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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
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
122 DanielChristlein & JossBland-Hawthorn
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
M easuring W arps with Spectroscopy 123
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
124 DanielChristlein & JossBland-Hawthorn
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
M easuring W arps with Spectroscopy 125
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