Con lusions - Title: Structure and substructure in the stellar halo of the Milky Way

aremaskedfrom ourtsandresidualswhenweimpose

|z| > 10

^kp^to^avoid^the

inuen eofthethi kdisk,andtherefore,they annotbedete ted.

TheTriangulum-Andromeda overdensity((Martin et al. 2007)) falls lose to

oneoftheeldsingroupF.Despitethisproximity,theresidualsshownoeviden e

foranoverdensityattheexpe teddistan eof

R GC ≈ 30

^kp
,^indi
ating^that^the

overdensitydoesnotextendfurtherin thisdire tion.

Comparingourdensityprolestothesmoothmodelts,were over the

pres-en eoftheSagittariusstream ingroupsGandH. TheSagittariusstream inthe

dire tion ofgroup H seemsto extendfurther out from theGala ti entre than

themodelshaveso farpredi ted,and onrmsprevious RRLyraedete tions

as-so iated withthestreamat su h distan es(Ibata et al.(2001 ), Totten&Irwin

(1998) and Dohm-Palmer et al. (2001)). We also nd eviden e of more modest

substru turesextendingoveralongrangeofdistan esingroupC(

35 ≤ R GC ≤ 60

kp ) andquite on entratedindistan ein groupD (

R GC ≈ 20

^kp
).

Ourpen ilbeamsurveyhasdemonstratedthatevenarelativelysmallnumbers

ofnarroweldsofview,providedtheyaresampledsu ientlydeepandwithan

abundant tra er, an pla e ompetitivelimits on the global density prole and

shape of the Gala ti halo. The advent of similarly deep, wide-area surveys

-like KiDS,VIKING andLSST- therefore promisesto enhan e substantiallyour

understandingofthehalo.

A KiDS view on the stru ture

of the Gala ti halo

Authors

B.Pila-Díez,J.T.A.deJongandK.Kuijken

Abstra t

We study the density prole and shape of the Gala ti halo using deep

multi- olourimagesfromtheKiloDegreeSurvey(KiDS)ontheVLTSurveyTeles ope,

in ombinationwithpreviousMENeaCSandCCCPsurveystostrengthenthe

on-strainingpowerthroughmultiplelinesofsight. Thesigni antphotometri depth

andtheSouthernlinesofsightofKiDSallowustoprobenewregionsofthehalo.

We build atalogues of near Main Sequen e Turno point stars by

homogeniz-ingthePSFoftheimages,separating starsfromgalaxiesthroughxed-aperture

photometryandusingamulti oloursele tionforhaloFstars. We al ulate

gala -to entri distan esforthese starsandbuilddensityprolesalongseverallinesof

sightoutto60kp . Wethenaddlinesofsightfromourearlieranalysisofthe

ME-NeaCSandtheCCCPsurveys,andtglobalhalomodelstothedensityproles.

Wendthatthestellarhaloisbestdes ribedbyapowerlawmodelwithabreak

inthepower index,lo atedwithin

[22 , 30)

^kp
. ^Theînner^power^lawîndexîs

de-pendentonthevalueofthebreakdistan e(

n in = [ −3.30, −3.90) ± 0.05

^),^whereas

theouter power law index is quite stable (

n in = −4.6 ± 0.1

^). ^We^nd ^a ^polar

axisratioof

q = 0.77 ± 0.05

^,ⁱⁿâgreement^with^previous^works,ândânîndi
ation

ofa very mildtriaxiality

w = 0.94 ± 0.05

^. ^We^also^re
overdata-to-model devia-tionsmat hingpreviouslyknownoverdensitiessu hastheSagittariusstreamand

theVirgoOverdensity,andpossibleextensionsoftheseorothersubstru tures to

greaterdistan esthanexpe ted.

3.1 Introdu tion

Hierar hi algalaxyformationisanatural onsequen eofthe urrent osmologi al

model. Manyoftheoldeststarsareexpe tedtohaveformedinsmallgalaxies,long

beforetheywereswallowedupintolargerobje ts,andsomeareexpe tedtohave

formedin-situorevaporatedfromearlylow-massglobular lusters. Consequently,

the oldest stars provide a fossil re ord of the beginnings of galaxy formation.

Eventhoughthesestarsonly ompriseatinyfra tionofthemassofpresent-day

galaxies,theirstudyisthereforeworthwhile. TheGalaxyisauniqueenvironment

in whi h to tra e old stars, be ause stellar populations an beisolated

star-by-star,eveninregionswherethesurfa ebrightnessiswellbelowobservationallimits

inexternalgalaxies.

Extensivestudies ofthestardistribution in theGala ti outskirts havebeen

undertaken(mostly in the Northern hemisphere) and have revealed a wealth of

substru tureintheformofstreamsandsatellites leareviden eofpasta retion

andmerger pro esses(York et al.(2000); Ahnet al.(2014) and Skrutskieet al.

(2006)). Also, the overall stru tural parameters of the stellar halo have been

measured, yielding a pi ture of an ellipsoidal distribution with a radial density

proleintheformofabrokenpower law. Asdatahaveimproved,ithasbe ome

possible to probethe halo with fainter, more abundantstars, lose to themain

sequen e turno: a signi ant advan e over older studies based on horizontal

bran horredgiantstars.

In this paper we usedeep photometry from theKilo DegreeSurvey (KiDS)

obtainedwiththeVLTSurveyTeles ope(VST)anditswide-eld amera

Omega-CAMto probemain sequen eturn-o(MSTO)stars outto distan esof60kp .

TheKiDS maindistinguishingattributes arebothitsfootprinton theSouthern

skyanditsoutstanding depth, wi h makeit ompetitivein termsofthedistant

andfaint orthe otherwiseun harted halo. Wesplit the urrentKiDS datainto

tenbroad linesof sightthrough theGala ti halo to onstrainitsgeneralstellar

stru tureandprobepotentialsubstru ture. We ombinethemwithpreviousdata

fromtheCanada-Fran e-HawaiiTeles ope(CFHT)MegaCamandtheWideField

Camera(WFC)attheIsaa NewtonTeles ope(INT)providingeightadditional

linesofsightin orderto in reaseits onstrainingpower(Pila-Díezet al.2015).

Inse tion2wedes ribetheobservationsandthedatapro essingrelevanttoour

stellar atalogues and to ouranalysis. Inse tion 3 we over the star sele tion,

thebuildingofthedensityprolesandthesmoothhalomodelsthatwe onsider.

Finally,in se tion4wedis ussandgive ontexttoourndings,andinse tion5

wesummarizeour on lusions.

In document Title: Structure and substructure in the stellar halo of the Milky Way (pagina 54-57)