IAUS,100,187

D ubinski,J.,K uijken,K .1995,ApJ,442,492


Hunter,C.,Toom re,A.1969,ApJ,155,747

Jiang,I.-G .,Binney,J.1999,M NRAS ),303,7

K uijken,K .1991,ApJ,376,467

Lynden-Bell,D .1965,M NRAS,129,299

Nelson,R.W .,Trem aine,S.1999,M NRAS,306,1

New,K .C.B.,Tohline,J.e.,Frank,J.,Vaeth,H.M .1998,ApJ,503,632

O striker,E.C.,Binney,J.1989,M NRAS,237,785

Revaz,Y.,Pfenniger,D .2004,A& A,425,67

Sancisi,R.1976,A& A,53,159

Sanchez-Saavedra,M .L.,Battaner,E.,Florido,E.1990,M NRAS,246,458

Sanchez-Saavedra,M .L.,Battaner,E.,G uijarro,A.,L�opez-Correidora,M .,Castro-Rodr��guez,

N 2003,A& A,399,457

Shen,J.,Sellwood,J.A.2006,M NRAS ),370,2

Sparke,L.S.,Casertano,S.1988,M NRAS,234,873

Toom re,A.1983,IAUS,100,177

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