The Sagittarius stream

Thephotometri turno pointdistan esarederivedfromthedistan e

modu-lus. Thereforetheun ertainties inthedistan es an be al ulatedasa

ombina-tionoftwosour esoferror: theun ertaintyderivedfromtheobservedbrightness

of the turno point (

E mag,CMD

^, ^dis
ussed ^above) ^and ^the un ertainty derived fromtheabsolutebrightnessoftheturnopoint,whi hdependsonthe hoi eof

iso hrone(andthusontheun ertaintyintheageor inthemetalli ity).

E µ,TO = q

E _mag,CMD ² + E _mag,isoch ² ;

(4.2)

Table4.1: Positionanddistan estotheSgrstream,togetherwitha tagforfaint

orbrightbran hmembership,a tentative lassi ationasleading ortrailingarm

and a number spe ifying the hierar hy of the dete tion in the CMD (primary,

se ondary,et .). The distan es are indi ated both as distan e modulus and as

helio entri distan e, withthedistan e un ertainty(

E d

⁾ⁱⁿ^kp
.

Field arm dete tion RA(deg) DEC(deg)

µ(mag) d

^(kp
)

E d

^(kp
)

A2104

f

lead 1 235.040644 -3.33158 18.8 56.6 3.1

RXJ1524

f

trail 1 231.170583 9.93498 16.2 17.1 2.0

A2050

f

lead 1 229.080749 0.08773 18.7 54.1 8.7

A1942

f

lead 1 219.654126 3.63573 18.7 54.1 3.7

A1882

b

lead 1 213.667817 -0.30598 18.5 49.3 5.7

A1835

b

lead 1 210.259355 2.83093 18.4 47.1 4.2

RXJ1347

b

? 1 206.889060 -11.80299 15.5 12.4 7.3

ZwCl1215

b

? 1 184.433196 3.67469 16.7 21.5 2.9

ZwCl1215

b

? 3 184.433196 3.67469 15.0 9.8 2.6

A1413

f

lead 1 178.842420 23.42207 17.5 31.1 2.7

A1413

f

trail 2 178.842420 23.42207 16.2 17.1 1.9

A1246

f

lead 1 170.987824 21.40913 17.6 32.6 9.2

A1185

f

? 1 167.694750 28.68127 16.3 18.7 12

ZwCl1023

b

? 1 156.489424 12.69030 17.4 29.7 11

A795

b

lead 1 141.030063 14.18190 16.0 14.2 2.8

A795

b

? 2 141.030063 14.18190 15.6 14.2 2.8

A763

b

? 1 138.150298 15.99992 16.7 21.5 2.6

A763

b

lead 2 138.150298 15.99992 15.8 14.2 1.0

RXJ0352

f

lead 1 58.263173 19.70387 15.7 13.6 0.7

RXJ0352

f

trail 2 58.263173 19.70387 17.7 34.1 4.3

A401

f

trail 1 44.759558 13.58975 17.4 29.7 3.4

A399

f

trail 1 44.478652 13.05185 17.6 32.6 11

A370

b

trail 1 39.963713 -1.65806 17.6 32.6 4.8

A223

b

trail 1 24.557005 -12.77010 17.0 24.7 1.7

RXJ0132

f

trail 1 23.169048 -8.04556 17.1 25.9 2.3

A133

b

trail 1 15.673483 -21.88113 16.6 20.6 2.4

A119

f

trail 1 14.074919 -1.23337 16.9 23.6 2.9

A85

b

trail 1 10.469662 -9.28824 16.9 23.6 1.6

A2670

f

trail 1 358.564313 -10.40142 16.6 20.6 1.1

RXJ2344

f

trail 1 356.059633 -4.36345 16.7 21.5 5.6

RXJ2344

f

lead 2 356.059633 -4.36345 15.6 13.0 1.2

A2597

f

trail 1 351.336736 -12.11193 16.9 23.6 1.4

b

Brightbran h

f

Faintbran h

Figure4.6: Absolutebrightnessoftheturnopointintherbandasafun tionof

metallli ityandageformetalpoorpopulations(green ir les). Thevaluesinthis

diagrammeettheage/metalli ityrelationfortheSgrdwarfgalaxy fromLayden

&Sarajedini (2000). Theiso hroneused inthispaper toderivedistan es tothe

Sgrstreamis representedwith a yellowstar, and itsmaximumdieren eto the

otherbrightnessvaluesin thisrangeis

∆ M = ±0.1mag

dete tion'tag and thedistan e modulus(

µ

^). În ^Figure ^4.7^weômpare ôur

re-sultsto valuesfromtheliterature 4

,splitin two diagrams(toppanelforthefaint

bran hesandbottompanelforthebrightbran hesin bothhemispheres).

Remarkablyourturnopointdistan esarenotonlyin agreementwith

previ-ousdistan emeasurementstoknownwraps,butalso ompatiblewiththedistan e

predi tionsfor nearbywrapsbythemodelsofPeñarrubia etal. (2010)and Law

&Majewski(2010b). Inthefollowingse tionwedis ussindetailthesendings.

4.3.2 Comparison with models of the Sgr stream

Usingthemodelpredi tionsshowningures4.8and4.9,we lassifyea heldas

belonging to the leading or trailing arm, by mat hing thedistan e and the sky

position. It is worth noting, however, that the distan e for a given parti le in

themodelsissensitivenotonlytotherightas ensionbutalsotothede lination

ofthe parti le. Whilewetakeinto a ount both oordinates when lassifyinga

dete tionasfaintorbrightbran h,wedonotspe i allya ountforthede lination

when omparing thedistan es ofthe modelled streamsand thedistan es ofour

TheSDSS-DR8measurementsshowninthispaperforthesouthernbrightarmhavebeen

orre tedforthedieren einthe alibrationofthered lumpabsolutemagnitude,aspointedout

inSlateretal.(2013)and orre tedinKoposovetal.(2013)).AndtheSDSS-DR5measurements

havebeende reasedby

0.15 mag

^to^mat
h^the^BHB^signal^from^SDSS,^as^pres
ribedⁱⁿ^Belokurov

etal.(2014).

Figure4.7: Photometri mainsequen eturnopointdistan esfortheSagittarius

stream along right as ension (northern-leading tailand southern- trailing tail).

Thetoppanelshowsresultsforthefaintbran h, whereasthebottompanel

or-respondstothebrightarm. Ourdata(blue ir lesforleadingtailsandred ir les

fortrailingtails) arebasedonthetheoreti aliso hrones byMarigo etal.(2008)

and the orre tions by Girardi et al. (2010), for a

10.2 Gyr

^old ^stellar

popula-tionwith

[ F e/H] = −1.0

^. Ôther^distan
e ^values ôrrespond ^to ^Belokurovêt âl.

(2006b)(greenandgreytriangles),Koposovetal.(2012,2013)(greenasterisks),

Belokurov et al. (2014) (pink triangles)and Slater et al. (2013) (yellow squares

for

> 3σ

^dete
tions^and^white^squares^for

< 3σ

^). ^White^ir
les^denote^dete
tions

that annotbeunambiguouslytaggedas leadingortrailing.

dete tionsingures4.8and4.9. Thismayhavesomeinuen einthesigni an e

of the osets betweenour distan e measurement and the mean distan e of the

N-body parti lesfora givenRA.

Of these two models,the model byPeñarrubia et al.(2010) was designed to

illustratetheee tofinternalsatelliterotationontheorbitoftheSgrdebris,and

itseemstore overbettertheseparationinstellardensitydistributionthatgives

risetothenorthernbifur ationintofaintandbrightbran hes(Figure4.8,upper

panel). Ontheotherhand,themodelbyLaw&Majewski(2010b)wasdesigned

toreprodu etheknownobservationalpropertiesofthestream(angularpositions,

distan es,radialvelo ities),anditseemstoreprodu ebettertheproje ted2MASS

stellardensitydistribution(Figure4.9, upperpanel). Inrelationto thesouthern

regionofthestream,althoughneitherofthetwomodelspredi tedthebifur ation

init,theysu eedinreprodu ingthegeneraldistribution ofthedebris.

Northernleading arm

From our eighteen measurements on the bright and the faint bran hes of the

northern-leadingarm(bran hesA andB,in theterminologyofBelokurovet al.

(2006b)),nine (blue ir les in Figure 4.7) learly reprodu e the distan e trends

of Belokurov et al. (2006b) and Belokurov et al. (2014) (green+grey and yan

trianglesinFigure4.7,respe tively)basedonredgiantandbluehorizontalbran h

stars. For the faint bran h, we extend westwards the distan e measurements

beyond thoseofSDSS, andweprovide itsmostdistantdete tionssofarout to

56kpc

^at ^RA

∼ 235 ^o

^. ^Comparing ^these ^most^distant^dete
tions ^to ^the ^distan
e

trendofthemodelsandtothebrightbran hatasimilarrightas ension,one an

arguethat these dete tions likely lie lose to theapo enter of the faint bran h

(orrepresenttheapo enterthemselves),and thereforetheyare probablyagood

estimationforitsdistan e.

For theother ninedete tions,wendthatthederiveddistan esareeitherin

mild disagreement with the trends of the leading arm (four ases, white ir les

in Figure 4.7) or in ompatible with the leading arm (ve ases, red and white

ir lesinFigure4.7). Inthesingle aseofmilddisagreementforthefaintbran h

(A1185,RA

∼ 168 ^o

⁾^the^distan
eîs^well^below^the^trendsôf^both^thisând

previ-ouswork (oset

≈ 10 kpc

^);^however ^its^largeun ertaintypreventsusfrom ruling outthat itbelongsto the faintbran h. Wewill dis uss analternative

member-ship in subse tion 4.3.2. The three ases of mild disagreement for the bright

bran h (ZwCl1023, A795-2and A763-1, RA

∼ 150 ^o

⁾ ^are^slightly ^above ^the

dis-tan etrend of this bran h. Parti ularly, elds A795 and A763also display two

additionaldete tions(primaryandse ondary,respe tively)slightlyunderthe

ex-pe ted distan e trend. Fields A795and A763 lie lose in thesky (less than

4 ^o

apart)and both yield primary and se ondary distan e measurements very

on-sistent with ea h other and with this di hotomy. Weinterpret this as possibly

indi atinga regionoftheskywherethebrightbran h runsbroaderindistan e.

Outof thevedete tionsin ompatible withthedistan e trendsof the

lead-ing arm, we will dis uss three (RXJ1524,A1413-2 and ZwCl1215-1) in

subse -Figure4.8: Ourdata omparedtothepredi tionsbythemodelfromPeñarrubia

et al.(2010). Top panel: Equatorial mapwith theposition ofourelds plotted

over the simulation. Bottom panel: Distan e vs RA diagram with our results

omparedtothemodelpredi tions. Fieldsonthefaintbran h aredenoted with

ir les,andelds onthebrightbran haredenoted withsquares. Measurements

mat hing theleading armaredenotedin pink,whereasthosemat hingthe

trail-ingarmare denoted in lightblue. White markers representdete tionsthat an

notbe unambiguously tagged as leading or trailing; grey markers in the upper

panel orrespondto elds withmore thanone MS dete tion(they unfoldin the

bottompanel). The olours alesrepresentthetime sin etheparti lesfrom the

simulationsbe ameunbound.

Figure4.9: SameasinFigure4.8butforthemodelfromLaw&Majewski(2010b).

tion4.3.2, together with theabove mentionedA1185. Regardingthe other two

(RXJ1347and ZwCl1215-3, RA

∼ 205 ^o

^and ^RA

∼ 185 ^o

respe tively), wehave studiedthemindividuallyandfoundthefollowing. Ontheonehand,ZwCl1215-3

mat hesthedistan etotheVirgoOverdensity(Bona aetal.2012b)whenusing

theappropriate ageand metalli ity values for the theoreti al iso hrone, so it is

likely a dete tionof this loud. On the other hand, RXJ1347 mat hes the

dis-tan eand position predi tedbythe model from Peñarrubia et al.(2010) for an

older northern-wrapof theleading arm, but alsothe distan espredi ted bythe

twomodelsforthenorthern-trailingwrap. Howeverwe annotdraw on lusions

regardingmembershipforanisolateddete tionandwela kkinemati data,soat

themomentwe an notdis riminatebetweenbothoptions(orevenathirdone).

Northerntrailingarm

Inthissubse tion werevisitfour dete tionsin thegala ti northernhemisphere

whi h yield distan es in ompatiblewith (or o) theleading arm. These

dete -tionsareRXJ1524, A1413-2(red ir lesin Figure 4.7),A1185 ( ompatible with

thefaint leading bran h thanks to its largeerror bars, but severely osetfrom

thedistan e trend)and ZwCl1215-1(white marker atRA

∼ 185 ^o

^on^the^bright

bran h). Thethreedete tionsinthefaintbran h(RXJ1524,A1413-2andA1185)

showdistan esstrongly onsistentwith ea hother (

∼ 17 kpc

^). ^And^the^three ^of

themaretheeldsmostapartfromtheSgrorbitalplaneinournorthernsample,

spreading

60 ^o

^along^the^orbit.

Remarkablyboththeirdistan esandtheirpositionsintheskyareinextremely

good agreement with the predi tions from the above mentioned models for the

Sgrdebris in thenorthern-trailingarm,and theyare roughly omparableto the

results in Correnti et al. (2010). However they are at odds with the laim in

Belokurov et al. (2014) that bran h C (at lower de linations and more distant;

see greytriangles in Fig. 4.7) is indeed a part of thenorthern-trailing arm and

the ontinuation of theGeministream/arm (Newberget al.(2003),Drakeet al.

(2013);seepinktrianglesfromBelokurovetal.(2014)in Fig.4.7)).

Giventhe onsisten yofourdistan emeasurementswithea hotherandwith

thesimulations,andgiventhedistribution oftheelds alongthefaintbran h of

thestream,webelievethese dete tionsarepart oftheSgrstream, mostlikelya

wrapofthenortherntrailingarm. Howeverkinemati dataoraspatiallybroader

photometri overageareneededto onrmthis.

Additionally, ZwCl1215-1, whi h lieson the bright bran h, yields a distan e

measurement ompatiblewith thetrendpredi tedforthenorthern-trailingarm.

Butitspositiononthesky(onthebrightbran h) annotbere on iledwiththe

urrentmodelsforthetrailingtail,neitherwiththeage,metalli ityanddistan e

valuesfortheVirgoOverdensity. Thus,itsmembershipandmeaninginthepuzzle

ofthehalo remainanopenquestion.

Southern trailingarm

Ourmeasurementsonthe bright andthefaint bran hesof thesouthern-trailing

armreprodu ethedistan etrendsofKoposovetal.(2012,2013)andSlateretal.

(2013) based on red lump and turno point stars. For the faint bran h, we

onrmthetrendsetbythe

< 3σ

^dete
tionsⁱⁿ^Slaterêtâl.^(2013),ând^we^briey

extendwestwardsandeastwardsthedistan emeasurements. Contraryto Slater

et al. (2013), we nd noeviden e fora dieren e in distan e betweenthe faint

andthebrightbran hesofthesouthern-trailingtail. However itispossiblethat

su hdieren eremainshiddeninourdistan eun ertainties.

When omparing tothe abovementioned models, wend thatthe measures

arein generalagreement with the predi tions for boththe faint and thebright

bran hes. Howeverthedistan egradientinthefaintbran hseemstobelesssteep

inthedatathaninthemodels,andthebran hseemstobethinnerindistan ethan

predi tedfor anyvalueoftheprobedRArange. Inthissense itisworthnoting

that, in ontrast to what happens to many of our northern hemisphere elds,

onlytwo of the CMDs in thesouthern gala ti hemisphere showse ondary MS

dete tions(RXJ0352and RXJ2344,atRA

∼ 58 ^o

^and^RA

∼ 356 ^o

^,respe tively).

Andthedieren ebetweentheturnopointbrightnessofthesedoubledete tions

doesnotfavourathi kbran h,butratherthedete tionofapreviouslyunknown

nearbywrap(seesubse tion4.3.2).

Southern leading arm

Inthis subse tion we revisit the double dete tions of 4.3.2, namely RXJ0352-1

andRXJ2344-2, (RA

∼ 58 ^o

^and^RA

∼ 356 ^o

^,^primary ^and^se
ondary dete tions, respe tively). WeshowtheirCMDsandtheir ross- orrelationdensitydiagrams

ingures4.10and4.11. Wendthat,usingthesameiso hronewehaveused to

derivedistan estoalltheSgrelds,bothyieldadistan eof

∼ 13 kpc

^. ^These

dis-tan esareinex ellentagreementwiththepredi tionsfromthetwosimulationsfor

theleadingarmintheSouthandalsowiththetrendsetbytheleading-northern

data. Wethus laimthatthesetwodete tionsarepartofthe ontinuationofthe

northern-leadingarmintothesouthernhemisphere(Chouetal.2007). The

posi-tionsoftheseelds,however,suggestthattheleadingarmdivesintothesouthern

hemisphereathigherde linationsthanpredi ted,overlappingin proje tionwith

thefaintbran h ofthetrailing arm.

Ifthedete tionofthesouthern-leadingarmor thenorthern-trailingarm

pro-posedin thispaperare onrmedinfuture works(withkinemati measurements

formembershiporphotometri follow-upforspatial overage),ourmeasurements

willbethe losestandtheoldestdebrisoftheSgrstreamdete tedtodate. Ifso,

thiswouldmeanthatourmethodhassu eededindete tingnearbysubstru ture

in regions of the skythat had already been explored. The explanation to su h

a performan e would lie on thefa t that weuse a sample ofstars (a largepart

ofthe main sequen e) to identify the overdensities in theCMD larger than the

sampleoftheusualhalotra ers(red lump,redgiantsorbluehorizontalbran h),

Figure 4.10: Left: Dereddened CMD for the westernmost pointing probing the

leading arm in the southern hemisphere; the template main sequen e fun tion

andtheturnopoint(green)areplottedforthemaximumoftheprimary

ross- orrelation. Right: Weighted-density diagram resultingfrom theprimary

ross- orrelation. Themaximum(whitebin,bla k ross)marksthetopleft ornerofthe

template-MSatthepositionofthesouthern-leadingarmmainsequen e,whereas

the red overdensity at fainter magnitudes orresponds to the southern-trailing

arm.

andthis ouldin reasethe ontrastinregionsoflow on entrationandthi kdisk

ontamination.

Presen e ofsparse nearby streams in SDSS

In the mu h shallower SDSS data the dete ted MS features are signi antly

smearedoutbyphotometri un ertaintiesandpoorerstar/galaxyseparation(see

CMD omparisonin g.4.12). Asa onsequen e,meagermain sequen eswould

bemoredi ulttoindentifyinSDSS(andmeagerBHBorRC ounterpartswould

beimpossibletoidentify). Thisleadsustothinkthattheabilitytodete tsparse

stellarpopulations liketheonesin se tions4.3.2and4.3.2isstronglydependent

ontheinitialimagequalityandthestarsele tionpro ess.

Thestrength of SDSS liesin thelargearea it overs, whi h might allowthe

dete tionof a nearby, dynami allyold stream in some statisti al way. However

thispursuitisbeyondthes opeofthispaper.

Figure 4.11: Left: Dereddened CMD for the easternmost pointing probing the

leading arm in the southern hemisphere; the template main sequen e fun tion

andtheturnopoint(green)areplottedforthemaximumofthese ondary

ross- orrelation. We have randomly removed

50%

^of ^the ^stars ontributing to the primary dete tion, whi h orresponds to the southern-trailing arm of the Sgr

stream. Right: Weighted-density diagram resulting from the se ondary

ross- orrelation. Themaximum(white bin, bla k ross)marksthe topleft orner of

thetemplate-MSfun tionatthepositionoftheOrphanstream'smainsequen e.

The primary dete tion has been partially removed, and the remainings an be

seenasa weaktailatfaintermagnitudesandslightlybluer olour.

Figure 4.12: Left: Smoothed CMD for eld RXJ1524 ontaining the CFHT

sour es lassied as stars by us. Right: Same as the diagram in the right for

theSDSSsour es lassied as starsin thedatabase. Theweaker featuresin the

leftgurearenotdete table in therightguredue tophotometri un ertainties

andthedierentstar/galaxyseparations.

In document Title: Structure and substructure in the stellar halo of the Milky Way (pagina 92-103)

E mag,CMD

E µ,TO = q

E mag,CMD 2 + E mag,isoch 2 ;

E d

µ(mag) d

E d

f

f

f

f

b

b

b

b

b

f

f

f

f

b

b

b

b

b

f

f

f

f

b

b

f

b

f

b

f

f

f

f

b

f

∆ M = ±0.1mag

µ

0.15 mag

10.2 Gyr

[ F e/H] = −1.0

> 3σ

< 3σ

56kpc

∼ 235 o

∼ 168 o

≈ 10 kpc

∼ 150 o

4 o

∼ 205 o

∼ 185 o

∼ 185 o

∼ 17 kpc

60 o

< 3σ

∼ 58 o

∼ 356 o

∼ 58 o

∼ 356 o

∼ 13 kpc

50%

E _mag,CMD ² + E _mag,isoch ² ;

∼ 235 ^o

∼ 168 ^o

∼ 150 ^o

4 ^o

∼ 205 ^o

∼ 185 ^o

∼ 185 ^o

60 ^o

∼ 58 ^o

∼ 356 ^o

∼ 58 ^o

∼ 356 ^o