Optical and IR Photometry of Globular Clusters in NGC1399: Evidence for Color-Metallicity Nonlinearity

2.3 MB·English

by  John P. Blakeslee

TOAPPEARINAPJ,JANUARY2012
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
OPTICALANDIRPHOTOMETRYOFGLOBULARCLUSTERSINNGC1399:
EVIDENCEFORCOLOR-METALLICITYNONLINEARITY*
JOHNP.BLAKESLEE1,HYEJEONCHO(조혜전)2,ERICW.PENG3,4,LAURAFERRARESE1,ANDRÉSJORDÁN5,&ANDRÉR.MARTEL1,6
ToappearinApJ,January2012
ABSTRACT
2 WecombinenewWideFieldCamera3IRChannel(WFC3/IR)F160W(H160)imagingdataforNGC1399,
1 thecentralgalaxyintheFornaxcluster,witharchivalF475W(g475),F606W(V606),F814W(I814),andF850LP
0 (z850) optical data from the Advanced Camera for Surveys (ACS). The purely optical g475- I814, V606- I814,
2 and g - z colors of NGC1399’s rich globular cluster (GC) system exhibit clear bimodality, at least for
475 850
magnitudes I >21.5. The optical-IR I - H color distribution appears unimodal, and this impression
n 814 814 160
is confirmed by mixture modeling analysis. The V - H colors show marginal evidence for bimodality,
a 606 160
J consistentwith bimodalityinV606- I814 and unimodalityin I814- H160. If bimodalityis imposedfor I814- H160
withadoubleGaussianmodel,thepreferredblue/redsplitdiffersfromthatforopticalcolors;these“differing
4
bimodalities” mean that the optical and optical-IRcolors cannotboth be linearly proportionalto metallicity.

Consistentwiththedifferingcolordistributions,thedependenceofI - H ong - I forthematchedGC
] 814 160 475 814
O sampleissignificantlynonlinear,withaninflectionpointnearthetroughintheg - I colordistribution;the
475 814
resultissimilarfortheI - H dependenceong - z colorstakenfromtheACSFornaxClusterSurvey.
C 814 160 475 850
These g - z colors have been calibrated empirically against metallicity; applying this calibration yields
475 850
.
h a continuous, skewed, but single-peaked metallicity distribution. Taken together, these results indicate that
p nonlinear color-metallicity relations play an importantrole in shaping the observed bimodal distributions of
- opticalcolorsinextragalacticGCsystems.
o
r Subjectheadings:galaxies: ellipticalandlenticular,cD—galaxies: individual(NGC1399)—galaxies: star
t clusters—globularclusters:general
s
a
[
1. INTRODUCTION mean metallicity. The bulk of the host galaxy’sstellar mass
1 would then likely originate from two distinct major forma-
The major star formation episodes in the history of any
v tion episodes. Ashman & Zepf (1992) originally predicted
largegalaxywillbeimprintedinthepropertiesofthegalaxy’s
1 such GC bimodality based on the idea that ellipticals form
starclusterpopulation.Interpretingtheobservedpropertiesto
3
fromgas-richmajormergersoflate-typegalaxiesthatalready
0 derivetheformationhistorieshasproventobeadifficulttask.
possessedextensivemetal-poorGCpopulations. Becauseel-
1 Allgiantellipticalgalaxiescontainlargepopulationsofglob-
lipticals are believed to have formed in a more stochastic,
. ular clusters (GCs), often numbering in the thousands (e.g.,
1 hierarchical fashion, a number of other merger or accretion
Harris1991). TheGCsystemsgenerallyfollowbimodaldis-
0 scenarioswerelaterproposedtoaccountfortheobservedbi-
tributionsinopticalcolors(Zepf&Ashman1993;Gebhardt
2 modality(Forbesetal.1997;Kissler-Patigetal.1998b;Côté
& Kissler-Patig 1999). For giant ellipticals in clusters, the
1 et al. 1998; Beasley et al. 2002; Kravtsov & Gnedin 2005).
twopeaksinthecolordistributionsareroughlyequalinsize
: However,thesedifferentscenariosareoftendifficulttodistin-
v (e.g., Peng et al. 2006; Harris et al. 2006), except at large
i radii where the color distribution is more strongly weighted guishobservationallyfromoneanother,andnoneappearsto
X accountnaturallyforallthedata(seePengetal.2006).
towardstheblue(e.g.,Dirschetal.2003;Harris2009).
r If the optical colors are interpreted as a direct proxy for More recently, the assumption of optical colors as a sim-
a ple,linearproxyformetallicityhasbeenreexamined(Richtler
metallicity, then the bimodality represents an extraordinary
2006; Yoon et al. 2006). It has been known for many years
constraint on the star formation histories of giant ellipti-
thattheslopeofthemetallicityasafunctionofopticalcolor
cals. In this case, the two color peaksrepresenttwo distinct
becomes shallower (i.e., color becomes more sensitive to
cluster populations, differing by roughly a factor of ten in
metallicity) at intermediate metallicities (Kissler-Patig et al.
*BasedonobservationswiththeNASA/ESAHubbleSpaceTelescope, 1998a). Yoonetal.(2006)showedthatthe“wavy”nonlinear
obtainedfromtheSpaceTelescopeScienceInstitute,whichisoperatedby color-metallicityrelationpredictedbytheirstellarpopulation
AURA,Inc.,underNASAcontractNAS5-26555. models matched, at least qualitatively, the color-metallicity
1Dominion Astrophysical Observatory, Herzberg Institute of Astro-
data assembled by Peng et al. (2006). They further pointed
physics, National Research Council of Canada, Victoria, BC V9E2E7,
Canada;[email protected] out that the “projection” from metallicity to color with such
2DepartmentofAstronomyandCenterforGalaxyEvolutionResearch, wavyrelationscanproducebimodalcolordistributionsfrom
YonseiUniversity,Seoul120-749,Korea unimodaldistributions in metallicity. Cantiello & Blakeslee
3DepartmentofAstronomy,PekingUniversity,Beijing100871,China
(2007) confirmed the Yoon et al. (2006) result, in the sense
4Kavli Institute for Astronomy and Astrophysics, Beijing 100871,
thatothersetsofmodelsthatincluderealisticprescriptionsfor
China
5Departamento de Astronomía y Astrofísica, Pontificia Universidad thebehaviorofthehorizontalbranchasafunctionofmetallic-
CatólicadeChile,7820436Macul,Santiago,Chile ityalsogivenonlinearcolor-metallicityrelationsthatcanpro-
6SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore, ducebimodalcolor distributions. More recently, Yoonet al.
MD21218,USA
2 Blakeslee,Cho,Peng,Ferrarese,Jordán&Martel
(2011a,b)findthattheGCmetallicitydistributionsderivedfor ing section summarizes the observational details and image
severalgalaxiesfromopticalcolorsusing their modelcolor- reductionsforboththeWFC3/IRnear-IRandACS/WFCop-
metallicity distributions are not bimodal, but are similar to ticaldata. Section3describesourphotometricmeasurements
the metallicity distributionsfoundfor stellar halos in ellipti- and selection of GC candidates. Section 4 presents the GC
calgalaxies. color-magnitudediagrams(CMDs)andcolordistributionsre-
Thisissue remainshighlycontroversial,butitis clear that sultingfromournewphotometry,aswell asmixturemodel-
additionalstudiesofGCmetallicitydistributionsareneeded. ing analysis results for the differentdistributions. The form
In particular, it is important to constrain independently the ofthecolor-colorrelationbetweenI - H andg - I is
814 160 475 814
form of the metallicity distributions in galaxies with promi- discussedindetailinSection5. InSection6,wecross-match
nently bimodal GC color distributions. Large samples of ourphotometryagainsttheACSFornaxClusterSurvey(here-
spectroscopicmetallicitiesinextragalacticGCsystems(more after, ACSFCS; Jordán et al. 2007)catalogue for NGC1399
thanafewpercentofthetotalpopulation)arenowbecoming anddiscusstheimplicationsfortheunderlyingmetallicitydis-
available(Beasleyetal.2008;Fosteretal.2010;Alves-Brito tribution.Thefinalsectionplacesourstudyinthelargercon-
etal. 2011; Caldwellet al. 2011; see Section 7 fora discus- text of GC color and metallicity studies, before listing our
sionoftheseresults). However,atthedistancesoftheVirgo mainconclusions.
and Fornax clusters, spectroscopic data of sufficient quality
remainsobservationallyexpensive, requiringmultiple nights
2. OBSERVATIONALDATASETS
often-meterclasstelescopetimetoamasssignificantsamples.
Another way to constrain GC metallicity distributions is NGC1399 was observed for one orbit in the F110W and
through a combination of near-infrared (near-IR) and opti- F160W bandpasses of WFC3/IR in 2009 December as part
calphotometry(e.g.,Kissler-Patigetal.2002;Beasley etal. of HST program GO-11712. The WFC3 IR Channel uses a
2002; Puzia et al. 2002; Kundu & Zepf 2007; Kotulla et al. 10242 pix HgCdTe detector with an active field of view of
2008;Chies-Santosetal.2011a,b). Hybridoptical-IRcolors 2.′05 2.′27, anda mean pixelscale ofabout0.′′128pix- 1
such as I- H or I- K for old stellar systems such as GCs are ∼(see Dr×essel et al. 2010 for more information). A primary
mainlydeterminedbythemeantemperatureofstarsonthered goal of GO-11712 is to obtain an empirical calibration for
giantbranch,whichinturndependsalmostentirelyonmetal- thesurfacebrightnessfluctuations(SBF)methodinthesetwo
licity (e.g., Bergbusch & VandenBerg 2001; Yi et al. 2001; WFC3/IR bandpasses similar to those derived for ACS in
Dotteretal.2007). Optical-IRcolorsthatinvolvebluerpass- F814WandF850LP(Blakesleeetal.2009,2010b;Meietal.
bands,suchasV- H orB- K,alsohavethestrongmetallicity 2005).Tothisend,GO-11712targeted16early-typegalaxies
dependencefromthegiantbranchtemperature,buttheyhave intheFornaxandVirgoclustersoverawiderangeinluminos-
additionalsensitivitytothemainsequenceturnoff,whichde- ityandcolor. Anotherimportantgoalofthe projectis to in-
pendsstronglyonage,andtothehorizontalbranchmorphol- vestigatetheoptical-IRcolorsoftheGCpopulationsinthese
ogy,whichbehavesnonlinearlywithmetallicityandalsode- galaxies,usingprimarilytheF160Wbandpass(whichaffords
pends on age (e.g., Lee et al. 1994; Sarajedini et al. 1997; thewiderbaseline)combinedwithexistingACSopticaldata.
Dotteretal.2010). Althoughthedependenceofgiantbranch NGC1399wasoneofthefirstlargegalaxiesobservedinthis
temperature(andthusofI- H andsimilarcolors)onmetallic- project, and we have used it to optimize our image process-
ityisnotnecessarilylinear,thereisnoevidenceforasharply ing pipeline. Analyses of the SBF propertiesand GC colors
nonlinear transition with metallicity, as occurs for the hori- forthefullsampleofGO-11712galaxieswillbepresentedin
zontalbranch. forthcomingworks.
The extensive GC system of NGC1399, the dominant el- Atotalof1197sofintegrationwasacquiredwithWFC3/IR
lipticalgalaxyin the Fornaxcluster,hasbeena frequenttar- in the F160W band. The data were retrieved multiple times
getforopticalphotometricandspectroscopicstudiessincethe from the STScI archive as the calibration reference files for
pioneeringworkofHanes&Harris(1986). Atadistanceof WFC3 were updated. The photometric results improved
20 Mpc (Blakeslee et al. 2009)Fornaxis the second nearest markedly after flat fields produced on-orbit (described by
galaxy cluster after Virgo, and NGC1399 is at its dynami- Pirzkaletal.2011)wereimplementedintheSTScIpipeline.
calcenter(Drinkwateretal.2001).NGC1399wasoneofthe Theresultspresentedherearederivedfromimagesretrieved
firstexternalgalaxiesreportedashavingabimodalmetallicity from STScI in 2011 January. We combined the individual
distribution,basedonopticalcolors(Ostrovetal.1993). In- calibratedWFC3/IRexposuresintoafinalgeometricallycor-
terestingly,thisgalaxyprovidedthefirstindicationoftheim- rectedimageusingtheMultidrizzle(Koekemoeretal.2002)
portanceofthedetailedshapeofthecolor-metallicityrelation: taskintheSTSDASpackage8. Afterexperimentingwithdif-
Kissler-Patigetal.(1998a)measuredspectroscopicmetallic- ferentDrizzle(Fruchter&Hook2002)parameters,wesettled
ities for a sample of GCs in NGC1399 and found that the onthesquareinterpolationkernelwitha“pixfrac”valueof0.8
slopeofmetallicityversus(V- I)colorwassignificantlyflat- andafinalpixelscaleof0.′′1pix- 1. Thisscaleisconvenient
terthantheextrapolationfromlow-metallicityGalacticGCs. becauseitisexactlytwicethatofACS/WFC.
NGC1399alsohasthelargestsampleofmeasuredGCradial NGC1399 has been observed many times before with
velocitiestodate(Schuberthetal.2010). HST, including several times with the ACS/WFC, allowing
Here we present new near-IR and optical photometry of us to investigate the optical-IR colors of its GCs. Cali-
GCs in NGC1399 using the Infrared Channel of the Wide brated observations in F606W from GO-10129 (PI: Puzia)
Field Camera 3 (WFC3/IR) and Wide Field Channel of the and F475W+F814W from GO-10911 (PI: Blakeslee) were
Advanced Camera for Surveys (ACS/WFC) on board the retrieved from the STScI archive and processed with Apsis
Hubble Space Telescope (HST). We take an empirical ap- (Blakesleeetal.2003)toproducesummed,geometricallycor-
proach in this work, comparing the color distributions and
examining the color-color relations, without trying to judge 8STSDASisaproductoftheSpaceTelescopeScienceInstitute,whichis
betweendifferentsetsofmodelsforthepresent. Thefollow- operatedbyAURAforNASA.
NGC1399Color-MetallicityNonlinearity 3
FIG.1.—HSTWFC3/IRF160WimageofNGC1399(top),andthesamefollowinggalaxyisophotalmodelandskysubtraction(bottom).The
fieldsizeisapproximately2.′3×2.′1,andtheimageisshownattheobservedorientation. Pixelsflaggedasbadinthedataqualityarraysare
zeroedinbothpanels.Thecentral1.′′2ofthegalaxywasnotmodeled,andissettozerointhelowerpanel.
4 Blakeslee,Cho,Peng,Ferrarese,Jordán&Martel
rected,cosmic-ray-cleanedimagesforeachbandpass.Reduc-
tionoftheGO-10911imagingdataisdescribedinmoredetail
byBlakesleeetal.(2010b);thesameprocedureswereapplied
fortheGO-10129program,whichobservedninecontiguous
fields in F606W. We processed only the four pointings that
overlappedwith the ACS/WFC imagesin otherbands; three
of these pointings overlap with our smaller WFC3/IR field.
NGC1399wasalso observedinF475WandF850LPaspart
ofHSTprogramGO-10217(Jordánetal.2007);inthiscase,
weusedthephotometrycatalogueproducedbythatprogram,
asdiscussedinSection6.
Throughout this study, we employ the natural photomet-
ric systems defined by the instrument bandpasses, rather
than converting to the Johnson system. We calibrated the
WFC3/IR photometry using the AB zero-point coefficients
given by Kalirai et al. (2009) and the ACS photometry us-
ing the AB zero points from Sirianni et al. (2005). We cor- FIG.2.—Theerrorinmagnitudeforpixelsabovetheisophotalde-
rected for Galactic extinction towards NGC1399 assuming tectionthresholdisplottedagainstI814MAG_AUTOvaluesfromSEx-
E(B- V)=0.0125mag(Schlegeletal. 1998),the ACS/WFC tractor. ThemagnitudesareontheABsystem. Theverticaldashed
extinctionratios fromSirianni et al. (2005),and the H-band lineatI814=23.5magisdrawnsomewhatfainterthanthemeanmag-
extinction ratio from Schlegel et al. (1998). Table 1 sum- nitude(or“turnover”)oftheGCluminosityfunction,whichoccurs
marizesthe observationaldetails of the data sets used in the atI814=23.5mag(seetext).Themedianisophotaldetectionerrorat
presentstudy,includinginthe lastcolumnthesymbolsused I814=23.5is0.028mag.
todenotemagnitudesinthevariousbandpasses.
3. OBJECTSELECTIONANDPHOTOMETRY
InordertoobtainphotometriccataloguesofGCcandidates,
wefirstconstructedellipticalisophotalmodelsforthediffer-
entbandpassesasdescribedinourpreviousworks(e.g.,Tonry
etal.1997;Jordanetal.2004;Blakesleeetal.2009,2010b)
and used these to subtract the galaxy light. Figure 1 shows
our WFC3/IR F160W image of NGC1399 before and after
the galaxy subtraction. The compact sources corresponding
toGCcandidates,aswellassomebackgroundgalaxiesanda
fewstars,areclearlyevident.
We performed object detection using SExtractor (Bertin
& Arnouts 1996) with an RMS weight image that included
the photon noise and SBF contributionsfrom the subtracted
galaxy. The full procedure is described in detail by Jordan FIG.3.—Full-widthhalfmaximum(FWHM)isplottedagainstI814
etal.(2004,2007)andBarberDeGraaffetal.(2007).Forthe MAG_AUTOparameterfromSExtractorforallobjectsdetectedinthe
mostpart,weuseF814Wtodefinethesamplelimitsbecause ACS/WFCF814Wimage.TheFWHMvaluesareinpixels,atascale
itisabroadbandpassattheredendoftheopticalspectrum, of0.′′05pix- 1.Thegrayshadedregionshowstheinitialselectionfor
and the signal-to-noise (S/N) of the data is high, despite the GCcandidatesinthisplane. TheturnoveroftheGCLFoccursnear
half-orbit integration. For object detection in the ACS im- I814≈23.2mag.
ages, we required an area of at least four connected pixels
abovea S/N thresholdof two; thus, a minimum S/N of four I814 is the SExtractor MAG_AUTO value; this would indicate
withintheisophotaldetectionarea.TheSBFisquitestrongin aturnoverin F814Wof 23.1mag. Thisisconsistentwith
∼
theWFC3 F160Wimage, andwethereforesetthe detection amagnitudehistogramoftheGCcandidatesselectedbelow,
threshold higher, requiring a total S/N of at least 6 to avoid althoughbasedontheVirgoGCLFandtherelativedistances
spuriousdetections. Objects were detected and measuredin ofthetwoclusters,theGCLFturnoverwouldbeexpectedto
each band separately (rather than “dual image mode”), then occurabout0.2magfainter. Inanycase,thisisstillbrighter
the catalogues were matched across bands; this process re- thanthedashedlineshownatI =23.5maginFigure2.
814
movesspurious objects from the combinedmulti-band sam- Figure 3 shows the full width at half maximum (FWHM)
ples. Figure2illustratesthedepthofthedetectioninF814W values measured with SExtractor as a function of I mag-
814
by plotting the magnitude error within the isophotal area as nitude. There is a “finger” of compactsourceswith FWHM
afunctionoftheSExtractor MAG_AUTO parameter,theesti- 2pixthatcanbedistinguishedfromthebackgroundpopu-
∼
matedtotalmagnitudefromSExtractor. lationdowntoI 24mag(shadedregion). Thisissimilar
814
≈
CandidateGCsinNGC1399mustbenearlypoint-likeob- to the FWHM of the point spread function, which is about
jectsinthe expectedmagnituderange. Villegasetal. (2010) 0.′′09, or 1.8 to 1.9 pix. At the 20 Mpc distance of Fornax,
foundthattheturnoverintheGCluminosityfunction(GCLF) 1′′ corresponds to about 96 pc, so the typical GC half-light
occursatz =22.802 0.044,wherez isthetotalmagni- radiusofr 3pccorrespondsto0.6pixfortheACS/WFC.
850 850 h
± ≈
tudeinF850LPfromtheACSFCS.WefindinSection6that Thus, the GCs are very marginally resolved, and some with
themeancoloroftheGCcandidatesisI - z 0.3,where smaller r will be indistinguishablefrom point sources. For
814 850 h
≈
NGC1399Color-MetallicityNonlinearity 5
our initial GC candidate selection, we therefore selected all
objectsinthemagnituderange19.5<I <23.5mag,with
814
1<FWHM<4. We also require the candidates to be rea-
sonablyround,withellipticity<1/3(inpractice,thisrejects
veryfew objects, since they are alreadyrequiredto be com-
pact). InSection6, wherewematchoursampleagainstthat
oftheACSFCS, we confirmthatthe overwhelmingmajority
oftheobjectsarehigh-probabilityGCs.
The catalogs produced by SExtractor include magnitudes
measured within many different apertures. In general, we
foundthatthemagnitudesofGCcandidatesmeasuredwithin
theSExtractor6pix(diameter)apertureprovedtobeoptimal,
inthesensethatthescatterbetweencolorswasnearthemin-
imum,yettheapertureenclosed 75%ofthetotallightfor
∼
point-likeobjects, thusmakinga goodcompromisebetween FIG.4.— Optical color-magnitude diagrams for GC candidates in
statistical and systematic errors (cf. Appendix F of Sirianni NGC1399fromACS/WFC imaging. Dashedhorizontal lines indicate the
etal.2005).ThisprovedtobethecaseforboththeACS/WFC rangeoverwhichthecolorhistogramsaresignificantlybimodal,andthever-
tical lines indicate the peaks in the color distributions for this magnitude
and WFC3/IR photometry. Althoughforthe ACS/WFC this
range. Thebrightest GCs, morethan ∼2magabovethe turnover, donot
aperturecorrespondstoaradiusr=0.′′15,whileforthedriz- exhibitdistinctbimodalitybecauseNGC1399isastrong“bluetilt”galaxy.
zled WFC3/IR data it is r =0.′′30, the FWHM of the point
spread function (PSF) of WFC3/IR is roughly twice that of in their brightest magnitude bin followed a unimodal color
ACS/WFC; thusthe aperturecorrectionsfromSiriannietal. distribution peaking at C- T 1.5 in the Washington sys-
1
≈
(2005) and Kalirai et al. (2009) are very similar when the tem, whereas GCs in the next two magnitude bins exhibited
WFC3/IRapertureistwicethatoftheACS/WFCaperturein aclearlybimodalcolordistributionwiththe“gap”occurring
angularunits. Asa result, thesystematic offsetsin theaper- near the sameC- T 1.5 color. This confirmed the earlier
1
≈
ture colors due to differential PSF effects across bands are resultbasedona smallersamplebyOstrovetal. (1998)that
relativelysmall. thebrightest 1magoftheGCpopulationhadabroadcolor
∼
Following Jordán et al. (2009), we calculated the color distribution, with a mean colorsimilar to the location of the
corrections due to differential aperture effects within our gap in the distribution of the fainter GCs. Dirsch et al. also
apertures (r=0.′′15 for ACS/WFC; r =0.′′30 for WFC3/IR) foundthattheredpeakwasonlyprominentatr.9′, andis
for a typical GC (King model with r = 3 pc and con- moreofaredtailatlargerradii.Bassinoetal.(2006)analyzed
h
centration c = 1.5) convolved with the PSF. For g - I , two additionalMosaic fields to extend the areal coverageof
475 814
V - I ,V - H ,andI - H ,theestimatedcorrections NGC1399GCsystemevenfurther,andfoundresultssimilar
606 814 606 160 814 160
are: +0.02,+0.01,- 0.01,and- 0.02mag,respectively. Since tothoseofDirschetal.(2003).
theseoffsetsaresmall, systematic,andsystematicallyuncer- Figure4showsourg - I andV - I versusI color-
475 814 606 814 814
tain at the 0.02 mag level, we have not applied them to magnitude diagrams (CMDs) for GC candidates selected in
∼
ourcolorsforthisanalysis, butsimplynotethatsuchoffsets I ,asdescribedabove,andmatchedwithobjectsdetectedin
814
wouldbeexpectedforsomeexternalcomparisons. Thecon- theGO-10911g data(takenatthesamepointingandwithin
475
clusionsofthisworkwouldnotchange. Incontrast,thecor- the same orbitas I ) and, separately, with objects detected
814
respondingcorrectionswouldbe 0.1magforcolorsinvolv- inV fromthemulti-pointingGO-10129observations. The
606
∼
ing z , since the PSF is significantly broader in that band; colorsaremeasuredwithintheadoptedr=3pixaperture,and
850
however, in that case (see Section 6), we use the ACSFCS weonlyconsiderobjectswithcolorerrors<0.2magforthis
photometry,for which the colorshave beenconvertedto the aperture. Many large galaxies exhibit a “blue tilt” in their
infinite-aperturevaluesassumingtheKingmodelprofilefora CMDs,atendencyforGCsinthebluepeaktobecomeredder
typicalGCconvolvedwiththePSF(seeJordanetal.2009). at higherluminosities, sometimesmergingtogetherwith the
redcomponent(Harrisetal.2006;Mieskeetal.2006;Strader
4. COLORDISTRIBUTIONS et al. 2006; Peng et al. 2009). In this sense, NGC1399 is
a very strong “blue tilt galaxy,” since the two color compo-
NGC1399wasoneofthefirstgalaxiessuggestedtoshow
nentsmergeforthebrightestGCs(Dirschetal.2003;Mieske
evidenceforbimodalityinitsGCmetaldistribution,basedon
etal.2010). However,Forteetal.(2007)findthatforfainter
Washingtonsystemphotometry(Ostrovetal.1993),although
GCs,intheluminosityrangewherethedistributionisclearly
there was no clear separation between the proposedcompo-
bimodal,thereisnosignificantslopeinthecoloroftheblue
nents in the color distribution of that early sample. The bi-
peakwithmagnitude.Thisisconsistentwithmodelsinwhich
modalityresult,againbasedonphotometriccolors,wasmore
thetiltisduetoself-enrichmentandrequiressomeminimum
evidentforsomeothergalaxies(e.g.,Zepf&Ashman1993).
GC mass threshold (Bailin & Harris 2009). Blakeslee et al.
It is now clear that the situation in NGC1399 is more com-
(2010a)showedthatasimplemass-metallicityrelation,com-
plex.Colorbimodalityispresentinthesystem,butnotforall
bined with a nonlinear color-metallicity transformation, can
subsamples,andwefirstaddressthisissue.
produce bimodal color distributions with a blue tilt in GC
populationswithbroadunimodalmetallicitydistributions. In
4.1. OpticalColorsbyMagnitude
Figure 4, the horizontal lines enclose the magnitude range
Dirsch et al. (2003) discussed the “striking” difference in in NGC1399 where the bimodality is most evident, and the
thecolordistributionofNGC1399’sbrightestGCscompared vertical lines show the approximate locations of the peaks,
tothosewithin.2magoftheGCLFturnover,basedonwide- foundbelow. The tilting of the blue GCs towards the red at
fieldCTIO4mMosaicimagingextendingto 20′. TheGCs I <21.5isclear.
814
∼
6 Blakeslee,Cho,Peng,Ferrarese,Jordán&Martel
FIG.5.—Histogramsofg475- I814colorsfordifferentI814magnituderanges.
FIG.6.—HistogramsofV606- I814colorsfordifferentI814magnituderanges.
NGC1399Color-MetallicityNonlinearity 7
distinctpeakswiththeredpeakcontainingroughly2/3ofthe
GC population. The predominance of red GCs occurs be-
causetheHSTfieldscoverarelativelysmallcentralarea,and
the NGC1399 GC system has a radial color gradient. The
I - H andV - H distributionsappearquite similar to
814 160 606 160
each other: unlike in the optical colors, there are no well-
separated peaks, although neither do they appear to be sim-
ple Gaussians. The V - H distribution shows some evi-
606 160
dence for a blue component; this is understandable because
I - H is sharply peaked (though, again, not symmetric),
814 160
whileV - I is bimodal, soV - H would be expected
606 814 606 160
toshowsomebimodality.
We have used the GMM code (“Gaussian Mixture Mod-
eling”) of Muratov & Gnedin (2010) to quantify the above
FIG.7.— Optical-IR color-magnitude diagrams. The dashed horizontal qualitativeimpressions. Theseauthorsprovideadetaileddis-
linesareshowntofacilitatecomparisonwiththeI814regionenclosedbysim- cussionoftheissuesandpitfallsinherentinbimodalitytests.
ilarlinesinFigure4;theyhavebeenshiftedbythemeanI814- H160colorof TheynotethatGaussian mixturemodelingsuchasGMM or
0.70mag.
KMM(Ashmanetal.1994),byitself,ismoreatestofGaus-
Figures5and6displayg - I andV - I histograms
475 814 606 814 sianitythanunimodality. Asobservedbymanyauthors,uni-
for GC candidatesin severaldifferentI magnituderanges
814 modal but skewed distributions will strongly favor a double
(notethatthereareonlyfourbrighterthanI =20). Inboth
814 Gaussianmodel,evenifthereisnotruebimodalityinthedis-
cases, the appearance of bimodality is greatest within the
tribution. To address this point, they emphasize the impor-
21.5<I814<22.5and22.5<I814<23.5magnituderanges. tanceofthekurtosis,statingthat“kurt<0isanecessarybut
Althoughtheg - I colorsinthebrightestmagnituderange
475 814 not sufficient condition of bimodality.” This is true because
ofFigure5appearsomewhatbimodal,theblue“peak”inthis
thesumoftwopopulationswithdifferentmeansisnecessar-
bright range coincides with the “gap” at g475- I814 0.95 in ilybroaderthanasinglepopulation;therefore,inthecaseof
≈
thecolorhistogramsofthefainterGCs. Thesituationissim-
Gaussians, which have kurt=0, the kurtosis must be nega-
ilar in Figure 6, although in that case, the histogram of the
tivefora validdoubleGaussian decomposition. Inaddition,
brightest GCs is essentially a broad red distribution that en-
following Ashman et al. (1994), Muratov & Gnedin define
compassesthegapatV606- I814=0.4forthefainterGCs. the quantity D as the separation between the means of the
Thus, although the Dirsch et al. (2003)and Bassino et al.
componentGaussiansrelativeto theirwidths; theystate that
(2006)studiescoveredamuchlargerareathanourHSTdata,
Gaussian splits that appear significant based on the p value
wefindthesamelackofobviousbimodalityinthecolordis-
buthaveD<2are“notmeaningful,”inthesensethattheydo
tribution of the brightest GCs. Our principal goal here is
notdemonstrateclearbimodality,butnon-Gaussianity.
to examine the optical-IR colors for GCs that define a dis-
Table 2 presents our GMM analysis results; the errors on
tinctlybimodaldistributioninpurelyopticalcolors.Thispro-
the tabulated quantitiescome from the bootstrap resampling
videsasimple,fairlydirecttestofwhetherthecolorslinearly
done in the GMM code. Note that the analysis is indepen-
trace metallicity and reflect true bimodality in the underly- dentofanybinning. Thefullg - I sampleforthe21.5<
475 814
ingdistribution. Inthe followingsection, we thereforecom-
I <23.5magnituderangecontains584objectsandstrongly
814
parethepurelyopticalandoptical-IRcolordistributionsover
prefers a double Gaussian model with peaks at 0.83 0.02
the21.5<I814<23.5magnituderangewheretheopticalbi- and 1.19 0.01 mag. (We remind the reader that the±small
modalityisstrongest. ±
offsetsfor aperturecorrections, as givenin Section3 should
beappliedforexternalcomparisons.) About70%ofthe GC
4.2. Optical-IRColorDistributions
candidates are assigned to the red component. The kurtosis
For comparison to Figure 4, we show the V - H and isnegative,andDissignificantlyabove2,confirmingavalid
606 160
I - H versusH CMDsinFigure7. Thecolorsaremea- bimodal Gaussian model. We therefore assign a “Y” in the
814 160 160
suredwithinaperturesofr=3pix,asdescribedinSection3. lastcolumnofTable2toindicatetheaffirmativeontheques-
Again, only objects with color errors<0.2 mag are shown. tion of bimodality. However, because the g - I distribu-
475 814
Thehorizontallinesherearetheequivalentofthoseshownin tion has a few outliers in tails, and mixture modeling codes
Figure 4, but shifted by the mean I - H =0.70 0.01 are generally sensitive to extended tails, we ran another test
814 160
mag. TheyaremerelyforcomparisohntothepireviousC±MDs, after restricting the range to 0.6<g - I <1.5. The re-
475 814
nottoselectamagnituderangewiththemostbimodality.Any sults, given in the second row of Table 2, are very similar
bimodalityintheseCMDsismuchlessevident. to prior run, but with the kurtosis being even more negative
Figure 8 shows the binned g - I , V - I , I - H , andthemodeluncertaintiesslightlyreduced. Thesituationis
475 814 606 814 814 160
andV - H colordistributions,alongwithsmoothdensity verysimilarforV - I ,whichfavorsawell-separateddou-
606 160 606 814
estimatesconstructedwithaGaussiankernel. Thecolorsin- bleGaussianmodelwithpeaksat0.34 0.01and0.54 0.01
± ±
volving I use the magnitude range selected above, while mag,and74 3%oftheGCsassignedtotheredpeak.Again,
814
forV - H , we use 22<V <24, asthe meanV - I weanswer“Y±”tothequestionofbimodality.
606 160 606 606 814
color is 0.49 mag. We did this for conveniencebecause the Consistentwiththevisualimpression,theGMMcodefinds
pairsofbandpasseswerematchedseparatelyatthisstage;in that the I - H distribution is not bimodal. When run on
814 160
Section6,weexaminethecolorsofasinglemergedsample. the full distribution in Figure 8, the few objects in the tails
Inanycase,theselectiononeitherI orV makesnodif- resultinapositivekurtosisandvirtuallyalloftheobjectsare
814 606
ference to the conclusionshere. The g - I andV - I assigned to the red peak. When we restrict the color range
475 814 606 814
colordistributionsappearstronglybimodal,bothhavingtwo to 0.1<I - H <1.3, the kurtosis becomes negative as
814 160
8 Blakeslee,Cho,Peng,Ferrarese,Jordán&Martel
FIG.8.—Histogramsofg475- I814,V606- I814,I814- H160,andV606- H160colorsforallmatchedGCcandidatesinthespecifiedmagnituderanges
andwithcolorerrorslessthan0.2mag.
required,butthe pvalueofthedoubleGaussianmodelisnot berealwere75%,97%,4%,and22%.
significant,andthefractionofobjectsassignedtothesecond Overall,we confirmthatthe g - I andV - I distri-
475 814 606 814
componentisconsistentwithzero. butionsforGCsinNGC1399arestronglybimodal,withcon-
ForV - H ,thesituationisabitmorenuanced. ForGC sistent proportions of red and blue GCs, when the brightest
606 160
candidates in the broad color range 0<V - H <2, the GCs are excluded. The I - H distribution is not signifi-
606 160 814 160
kurtosisisnegative,andthe pvalueof0.01ismarginallysig- cantly bimodal, but there is marginal evidence for bimodal-
nificant,buttheseparationDisnotmeaningful.Moreover,the ity in V - H consistent with the proportions found for
606 160
weaklyfavoreddecompositionhasonly39 22%oftheGCs g - I and V - I . This is understandable if the opti-
475 814 606 814
±
intheredpeak,ascomparedto 70%fortheopticaldecom- cal color bimodality is partly the result of a changing hori-
∼
position.Ifweremovethetwobluestoutliers,the pvaluebe- zontal branch morphology, since the colors involving bluer
comesslightlylesssignificant,butnowDismarginallyabove bandpasses would be more affected by the behavior of the
2,andthefractionofobjectsinthesecond(red)peakiscon- hothorizontalbranchstars,whereascolorssuchasI - H
814 160
sistentwiththatfoundin theoptical,althoughitisalso con- wouldnot. Thus,someevidenceforbimodalityinV - H
606 160
sistent with 100% at the 2-σ level. We therefore answer an would be expected, but weaker than that foundforV - I
606 814
uncertain“Y?”tothequestionofbimodalityhere,consistent (Cantiello&Blakeslee2007).
withunimodalityinI - H butbimodalityinV - I .
814 160 606 814
Asanothertestofthebimodalityinthecolordistributions, 5. COLOR-COLORCURVATURE
we ran the “Dip” test (Hartigan & Hartigan 1985) supplied The broadest baseline color-color combination we have
with GMM, which nonparametrically measures the signifi- availableforinvestigatingstellarpopulationissuesisg - I
475 814
canceofanygaps,or“dips,”inadistribution. SincetheDip versusI - H . Thesecolorindicesspanfactorsof1.7–2.0
814 160
testisinsensitivetotheassumptionofGaussianity,itismuch in wavelength. The g - I index for GCs is affected by
475 814
morerobust, butasdiscussed byMuratov& Gnedin, the re- thepropertiesofstarsnearthemainsequenceturnoff,onthe
turned significance levels are much lower than for the para- horizontal branch, and on the giant branch. It is therefore
metric GMM analysis. Probabilities above 50% may be sensitive to metallicity, age, and any other parameters (e.g.,
∼
taken as indicative of likely bimodality. From this test, we helium)thataffectthetemperatureofthemainsequenceand
foundthattheprobabilityforthemostsignificantgapsinthe themorphologyofthehorizontalbranch.Bycomparison,the
g475- I814,V606- I814,I814- H160, andV606- H160 distributionsto V606- I814 index spans a much smaller factor (1.3) in wave-
NGC1399Color-MetallicityNonlinearity 9
FIG.9.—TheACS+WFC3/IRI814- H160 versusg475- I814 color-colorplots. Intheleftpanel,theinvidualmatchedGCcandidatesareshown,
with error bars estimated from the RMS images mentioned in Section 3. The red curve represents a 4th-order robust polynomial fit with
coefficientsgiveninthetext. Therightpanelshowsthemedianvalueswithintenbins(40objectsperbin)orderedbyg - I . Wealsoplot
475 814
thesamepolynomialfitasintheleftpanel(reddashedcurve),a4th-orderpolynomialfittothemedianpoints(bluecurve),andalinearfitto
thesepoints(blackdashedline). Errorbarsonthepointsareplotted,butinmostcasestheyaresmallerthanthepointsize. Theplotscaleis
expandedintherightpaneltoclarifythedifferencesbetweenthefits.
length,andislesssensitivethang - I tofeaturessuchas galaxies, or to real stellar population variations. In particu-
475 814
thebluehorizontalbranch. AsdiscussedintheIntroduction, lar,objectsthatlieaboveortotheleftoftherelationcouldbe
colors such as I - H are mainly sensitive to the temper- youngclusters,ortheymayhaveextremehorizontalbranches.
814 160
ature of the giant branch, which is controlled by metallicity Despitesomeoutliers,thereisatightlocusofobjectsthatde-
with very little age dependence. Colors such as g - H fine the curved sequence in color-color space traced by the
475 160
andV - H arealsosensitivetogiantbranchtemperature, abovefit.
606 160
but they complicate the metallicity dependence with addi- Tofurtherillustratethecurvature,wehavebinnedthedata
tionalsensitivitytothemainsequenceturnoffandhorizontal by g - I into 10 groups of 40 objects, and we plot the
475 814
branch.Wethereforeconcentratenowontherelationbetween medianvaluesofthecolorswithinthebinsintherightpanel
g - I andI - H . ofFigure9. Thepolynomialfitfromtheleftpanelisplotted
475 814 814 160
Figure 9 (left panel) plots I - H as a function of again,aswellaslinearandquarticfitstothebinneddata.The
814 160
g - I for401NGC1399GCcandidatesinourcombined deviationfromalinearrelationishighlysignificant.Ifweuse
475 814
ACS-WFC3 data set with 19.5 < I814 < 23.5 and 0.5 < thescattertoestimatetheerrors(σ/√N)inthemedians,the
g475- I814<1.6. ThecorrespondingdataarelistedinTable3. value of the reduced χ2ν is 2.7 for the linear fit, and 0.1 for
Althoughwe deal throughoutthis work with aperturecolors the quartic fit. Thus, the linear fit is strongly rejected. The
withina3-pixelradius,inordertofacilitateexternalcompar- lowvalueofχ2 forthequarticfitprobablyindicatesthatthe
ν
isons,thelasttwocolumnsofTable3providecolorsthathave
uncertainties in the medians have been overestimated, most
beencorrectedfordifferentialapertureeffects, assuming the
likelybecause the smallfractionof outliersmakethe scatter
correctionsforatypicalGC,asgiveninSection3. Sincewe
areinterestedhereintherelationshipbetweenthesecolorin- estimatetoohigh.Thisalsosuggeststhatχ2ν shouldbelarger
than2.7forthelinearfit. The4thordermodelcloselytraces
dices,ratherthansimplythepresenceorabsenceofbimodal-
thecurvatureofthebinnedpoints;addinganothertermtothe
ity, we improvethe statistics by includingthe GCs from the
brightestmagnituderangeinFigure5,increasingthesample polynomialdoes not reduce χ2ν further. The polynomialfits
to the binned and unbinned data diverge near the endpoints
by 17%. The relation appears nonlinear. The plotted curve
because of the lack of constraints, but they agree well over
represents a quartic (4th order) polynomial, which has been
the 0.8–1.3colorrange.
fittedtothedatausingrobustorthogonalregression(Jefferys ∼
Figure10showsthefitresidualsasafunctionofthemean
etal.1988). Thisapproachminimizestheresidualsinthedi-
of the g - I and I - H colors, which corresponds to
rectionorthogonalto the fitted relation, andit is appropriate 475 814 814 160
half of the g - H color. Since the color-color relation
here because of the significant observational scatter in both 475 160
has an average slope of about one (the linear fit in Figure 9
coordinates.Thefittedcoefficientsaregivenby
hasslope1.01 0.06),thismeancoloris, toagoodapprox-
±
I - H = - 14.54+49.23x- 59.02x2+30.14x3 imation, proportionalto the distance along the relation (and
814 160
therefore in the direction orthogonal to the residuals). The
- 5.24x4, (1)
small squares in this figure represent individual GC candi-
wherex (g - I ).Severaloftheobjectsappeardiscrepant dates, and their residuals are with respect to the fit given in
475 814
with res≡pect to this relation: these may be due to contami- equation(1).Opensquaresareusedforthefaintestmagnitude
nation in the sample from Galactic stars and/or background of GC candidatesincludedin the fitting, while solid squares
10 Blakeslee,Cho,Peng,Ferrarese,Jordán&Martel
sistent with GCs in the program galaxies were fitted with
King (1966) models using the methodology of Jordán et al.
(2005)to derivehalf-lightradii r and total magnitudes. As
h
described in Jordán et al. (2009), objects with colors in the
range0.5<g - z <1.9andradii0.75<r <10pcwere
475 850 h
assigned probabilities p of being GCs based on their z
GC 850
magnitudesandhalf-lightradii,ascomparedwiththosefound
forobjectsinbackgroundfields. Thosewith p >0.5were
GC
accepted as likely GCs. For the g - z colors considered
475 850
inthissection,weusedtheNGC1399photometriccatalogue
producedasdescribedinJordánetal.(2009)andalreadyused
inseveralACSFCSpublications(Mastersetal.2010;Mieske
etal.2010;Villegasetal.2010;Liuetal.2011).
We used the object positions to match the ACSFCS pho-
tometry for NGC1399 with our g , I , and H mea-
475 814 160
surements. We keep high probability GCs with p 0.9
GC
FIG.10.— Color-color fit residuals are plotted as a function of 0.5× (in practice, there was only one matched object with ≥0.5<
(g475- H160),whichcorrespondstothemeanofg475- I814andI814- H160.We p <0.9).Thismergeddatasetprovidestwoimportantben-
usethismeanforthehorizontalaxisbecauseitisapproximatelyorthogonal GC
totheresidualsthatwereminimizedintherobustregressionfitting(seetext). efits. First, the probabilistic selection based on rh and z850
SquaresymbolsshowtheorthogonalresidualsforindividualGCcandidates should remove most of the remaining contaminants in our
withrespecttothecurveintheleftpanelofFig.9. Filledsquaresareused sample. Second, the ACS g - z colors have been cali-
forobjects withI814<22.5, while opensquares areusedforobjects with bratedempiricallyagainstme4t7a5llic8it5y0. Althoughthethrough-
22.5≤I814<23.5.Thelargecirclesindicatethequarticfitresidualsforthe
medianpointsshownintherightpanelofFig.9. put for objects with GC spectra is about twice as high in
F814Was in F850LP,allowingmore precisecolor measure-
ments with I for a given exposure time, the longer base-
areusedforbrighterobjects.Thefainterobjectsscattermore, 814
but otherwise do not differ systematically from the brighter lineaffordedbyz850improvesthemetallicitysensitivity(Côté
ones. The large circles in Figure 10 show the residuals for et al. 2004). Peng et al. (2006) found that the relation be-
tween[Fe/H] andg - z wasnotadequatelydescribedby
the median-filtered points with respect to the quartic fit for 475 850
thesepointsplottedasasolidcurveintherightpanelofFig- a linear fit; they used a broken linear model with a shal-
ure9. Inthiscase,themedianvalueshaveverylittlescatter, lower slope for g475- z850 > 1.05. This was adequate over
and the fit is essentially identical regardless of whether or- therangeg475- z850=0.7–1.4,butunder-predictedthespectro-
thogonalregressionorasimpleleast-squaresapproach(min- scopicmetallicitiesofM87andM49GCsinthe1.4–1.6color
imizing residuals in I - H ) is used. From Figure 10, we range. Additionalcurvature was needed to match these. To
814 160
accommodatethemeasuredmetallicitiesoftheseredderGCs,
concludethatthe formof the color-colorrelationis wellap-
proximatedbyaquarticpolynomialoverarangeing - H Blakesleeetal.(2010a)usedapolynomialfitthatfollowedthe
from 1.3to 2.2mag,whichcorrespondsapproxim47a5tely1t6o0 data over the full0.7–1.6range in g475- z850. This empirical
the0.∼8.g ∼- I .1.3magrangequotedabove. calibrationprovidesuswithsomehandleonthemetallicities
475 814
We emphasize that not only is the relation between foroursample.
g - I andI - H nonlinear,buttheslopeattainsalocal Figure 11 shows the color and estimated metallicity his-
475 814 814 160
tograms for our sample of ACS+WFC3 data after merging
minimumwithin the colorrangewhere itis particularlywell
constrained. In fact, the inflection point of the quartic fit to with the ACSFCS dataset. Again we use the 21.5<I814 <
the unbinned data occurs at g - I =1.002, which corre- 23.5magnituderangeforthehistogramstoselecttheregime
475 814
spondscloselytothedipintheg - I colordistributionin of optical color bimodality. After the merging with ACS-
Figure 8. If I814- H160 is a good,47r5elat8i1v4ely simple, indicator FCS, there was one object at g475- I814 =0.45 mag, whereas
of metallicity, in line with theoreticalexpectations, then this all the others were in the 0.64–1.52range; we removed this
singleoutliering - I color. Incontrasttotheapproachin
type of “wavy” or inflected relation could produce double- 475 814
peakedopticalcolorhistogramsfrommetallicitydistributions Figure8,theexactsame322high-probabilityGCsarerepre-
that have a very different form. The metallicity distribution sentedinallofthecolorhistogramsinFigure11. Forunifor-
mity, we have used the same binsize of 0.07 mag for all the
doesnotneedtobeunimodalorevensymmetric;chemicalen-
richmentscenariostend to produceasymmetric distributions colors. The change in binning is mainly responsible for the
(seeYoonetal.2011bforadetaileddiscussionofthisissue). differentappearanceoftheI814- H160 histogramsinFigures8
and11,butthekerneldensitycurvesreflectthesamefeatures.
Weaddressthemetallicitiesbrieflyinthefollowingsection.
The observed wavy relation between g - I and Asdescribedabove,themetallicityhistograminthelower
475 814
I - H accounts for the marked difference between the rightofFigure11isbasedonthepolynomialfitbyBlakeslee
814 160
etal.(2010a)tothePengetal.(2006)data.Weusethispoly-
purelyopticalandoptical-IRcolordistributionsfoundinFig-
ure8. Thesignificanceofthecurvaturedemonstratesthatthe nomial transformation only for objects with g475- z850 <1.6
apparentdisparity was notthe result of underestimatedpho- mag,sinceitisnotconstrainedbeyondthis;objectswithred-
tometricerrorsobscuringbimodalityinI - H . der g475- z850 colors (7.5% of our merged sample) are likely
814 160
affected by observational scatter. Thus, there are fewer ob-
jects in the [Fe/H] histogram, and the truncationat [Fe/H]=
6. ACSFCSCOMPARISON:IMPLICATIONSFORMETALLICITY 0.76dexisartificial.However,thepeakat[Fe/H] - 0.3dex,
≈
TheACSFCS(Jordánetal.2007)isasurveywithACSin andthe long tail to lower metallicities, accuratelyreflectthe
theF475WandF850LPbandsof43early-typeFornaxclus- g - z distributioncombinedwiththeempiricalmetallicity
475 850
ter galaxies. All objects with colors and magnitudes con- calibration.