Sear h for overdensities - Title: Structure and substructure in the stellar halo of the Milky W

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

foot-print(in olour)andseveralofthehalostellarstreams(greys ale). Bottompanel:

equatorialmapshowingtheKiDS DR-1andDR-2footprint(in olour)together

withtheSagittariusstreamasseenbySDSS(ba kgroundimage)andby2MASS

(s attered bla k dots). The SDSS density map is from Koposov et al. (2012)

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

thepositionofthestream'sprogenitor, theSagittariusdwarfgalaxy.

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

totalarea probed byKiDS DR-1and DR-2for ea h eld and the

a

^fa
tor ^used

to al ualte thekernel'sbandwidthforthestellar density maps. Parameter

a

^is

amagi number 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

distan e sli es (from

[10, 15]

^kp ^to

[40, 55]

^kp
) ⁱⁿ ^two ^forms: ⁱ⁾ s atterplots showingindividualstars,andii)densitymapsbuiltwithak-NearestNeighbours

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

bandwidthistailoredtoea hKiDSeld,andis al ulatedas

a/std(map)

^,^where

a

îs â ônstant ^(see ^table ^6.1) ând

std(map)

^is ^the ^standard ^deviation ^of ^the

stellar ountsinthateldatthespe i distan erangeofea hmap. Thedensity

for ea h map is normalized over the mean density for that parti ular eld and

distan erange,resultingin apixelbypixelsignal-to-ba kgroundfun tion.

Onthesemapswelookfortwothings: rst,welookforbroaddensitygradients

a rossthemaps,asasignoflargesubstru turesspanningseveraldegreesinwidth

andlength. Se ond,welook forspe i smalloverdensitiesspanning onlya few

degreesinwidth(iftheyareelongated)or indiameter(iftheyarerounded). We

requestthese features to havehigh signal-to-ba kgroundvalues andto fadeout

intomorethanonedistan e sli e.

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

known overdensities given that mostof our ontinuous overage is in theNorth

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

overdensities. Wealso produ emagnitude(distan e) vsRAmaps fortheNorth

and the South elds as a way to re over again these features and tra e their

evolutionalongRA(Figures6.12to6.15).

For the andidatesmalloverdensities, weplottheindividualCMDofthetile

whereea hone islo atedandlookfordistin tmain sequen esorred lumps. A

plotofthepositionsofjustthesestarsinanequatorialmapofthetile anthenbe

usedtoassesswhetherthisisadistin tobje tora han eenhan ement. Results

ofouranalysisofthemostpromisingoverdensities arepresentedin Table6.2).

6.3.2 Results

Weidentify large stru tures in all the KiDS elds ex ept in KiDS-South45. In

parti ular, Figure 6.12 and 6.13 show the Eastern Band Stru ture (Li et al.

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

KiDS-North180and boththeGala ti diskandtheSagittariusstream in

KiDS-Figure6.2: Stellars attermapsand stellardensitymapsforthe losest distan e

sli esineld KiDS-North220.

Figure6.3: Stellars attermapsandstellardensitymapsforthefurthestdistan e

sli esineld KiDS-North220( ontinuationofFigure 6.2).

Figure6.4: Stellars attermapsand stellardensitymapsforthe losest distan e

sli esineld KiDS-North180.

Figure6.5: Stellars attermapsandstellardensitymapsforthefurthestdistan e

sli esineld KiDS-North180( ontinuationofFigure 6.4).

Figure6.6: Stellars attermapsand stellardensitymapsforthe losest distan e

sli esineld KiDS-North135.

Figure6.7: Stellars attermapsandstellardensitymapsforthefurthestdistan e

sli esineld KiDS-North135( ontinuationofFigure6.6).

Figure6.8: Stellars attermapsand stellardensitymapsforthe losest distan e

sli esineld KiDS-South45.

Figure6.9: Stellars attermapsandstellardensitymapsforthefurthestdistan e

sli esineld KiDS-South45( ontinuationofFigure 6.8).

Figure6.10: Stellars attermapsandstellardensitymapsforthe losestdistan e

sli esineld KiDS-South-15.

Figure6.11: Stellars attermapsandstellardensitymapsforthefurthestdistan e

sli esineld KiDS-South-15( ontinuationofFigure 6.10).

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

easilyre ognizableinthedistan e-RA mapsthaninthemag-RAmaps,whereas

theouterhalostru turesshowtheoppositeee t. Thereasonforthisisthe

log-arithmi on entratingpower ofmagnitudesin relationtodistan esas distan es

in rease, whi h be omes very relevant when large volumes are probed and the

average stellar densities de rease. Conversely, thesame ee t washes outshort

s aleoverdensitieswhentheseones arelo atednearby.

In the KiDS-North220 eld, the presen e of disk stars is dete ted at least

outto

12 − 15

^kp
. ^The^VirgoOverdensitystret hesoutto20kp inhelio entri distan esand,initswestern-mostregions,possiblyoutto

25 −30

^kp
. ^The^EBS^is

mostly on entratedatunder15kp ,withsomepotentialdebrisextendingfurther

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

KiDS-North220 learlypeaksat

22 .0 −22.2

^magⁱⁿ

r

^but^extends^from

∼ 21.6

^to

∼ 22.9

indi atingabroadbran handpossiblyadependen eofdistan ewithde lination.

Assuminganaverage absolutemagnitudeof

M r,T O = 4 .00

^for^the^MSTO ^of^the

Sagittariusstream(Pila-Díezetal.2014),thesemagnitudestranslateintoapeak

distan eof

40

^kp^and

[33 , 60]

^kp^(soft)boundaries.

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

agree-mentwithitslo ationonthe2MASSmaps. Attheselatitudes,thestreamsitsat

adistan ebetween

∼ 15

^kp^and

∼ 30

^kp
,ⁱⁿ ^agreement^with^thepredi tions of Law&Majewski(2010b)andPeñarrubiaetal.(2010). Su hawiderangeof

dis-tan essuggests thepossiblepresen eoftwo wraps(theleading andthetrailing)

oraquitethi kbran hinthisregionofthesky.

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

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

The still pat hy overage of KiDS in the

RA > 225 deg

^range unfortunately prevents us from fully tra ing the stream. Nonetheless, we follow the

pro e-dure des ribed above to analyse the CMD of this overdensity and demar ate

the ex ess of stars within the tiles area. A lear main sequen e and main

se-quen eturno pointarevisiblein theCMD orrespondingto thetile entredat

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

^(left ^panel ôn ^Figure ^6.16), ând êven â ^se
ondary

main sequen e turno point is lo ated at fainter magnitudes. As dis ussed in

Pila-Díezet al.(2014),theseoriginate inthePal5tailandtheunderlying

Sagit-tariusstream,respe tively. WeisolatethestarsinthePal5tail'smain sequen e,

andbuildastellardensitymapofthetilespe i tothissubstru ture(rightpanel

onFigure6.16). Thismapni elyshowsthePal5tail rossingthetilethroughits

North-Eastquadrant.

For the rest ofthe smalloverdensities (Table6.2) wefollowthesame

pro e-dure. FromtheirCMDandstampmapswendthattheseareeither i)spurious

enhan ements in the distan e/magnitude-sli ed maps (this is,

RC/nearMSTO- olour overdensities in the CMD without the ompanion RGB/main sequen e

overdensity),orii)apparentmainsequen esintheCMDbut withouta oherent

spatialfeature in thestamp map (meaningthat,whenplotting dierent

olour-magnitudese tionsoftheapparentmainsequen eonthestampmap,they

popu-Figure6.12: Distan e vsRA (top)andmagnitudevsRA(bottom)s attermaps

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

halostru turesliketheSagittariusstream,theVirgoOverdensityandtheEBSare

visible. Thisgureillustratesthatthenearbystru turesaremoreeasily

re ogniz-ablein thedistan espa ethanthefarstru tures,whereasthefarstru turesare

moreeasily re ognizablein themagnitudespa e. This isdue to thelogarithmi

on entratingpower ofmagnitudesin relationtodistan es.

Figure6.13: Sameas in Figure6.12butshowingthestellardensitymapinstead

ofthestellar ountsmap. Thedensityhasbeen al ulatedwithagaussiankernel

ofvariablebandwidth.

Figure6.14: Distan e vsRA (top)andmagnitudevsRA(bottom)s attermaps

forthenearMSTO starsin theKiDS Southern elds. TheSagittariusstream is

visibleintheeasternpart oftheeld.

Figure6.15: Sameas in Figure6.14butshowingthestellardensitymapinstead

ofthestellar ountsmap.Thedensityhasbeen al ulatedwitha gaussiankernel

ofvariablebandwidth.

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

sequen eofthe CMD(right),pointing entered at

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

The main sequen e at

20.0 < r < 22.0

^represents ^the ^Palomar ⁵ ^stream. ^The

se ondarymainsequen evisibleat

r ≈ 23

^is^theSagittariusstream.

Table 6.2: Potential small overdensities identied in the

distan e-sli ed/magnitude-sli ednearMSTOorRCdensitymaps. Thetable indi atesthe

entral oordinatesofthetilewhereea h 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). Examplesforthetwotypesof asesareprovided

inFigures6.17and6.18,respe tively, orrespondingtooverdensitiesAandE.We

on ludethatallAto Eoverdensitiesarefalsepositivehalo substru tures.

In document Title: Structure and substructure in the stellar halo of the Milky Way (pagina 156-174)