SDSS spectroscopic survey of stars

1.3 MB·English

by  Z. Ivezic

Mem.S.A.It.Vol.77,0
(cid:13)c SAIt 2004 Memoriedella
SDSS spe
tros
opi
survey of stars
Zˇ.Ivezic´1,D.Schlegel2,A.Uomoto3,N.Bond4,T.Beers5,C.AllendePrieto6,R.
Wilhelm7,Y.SunLee5,T.Sivarani5,M.Juric´4,R.Lupton4,C.Rockosi8,G.Knapp4,J.
Gunn4,B.Yanny9,S.Jester9,S.Kent9,J.Pier10,J.Munn10,G.Richards11,H.
7 Newberg12,M.Blanton13,D.Eisenstein14,S.Hawley1,S.Anderson1,H.Harris10,F.
0
0 Kiuchi1,A.Chen1,J.Bushong1,H.Sohi1,D.Haggard1,A.Kimball1,J.Barentine15,H.
2 Brewington15,M.Harvanek15,S.Kleinman15,J.Krzesinski15,D.Long15,A.Nitta15,S.

n
Snedden15,fortheSDSSCollaboration
a
J

7 1 University of Washington, 2 Lawrence Berkeley National Laboratory, 3 Johns Hopkins
1 University, 4 Princeton University, 5 Michigan State University (JILA), 6 University of
Texas,7 TexasTechUniversity8 UniversityofCaliforniaatSantaCruz,9 FermiNational

1 Accelerator Laboratory, 10 U.S. Naval Observatory, 11 Drexel University, 12 Rensselaer
v PolytechnicInstitute,13NewYorkUniversity,14UniversityofArizona,15SpaceTelescope
9 ScienceInstitute,16ApachePointObservatory;e-mail:[email protected]
0
5
1
Abstract. Inadditiontoopticalphotometryofunprecedentedquality,theSloanDigitalSky
0
Survey(SDSS)isalsoproducingamassivespectroscopicdatabase.Wediscussdetermina-
7
0 tionofstellarparameters,suchaseffectivetemperature,gravityandmetallicityfromSDSS
/ spectra,describecorrelationsbetweenkinematicsandmetallicity,andstudytheirvariation
h as a function of the position in the Galaxy. We show that stellar parameter estimates by
p Beersetal.showagoodcorrelationwiththepositionofastarintheg−rvs.u−gcolor-
- colordiagram,therebydemonstratingtheirrobustnessaswellasapotentialforphotometric
o
parameter estimation methods. Using Beers et al. parameters, we find that the metallic-
r
t ity distribution of the Milky Way stars at a few kpc from the galactic plane is bimodal
s
with a local minimum at [Z/Z ] ∼ −1.3. The median metallicity for the low-metallicity
a ⊙
: [Z/Z⊙]<−1.3subsampleisnearlyindependentofGalacticcylindricalcoordinatesRandz,
v whileitdecreaseswithzforthehigh-metallicity[Z/Z ] > −1.3sample.Wealsofindthat
⊙
i the low-metallicity sample has ∼2.5 times larger velocity dispersion and that it does not
X
rotate(atthe∼10km/slevel),whiletherotationalvelocityofthehigh-metallicitysample
r decreasessmoothlywiththeheightabovethegalacticplane.
a
1. Introduction cess that created a smooth distributions of
stars, with the standard view exemplified by
The formation of galaxies like the Milky
the models of Bahcall&Soneira (1980) and
Way was long thought to be a steady pro-
Gilmore,Wyse,&Kuijken (1989), and con-
strainedindetailbyMajewski (1993).Inthese
Sendoffprintrequeststo:Zˇ.Ivezic´
Ivezic´etal.:SDSSspectroscopicsurveyofstars 1
models,theMilkyWayisusuallymodeledby encestherein).To brieflysummarizehere,the
three discrete components: the thin disk, the fluxdensitiesofdetectedobjectsaremeasured
thick disk, and the halo. The thin disk has a almost simultaneously in five bands (u, g, r,
cold(σ ∼ 20kms−1)stellarcomponentanda i, and z) with effective wavelengths of 3540
z
scale height of ∼300 pc, while the thick disk Å, 4760 Å, 6280 Å, 7690 Å, and 9250 Å.
is somewhatwarmer(σ ∼ 40kms−1), with a The completeness of SDSS catalogs for point
z
largerscaleheight(∼1kpc)andloweraverage sourcesis∼99.3%atthebrightendanddrops
metallicity ([Z/Z ] ∼ −0.6). In contrast, the to95%atmagnitudesof22.1,22.4,22.1,21.2,
⊙
halo component is composed almost entirely and 20.3 in u, g, r, i and z, respectively. The
of low metallicity ([Z/Z ] < −1.5) stars and finalsurveyskycoverageofabout10,000deg2
⊙
has little or no net rotation. Hence, the main will result in photometric measurements to
differences between these components are in the above detection limits for about 100 mil-
their rotational velocity, velocity dispersions, lion stars and a similar number of galaxies.
andmetallicitydistributions. Astrometricpositionsareaccuratetoabout0.1
As this summary implies, most studies of arcsecpercoordinateforsourcesbrighterthan
the Milky Way can be described as investiga- r ∼20.5m, and the morphologicalinformation
tions of the stellar distribution in the seven- from the images allows robust point source-
dimensionalspacespannedbythethreespatial galaxy separation to r ∼ 21.5m. The SDSS
coordinates, three velocity components, and photometricaccuracyis0.02mag(root-mean-
metallicity. Depending on the quality, diver- square,atthebrightend),withwellcontrolled
sityandquantityofdata,suchstudiestypically tailsoftheerrordistribution.Theabsolutezero
concentrate on only a limited region of this pointcalibrationoftheSDSSphotometryisac-
space (e.g. the solar neighborhood), or con- curatetowithin∼0.02mag.Acompendiumof
sider onlymarginaldistributions(e.g. number technical details about SDSS can be found in
densityofstarsirrespectiveoftheirmetallicity Stoughtonetal. (2002)andontheSDSSweb
orkinematics). site(http://www.sdss.org),whichalsoprovides
Toenablefurtherprogress,adatasetneeds interfaceforthepublicdataaccess.
to be both voluminous (to enable sufficient
spatial, kinematic and metallicity resolution)
2.1. SDSSspectroscopicsurveyof
and diverse (i.e. accurate distance and metal-
stars
licity estimates, as well as radial velocity and
propermotionmeasurementsareneeded),and Targetsforthespectroscopicsurveyarechosen
thesamplesneedtoprobeasignificantfraction fromtheSDSSimagingdata,describedabove,
of the Galaxy. The Sloan Digital Sky Survey basedontheircolorsandmorphologicalprop-
(hereafter SDSS, York et al. 2000), with its erties.Thetargetsinclude
imaging and spectroscopic surveys, has re-
cently provided such a data set. In this con- – Galaxies: simple flux limit for “main”
tribution,wefocusontheSDSSspectroscopic galaxies, flux-color cut for luminous red
surveyof stars andsome recentresults on the galaxies(cD)
MilkyWaystructurethatitenabled. – Quasars:flux-colorcut,matchestoFIRST
survey
– Non-tiled objects (color-selected): cali-
2. SloanDigitalSkySurvey brationstars(16/640),interestingstars(hot
white dwarfs, brown dwarfs, red dwarfs,
The SDSS is a digital photometric and spec-
carbonstars,CVs,BHBstars,centralstars
troscopic survey which will cover up to
ofPNe),sky
one quarter of the Celestial Sphere in the
North Galactic cap, and produce a smaller Here, (non)-tiledobjects refers to objects that
area (∼225 deg2) but much deeper sur- are (not) guaranteed a fiber assignment. As
vey in the Southern Galactic hemisphere an illustration of the fiber assignments, SDSS
(Adelman-McCarthyetal. (2006) and refer- Data Release 5 contains spectra of 675,000
2 Ivezic´etal.:SDSSspectroscopicsurveyofstars
galaxies, 90,000 quasars, and 155,000 stars.
A pair of dual multi-object fiber-fed spectro-
graphson the same telescope are used to take
640simultaneousspectra(spectroscopicplates
havearadiusof1.49degrees),eachwithwave-
lengthcoverage3800–9200Åandspectralres-
olutionof∼2000,andwithasignal-to-noisera-
tioof>4perpixelatg=20.2.
Thespectraaretargetedandautomatically
processedbythreepipelines:
– target:Targetselectionandtiling
– spectro2d:Extractionofspectra,skysub- Fig.1. Atestofthequalityofspectrophotometric
traction, wavelength and flux calibration, calibration.Eachthincurveshowsaspectrumofa
combinationofmultipleexposures hotwhitedwarf(whichwerenotusedincalibration)
– spectro1d: Object classification, red- dividedbyitsbest-fitmodel.Thethickredcurveis
shifts determination, measurement of line themedianofthesecurves.
strengthsandlineindices
Foreachobjectinthespectroscopicsurvey, similar in spectral type to the SDSS primary
a spectral type, redshift (or radial velocity), standardBD+17 4708(an F8 star). The spec-
and redshift error is determined by matching trum of each standard star is spectrally typed
the measured spectrum to a set of templates. by comparing with a grid of theoretical spec-
The stellar templates are calibrated using the trageneratedfromKuruczmodelatmospheres
ELODIEstellarlibrary.Randomerrorsforthe usingthespectralsynthesiscodeSPECTRUM
radialvelocitymeasurementsareastrongfunc- (Grayetal. 2001). The flux calibration vec-
tionofspectraltype,butareusually<5kms−1 tor is derived from the average ratio of each
forstarsbrighterthang∼18,risingsharplyto star and its best-fit model, separately for each
∼25kms−1forstarswithg=20.Usingasam- of the 2 spectrographs, and after correcting
ple of multiply-observedstars, Pourbaixetal. forGalacticreddening.Sincetheredandblue
(2005) estimate that these errors may be un- halvesofthespectraareimagedontoseparate
derestimatedbyafactorof∼1.5. CCDs, separate red and blue flux calibration
vectorsare produced.Thespectrafrommulti-
pleexposuresarethencombinedwithbadpixel
3. TheutilityandanalysisofSDSS
rejectionandrebinnedtoaconstantdispersion.
stellarspectra
The absolute calibration is obtained by tying
TheSDSSstellarspectraareusedfor: ther-bandfluxesofthestandardstarspectrato
the fiber magnitudes output by the photomet-
1. Calibrationofobservations ric pipeline (fibermagnitudesare correctedto
2. More accurate and robust source identifi- aconstantseeingof2arcsec,withaccounting
cationthanthatbasedonphotometricdata for the contribution of flux from overlapping
alone objectsinthefiberaperture).
3. Accuratestellarparametersestimation To evaluate the quality of spectrophoto-
4. Radialvelocityforkinematicstudies metric calibration on scales of order 100Å,
the calibrated spectra of a sample of 166 hot
DA white dwarfs drawn from the SDSS DR1
3.1. CalibrationofSDSSspectra
White Dwarf Catalog (Kleinmanetal. 2004)
Stellar spectra are used for the calibration of arecomparedtotheoreticalmodels(DAwhite
allSDSSspectra.Oneachspectroscopicplate, dwarfsare usefulforthiscomparisonbecause
16 objects are targeted as spectroscopic stan- they have simple hydrogen atmospheres that
dards. These objects are color-selected to be canbeaccuratelymodeled).Figure1showsthe
Ivezic´etal.:SDSSspectroscopicsurveyofstars 3
results ofdividingeach white dwarfspectrum thebluetipofthestellarlocus(u−g<1),the
byitsbestfitmodel.Themedianofthecurves expression
showsa netresidualof order2%atthebluest
[Z/Z ]=5.11(u−g)−6.33 (2)
⊙
wavelengths.
Another test of the quality of spectropho- reproduces the spectroscopic metallicity with
anrmsofonly0.3dex.
tometric calibration is provided by the com-
parison of imaging magnitudes and those These encouraging results are important
synthesized from spectra, for details see for studies based on photometric data alone,
andalsodemonstratetherobustnessofparam-
VandenBerketal. (2004) and Smolcˇic´etal.
(2006). With the latest reductions1 the two etersestimatedfromspectroscopicdata.
types of magnitudes agree with an rms of
∼0.05mag.
3.4. MetallicityDistributionand
Kinematics
3.2. SourceIdentification
Due to large sample size and faint limiting
magnitude (g ∼ 20), the SDSS stellar spec-
SDSS stellar spectra have been success-
tra are an excellent resource for studying the
fully used for confirmation of unresolved bi-
MilkyWaymetallicitydistribution,kinematics
nary stars, low-metallicity stars, cold white
andtheircorrelationallthewaytothebound-
dwarfs, L and T dwarfs, carbon stars, etc.
ary between the disk and halo at several kpc
For more details, we refer the reader to
above the Galactic plane (Juric´etal. 2006).
Adelman-McCarthyetal. (2006) and refer-
Herewepresentsomepreliminaryresultsthat
encestherein.
illustratetheongoingstudies.
3.3. StellarParametersEstimation
3.4.1. TheBimodalMetallicity
SDSS stellar spectra are of sufficient qual- Distribution
ity to provide robust and accurate stellar pa-
rameters such as effective temperature, grav- Inordertominimizevariousselectioneffects,
we study a restricted sample of ∼10,000 blue
ity, metallicity, and detailed chemical compo-
main-sequence stars defined by 14.5 < g <
sition. Here we study a correlation between
19.5,0.7<u−g<2.0and0.25<g−r<0.35.
thestellarparametersestimatedbyBeersetal.
The last condition selects stars with the ef-
group(AllendePrietoetal. 2006)andthepo-
sitionofastarintheg−rvs.u−gcolor-color fectivetemperatureinthe narrowrange6000-
6500K.Thesestarsarefurtherconfinedtothe
digram.
main stellar locus by |s| < 0.04, where the s
Figure 2 shows thatthe effective tempera-
color,describedbyIvezic´etal. (2004),isper-
turedeterminestheg−rcolor,buthasnegligi-
pendicular to the locus in the g− r vs. u− g
bleimpactontheu−gcolor.Theexpression
color-color diagram (c.f. Fig. 2). We estimate
log(T /K)=3.877−0.26(g−r) (1) distancesusingaphotometricparallaxrelation
eff
derivedbyJuric´etal.(2006).
provides correct spectroscopic temperature The metallicity distribution for stars from
withanrmsofonly2%(i.e.about100-200K) this sample that are at a few kpc from the
forthe−0.3 < g−r < 1.0colorrange.While galacticplaneisclearlybimodal(seethemid-
the medianmetallicityshowsa morecomplex dle panel in Fig. 3), with a local minimum at
behaviorasfunctionoftheu−gandg−rcol- [Z/Z⊙] ∼ −1.3. Motivated by this bimodality,
ors,itcanstillbeutilizedtoderivephotometric we split the sample into low- (L) and high-
metallicity estimate. For example, for stars at metallicity (H) subsamples and analyze the
spatialvariationoftheirmedianmetallicity.As
1 DR5/products/spectra/spectrophotometry.html, shown in the bottom panel in Fig. 3, the me-
whereDR5=http://www.sdss.org/dr5 dian metallicity of the H sample has a much
4 Ivezic´etal.:SDSSspectroscopicsurveyofstars
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
0.8 0.9 1 1.1 1.2 1.3 0.8 0.9 1 1.1 1.2 1.3
Fig.2. Thetopleftpanelshowsthemedianeffectivetemperatureestimatedfromspectraof∼40,000stars
as a function of the position in the g−r vs. u−g diagram based on imaging data. The temperature in
eachcolor-colorbinislinearlycolor-codedfrom4000K(red)to10,000K(blue).Thebottomleftpanelis
analogousexceptthatitshowsthebluetipofthestellarlocuswiththeeffectivetemperatureintherange
5300Kto6700K.Thetworightpanelsareanalogoustotheleftpanels,exceptthattheyshowthemedian
metallicity,linearlycolor-codedfrom-0.5(red)to-2.5(blue).
largergradientinthezdirection(distancefrom distribution appears Gaussian, with the width
the plane),than in the R direction(cylindrical of0.35dexandcenteredon[Z/Z ]=−1.75.
⊙
galactocentricradius). In contrast, the median The decrease of the median metallicity
metallicity of the L sample shows negligible with z for the H sample is well described by
variationwiththepositionintheGalaxy(<0.1 [Z/Z ] = −0.65−0.15Z/kpcforZ < 1.5kpc
⊙
dexwithin4kpcfromtheSun)andthewhole and[Z/Z ]=−0.80−0.05Z/kpcfor1.5<Z <
⊙
Ivezic´etal.:SDSSspectroscopicsurveyofstars 5
Fig.4. The dots in the top panel show the radial
velocity as a function of metallicity for stars with
0.25 < g−r < 0.35 and 160 < l < 200 (towards
anticenter,wheretheradialvelocitycorrespondsto
thev velocitycomponent). Thelargesymbolsare
R
the medians evaluated in narrow metallicity bins,
andthedashedlinesshowthe2σenvelopesaround
themedian.Theradialvelocitydistributionsforthe
low-andhigh-metallicitysubsamples(separatedby
5 6 7 8 R (kp9c) 10 11 12 13 [Z/Z⊙] = −1.3)areshowninthebottompanel,to-
Fig.3. Thedotsinthetoppanelshowthemetallic- getherwiththebest-fitGaussians(dashedlines)and
ity of stars with 0.25 < g−r < 0.35 as a func- theirparameters.Notethatthevelocitydispersionis
tion of the height above the Galactic plane. The ∼2.5timeslargerforthelow-metallicitysubsample.
largesymbolsarethemediansevaluatedseparately
for the low-metallicity ([Z/Z ] < −1.3) and high-
⊙
metallicity ([Z/Z ] > −1.3) subsamples, and the
⊙
dashedlinesshowthe2σenvelopesaroundtheme- 4 kpc (see the top panelin Fig. 3). For Z < 1
dian. Thehistogram inthe middlepanel illustrates kpc, most stars have [Z/Z⊙] > −1.3 and pre-
the bimodality of metallicity distribution for stars sumably belong to thin and thick disks (for a
with heights above the galactic plane between 1 recentdeterminationofthestellarnumberden-
kpc and 2 kpc. The two solid lines are the best-fit sity based on SDSS data that finds two expo-
Gaussians, and the dashed line is their sum. The nentialdisks,seeJuric´ 2006).Thedecreaseof
dependence ofthemedianmetallicityforthehigh-
themedianmetallicitywithzfortheHsample
metallicitysubsampleonthecylindricalgalacticco-
couldthusbeinterpretedasduetotheincreas-
ordinatesRandzisshowninthebottompanel(lin-
ingfractionofthelower-metallicitythickdisk
early color-coded from −1.2 to −0.5, blue to red).
stars. However, it is puzzling that we are un-
Note that the z gradient is much larger than the R
gradient. For the low-metallicity subsample, these abletodetectanyhintofthetwopopulations.
gradientsarenegligible. Ananalogousabsenceofacleardistinctionbe-
tween the thin and thick disks is also found
whenanalyzingtheradialvelocitydistribution.
6 Ivezic´etal.:SDSSspectroscopicsurveyofstars
Low-metallicity, velocity dispersion, 40-160 km/s
Fig.6. AnalogoustoFig.5,exceptthatthevelocity
dispersion is shown (color-coded from 40 km/s to
160km/s).
Fig.7. A comparison of the radial velocity dis-
tributionforlow-metallicitystarsobservedtowards
l ∼ 90 and l ∼ 180. Note that the subsample ob-
served towards the anti-center has a large velocity
dispersion,inagreementwithFigure6.
Fig.5. Thetoppanelshowsthemedianradialve-
locityinLambertprojectionforstarsfromthelow-
metallicity [Z/Z ] < −1.3 subsample, which have
⊙ component has about 2.5 times larger veloc-
b>0andareobservedatdistancesbetween2.5kpc
ity dispersion than the high-metallicity com-
and 3.5 kpc. The radial velocity is linearly color-
ponent.Ofcourse,thismetallicity–kinematics
coded from -220 km/s to 220 km/s (blue to red,
correlationwasknownsincetheseminalpaper
green corresponds to0km/s). Thebottompanel is
analogous, except thattheradialvelocitymeasure- by Eggen,Lynden-Bell&Sandage (1962),
ments are corrected for thecanonical solar motion but here it is reproduced using a ∼100 times
of220km/stowards(l=90,b=0). largersamplethatprobesasignificantlylarger
Galaxyvolume.
3.4.2. TheMetallicity–Kinematics
Correlation 3.4.3. TheGlobalBehaviorof
Kinematics
Inadditiontothebimodalmetallicitydistribu-
tion, the existence of two populations is also The large sample size enables a robustsearch
supported by the radial velocity distribution. for anomalous features in the global behavior
As illustrated in Fig. 4, the low-metallicity ofkinematics,e.g.Sirkoetal. (2004).Forex-
Ivezic´etal.:SDSSspectroscopicsurveyofstars 7
ample,whilethevariationofthemedianradial large velocity dispersion and a non-zero rota-
velocity for the low-metallicity subsample is tionalcomponentinawell-defined∼1000deg2
well described by the canonical solar motion largeregion,perhapsduetostellarstreams.
(Fig. 5), wefind anisolated∼1000deg2 large Thesepreliminaryresultsareonlybriefil-
region on the sky where the velocity disper- lustrations of the great potential of the SDSS
sion is larger (130 km/s) than for the rest of stellar spectroscopic database. This dataset
the sky (100 km/s), see Figs. 6 and 7. This is willremaina cuttingedgeresourcefora long
probablynotadataartefactbecausethedisper- time because other major ongoing and up-
sion for the high-metallicity subsample does comingstellarspectroscopicsurveysareeither
not show this effect. Furthermore, an analy- shallower(e.g.RAVE),orhaveasignificantly
sis of the proper motion database constructed narrowerwavelengthcoverage(GAIA).
by Munnetal. (2004) finds that the same
starsalsohaveanomalous(non-zero)rotational Acknowledgements. Funding for the SDSS and
velocity in the same sky region (Bondetal. SDSS-II has been provided by the Alfred P.
SloanFoundation,theParticipatingInstitutions,the
2006). This kinematic behavior could be due
National ScienceFoundation, theU.S.Department
to the preponderanceof stellar streams in this
of Energy, NASA, the Japanese Monbukagakusho,
region (towards the anti-center, at high galac-
theMaxPlanckSociety,andtheHigherEducation
tic latitudes, and at distances of several kpc).
FundingCouncilforEngland.
Bondetal. (2006) also find, using a sample
of SDSS stars for which all three velocity
components are known, that the halo (low- References
metallicitysample)doesnotrotate(atthe∼10
Adelman-McCarthy, J.K., et al. 2006, ApJS,
km/s level), while the rotational velocity of
162,38
thehigh-metallicitysampledecreaseswiththe
AllendePrieto,C.,etal.2006,ApJ,636,804
heightabovethegalacticplane.
Bahcall, J.N. & Soneira, R.M. 1980, ApJSS,
44,73
4. Conclusions Bond,N.,etal.2006,inpreparation
Eggen,O.J.,Lynden-Bell,D.&Sandage,A.R.
We show that stellar parameter estimates by 1962,ApJ,136,748
Beers et al. show a good correlation with the Gilmore, G., Wyse, R.F.G. & Kuijken, K.
position of a star in the g−r vs. u−g color- 1989,ARA&A,Volume27,pp.555-627
colordiagram,therebydemonstratingtheirro- Gray,R.O.,Graham,P.W. &Hoyt,S.R. 2001,
bustnessaswellasapotentialforphotometric AJ,121,2159
stellar parameterestimationmethods.We find Ivezic´,Zˇ.,etal.2004,AN,325,583
that the metallicity distribution of the Milky Juric´,M.,etal.2006,submittedtoAJ
Waystarsatafewkpcfromthegalacticplane Kleinman,S.J.,etal.2004,ApJ,607,426
is clearly bimodal with a local minimum at Majewski,S.R.1993,ARA&A,31,575
[Z/Z⊙] ∼ −1.3.Themedianmetallicityforthe Munn,J.A.,etal.2004,AJ,127,3034
low-metallicity [Z/Z⊙] < −1.3 subsample is Pourbaix,D.,etal.2005,A&A,444,643
nearly independentof Galactic cylindricalco- Sirko,E.,etal.2004,AJ,127,914
ordinatesRandz,whileitdecreaseswithzfor Smolcˇic´,V.,etal.2006,acceptedtoMNRAS
thehigh-metallicity[Z/Z⊙]>−1.3sample.We Stoughton,C.,etal.2002,AJ,123,485
also find that the low-metallicity sample has VandenBerk,D.E.,etal.2004,ApJ,601,692
∼2.5timeslargervelocitydispersion. York,D.G.,etal.2000,AJ,120,1579
Thesamplesdiscussedherearesufficiently
largeto constrainthe globalkinematicbehav-
iorandsearchforanomalies.Forexample,we
findthatlow-metallicitystarsobservedathigh
galactic latitudesat distancesof a few kpcto-
wards Galactic anticenter have anomalously