Survey and stellar atalogues

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.

of imaging 1500 square degrees of extragala ti sky in four opti al broad-band

lters (

u

^,

g

^,

r

^and

i

^{). The survey area is split between two elds, one in the}

northernandone inthesouthernGala ti ap. Aimedprimarily at onstraining

thedark matterdistribution in theuniverse through weakgravitationallensing,

KiDS delivers deep and high quality images with typi al image quality (PSF

FWHM) ranging from

0 .7 ^′′

ⁱⁿ

r

^to

1 .1 ^′′

ⁱⁿ

u

^{. KiDS limiting magnitudes are}

approximately2magnitudesfainterthanthoseofSDSS:

24 .3

ⁱⁿ

u

^,

25 .1

ⁱⁿ

g

^,

24 .9

in

r

^and

23 .7

ⁱⁿ

i

^.

Theindividualpointingsare1

×

¹

deg ²

. Theyarebuiltupfromfour(

u

^)orve

(

gri

^{)ditherstollinter-CCDgaps,withea hpositionvisitedon eforea hlter,}

yielding the nal survey depth in one go. During the rst years of operation,

the GalaxyAnd Mass Assembly (GAMA, Driver et al. 2011) elds have been

prioritizedtomaximizethesynergywiththesedeepspe tros opi data.

The KiDS data used in this work are from the rst and se ond publi data

releases. Figure3.1 showsthelo ation ofthein ludedelds onthe skyandthe

planned nal overage. A detailed des ription of thedata redu tion isprovided

in therelease notes oron theKiDS website 1

, but here followsa brief summary.

Following ross-talk orre tion, satellite tra k removaland at-elding, an

illu-mination orre tion is appliedin orderto atten the photometryover the

eld-of-view. Thephotometri alibrationisbasedonnightlyzeropoints,after whi h

theoverlapsbetweenCCDsfromthedierentditherpositionsareusedtotieall

CCDsanddither together,resultingin photometry onsistent tothe12% level

overthefulleld-of-view. Smallabsolutezeropointosetsbetweenpointings

per-sistsin ethe pat hy distribution of thein luded surveytiles yet preventsa full

ross- alibration. Aftersolvingfortheastrometri solutionofallditherstogether,

sta ked images are produ ed togetherwith weightmaps, masksfor bright stars

andotherimagedefe ts,andsour e atalogs. Thesta kedimagesprovidedinthe

publi datareleases formtheinputforourfurtheranalysis.

We arry outa point-spread fun tion(PSF)homegenization a ross ea h

im-age.Thishomogenizationprovidesimprovedxedaperturephotometry, orre ted

shape measurements and, indire tly, rened olours and enhan ed star-galaxy

separation. The odeforthePSFhomogenizationdes ribedin(Pila-Díezetal.

2014) measures the shapes of the bright stars a ross a given image, maps the

varying PSF and nally onvolves the map with a spatially variable kernel in

ordertoreturngaussianPSFs.

Fromtheseimages,photometri "GaussianApertureandPSF(GAaP)"

ata-loguesareprodu edusingGaussianapertureweightfun tions

ω ² ·exp[−r ² /2(ω ² − p ² )] /(ω ² −p ² )

atthepositionsofSExtra tor-dete tedsour es. Aslongasthe aper-tureradius

ω

^{islargerthantheGaussianPSFdispersion}

p

^{,thisaperturefun tion}

yields the orre t total ux for isolated point sour es (irrespe tive of

ω

^{). For}

extendedsour es,however,these apertureuxesin reasewith

ω

^.

1

http://kids.strw.leidenuniv.nl/DR2

Figure 3.1: Equatorial map showing the position of all theKiDS elds used in

this work (Data Releases 1 and 2). The dierent olours indi ate the lines of

sight in whi h the elds have been grouped to al ulate the dierent density

proles. The ba kground image is the SDSS-DR8 density map from Koposov

etal.(2012),whi hshowsthefootprintoftheSagittariusstreamandthelo ation

fotheSagittariusdwarfgalaxy. Theredlinedenotestheexpe tedKiDSfootprint

upon ompletion ofthesurvey.

3.2.2 Catalogues

Starsandgalaxiesareseparatedbymeasuringtheiruxat

ω = 0.5”

^and

0 .7”

aper-turesintherband,andkeepingonlythosesour eswith

F 0.5 /F 0.7 ∈ [0.975, 1.025]

^,

that aredete ted at

> 5σ

signi an e. A stellar ompleteness limitof

r = 23.2

magisimposedtoavoid ontaminationbythesmall,round,faintergalaxies. And

dupli ateobje tsonoverlappingtilesareremoved.

The apparent magnitudes are dereddened using the interstellar extin tion

maps from S hlegel et al. (1998), and the GAaP photometry is orre ted for

a tile-based seeing dependen y dete ted on the KiDS-to-SDSS osets (see

Fig-ure 3.2). This dependen y, a sign of residual ux at large radii after the PSF

Gaussianization, is orre ted through dire t measurements (if the tile overlaps

with SDSS data) or through a lter-spe i interpolation. On e orre ted for

theseosets,GAaPprovidesamu hsmootherandatterphotometryona

star-by-star a ountthanother photometrymeasuringtools,sin eit orre tsforthe

PSFvariationa rossea heldofview. LastwetransformtheKiDSmagnitudes

totheSDSSsystembyappyingthe olourterms:

u SDSS = u KiDS + 0 .053 · (u ^KiDS − g ^KiDS ) − 0.0028 · (g ^KiDS − r ^KiDS ) g SDSS = g KiDS + 0 .053 · (g ^KiDS − r ^KiDS )

r SDSS = r KiDS + 0 .336 · (g ^KiDS − r ^KiDS )

i SDSS = i KiDS − 0.012 · (r ^KiDS − i ^KiDS ) − 0.0004 · (g ^KiDS − r ^KiDS )

Table3.1: GroupsofpointingsofKiDSasshowninFigures3.1,3.4,3.5aand3.6a.

Thetableshowsthe entral oordinatesforea hgroup,thenumberofindividual

eldsof view ontributingto it, itstotalarea andthestellar ompletenesslimit

intherband.

GroupKiDS- RA(deg) De (deg)

l

^(deg)

b

^(deg)

n _fields Σ

^(deg

²

^{) mag}

_lim,r,∗

North220W 233.395758 0.12301 4.996485 42.63794 5 5.55 23.2

North220E 218.282384 -0.30425 348.794941 53.26131 24 24.45 23.2

North180W 183.806111 -0.02812 283.687780 61.48308 18 18.23 23.2

North180E 176.151782 -0.38023 269.928812 58.14329 19 18.29 23.2

North135W 138.175020 0.10505 230.730761 31.01870 17 18.49 23.2

North135E 131.554071 0.76375 226.214265 25.68091 20 19.78 23.2

South45W 46.602118 -32.24177 231.091424 -60.36720 8 9.68 23.2

South45E 35.744934 -31.99697 232.203287 -69.55028 6 5.52 23.2

South-15W 351.902387 -31.61901 14.081849 -71.22605 4 4.93 23.2

South-15E 342.302882 -31.81892 15.507201 -63.07876 12 13.29 23.2

Thenalphotometryfollowstheexpe ted olour- olourstellarlo ifromCovey

etal.(2007)(seeFigure3.3).

FinallytheKiDS eldsaresplitintenbroadlinesofsightbasedontheirsky

distribution (seeFigure 3.1). Thenumber ofelds per line of sight, the entral

oordinatesoftheselinesofsightandtheirae tiveareaarere ordedinTable3.1.

Theee tive areas havebeen al ulated basedon a nely spatially binned grid

wherewe ountthenumber ofstarsper binas awayto determinethe ombined

ee tofmasksandweightsandtilesedgesoverthenal atalogue ofstars.

In document Title: Structure and substructure in the stellar halo of the Milky Way (pagina 57-60)