Dis ussion

theSgrstream). Thederiveddistan es(Table5.4)are onsistentwiththose

ob-tainedusingtheiso hronettingmethodgiveninSe . 3.4,withoutanyeviden e

ofsystemati osetortrend. Wethus on ludethatthe ross- orrelationmethod

independently onrms(withintheun ertainties)thedistan emeasurementsfor

theGCs lassiedasgroup D.

Figure5.10: CMDs orrespondingtothe lustersWhiting1,NGC1261,NGC1851and

NGC1904 (middle olumn) and to those obje ts beyond

r bg

^from ^the ^luster ^entre

(right olumn). Amapshowingthedistributionofthestarsinthe atalogwithrespe t

tothe luster entreisalsoin luded(left),where

r bg

îsîndi
ated^byâ^red^line.

Figure5.11: CMDs orrespondingtothe lusters NGC2998, NGC4147, Rup106and

NGC4590 (middle olumn) and to those obje ts beyond

r bg

^from ^the ^luster ^entre

(right olumn). Amapshowingthedistributionofthestarsinthe atalogwithrespe t

tothe luster entreisalsoin luded(left),where

r bg

îsîndi
ated^byâ^red^line.

Figure5.12: CMDs orrespondingtothe lustersNGC5024,NGC5053,NGC5272and

AM4(middle olumn)and to thoseobje ts beyond

r bg

^from^the ^luster ^entre ^(right

olumn). Amapshowingthedistributionofthestarsinthe atalogwithrespe ttothe

luster entreisalsoin luded(left),where

r bg

îsîndi
ated^byâ^red^line.

Figure 5.13: CMDs orresponding to the lusters NGC5466, NGC5634, NGC5694

andNGC5824(middle olumn)andtothoseobje tsbeyond

r bg

^from^the^luster^entre

(right olumn). Amapshowingthedistributionofthestarsinthe atalogwithrespe t

tothe luster entreisalsoin luded(left),where

r bg

îsîndi
ated^byâ^red^line.

Figure5.14: CMDs orrespondingtothe lustersPal5,NGC6229,Pal15andNGC6864

(middle olumn)andtothoseobje tsbeyond

r bg

^from^the^luster^entre^(right^olumn).

Amap showing thedistributionof thestarsinthe atalog withrespe ttothe luster

entreisalsoin luded(left),where

r bg

îsîndi
ated^byâ^red^line.

Figure5.15: CMDs orrespondingtothe lustersNGC7006,NGC7078andNGC7492

(middle olumn)andtothoseobje tsbeyond

r bg

^from^the^luster^entre^(right^olumn).

Amap showing thedistributionof thestarsinthe atalog withrespe ttothe luster

entreisalsoin luded(left). Notethatinthe aseofNGC7006,onlyoneofthepointings

hasbeenin ludedinthatmap,where

r bg

îsîndi
ated^byâ^red^line.

Figure5.16: Density diagramsresultingfrom ross- orrelatingtheCMDs ofthe

outer regions with the MS template. From left to right and top to bottom:

NGC1261,NGC1851,NGC4147,NGC5634,Pal5 (twi e)andWhiting1.

Figure5.17: TheSgrtidalstreamaspresentedinthemodelbyP10. Theupperpanel

shows the predi tedorbit of thestream inthesky where the olour indi atedierent

a retiontimes for the parti les inranges of 0.25Gyrlong. The middleand bottom

panels show the helio entri distan e and radial velo ity distribution of the stream,

usingthesame olours heme. Thepositionand radialvelo ity oftheglobularsinour

sampleareover-plottedasstars. Onlythefra tionofthesubstru turewith

d _⊙ <

⁶⁰^kp

hasbeen onsidered.

Figure5.18: CMDsforthe surroundingarea oftheGCs possiblyasso iated withthe

Sgrtidalstream,withstellarover-densitiesthatstandoutinthe omparison withthe

syntheti diagrams. TheSgr iso hrone orrespondingtothe radialdistan ederivedis

over-plotted.

theGCformationpro essinSgr,giventhatWhiting1mightbetheyoungestGC

amongthe lustersalreadyasso iatedwiththatdSph(

∼ 1

^Gyr^younger^than^the

intermediateageGCs Arp2, Ter7 andPal12, already asso iated to Sgr). This

wouldindi atethatSgrwasabletoformGCsduringaperiodof6Gyraspointed

byCarraroetal.(2007).

NGC5634isoneofthe losest lusterstotheplanethat ontainstheorbitof

theSgr dSph (L10) and stream stars were identied by Majewski et al. (2003)

in that line-of-sight. Our photometryshows forthe rsttime a CMD

morphol-ogy ompatible with that of the Sgrstream in thesurroundings of this luster.

It however does not reveal any underlying population at a similar distan e of

this luster. Amore important ontribution in theba kgroundisdete ted, ata

distan e nearly twi e thedistan e to NGC5634, as onrmed by both distan e

determinationmethods. OnthebasisoftheP10model, weidentify thatsystem

intheba kgroundasadistantse tionoftheleadingarmoftheSgrtidalstream.

TheCMDofPal5 presentsthemost omplexmorphologyin oursurvey,

dis-playingtwo MS-likefeatures atdierentdistan es asshownin Figure5.18. The

rstliesis a high signi an estellar population in theba kgroundof Pal5 at a

similardistan eofthe luster. Thesestarsarelikely lustermembers populating

thewell-studiedmassivetidaltailsemergingfromthis luster(Odenkir henetal.

2001;Ro kosiet al.2002;Grillmair&Dionatos2006a). Ase ondandsigni ant

(

S ∼ 8

⁾^MS^is^dete
ted^below^the^feature^asso
iated^to ^the^tidal^tails^(see^Fig.¹²

in Pila-Díezetal.2014)ataradialdistan e ompatiblewiththatoftheSgrtidal

streama ordingtoP10. Interestingly,Sbordoneet al.(2005) derived

α

^-element

abundan esforPal5,resemblingthose obtained forM54and Ter7,members of

theSgrGCsystem.

Bellazziniet al. (2003) arguedforthe asso iationof NGC4147with theSgr

streamfromitsradialvelo ityandthedete tionofMgiantSgrstarsaroundthis

luster. The dete tion of a MS feature from the Sgr stream stellar population

aroundNGC4147 in ourpen il-beamsurveywas already reported in

Martínez-Delgadoet al.(2004a), beforethemappingofthisstru ture withlarge-s ale

sur-veys(e.g. Majewskietal.2003;Belokurovetal.2006b;Koposovetal.2012). We

dete tanunderlyingstellarpopulationlikelyasso iatedtothathalosubstru ture

d ⊙ ∼

³⁵^kp
, ^separated ^from ^the ^GC^along ^our line-of-sight by

∼ 15

^kp
, ⁱⁿ

agreement with the position of the leading arm predi ted by P10. Our results

indi atethatthis lusterisnotimmersedintheSgrtidalstream,asalsopointed

outinMartínez-Delgadoetal.(2004a),wheretheintegralsofmotionsofboth

sys-temswereanalyzed. SDSSmapping hasalsoshowedthatthepathofthestream

rossesthesurroundingsofNGC5024andNGC5053,whi hareinthevi inityof

NGC4147inproje tedposition(see Koposovetal.2012).

Around NGC5053 ( lassiedin the group C)wehavefound anover-density

initsba kgroundCMDsuggesting asubja entpopulationat

∼

⁴⁰^kp
,

ompat-iblewith theradialdistan es predi tedbyP10forthe Sgrleadingarmsonthat

dire tionofthesky. Thesigni an efortheover-densityin NGC5024is

S < 5

However,the ross- orrelationmethodreturnsun on lusiveorambiguous

dete -tionsin the ase of both globulars, likely produ ed by the presen e of blended

populationsinthediagrams,withase ondMSpossibly orrespondingtoamore

distant wrap of Sgr with a low

S/N

^. ^From ^these ^globulars, ^only ^NGC⁵⁰⁵³ ^is

presentedbyL10as agenuine andidatetobelongtotheSgrGCsystem.

NGC7492istheonly lusterofoursampleforwhi han"un ertain"dete tion

ofanunderlying debris(group B)hasbeenfound, anditis oneof theglobulars

with low probability of belonging to the Sgr GCsystem a ording to L10. We

identiedasubja entMSfeatureatadistan e ompatiblewiththatofthe luster,

whi hisnotpredi tedinthesyntheti TRILEGALCMDs,whilethesigni an e

ofsu h a featuredrops belowtheadopted treshold whentheBesançon model is

adopted. With our photometry only, it is not possible to address the question

ofwhether this dete tionis real or asso iatedto tidal tails originatingfrom the

luster. However,intheradialproleobtainedforthis luster(see Carballo-Bello

etal.2012),thestellardensitybeyond

r bg

⁽

∼ 14

^ar
min)^suggests^the^presen
e^of

ahomogeneouslydistributedpopulation. Thissuggeststhattheeventual

underly-ingpopulationisasso iatedtoadierentsystemintheba kgroundofNGC7492.

Figure 5.17 shows that the proje ted position and distan e of the most re ent

a retedfra tion ofthe Sgrstreamtrailing arm(

t accr < 0.25

^Gyr) ^is^ompatible

with the position of this globular. Interestingly, the region around this luster

fallsinaskyareawithoutSDSSdata(seeFigure5.19),butwitheviden eofSgr

starsin itsvi inity, whi hstrenghtensthehypothesisthat this GCisembedded

intheSgrstream.

Thenegativedete tionsinthesurroundingsoftheother andidatespreventus

fromobtaining anal on lusion aboutthepossible asso iationofthose lusters

withtheSgrtidalstream, withinoursurfa e-brightnessdete tionlimits. Among

them,onlyAM4hasbeensuggestedasmemberoftheSgrGCsystembyCarraro

(2009)but,a ordingtotheba kgroundCMDobtained,thereisnoeviden eofa

subja entstellarpopulationasso iatedtothatstream. Thesenegativedete tions,

even in ases where the proje ted positions are favouring the dete tion of Sgr

streamstarsspatially oin identwiththeglobulars(e.g. NGC5053orNGC5634),

mightbeusedtoestablishthelimitationsofourphotometri survey. Indeed,the

absen e of tidal remnants might be related to the evolution of the Sgr dSph

and its intera tion with the Milky Way. A ording to the model of P10, while

Whiting1 and NGC7492 arespatially oin identwith theSgr stars a retedin

thelast0.75Gyr,NGC5053,NGC5634andPal5aresurroundedbythematerial

a reted from the satellite

>

²^Gyr âgo. ^This îs â onsequen e of the fa t that se tionsofthestreamgeneratedalongtimeagoaremoredispersed,withalower

surfa e-brightness,andonlythemostre entarmsoftheSgrtidalstream ouldbe

dete tedbyoursurvey. Thiss enarioisalsovalidforPal12,a lusterpreviously

asso iatedtoSgrbyMartínez-Delgado et al.(2002),whi h inthe ontext ofthe

P10modelseemstobeasso iatedwiththese tionofthestreama retedin the

last0.75Gyr.

Inthebottom panelof Figure5.17we omparethe predi tedradial velo ity

ofthe streamwith those values measuredfor the lusters in oursample (Harris

2010). The globularsthat arekinemati ally ompatible and oin ident withthe

position of the P10 tidalstream are Whiting1, NGC5053, NGC5634 andPal5

(suggestedasmembersoftheSgrGCsystembyL10). Ontheotherhand,thereis

adieren eof

∆ v r ∼ 100

^km^s

⁻¹

ⁱⁿ^the^ase^of^NGC^7492. ^So,^for^this^stellar

sys-tem, lusterandstreamseemtobeindependentsystems,althoughtheorbitand

stru tureoftheSgrstream inthesouthernskyare notwell onstrainedbe ause

of the la k of a deep full-sky photometri database as the one available in the

northernhemisphere(seedis ussioninL10,P10). Furtherfollow-upspe tros opy

isrequired toinvestigate thenature of thestellar population dis overed around

NGC7492.

5.5.2 Other over-densities

The analysis of the CMDs orresponding to the GCs not asso iated with Sgr

suggeststhepresen eofMSfeatureslikelyasso iatedwithsubja entstellar

pop-ulationsin three of them: NGC1261, NGC1851and NGC7006. In this se tion,

wedis ussthepossibleoriginofthesetentativeremnantsandtheirpossible

asso- iationwithotherknownover-densitiesorstellarstreamsalreadyreportedinthe

MilkyWay.

Anextended stellar over-densityaround NGC1851?

Oneof themost onspi uous over-density ofoursurvey,notasso iated withthe

Sgrstream,wasdete ted aroundNGC1851,rst dis overed byOlszewskiet al.

(2009),whointerpretedthisfeatuareasanextendedhalo surroundingthis luster

uptodistan esof75ar min(

∼ 6.5r ^t

⁾^from^the^luster^entre,^andindependently reportedby(Carballo-Bello&Martínez-Delgado2010).

NGC1851 is one of the most interesting andidates in our sample be ause

ofits multiple stellar populations (Milone et al. 2008; Han et al. 2009)and the

well-studied star-to-star abundan e variations (e.g. Milone et al. 2009; Zo ali

etal.2009;Carrettaetal.2010,2011;Campbelletal.2012;Carrettaetal.2012),

whi hsuggest a s enarioin whi h this lusteristheresultofthemergingof two

previousGCs,formed inthenu leusofana reteddwarfgalaxy(Carretta etal.

2010; Bekki & Yong 2012). This luster is member of a group of GCs formed

by NGC1851,NGC1904, NGC2298andNGC2808,whi h seemstobe onned

in a sphere with a radius of 6kp . That spatial distribution resembles that of

M54,Terzan7,Terzan8 andArp2, globularsfoundin themain body oftheSgr

dSph(Bellazzinietal.2004;Martin etal.2004). Inaddition,all4 lusters show

extendedHBmorphologiesin theirCMDs,featurethathasbeensuggestedasan

indi atorofanextra-Gala ti originin GCs(Lee etal.2007).

Figure 5.10 shows the presen e of theprominent MS population in the

sur-roundingsofthis luster, whi h isthe samereportedbyOlszewskiet al.(2009).

This feature is not predi tedby the TRILEGAL or Besançon models and it is

alsodete tedwhen the ross- orrelationmethod is used(Table5.4)at a similar

helio entri distan ethanthe luster. Usinglowresolution spe tra fora sample

of107 starssele ted from thesame photometry presented in this work, Sollima

etal.(2012)dete tedaunexpe teddistin tstellar omponentwitharadial

velo -itydistributionthat annotbeasso iatedneitherwiththeGala ti velo ityeld

norNGC1851outliers,witha meandieren ewithrespe ttothose omponents

∆ v r ∼ 150

^km^s

⁻¹

^and

∆ v r ∼ 200

^km^s

⁻¹

^,respe tively. Theseauthorsdis uss the possible asso iation of this feature with the Mono eros ring, showing that

theobservedvelo itydistributionandthepredi tionmadebytheP05modelfor

that ring-like stru ture are slightly dierent, although not ompletely

in onsis-tentgiventheun ertaintiesintheadoptedGala ti potential. However,are ent

spe tros opi analysisbyMarinoetal.(2014)analysedasetofmedium-resolution

spe tra forasampleofstarsintheouterhalo ofNGC1851reportingthela kof

anysigni antover-densityofstarsatthevelo ityofsu hsupposedstream.

Sum-marizing,with the present dataset it is not learif the dete tedover-density is

linkedtothepre en eofanextendedhalo of lustermemberstars(assuggested

by Olszewskietal.2009)ortoasubja entstream(possiblytheMono erosring).

Deepdataextending over a widerFOV areneeded todistinguishbetweenthese

two hypotheses.

NGC7006

NGC7006 is a luster slightly younger than other similar lusters in the inner

Galaxy(Dotter et al. 2011). In addition, this GC is one of the most energeti

lustersin theMilkyWaywitha very e entri orbit(Dines uet al.2001),

sug-gestingandextra-Gala ti origin forthat system. Figure5.15showthepresen e

of a signi ant MS feature in the outer region of NGC7006 (rst reported in

Martínez-Delgadoetal.2004a). Sin eour ross- orrelationmethodfailstodete t

these features due to the rowding of the elds (this luster is lassied in the

groupC),ourdistan eestimatesareonlybasedoniso hronetting(Table5.3).

Ourresultsshowthat thehypotheti subja ent stellarpopulation isat dierent

distan efrom the luster. Inparti ular, wederiveda dieren eindistan e of

∼

8kp forthispossibletidaldebrisfromthemainbodyofNGC7006. However,an

inspe tionoftheCMDofthis luster(Figure5.15)showsthattheMSTOofthis

featureisseverelyae tedbythepresen eofbrightMilkyWaydis starsat

V ∼

2021,makingthe estimateofitspositionvery un ertain. Therefore,webelieve

thatthispopulationliesata distan e

d ⊙ = 15 − 20

^kp
.

Figure5.19showsastellardensitymapofMS-starsinaregionoftheskyfrom

the SDSS photometri database, whi h in ludes both NGC7006 and NGC7078

(markedasopen ir les). Theseglobularsseemtobeinmersedinaregionofhigh

density of halo stars, that extends up to Gala ti latitudes

b ∼ −40 ^◦

^(see ^also

Deason et al. 2014)and that might bethe best explanation forthe presen e of

thisfeature intheCMDofNGC7006. However,ana uratemodelfortheshape

ofthestellarhalo isneededto onrmthispossibility.

Analternatives enario might bethepresen e ofthe southern omponent of

theHer ulesAquilaover-densityinthepositionsofthis luster. Re entresultsby

Simionetal.(2014)supportsthepresen eofaprominentover-densityofRRLyrae

starsasso iatedtothisvastover-densityinthisregionofthesky,withadistan e

Figure5.19: Densitymapsgeneratedfrom SDSSdataofthesky areawherethe GCs

NGC6229, NGC7006, NGC7078 and NGC7492 are lo ated. The huge stellar

over-densityobservedmightbeasso iatedwhetherwiththeHer ules-Aquila loud(Belokurov

etal.2007a;Simionetal.2014)orwiththeregionofhigherdensityofhalostarsreported

byDeasonetal.(2014). Notethepresen eoftheSgrstreaminthebottomleft orner

ofthemap.

rangeof

10 < d ⊙ < 25

^kp^(see ^their ^Fig. ^9), strengthing thehypothesis of its originfrom thetidaldisruptionof anan ientdwarf galaxy. That distan erange

is ompatiblewiththeone derivedfrom ourCMDsand suggeststhat NGC7006

mightbewellembeddedin(andpossiblyasso iatedto)thisgiant loudofdebris.

Interestingly, Simion et al. (2014) also found that the Her ulesAquila loud is

barelyvisibleasaRR Lyraeover-densityinthenorthernhemisphere,suggesting

thatthis loudispossiblynotsymmetri withrespe ttotheGala ti plane. This

is onsistentwiththelowsigni an eoverdensity(

S < 2.5

⁾^of^this^omponentⁱⁿ

thesurroundingsofNGC6229(seeFigure 5.14).

NGC1261

NGC1261,liesinaproje tedpositionalignedwithtwoothermassiveGCs

show-inganextended-HBmorphologyintheirCMDs,NGC1851andNGC1904. Around

this lusterwehaveunveiledastellarpopulation(seeFigure5.10)thatstandsout

signi antlywhentheba kgrounddiagram is omparedwiththeones generated

withthe onsideredGala ti modelsanditisalsoapparentintheresultsobtained

throughthe ross- orrelationmethod(seeTable5.4). Theradialdistan e tothe

underlying omponentissimilartothatofthe luster,suggestingthateitheritis

omposedof luster membersor ofanunknownstellarpopulation. Thepossible

relationwiththegroupof lustersdes ribedinSe tion5.5.2en ouragestoexplore

thearea betweenNGC1261andthose GCs.

5.5.3 Negative dete tions

Therearenosignaturesofthepresen eofsigni antsubja entpopulationsaround

the remaining andidates (AM4, NGC1904, NGC2298, NGC4590, NGC5024,

NGC5272, NGC5466, NGC5694, NGC5824, NGC6229, NGC6864, NGC7078,

Pal15 and Rup106) as we nd no eviden e of distin t stellar population

on- entratedat a spe i distan e within theprobed olour-magnitude rangeusing

both the ross- orrelation and the iso hrone tting methods. The photometri

non-dete tionoftidal debrisaroundthe haloGCs in this studyis animportant

result to onsider in the ontext of hierar hi al stellar halo assembly theories.

Whether or not su h non-dete tions an rule out an a retion origin for these

GCs(andaportionoftheMilkyWaystellarhalo)dependsontwo mainfa tors:

1)howmassiveweretheprogenitordwarfgalaxiestheseGCswerea retedwith,

and 2) when were these dwarf galaxies and their GCs a reted into the Milky

Way? Indeed, GCs hosted in lowluminosity dwarfs whi h were a reted early,

mayshowminimalasso iatedstellardebriswhenobservedatpresent.

In document Title: Structure and substructure in the stellar halo of the Milky Way (pagina 136-151)

r bg

r bg

r bg

r bg

r bg

r bg

r bg

r bg

r bg

r bg

r bg

r bg

d ⊙ <

∼ 1

S ∼ 8

α

d ⊙ ∼

∼ 15

∼

S < 5

S/N

r bg

∼ 14

t accr < 0.25

>

∆ v r ∼ 100

−1

∼ 6.5r t

∆ v r ∼ 150

−1

∆ v r ∼ 200

−1

∼

V ∼

d ⊙ = 15 − 20

b ∼ −40 ◦

10 < d ⊙ < 25

S < 2.5

d _⊙ <

⁻¹

∼ 6.5r ^t

⁻¹

⁻¹

b ∼ −40 ^◦