Searh for overdensities - Structure and substructure in the stellar halo of the Milky Way

6.3.1 Methodology

WeusethenearMSTO,theRCandtheBHBstarsataloguestobuildstellarmaps

and stellar density maps for dierent distane or magnitude ranges. TheBHB

mapsresultinveryfewountswithnolearoverdensities,sowedonotusethem

further. Ontheotherhand,thenearMSTOandtheRCmapsreturnsuiently

populatedmapstobeusedinthesearhforspatialenhanements. Thedistane-

slied stellar and stellar density maps for the nearMSTO stars in the dierent

Figure6.1: Toppanel: equatorial map showingthe KiDS DR-1and DR-2foot-

print(inolour)andseveralofthehalostellarstreams(greysale). Bottompanel:

equatorialmapshowingtheKiDS DR-1andDR-2footprint(inolour)together

withtheSagittariusstreamasseenbySDSS(bakgroundimage)andby2MASS

(sattered blak dots). The SDSS density map is from Koposov et al. (2012)

whilethe 2MASS data are from Majewski et al.(2003). The red starindiates

Table 6.1: KiDS elds of view as shown in Figure 6.1. Thetable indiates the

totalarea probed byKiDS DR-1and DR-2for eah eld and the

a

fator used toalualte thekernel'sbandwidthforthestellar density maps. Parameter

a

is amaginumber hosento optimizethegranularityofthedensitymaps.

Group

a

Area

(deg

2

) KiDS-North220 4.8 29 KiDS-North180 6.6 37 KiDS-North135 5.4 37 KiDS-South45 3.6 14 KiDS-South-15 14.1 16

distane slies (from

[10,15]

kp to

[40,55]

kp) in two forms: i) satterplots showingindividualstars,andii)densitymapsbuiltwithak-NearestNeighbours

algorithm that uses a gaussian kernel with spatially variable bandwidth. The

bandwidthistailoredtoeahKiDSeld,andisalulatedas

a/std(map)

,where

a

is a onstant (see table 6.1) and

std(map)

is the standard deviation of the stellarountsinthateldatthespeidistanerangeofeahmap. Thedensity

for eah map is normalized over the mean density for that partiular eld and

distanerange,resultingin apixelbypixelsignal-to-bakgroundfuntion.

Onthesemapswelookfortwothings: rst,welookforbroaddensitygradients

arossthemaps,asasignoflargesubstruturesspanningseveraldegreesinwidth

andlength. Seond,welook forspei smalloverdensitiesspanning onlya few

degreesinwidth(iftheyareelongated)or indiameter(iftheyarerounded). We

requestthese features to havehigh signal-to-bakgroundvalues andto fadeout

intomorethanonedistane slie.

For the large gradients and strutures, we look for a spatial overlap with

known overdensities given that mostof ourontinuous overage is in theNorth

elds (where SDSS has already probed the halo). This helps us identify the

overdensities. Wealso produemagnitude(distane) vsRAmaps fortheNorth

and the South elds as a way to reover again these features and trae their

evolutionalongRA(Figures6.12to6.15).

For theandidatesmalloverdensities, weplottheindividualCMDofthetile

whereeahone isloatedandlookfordistintmain sequenesorredlumps. A

plotofthepositionsofjustthesestarsinanequatorialmapofthetileanthenbe

usedtoassesswhetherthisisadistintobjetorahaneenhanement. Results

ofouranalysisofthemostpromisingoverdensities arepresentedin Table6.2).

6.3.2 Results

Weidentify large strutures in all the KiDS elds exept in KiDS-South45. In

partiular, Figure 6.12 and 6.13 show the Eastern Band Struture (Li et al.

2012, EBS) in KiDS-North135, the Virgo Overdensity (Bonaa et al.2012b) in

Figure6.2: Stellarsattermapsand stellardensitymapsforthelosest distane

Figure6.3: Stellarsattermapsandstellardensitymapsforthefurthestdistane

Figure6.4: Stellarsattermapsand stellardensitymapsforthelosest distane

Figure6.5: Stellarsattermapsandstellardensitymapsforthefurthestdistane

Figure6.6: Stellarsattermapsand stellardensitymapsforthelosest distane

Figure6.7: Stellarsattermapsandstellardensitymapsforthefurthestdistane

Figure6.8: Stellarsattermapsand stellardensitymapsforthelosest distane

Figure6.9: Stellarsattermapsandstellardensitymapsforthefurthestdistane

Figure6.10: Stellarsattermapsandstellardensitymapsforthelosestdistane

Figure6.11: Stellarsattermapsandstellardensitymapsforthefurthestdistane

North220. It is worth noting that the inner-halo or disk strutures are more

easilyreognizableinthedistane-RA mapsthaninthemag-RAmaps,whereas

theouterhalostruturesshowtheoppositeeet. Thereasonforthisisthelog-

arithmionentratingpower ofmagnitudesin relationtodistanesas distanes

inrease, whih beomes very relevant when large volumes are probed and the

average stellar densities derease. Conversely, thesame eet washes outshort

saleoverdensitieswhentheseones areloatednearby.

In the KiDS-North220 eld, the presene of disk stars is deteted at least

outto

12−15

kp. TheVirgoOverdensitystrethesoutto20kpinhelioentri distanesand,initswestern-mostregions,possiblyoutto

25−30

kp. TheEBSis mostlyonentratedatunder15kp,withsomepotentialdebrisextendingfurther

outto 20 kp. The Sagittarius stream's nearMSTOstars overdensity in KiDS-

North220learlypeaksat

22.0−22.2

magin

r

butextendsfrom

∼21.6

∼22.9

, indiatingabroadbranhandpossiblyadependeneofdistanewithdelination.

Assuminganaverage absolutemagnitudeof

Mr,T O= 4.00

fortheMSTO ofthe Sagittariusstream(Pila-Díezetal.2014),thesemagnitudestranslateintoapeak

distaneof

40

kpand

[33,60]

kp(soft)boundaries.

Figure6.14and6.15showtheSagittariusstreaminKiDS-South-15,inagree-

mentwithitsloationonthe2MASSmaps. Attheselatitudes,thestreamsitsat

adistanebetween

∼15

kpand

∼30

kp,in agreementwiththepreditions of Law&Majewski(2010b)andPeñarrubiaetal.(2010). Suhawiderangeofdis-

tanessuggests thepossiblepreseneoftwo wraps(theleading andthetrailing)

oraquitethikbranhinthisregionofthesky.

Among the small andidate overdensities, we identify one as a fragment of

thePalomar 5 (Pal5) globular luster tidal tails (Grillmair &Dionatos 2006a).

The still pathy overage of KiDS in the

RA >

225 deg

range unfortunately prevents us from fully traing the stream. Nonetheless, we follow the proe-

dure desribed above to analyse the CMD of this overdensity and demarate

the exess of stars within the tiles area. A lear main sequene and main se-

queneturno pointarevisiblein theCMDorrespondingto thetileentredat

(RA, Dec) = (230.0,0.5) deg

(left panel on Figure 6.16), and even a seondary main sequene turno point is loated at fainter magnitudes. As disussed in

Pila-Díezet al.(2014),theseoriginate inthePal5tailandtheunderlying Sagit-

tariusstream,respetively. WeisolatethestarsinthePal5tail'smain sequene,

andbuildastellardensitymapofthetilespeitothissubstruture(rightpanel

onFigure6.16). ThismapnielyshowsthePal5tailrossingthetilethroughits

North-Eastquadrant.

For the rest ofthe smalloverdensities (Table6.2) wefollowthesame proe-

dure. FromtheirCMDandstampmapswendthattheseareeither i)spurious

enhanements in the distane/magnitude-slied maps (this is, RC/nearMSTO-

olour overdensities in the CMD without the ompanion RGB/main sequene

overdensity),orii)apparentmainsequenesintheCMDbut withouta oherent

spatialfeature in thestamp map (meaningthat,whenplotting dierentolour-

Figure6.12: Distane vsRA (top)andmagnitudevsRA(bottom)sattermaps

for the nearMSTOstars in the KiDS Northern elds. Both the disk and large

halostruturesliketheSagittariusstream,theVirgoOverdensityandtheEBSare

visible. Thisgureillustratesthatthenearbystruturesaremoreeasilyreogniz-

ablein thedistanespaethanthefarstrutures,whereasthefarstruturesare

moreeasily reognizablein themagnitudespae. This isdue to thelogarithmi

Figure6.13: Sameas in Figure6.12butshowingthestellardensitymapinstead

ofthestellarountsmap. Thedensityhasbeenalulatedwithagaussiankernel

Figure6.14: Distane vsRA (top)andmagnitudevsRA(bottom)sattermaps

forthenearMSTO starsin theKiDS Southern elds. TheSagittariusstream is

Figure6.15: Sameas in Figure6.14butshowingthestellardensitymapinstead

ofthestellarountsmap.Thedensityhasbeenalulatedwitha gaussiankernel

Figure6.16: CMD(left)andstellardensitymap(right)forthestarsinthemain

sequeneofthe CMD(right),pointingentered at

(RA, Dec) = (230.0,0.5) deg

. The main sequene at

20.0 < r <

22.0

represents the Palomar 5 stream. The seondarymainsequenevisibleat

r≈23

istheSagittariusstream.

Table 6.2: Potential small overdensities identied in the distane-

slied/magnitude-sliednearMSTOorRCdensitymaps. Thetable indiatesthe

entraloordinatesofthetilewhereeah was identied,tagsthemand provides

adiagonisis(true orfalsepositive).

Overdensity Field RA

tile

(deg) De

tile

(deg) positive

Pal5 KiDS-North220 230.0 0.5 true

A KiDS-North180 179.0 -0.5 false

B KiDS-North135 135.0 0.5 false

C KiDS-North135 132.0 -0.5 false

D KiDS-South45 47.8 -31.2 false

E KiDS-South-15 350.6 -32.1 false

latedierentregionsofthetile). Examplesforthetwotypesofasesareprovided

inFigures6.17and6.18,respetively,orrespondingtooverdensitiesAandE.We

onludethatallAto Eoverdensitiesarefalsepositivehalo substrutures.

6.4 Disussion

The methods used in this work are tehnially robust (k-Nearest Neighbours,

CMDs, distane or magnitudesliing, olour-olour star seletion) and their ef-

fetivityforthehalo hasbeenamplytestedintheliterature. Indeedweareable

toreoverlargeknownhalostruturesandalsoafragmentofthePalomar5tidal

tails.

Thereforeweassoiatethe lakof a newdisovery in theKiDS DR1/2 data

Figure 6.17: CMD (left) and stellar density map (right) orresponding to the

pointingentered at

(RA, Dec) = (179.0,−0.5) deg

(labelledas A in Table6.2). Thestellardensity mapontainsthestarsin theapparentmain sequeneofthe

whole pointing (not shown), and the CMD is built of the stars loated in the

viinityof theoverdensity at

(RA, Dec)≈(178.9,−0.7) deg

. Thelakof amain sequeneandanRGBonthisCMDindiatesthatthisoverdensityinnearMSTO

starsisaspuriousenhanementand,therefore,isafalsepositive. Similaranalysis

onweaker spatial overdensities inthis pointingleadtoonlude thatthere isno

loalizedmainsequeneinthispointing.

Figure6.18: CMD(left)andstellardensitymapforthestarsinthemainsequene

ofthe CMD (right). Theyorrespond to the pointingentered at

(RA, Dec) =

(350.6,−32.1) deg

and totheoverdensitylabelledasE in Table6.2. Thelakof aoherentspatialfeature indiatesthisisa falsepolsitive.

probedyetby othersurveys. Inpartiular, inthe KiDS'northernelds there is

atotaloverlapwiththefootprintofthelatestSDSSdatareleases,andtherewas

originally a

∼

80%

overlap planned. Of the urrent

133 deg

2

sanned by KiDS

DR-1andDR-2inthefourbands,only

23%

belongstothesouthernhemisphere. This limitsKiDS urrent hanes of deteting new globular lusters or satellite

galaxies,andlimitsitsabilitytodetetandtraeweakstreams.

Basedbothonthelaimsofa fundamentalplaneof satellitegalaxies (Palma

etal.(2002b),Zentneretal.(2005),Pawlowskietal.(2012))andonthepossibility

ofsuhafundamentalplanebeingprobedwrong,butbasedmainlyonthesatellite

populationinthenorthernsky,thenumberofsatellitegalaxiesexpetedtoinhabit

thesouthernskyishigh. FurthermorethereentresultsbyDESandATLASshow

good prospets, and donot suggest a fundamental asymmetry in the density of

satellites between the northern and southern Galati hemispheres. Given the

urrent estimates for the Galati satellite luminosity funtion (Koposov et al.

2008)and KiDS unpreedented power to probe faint objets, the prospets for

KiDS are partiularly good at the faint end. Therefore there is no reason for

disouragement and future data releases of KiDS should unover new satellites

andstellardebris.

6.5 Conlusions

TheKiloDegreeSurveyisurrentlymapping

1500 deg

2

oftheskyatanunpree-

denteddepthamonglargeareasurveys. Itsgoodimagequality,seeingonstraints

andmulti-olour photometrymakeit partiularlysuited toexplore theGalati

halo in depth. It targets boththe North and the South Galati hemispheres,

overlapping withSDSS and ATLASin eah region respetively, but probing an

average of two magnitudes deeper than any of the two surveys in any of the

four photometri lters. The PSF-homogenization that we arry out allows us

to auratelyseparate stars from galaxies, reahing a stellar magnitudelimit of

magr= 23.1

withoutsigniantgalaxyontamination. Thisallowsustoproperly exploitmainsequeneturnopointstarsouttolargehelioentridistanes(

∼60

kp).

In this work wepresentmagnitude-slied or distane-slied maps forphoto-

metrially seletedred lump starsand near main sequene turno point stars.

WeidentifybroadhalostruturessuhastheSagittariusstream,theVirgoOver-

density and the Eastern Band Struture in the northern hemisphere, and the

Sagittariusstream in the southernhemisphere. In the northern hemisphere we

alsoidentifythediskstarsatlongitudes andlatitudesneighbouringtheGalati

entre. Wetraethesefeaturesindistanespaeandmagnitudespaeandreport

helioentridistanesompatible withpreviousworks.

Wealso identify in these distane/magnitude-slied maps a number of small

overdensities potentiallyrepresentativeof halo substruture. Ofthe seveniden-

tiations,wereognizeone asa piee ofthePalomar5tidal tails. Therest are

distributionofanyCMDoverdensities.

TheKiloDegreeSurveyispromisingin thesearh forsmall,sparseandfaint

substruture,givenbothitstehnialapabilitiesand thefat thatit isprobing

unharted halo territory in the Southernhemisphere. Theurrentmethodology

presentedinthishapter,togetherwithstate-of-the-artmathed-lteralgorithms,

shouldsue to identify newsubstruturein thehalo one thespatial overage

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