Multiplicity of nuclear dust lanes and dust lane shocks in the Milky Way bar

0.21 MB·English

by  H. S. Liszt

Astronomy&Astrophysicsmanuscriptno.09748-xar (cid:13)c ESO2009
January23,2009
Multiplicity of nuclear dust lanes and dust lane shocks in the Milky
Way bar
H.S.Liszt
NationalRadioAstronomyObservatory,520EdgemontRoad,Charlottesville,VA,USA22903-2475
receivedJanuary23,2009
ABSTRACT
Aims.Weshowtheexistenceofasmallfamilyofinner-galaxydustlanesanddustlanestandingshocksbeyondthetwomajorones
9 thatwerepreviouslyknowntoexist
0 Methods.WeanalyzeimagesofCOemissionintheinnerregionsoftheGalaxy
0 Results.Thepeculiarkinematicsofthemajordustlanefeaturesarerepeatedinseveralotherdistinctinstancesatl>0o,inonecase
2 atacontrarylocation100pcabovethegalacticequatoratl>3oattheupperextremityofClump2.Likethepreviously-knowndust
lanes, these new examples are also associated with localized, exceptionally broad line profiles believed to be characteristic of the
n shreddingofneutralgasatthestandingdustlaneshocks.
a Conclusions. There may be secondary dust lane and standing shocks in the Milky Way bulge. The vertical structure provides a
J
temporalsequenceforunderstandingthesecularevolutionofgasflowinthebar.

8
Keywords. interstellarmedium–molecules

]
A
G 1. Introduction.
.
h Constructing a coherent picture of the inner-galaxy gas and its
p kinematics from our vantage point within the disk has proved
- challenging.IntheearliestdaysthegaswasfirstobservedinHI
o
anddecomposedinto“features”(vanderKruit,1970;Cohen&
r
t Davies,1976;Oort,1977)thensubsequentlyobservedinmolec-
s ularlines(Fig.1)revealingsomewhatdifferent,oftenseemingly
a
[ disparatebehavior.Thevariousviewswerecomparedandrecon-

ciled (Burton & Liszt, 1983), largely through realization of the
1 differences in the conditions of observing of various gas con-
v
stituents,andthe“features”wereassociatedwitheachotherand
3
then with an underlying physical entity, namely a galactic bar.
1
1 Theprocessofobserving,comparingandassimilatingothercon-
1 stituents of the bar is ongoing, for instance with derivation of
. the distribution of extinction and dust from the bar dust lanes
1
(Marshalletal.,2008)andstudyofthechemistrybasedonady-
0
namical separation of consituents (Rodriguez-Fernandez et al.,
9
2006).
0
: Along the way there were many false starts but the notion
v
(whichtookholdveryearly)thattheobservationsmustberec-
i
X onciled with the existence of a galactic bar in the presence of
an overall tilt of the gas layer (Cohen & Davies, 1976; Burton
r
a &Liszt,1978)certainlyprovedcorrect.Fig.1,ascanalongthe
inner galactic plane in CO, integrated over -1o≤ b ≤1o to sup-
pressthewealthofverticalstructure,illustratesthecomplexities
which must be overcome in order to gain a view of the under- Fig.1. Longitude-velocity diagram of 12CO emission from the
lyingphysicalphenomena.Italsohelpstoexplainwhyourun- datasetofBitranetal.(1997)ona0.125o grid,separatelyaver-
derstanding of the observations has improved as models of the aged over −1o ≤ b ≤ 1o at each longitude. Features a (aka the
bar gas flow have matured, from simple recognition of the x2- “connectingarm”)anddareassociatedwithprominentgalactic
x1 orbit separation in fast bars (Binney et al., 1991) to holistic dustlanes;bandcareClump2andClump1ofBania(1977),e
views of the hydrodynamic gas flow over an entire bar (Fux, isthe“rotatingnucleardisk”ofRougourandOort(Oort,1977),
1999; Regan et al., 1999; Regan & Teuben, 2003). For recent fisthe“expandingmolecularring”ofKaifuetal.(1972)andg
arisesinthe3-kpcarmBania(1980)
Sendoffprintrequeststo:H.S.Liszt
.
Correspondenceto:[email protected]
2 H.S.Liszt:MultiplicityofnucleardustlanesanddustlaneshocksintheMilkyWaybar
views of the galactic center gas distribution which incorporate SeveralotherprominentfeaturesinFig.1aredirectlyrelated
notionsofbargasflow,seeSawadaetal.(2004); Liszt(2006); tothedustlanes,inparticulartheb-feature,Clump2of(Bania,
Rodriguez-Fernandezetal.(2006)andMarshalletal.(2008). 1977; Stark & Bania, 1986), and the broad, localized, anony-
Itisaccesstotheratherextravagantverticalstructureofthe mousfeatureatl=5.5o.Theseweremappedrecentlyinseveral
inner-galaxy gas which is the truly unique aspect of observing moleculesat1(cid:48)resolution(Liszt,2006)anddiscussedintermsof
thegalacticnucleusfromthevantagepointoftheSun,andthis theshreddingoflargequantitiesofmoleculargasatthestanding
shouldfeedbackintomodelsofthegasflow.However,theverti- dust lane shock (Fux, 1999): various estimates yield some 106
calstructurehasprovedlargelyintractabletheoreticallyandhas M ineitherfeature.ThebroadlinesinFig.1arespatiallyre-
sun
oftenbeensuppressed(asinFig.1),ignoredordenied,evenin solved at arcminute (2.4 pc) resolution, with velocity gradients
purelyobservationaldescriptions.Yet,itistheverticalstructure strongerthanseenacrossthecircumnucleardiskfaroundSgrA∗.
of the gas which often presents the sharpest and most surpris- Theweaknessofthefar-sidedustlanefeaturedatnegative
ingstructure(Liszt,2006).Marshalletal.(2008)demonstrated longitude may be related to the absence of similar broad-lined
a vertical separation between the tilts of the dust and molecu- featuresatnegativelongitude,indicatingthatthenegativelongi-
largasintheMilkyWaydustlanewhichwouldbeimportantin tudedustlaneis(temporarily?)somewhatstarvedofgas.
interpretingobservationsofothersystems.
Herewenotethatthepeculiarkinematicswhichdistinguish
thestandingshocksofthelarge-scaleinner-galaxybardustlanes 2.2. Otherdustlanefeatures
are repeated several times over the inner regions, in one case
with a tilt quite opposite to that which is usually recognized. Fig. 2 shows the higher-velocity portions of two composite l-v
Apparently, the Milky Way has secondary bar/dust lane struc- diagrams constructed below and above the galactic equator, at
tures,asdiscussedinSect.2. leftandright,respectively.LikeFig.1thesehavealsobeeninte-
gratedinlatitude,butoversmallerregions.Atnegativelatitudes
intheleft-handpanel,alocuslabelledL3,nearlyparalleltothe
2. Kinematicfeaturesassociatedwiththedust connectingarmfeature,isprominent;itisofcoursefaintlyvis-
lanesanddustlaneshocks ible in Fig. 1 as well and was labelled feature J by Rodriguez-
Fernandezetal.(2006).Itfollowsthemostprominentgastiltto
The dataset employed here is that of Bitran et al. (1997), with negativelatitudeatl>0.Furthermimicingthelargerdustlane,
12CO spectra on a 0.125o grid at |b| ≤ 1o, |l| ≤ 13o and on a there also appears to be a localized broad line over the interval
0.25o gridat1o ≤ |b| ≤2o.ToproduceFig.1weaveraged(un- downto0-velocityatl=3o.
weighted)overalllatitudesateachlongitude;thisfigureissim- The span of L3 appears contained between the l=5.5o fea-
ilar to one panel of Fig. 3 of Marshall et al. (2008). Several of ture at one end (l=5o, v = 0 kms−1) and the strong-lined gas
themostprominentkinematicfeaturesarisingexteriortotheSgr stretching back to Sgr A at the other (l=1.5o, v = 140 kms−1).
A-EsourcecomplexarecalledoutinFig.1. Theregionatl=1.3oiswell-knownforhostingacopiousamount
ofgasandhasbeensuggestedasthecontactpointforinflowing
2.1. Kinematicsofdustlinefeatures material(Fux,1999).ItwasrecentlymappedbyKunihikoetal.
(2007) and attributed to a nascent superbubble, although there
Despitetheirobviousasymmetriesinintensity,velocity,andlon- is no evidence for recent or current star formation activity. A
gitudeextent,itisthepairedfeaturesaanddwhichtraditionally broad-lined,vertically-extendeddustlanestandingshockfeature
havebeenassociatedwitheachotherandwiththenearandfar- formsthecoreofthegasconcentrationatthislongitude(Liszt,
side galactic dust lanes, respectively (Cohen & Davies, 1976; 2006).
Liszt & Burton, 1980; Fux, 1999; Rodriguez-Fernandez et al., At positive latitude b= 0.5o-1.0o in the right panel, Fig. 2b
2006;Marshalletal.,2008).Featuresaanddareoppositelydis-
is much leaner and dominated by several (3-4) of the localized
placedbelowandabovethegalacticequator,respectively,inthe
broad-lined features. At l> 0o, all of the behavior in Fig. 2b is
senseofthemostpervasivegastilt(Liszt&Burton,1980).
spectacularlycontrarytotheoverallgeneraltiltofthegaslayer.
Thea-featurewassufficientlyprominentinHItohavebeen
ThelocusL2,whichisnotreadilyapparentinFig.1,isclearly
known originally as the “connecting arm” while the d-feature
associated with the b-feature (Clump 2), in fact at its northern
was not always remarked as a separate entity (see Fig. 1 of terminus.Abroad-lineddustlaneshockfeatureatl=1.3oismore
Cohen&Davies(1976)).Itisasomewhatofanaccidentofthe
prominentthaninFig.2abecauseofitsverticalextension.The
Sun’slocationthatthedustlaneshockappearsinsuchawayas gasatl<−2o,includingClump1ofBania(1977),isacompos-
to form the terminal velocity. Incorporating the contrary kine-
ite projection of a rear-side dust lane and the base of a far-side
matics of the a and d-features into the derivation of a galactic
spiralarm,accordingtoFux(1999).
rotation curve directly from the observed terminal velocity, as-
sumingpurecircularmotion,causesanapparentdeclineinequi-
librium circular velocity with galactocentric radius, corrupting 2.3. Locatingthedustlanefeaturesinspace
the mass model (Caldwell & Ostriker, 1981; Burton & Liszt,
1993). Tolocatethedustlanefeatures,wecreatedmapsofthebright-
Feature f (the expanding molecular ring) and its positive- ness integrated across the velocity ranges of their ridges in Fig
velocity counterpart crossing zero-longitude at +165 kms−1 2,varyingthespanoftheintegrationwithlongitude(only).This
ariseinthesprayofmaterialflowinginwardalongthedustlanes is most nearly definitive for the connecting arm feature L1 be-
wheretheycontactthenuclearstar-formingringcontainingthe cause it occurs at the extremity of the gas kinematics. For the
Sgr A-E radiocontinuum sources, see Regan et al. (1999), Fig. otherdustlanefeatures,gasassociatedwithotheraspectsofthe
9,orFux(1999),Fig.17.SgrB2,whichhasbeendescribedas distributionisunavoidablycaptured,butthishasthevirtuethat
resulting from a cloud-cloud collision (Hasegawa et al., 2008), itexposestherelationshipsbetweenthedustlanesandtheshred-
isprobablyatsuchacontactpoint. dedmoleculargas.
H.S.Liszt:MultiplicityofnucleardustlanesanddustlaneshocksintheMilkyWaybar 3
Fig.2. Positive-velocityportionsofl-vdiagrams.Left:atnegativelatitudesaveragedover−1o ≤b≤0o.Right:atpositivelatitudes,
averagedover0.5o ≤b≤1o.ThebandcnotationisrepeatedfromFig.1.
Fig3hasthreepanelscorrespondingtointegrationoverthe strongerorricherdustlaneononesideoftheGalaxytowardthe
velocity spans of the L1-L3 features in Fig. 2. The L1 panel at nearerendofthebar.
top is similar to one panel of Fig. 3 of Marshall et al. (2008) Presumably there are also a direction and sequence to the
becauseitrepresentsthegasinthemajordustlane.Atitssouth- verticalstructureaswell.Tointerpretthebroad-linedfeatureat
ernmost terminus at l = 5.5o, b=-1.8o, the gas is 250 pc below l=5.5o it was suggested that material is falling down from re-
the galactic equator at a distance of 7.5 kpc from the Sun (the gionsneartheequatorattheupperextremityofthefeature,into
dustlanepointsmostlytowardus). theconnecting-armdustlaneL1belowtheplane(Liszt,2006).
Themiddlepanelintegrated overthespanofL2isnot par- However no source of material at the upper extremity was dis-
ticularlyrevealingofstructureinL2butitinadvertentlycaptures cussed.Bycontrast,broad-linedClump2atl=3.2ohasL2atits
theoverallverticalarcofthebroad-linedfeatureatl=5.5o from upperextremityandL3below,andL2doesnotextendinwardof
Fig.1and2a,showingthatitterminatesbelowatthelocusofthe l=3o.Thissuggeststhatthesourceofthegasbeingshreddedat
L1dustlanefeature(Liszt,2006).Someofthebrightvertically- Clump2isstillvisibleinL2,thatthesequenceisalsodownward
extendedemissionatl=8o-9oislow-velocitylocalgas. andthatitisindeedpossibleforgastoemergefromadustlane
The lowest panel, representing integration over the L3 fea- aswell.Whythisshouldoccursofarfromthegalacticequator
ture, shows that Clump 2 at l=3.2o extends vertically just be- is a mystery but it should hardly be a surprise if the extremi-
tween L2 and L3. This seems unlikely to be an accident, espe- tiesofClump2andthefeatureatl=5.5o arethesiteofunusual
cially considering the abruptness with which L2 terminates in phenomena.
thevicinityofthebroad-linedfeaturefromClump2inFig.2b. Understandingtheverticalstructureofthegasisstillachal-
Apparently, L3 underpins the vertically extended Clump 2 at l lenge,bothobservationallyandtheoretically.Yet,itappearsthat
= 3.2o in just the same way that L1 underpins the equivalent thesecularevolutionoftheinnergalaxyisactuallyvisibleinthe
vertically-extendedbroad-linedfeatureatl=5.5o. vertical structure and that the tilt of the gas is the result of this
evolution.
Acknowledgements. The NRAO is operated by AUI, Inc. under a coopera-
3. Verticalstructureandsecularevolution tiveagreementwiththeUSNationalScienceFoundation.IthankTomDame
(CfA)forprovidingthedatacubeofBitranetal.(1997)andthankhimandPat
In the bar flow picture, the kinematics are spatially and tem- Thaddeus (CfA) for enabling that work. Thanks also to Nemesio Rodriguez-
porally sequenced as material streams inward from larger radii Fernandezandtherefereeforhelpfulcomments.
at higher longitudes. The localized broad-lined features gener-
ally represent the uptake and shredding of dense material; the
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