Observations and data pro essing

2.2.1 Survey and observations

Weuse

g

^and

r

^{images from theMENeaCSand theCCCPsurveys(Sand et al.}

2012; Hoekstra et al. 2012; Bildfell et al. 2012) together with several ar hival

−50 0

50 100

150 200

250 RA (degrees)

−40

−20 0 20 40 60 80

D EC ( de gr ee s) A

B C

D

E

G F H

Figure2.1: Equatorialmapshowingthepositionofalltheeldsusedinthiswork.

Thedierent oloursand symbols indi ate how theelds havebeengrouped to

al ulatethedierentdensity proles. The ba kgroundimage isthe SDSS-DR8

map from Koposov et al. (2012), whi h shows the footprint of the Sagittarius

stream and the lo ation of the Sagittarius dwarf galaxy. When grouping the

elds,wehavealsotakenintoa ountthepresen eofthisstream,theT

riangulum-Andromedaoverdensity,andtheanti entresubstru tures(ACS,EBS,and

Mono- eros),intrying to ombine theiree t in ertainprolesandavoiditin others.

lustereldsfromtheCFHT-MegaCaminstrument. We ombinethesedatawith

U

^and

i

^{imagesfrom a follow-up ampaign withtheINT-WFC instrument(van}

derBurgetal.,inprep.). Whereasthesesurveystargetedapresele tedsampleof

galaxy lusters,thepointings onstitutea"blind"surveyoftheMilkyWaystellar

halosin etheirdistribution is ompletelyindependentofanypriorknowledgeof

thehalo's stru tureandsubstru ture.

Our pointings are distributed over the region of the sky visible to both the

CFHT and the INT (see Figure 2.1). To optimize the star-galaxy separation

(see se tion 2.2.2) we restri t our analysis to exposures with image quality of

subar se ond seeing, typi ally

<≈ 0.9 arcsec

^{in the}

r

^{band. This} limitation, ombinedwith thevarying elds of view and observing onditions between the

datasets,leadstopointingfootprintsizesthatrangebetween

0.24

^and

1.14 deg ²

^.

2.2.2 Image orre tion of the PSF distortion [and

impli a-tions for the star-galaxy separation℄

Previousresear hbyourgrouphasshownthattheperforman eofstandard

star-galaxyseparationmethodsbasedonthesizeandellipti ityofthesour es anbe

improvedbyhomogenizingthepoint-spreadfun tion(PSF)a rossanimageprior

toitsphotometri analysis(Pila-Díezet al.2014). Inaddition,su h a orre tion

alsoprovidesthebenetofallowingustoperformxedaperturephotometryand

olourmeasurements.

InordertohomogenizethePSFofourimages,weusea ode(Pila-Díezetal.

2014)that,asarststep,takestheshapeofthebrightstarsinagivenimageand

usesittomapthevaryingPSFand, asase ondstep, onvolvesthismapwitha

spatiallyvariable kerneldesignedto transformeverywheretheoriginalPSFinto

agaussianPSF.

2.2.3 Catalogues

From the PSF-homogenized exposures we reate photometri atalogues using

Sour eExtra tor (Bertin &Arnouts 1996). For the

g

^{and the}

r

^{data, we sta k}

thedierentexposures in ea h band to reatea single alibratedimage, and we

extra t the band atalogues from them. We perform a star-galaxy separation

based on the brightness, size and ellipti ity of the sour es and we mat h the

surviving sour es in the two atalogues to produ e a

gr

^{- atalogue of stars for}

ea held ofview (seePila-Díezetal.(2014)). Thelimitingmagnitudesof these

gr

^{star atalogues rea h}

m AB ∼ 25.0

^atthe

5 .0σ

^{levelin the}

r

^band.

Forthe

U

^andthe

i

^{eldsofview,weprodu eseveral}photometri atalogues, oneforea hindividualexposure. We orre tthemagnitudesinthe

i

^ataloguesfor

thedependen yoftheilluminationonpixelposition. Forea hpointingandband,

theexposure ataloguesare alibratedto a ommonzeropointand ombinedto

produ e a single-band atalogue. Inthese single-band atalogues, the resulting

magnitudeforea h sour eis al ulatedas themedianof the ontributionsofall

theindividualexposures. Atthispointthe

U

^andthe

i

^{magnitudesare onverted}

from the INT to the CFHT photometri system using the following equations,

whi hwederiveby alibratingourmixedINT-CFHT olourstothe olourstellar

lo ioftheCFHTLega ySurvey(Erbenetal.(2009),Hildebrandt etal.(2009)):

i M egaCam = i IN T − 0.12 ∗ (r M ega − i IN T )

(2.1)

u M egaCam = u IN T − 0.15 ∗ (u IN T − g M ega ) .

^(2.2)

Finallyweposition-mat hthesour esfromthe

U

^-,the

i

^-andthe

gr

- atalogues to reateanal atalogueofstellarsour esforea heldofview. Thesenal

ugri

- ataloguesareshallowerthanthe

gr

- ataloguesbe auseofthelesserdepthofthe

i

^andthe

U

observations(seeTable2.1). Figure2.2showsthe olour-magnitude diagrams (CMDs) for the nal

ugri

^and

gr

^{atalogues (top and entre,}

respe -tively),andthedieren ebetweenthem(bottom). Thebottompanelhighlights

that,inthe olourregimeofthehalo(

0 .2 < g − r < 0.3

^),the ombinationofthe fourbandsremovesmainlyveryfaint,unresolvedgalaxies.

We orre tforinterstellarextin tionusingthemapsfromS hlegeletal.(1998)

andtransform themagnitudes in the

ugri

^{-stellar atalogues from theCFHT to}

theSDSS photometri system. For this we use the equations on the Canadian

Table2.1: GroupsofpointingsasshowninFigures2.1,2.5,2.6and2.8. Thetable

showsthe entral oordinates for ea h group,the number of individual elds of

view ontributingto it, itstotalarea and thestellar ompleteness limitin ther

band.

Group RA(deg) De (deg)

l

^(deg)

b

^(deg)

n fields Σ

^(deg

²

^{) mag}

_lim,r,∗

A 160.654338 43.98310 171.335811 59.15040 8 5.60 22.8

B 231.593130 29.13513 45.577138 55.93598 5 3.98 22.7

C 229.347757 6.91624 9.425402 49.92775 4 3.44 24.1

D 210.062933 51.67173 99.735627 62.24580 2 0.64 23.4

E 121.918411 41.20348 179.233500 31.26694 5 2.73 22.7

F 342.735895 17.09581 86.019738 -36.99391 3 2.17 23.2

G 157.028363 17.15674 222.142793 55.48268 3 2.02 23.1

H 220.659749 2.00187 354.337092 53.38989 3 2.04 24.2

AstronomyData CenterMegaCamwebsite 1

u M egaCam = u SDSS − 0.241 · (u SDSS − g SDSS )

(2.3)

g M egaCam = g SDSS − 0.153 · (g SDSS − r SDSS )

(2.4)

r M egaCam = r SDSS − 0.024 · (g SDSS − r SDSS )

(2.5)

i M egaCam = i SDSS − 0.003 · (r SDSS − i SDSS )

(2.6) andinvertthemtoturn ourmeasurementsintoSDSSmagnitudes. Subsequently

we alibrate ea helddire tlytoSDSSusingstellarphotometryfromDR8. The

resultingphotometrymat hesthe olour- olourstellarlo iofCoveyetal.(2007)

asshownin Figure2.3. Unlessexpli itlystatedotherwise,allmagnitudesin this

paper areexpressedin theSDSSsystem.

Inordertoredu ethenoisewhenanalysingtheradialstellardensity

distribu-tionofthehalo,we ombinethe ataloguesfromnearbypointings,groupingthem

a ordingto theirposition in thesky. Thisstepisimportantbe auseofthe

na-tureofoursurvey,whi his omposedofrelativelysmall,s atteredeldsofview.

Weusea friends-of-friends(FoF)algorithmto groupthedierentpointings. We

requesttwofriendsnotto beapartbymore than20degrees,andin a few ases

we leanorsplitaresultinggroup(redpentagonsorblueandorangetrianglesin

Figure2.1) or ombineothers (purple diamonds)to a ountforthepositions of

thegala ti diskor majorhalosubstru tures. Be ause thedierentpointingsin

oursurveys havedierent ompletenesslimits,these groupedor ombined

ata-logueswhi hwenameA,B,C,... Harenallylteredtomeetthe ompleteness

magnitudethresholdoftheirmostrestri tive ontributor 2

.

1

www2. ad - da.hia-iha.nr - nr .g . a/megapipe/do s/lters.html

2

Todeterminethe ompletenesslimitofea heldofview,wetitsmagnitudedistribution

to a gaussian representing thepopulation of faint galaxies and another variable fun tion

representingthe stellar distribution alongthe whole magnituderange. We hoose as the

ompletenesslimiteitherthetransitionpointbetween thetwodistributions(thevalley)or,if

insteadthereisaplateau,theturningpointofthewholedistribution(theknee).

Figure2.2: Hessdiagrams showingthe number ofsour esper olour-magnitude

binin the

ugri

^{atalogue (top), in the}

gr

^{atalogue ( entre) and the dieren e}

betweenboth(bottom)foreld A1033. Mostofthesour eslostwhen ombining

the atalogues orrespondtofaintmagnitudes,be ausethe

i

^andthe

U

observa-tionsareshallower. Theee tistheremovalofmostofthefaintgalaxies(lo ated

in the

−0.2 < g − r < 0.7

^and

r > 23

^{region in the entral panel), mostof the}

faintestdiskMdwarves(

1 .1 < g − r < 1.3

^{)andanumber offaintobje ts(inthe}

i

^orthe

U

^{bands)s atteredthroughoutthe}

( g − r, r)

^diagram.

Figure2.3: Colour- olourdiagrams(CCDs) orrespondingto theelds ingroup

A(pointingsmarkedaslightgreen ir lesinFigure2.1). Thesour esinthe

ugri

atalogues(bla k)andthesubsetofnear-MSTOstars(red)havebeen alibrated

to SDSS using DR8 stellar photometry. The main sequen e stellar lo i (green

dashedlines)aretheonesgiveninTables3and4ofCoveyetal.(2007). Quasars

andwhitedwarf-Mdwarfpairsareabundantinthe

u − g < 1

^,

−0.3 < g − r < 0.7

spa e.

0.0 0.2 0.4 0.6 0.8 1.0

g - i

−10

−5 0 5 10

M ag r

−7 −6 −5 −4 −3 −2 −1 0 1

[Fe/H]

−10

−5 0 5 10

M ag r

Figure 2.4: Estimated absolute magnitude in the

r

^{band (}

M r

^{) and estimated}

metalli ity(

[ F e/H]

^{)forgroupAforthesour estypi ally onsideredashalostars}

(blue)and those that we havesele ted as near-MSTO stars(red). The sour es

sele tedas halo members meet

0 .2 < g − r < 0.3

^and

g, r, i > 17

^{. The subset}

of near-MSTO stars, additionally meets

M r > −2

^,

−2.5 ≤ [F e/H] ≤ 0

^and

0 .1 < g − i < 0.6

^.

In document Title: Structure and substructure in the stellar halo of the Milky Way (pagina 31-38)