Con lusions

Theoverdensity(s) in KiDS-South-15Esimilarlyextendfrom

R GC ≈ 15

^kp^out

55

kp ,but displays a lessstrongly peakeddistribution and, potentially, two

possible rests. Theseresidualsarelesssigni antthanthoseinKiDS-South-15E,

withthehighestoverdensitylevelrea hingafa torof

4 ⁺³

−2

Amoreextensiveinvestigationofalloverdensities(expe tedandunexpe ted),

usingadditional tools otherthan stellar density proles, isplanned fora future

publi ation.

thein lusionoflargesubstru turessu hastheSagittariusstreaminhalots an

signi antlyinuen ethere overedparameters. Basedontheex lusionofknown

substru ture,togetherwiththe onstrainingpowerofourdataatlargedistan es,

we on lude that the smooth outer stellar halo has a power law index lose to

−4.6

We havefound a disk axis ratio of

w = 0.94 ± 0.05

^, ^suggesting ^a ^very ^mild

triaxiality. Wedonottesttriaxialityin ombinationwiththebrokenpower law

to avoid overparametrization problems. Few other works have reported on the

triaxiality of the halo, and those who did agreed on

w > 0.8

^. ^We ^have ^also

found a polar axis ratio of

q = 0.77 ± 0.05

^, ^where ^most ^of ^the ^previous ^works

found

0 .55 ≤ q ≤ 0.70

^and^some

q ≤ 0.8

^. ^Overall ^there^seems^to^be^a ^signi
ant

onsensusonthestellarhalo beingmoderatelytoquiteoblate.

Wehavebeenabletore over a number ofknown stellaroverdensitiesin our

data-to-modelresiduals. Parti ularly,we learlyre overastrongsignalmat hing

the Sagittariusstream in the KiDS-North220E and KiDS-South-15E regions, a

moremoderatesignal in KiDS-South-15Wanda possible mat h(yetweak

over-density) in KiDS-North220W. Similarly, we re over overdensities mat hing the

lo ationandextentoftheVirgoOverdensityintheKiDS-North180Wand

KiDS-North180E regions; in these two regions the overdensities seem to fade slowly,

indi atingeither thatthefeatureextendsfurther outthanpreviouslythoughtor

thatitslowlyblendswithasmoothhalothatdoesnotexa tly followthemodels

attheselo ations. Finally,wealsoseemtondoverdensitiesinthealready

men-tionedregionsofKiDS-South-15EandKiDS-South-15Watfurtherdistan esthan

those expe ted forthe Sagittarius stream; this is suggestive of a previously

un-knownstru tureoradepartureoftheSagittariusstreamfromthemodels. These

overdensities will be further explored in future work, with toolsmore powerful

thandensityprolesfor hara terizingsubstru turesu h as Colour Magnitude

Diagramsormat hedlters.

Finding halo streams with a

pen il-beam survey new

wraps in the Sagittarius

stream

Authors

B.Pila-Díez,K.Kuijken,J.T.A.deJong,H.HoekstraandR.F.J.vanderBurg

Abstra t

Weusedatafrom twoCFHT-MegaCam photometri pen il-beamsurveys inthe

g'andther'bandstomeasuredistan estotheSagittarius,thePalomar5andthe

Orphanstream. Weshow that,using a ross- orrelationalgorithmto dete tthe

turnopointofthemain sequen e,itispossible toover omethemainlimitation

of a two-bands pen il-beam survey, namely the la k of adja ent ontrol-elds

that an be used to subtra t the foreground and ba kground stars to enhan e

the signal on the olour-magnitude diagrams (CMDs). We des ribe the

ross- orrelation algorithm and its implementation. We ombine the resulting main

sequen eturnopointswiththeoreti aliso hronestoderivea uratephotometri

distan es to the streams. Ourresults onrm the ndings by previous studies,

expandthedistan etrendfortheSagittariusfaintsouthernbran handtra ethe

Sagittariusfaintbran h ofthenorthern-leadingarm outto56kp . Inaddition,

they show eviden e for nearby substru tures. We argue that these dete tions

tra ethe ontinuation oftheSagittariusnorthern-leading armintothesouthern

hemisphere,andthenorthernwrap oftheSagittariustrailingarm.

PublishedinAstronomy&Astrophysi s Volume564,A18(2014)

Preprintin arXiv:1311.7580[astro-ph.GA℄

4.1 Introdu tion

Inthepast de ade ourpi ture ofthe MilkyWay's stellar halo hasdramati ally

hangedthankstotheadventofseveralobservationalsurveys,whi hhaveshown

theri hnessand omplexityofthesubstru tureintheGala ti halo (Ibataetal.

2001b;Newbergetal.2002;Majewskietal.2003;Yannyetal.2003;Martinetal.

2004;Grillmair&Dionatos2006b;Belokurovetal.2006b,2007b, ). OurGalaxy

is still undergoing an assembling pro ess, where part of the infalling material

hasalready been a reted and be ome dynami ally relaxed (Helmi et al. 1999;

Sheeldet al.2012), part of it is still dynami ally old (Bell et al.2008; Juri¢

et al.2008)and anotherpart is in thepro essof beingdynami allystrippedor

evenapproa hingitsrstdynami alen ounterwithourGalaxy(Kallivayaliletal.

2006b,a;Piatek etal.2008;Beslaet al.2010;Ro haetal.2012).

Themostprominentexampleofa urrentlyongoingdisruptionisthat ofthe

Sagittariusstream (Sgrstream). Sin e itsdis overy in1996(Mateoet al.1996),

thestream hasbeen mapped over roughly

2 π

^radians ^on ^the^sky, ^rst ^through

2MASS(Majewskietal.2003)and laterthroughSDSS (Belokurovetal.2006b;

Koposovetal.2012). Thereisgeneralagreementthatitisthestellardebrisofa

disruptingsatellitegalaxy,theSagittariusdwarfgalaxy(Ibataetal.1994),whi h

is urrently being a reted by the Milky Way (Velazquez & White 1995; Ibata

etal.1997;Niederste-Ostholtetal.2010). Thestreamis omposedoftheleading

andthetrailingtailsofthisdisruptionevent(Mateoetal.1996;Ibataetal.2001b;

Dohm-Palmeret al. 2001; Martínez-Delgado et al.2001, 2004b; Majewski et al.

2003;Belokurovetal.2006b,2014),whi hwrapatleaston earoundtheGalaxy

buthavebeenpredi tedtowrapmore thanon e(Peñarrubiaet al.2010;Law&

Majewski2010b). Inaddition,a bifur ationandwhatresemblesanextrabran h

parallelto the main omponent ofthe Sgrstream havebeendis overed bothin

thenorthernhemisphere(Belokurovetal.2006b)andinthesouthernhemisphere

(Koposovetal.2012). Theoriginofthisbifur ationandthemeaningofthetwo

bran hesarestilldebated: they ouldrepresentwraps ofdierentage(Fellhauer

etal.2006),they ouldhavearisenduetotheinternaldynami softheprogenitor

(Peñarrubiaetal.2010,2011)orthey ouldindeedbeduetodierentprogenitors

andamultiple a retionevent(Koposovet al.2012).

Ontheother hand,oneof thesimplestand neatestexamples ofa disrupting

satellite is that of the Palomar 5 globular luster (Sandage & Hartwi k 1977;

Odenkir henet al.2002;Dehnenet al.2004)andits stream(Odenkir henet al.

2001,2003). This streamextends over

20 ^o

alongitsnarrowleading andtrailing tails. It displays an inhomogeneous stellar density in what resembles gaps or

underdensities(Grillmair&Dionatos2006a);theoriginofthisstellardistribution

hasbeenattributedbothtointera tionswithdark satellites(Carlberg2012)and

toepi y li motionsof starsalongthetails(Mastrobuono-Battisti etal.2012).

Finally, there are also ases of streams with unknown progenitors, su h as

the so- alled Orphan stream (Grillmair 2006a; Belokurov et al. 2006b, 2007b;

Newberg et al. 2010). This stream extends for

50 ^o

in the North gala ti ap,

andthe hemi alsignatures from re ent spe tros opi observationsasso iateits

progenitor with a dwarf galaxy (Casey et al. 2013b,a). A number of plausible

progenitorshavebeensuggested(Zu keretal.2006;Fellhaueretal.2007b;Jin&

Lynden-Bell2007;Salesetal.2008),butitisstillpossiblethatthetrueprogenitor

remainsundis overed inthesouthernhemisphere(Caseyet al.2013b).

Ingeneral,thedis overyofmostofthesubstru tures inthehalooftheMilky

Wayhasbeenpossiblethankstophotometri multi- olourwideareasurveys. Su h

surveysposeseveraladvantagesforthiskindofsear h. First,theirmultiple-band

photometryallowsforstellarpopulationsele tions(haloorthi kdisk;red lump,

main sequen e turno point, et .) based on olour- olour stellar lo i. These

sele tion riteria anbeusedtomakestellardensitymapsthattra kthestreams

all through the survey's overage area (Majewski et al. 2003; Belokurov et al.

2006b). Se ond,their ontinuous overageofalargeareaallowtheeldsadja ent

to the substru ture to a t as ontrol elds. In this way, the olour-magnitude

diagrams (CMDs) of the ontrol elds an be used to statisti ally subtra t the

foreground and the ba kground stars from the elds probing the substru ture.

Thisenhan esthesignatureofthestellar population belongingtothestreamor

satellite(byremovingthenoise),andmakesitpossibletoidentifyageanddistan e

indi atorssu has the red lumpor themain sequen e turno point(Belokurov

etal.2006b;Koposovetal.2012;Slateret al.2013).

Inthispaper weexplorethepossibilitiesofusingdeeptwo-bandpen il-beam

surveys instead of the usual wide-area multi- olour surveys in order to dete t

and hara terize stellar streams of the halo and, in parti ular, we revisit the

Sagittarius, the Palomar 5 and the Orphan streams. We derive photometri

distan esusingpurelythemainsequen eturnopointandunlikeotherworks

regardlessofthegiantbran h anditsred lump.

In document Title: Structure and substructure in the stellar halo of the Milky Way (pagina 79-84)