The merger remnant NGC 3610 and its globular cluster system: a large-scale study

1.9 MB·

by  Lilia P. Bassino

MNRAS000,1–14(2016) Preprint10January2017 CompiledusingMNRASLATEXstylefilev3.0
The merger remnant NGC3610 and its globular cluster system:
a large-scale study
Lilia P. Bassino1,2⋆ and Juan P. Caso1,2
1FacultaddeCienciasAstronómicasyGeofísicasdelaUniversidadNacionaldeLaPlata,andInstitutodeAstrofísicadeLaPlata
(CCTLaPlata–CONICET,UNLP),PaseodelBosqueS/N,B1900FWALaPlata,Argentina
2ConsejoNacionaldeInvestigacionesCientíficasyTécnicas,Rivadavia1917,C1033AAJCiudadAutónomadeBuenosAires,Argentina
7
1
0
AcceptedXXX.ReceivedYYY;inoriginalformZZZ
2

n
a
ABSTRACT
J

9 WepresentaphotometricstudyoftheprototypemergerremnantNGC3610anditsglob-

ular cluster (GC) system, based on new GEMINI/GMOS and ACS/HST archival images.
]
A Thanks to the large FOV of our GMOS data, larger than previous studies, we are able to
detecta ‘classical’ bimodalGC colourdistribution,correspondingto metal-poorandmetal-
G
richGCs,atintermediateradiiandasmallsubsampleoflikelyyoungclustersofintermediate
. colours,mainlylocatedintheoutskirts.TheextentofthewholeGCsystemissettledasabout
h
p 40kpc.TheGCpopulationisquitepoor,about500±110membersthatcorrespondstoalow
- totalspecificfrequencySN ∼ 0.8.Theeffectiveradiiofaclustersamplearedetermined,in-
o
cludingthoseoftwospectroscopicallyconfirmedyoungandmetal-richclusters,thatareinthe
r
t limit betweenGC and UCD sizes and brightness.The large-scalegalaxysurface-brightness
s profilecanbedecomposedasaninnerembeddeddiscandanouterspheroid,determiningfor
a
[ both larger extents than earlier research (10kpc and 30kpc, respectively). We detect boxy
isophotes,expected in mergerremnants, and show a wealth of fine-structurein the surface-

1 brightnessdistributionwithunprecedenteddetail,coincidentwiththeouterspheroid.Thelack
v
ofsymmetryinthegalaxycolourmapaddsanewpieceofevidencetotherecentmergersce-
6
narioofNGC3610.
5
0 Keywords: galaxies:clusters:individual:NGC3610-galaxies:ellipticalandlenticular,cD
2 -galaxies:evolution
0
.
1
0
7
1 INTRODUCTION ‘fine-structure’ofshells,plumes,boxyisophotes,and‘X-features’
1
is widely considered as typical of disc-disc mergers of similar
: Globular cluster systems (GCSs) have been extensively studied
v massgalaxies(e.g.Hernquist&Spergel1992;Barnes&Hernquist
i in the past two decades, but a great effort has been focused 1992). Located in a low-density environment, NGC3610 is in-
X mainly in early-type galaxies in dense environments. The large
cludedinthegroupLGG234(Garcia1993)ortheNGC3642group
r number of globular clusters (GCs) present in these systems (e.g.
a (Fouqueetal.1992),inbothcasesafive-membergroupcomposed
Dirschetal.2003;Bassinoetal.2006;Harrisetal.2013,andref-
of the same galaxies. In fact, Madoreetal. (2004) identified four
erences therein) favours the statistical study of GCs, but restricts
physical companions (with known redshifts) within 300kpc and
ourknowledgetotheseparticularlyevolvedsystems.Despiteearly- 225kmsec−1fromthecentralgalaxy.
typegalaxiesinlowenvironmentsusuallyhaveratherpoorGCSs ±
(e.g.Spitleretal.2008;Laneetal.2013;Casoetal.2013b),their The complex structure of NGC3610 has been studied for
characteristics might provide clues to figure out the evolutionary decades. In their analysis of about 160 early-type galaxies,
historyofthegalaxyitself(e.g.Salinasetal.2015;Escuderoetal. Ebneteretal. (1988) had already detected that NGC3610 posses
2015;Casoetal.2015). boxy isophotes. The structure of NGC3610 has originally been
NGC3610 is considered as a prototype of an intermediate- studied by Scorza&Bender (1990), who pointed out at the un-
age merger remnant (Howelletal. 2004), classified as peculiar usual presence of an inner disc in an elliptical galaxy, while
lenticularorshellellipticalgalaxy(e.g.deVaucouleursetal.1991, Schweizer&Seitzer (1992) selected this galaxy as the one pre-
andNED,NASA/IPACExtragalacticDatabase).Thepresenceofa sentingtherichest fine-structureamongtheir sampleof 69Eand
S0mergercandidates.Michard&Marchal(1994)studiedthemor-
phology of over 100 E-S0 galaxies and pointed that NGC3610
⋆ E-mails:[email protected](LPB);[email protected] has astrongly twisted and peculiar envelope aswell as an asym-
(JPC) metric structure, presenting an embedded inner disc. Later on,
(cid:13)c 2016TheAuthors
2 L. P. Bassinoet al.
Whitmoreetal. (1997) performed a complementary study of the
galaxyanditsGCSbasedonHSTdata,presentingevidencethatit
isadynamicallyyoungelliptical.
Whitmoreetal.(1997)alsopointedtotheexistenceofanin-
termediate age population of GCs in NGC3610, that might have
originated in a past event related to the peculiar structure of this
galaxy.Afterwards,theinnerGCSwasstudiedbyWhitmoreetal.
(2002) and Goudfrooijetal. (2007) on the basis of images ob-
tainedwiththeWideFieldPlanetaryCamera2andtheAdvanced
Camera for Surveys (ACS), respectively, both on board the Hub-
bleSpaceTelescope(HST).Theyrevealedanunusualbehaviourof
theredmetal-richGCluminosityfunction(GCLF),beingthissub-
population of intermediateage ( 1.5 4Gyr) and formeddur-
∼ −
ing agas-rich merger. The inner half of such sub-population (for
radii smaller than 45arcsec) showed a flattening in theGCLF
∼
that is consistent with the predictions of GC disruption models
(e.g. Fall&Zhang 2001). Thus, it is a likely consequence of the Figure1.TwoGMOS-Sfieldsfromourprogramme,andthesmallerACS
stronger tidal field in the inner regions, that cause a more effec- fieldobtainedfromtheHSTDataArchive,superimposedonanRimage
tivelow-massclusterdisruptionthanfurtherout.Alternatively,the fromthePalomarObservatorySkySurvey.Theimagesizeis11arcmin×
outerhalf(uptoagalactocentricdistance 100arcsec)isconsis- 7.5arcmin.Northisup,Easttotheleft.
∼
tentwithapower-lawGCLF(seealsoGoudfrooijetal.2004).
Inaddition,asmallsampleofGCshasbeenspectroscopically 2 OBSERVATIONSANDREDUCTION
confirmed(Straderetal.2003,2004),includingacoupleofyoung
2.1 Observationaldata
andmetal-richones.TheyareidentifiedasW6andW11,withages
of1–2Gyrand1–3Gyrold,andmetallicities[Z/H] =+0.4and The data set consists of images obtained with GMOS (Gem-
+0.7,respectively.Theseagesareinagreementwiththeestimateof ini North) in g′, r′, and i′ filters, during semester 2013A (pro-
1.6+/-0.5Gyrobtained forNGC3610byDenicolóetal.(2005). grammeGN2013A-Q-42,PI:J.P.Caso),plusACS(HST)imagesin
According to such result, it is inferred that the intermediate-age F555W andF814W filters(programme9409,PI:P.Goudfrooij)
clusters formed in the disc-disc merger that has probably origi- obtained from the HST Archive, and originally observed during
natedthegalaxy.Lateron,Georgievetal.(2012)obtainedphoto- June2003.
metricalagesandmetallicitiesforasampleof50brightGCcandi- The GMOS images correspond to two slightly overlapped
dates(someofthemalreadystudiedbyStraderetal.),usingoptical fields(Fig.1),oneof themcentredonthegalaxyNGC3610, and
and near-IR imaging. By means of colour-colour diagrams based theotheronelocatedtotheWest(hereafter‘N3610F’and‘WestF’,
on that photometric combination, it is possible to break the age- respectively).Thepoint-sourceslocatedintheoverlappingregion
metallicitydegeneracy. Comparing withthespectroscopically de- willallowanypossiblezero-pointdifferencesinthecorresponding
rived parameters, the metallicities are in agreement while photo- magnitudestobedetermined.Theexposuretimeswere4 450s
metricagesare 2Gyrolderthanspectroscopicones,thoughthe ing′,4 210sinr′,and4 270sini′.Eachsetofexposu×reswas
agedifferenceb∼ecomessmallerformoremetal-richGCs.Theage slightly×dithered,inorderto×fillinthegapsoftheGMOSfieldand
andmetallicitydistributionsobtainedbyGeorgievetal.(2012)also toefficientlyremovecosmicraysandbadpixels.
pointtotheirbrightclustersamplebeingdominatedbyametal-rich The ACS field (‘ACSF’) is also centred on NGC3610 (see
andintermediate-agesub-population. Fig.1). The exposure times were 6410s in filter F555W (two
Thispaperhasbeenplannedasacomplementtotheprevious 2330s observations plus a 1770s third one) and 6060s in filter
ones, as our observational data will allow us to cover one of the F814W (two 2330s observations plus a 1770s third one). The
processed images weredownloaded fromthe HSTData Archive.
largestfield-of-view(FOV)usedtostudythisgalaxyanditsGCS
sofar.Thus,wewillbeabletoestablishthewholeGCSextension, IRAFtasksGEOMAPandGEOTRANwereusedtoregisterthese
images,duetodifferencesinthepositionoftheirFOV.Addition-
perform an homogeneous comparison of the GC colour distribu-
ally,another ACSfieldofthe47Tucoutskirtswasusedtomodel
tionsindifferentradialregimes,aswellasanalysetheouterregions
the point-spread function (PSF).These observations were carried
ofthesurface-brightnessdistributionofthegalaxyitself.
outinthesamefiltersastheNGC3610images,duringJune2003
The most recent distance determinations for NGC3610,
(programme9656),beingtheexposuretimeof30s.
based on surface-brightness fluctuations (SBF) gives 35Mpc
∼
(Cantielloetal. 2007; Tullyetal. 2013), remarkably larger than
previous ones(Tonryetal.2001;Blakesleeetal.2001).Itshelio- 2.2 Photometryandpoint-sourceselection
centricradialvelocityis1707 5kms−1 (Cappellarietal.2011),
± First,thesurface-brightnessprofileofNGC3610wasobtainedwith
lowerthanexpectedfortheredshiftindependent distances.Inthe
the ELLIPSE and the corresponding synthetic galaxy, generated
following,wewilladopttherecentSBFestimates,henceadistance
with BMODEL (both IRAF tasks), was subtracted from the orig-
modulusm M =32.7 0.1.
− ± inal image in order to improve the detection of point-sources as
Thispaperisorganizedasfollows.Theobservationsanddata muchaspossible.
reductionarepresentedinsection2,theresultsaredescribedinsec- For the GMOS fields, the software SEXTRACTOR
tion3, and section4 is devoted to the discussion. Finally, a sum- (Bertin&Arnouts 1996) was applied to the i′ image, which
maryandtheconcludingremarksaregiveninsection5. wasselectedbecauseithasthehighersignal-to-noiseratio(S/N),
MNRAS000,1–14(2016)
NGC3610anditsglobularclustersystem 3
inordertoobtainaninitialpoint-sourcecatalogue.Astheeffective InthecaseofACSdata,thecalibratedmagnitudes wereob-
radii (Reff) of classical GCs is usually smaller than 10pc (e.g. tainedusingtherelation:
Brüns&Kroupa 2012), at the adopted distance the NGC3610
GCsare seen aspoint-sources on our images. Then, we used the
SEXTRACTORparameterCLASS_STARtoeliminatetheextended mstd =minst+ZP
sources from our catalogue. The photometry was performed
with the DAOPHOT package (Stetson 1987) within IRAF. A foreachfilter,withzero-pointsZPF555 =25.724andZPF814 =
second-ordervariablePSFwasgeneratedforeachfilter,employing
25.501,takenfromSiriannietal.(2005),sothattheresultingcali-
a sample of bright stars, well distributedover the field. The final
bratedmagnitudescorrespondtoV andIfilters,respectively.
point-sourceselectionwasmadewiththeχandsharpparameters,
TheACSdatahasalreadybeenusedtostudythepropertiesof
fromthetaskALLSTAR.
theGCcandidatesbyGoudfrooijetal.(2007),whoappliedalong
InthecaseofACSdata,GC-likeobjectsmightbemarginally
their work corrections for contamination by background objects.
resolvedatthedistanceofNGC3610(e.g.Casoetal.2013a,2014).
Suchcontaminationwascalculatedasthecompactobjectsdetected
Hence, we applied SEXTRACTOR to images in both filters, but beyond a galactocentric radius (hereafter designated with ‘Rg’)
considered as likely GC candidates those sources with elonga-
Rg =100arcsec (i.e. 1.67arcmin), which they considered might
tion smaller than 2 and FWHM smaller than 5pixel. Similar cri-
beslightlyoverestimated.Infact,thankstothelargerareacovered
teria have already been used for identifying GCs on ACS im-
by our GMOS data, we will show in the following that GCs are
ages (e.g. Jordánetal. 2004, 2007). Approximately 40 to 50 rel-
presentuptoRg =4arcmin.Thus,itisworthdoinganewanaly-
atively isolated bright stars fromthe 47Tuc images were used to
sisoftheACSdata,togetherwiththenewonesfromGMOS,but
obtainthePSFforeachfilter.Then,aperturephotometrywasper-
takingintoaccountamorepreciseestimationofthecorrectionfor
formedontheNGC3610field,withanapertureradiusof5pixel,
backgroundcontamination.
whichisalmostthreetimestheFWHMobtainedfortheforeground
In order to estimate the background contamination for the
stars.ThesoftwareISHAPE(Larsen1999)wasusedtocalculate
GMOS data, we considered the point-sources within an area of
structural parameters for the GC candidates, considering a typi- 16.5arcmin2 at the Western side of field WestF, located at
calReff of0.35pixel(seeSection3.5).Approximately16GCand 5arcminfromthecentreofNGC3610.Itwillbeidentifiedinth∼e
ultra-compact dwarf (UCD) candidates brighter than F814W =
following as the ‘comparison region’. As we lack an appropriate
23mag, relatively isolated and with 0.3 < Reff < 0.4pixel in comparisonfieldfortheACSdata,wewillusethecorrectionsob-
bothfilterswereusedtocalculateaperturecorrections,resultingin
tainedfromthecomparisonregionoftheGMOSdata,takinginto
0.07magforF555W and 0.12magforF814W.
− − accountthedifferentdepthsandcompletenesscorrectionsofboth
photometries.
2.3 Photometriccalibrationandbackgroundestimation
AfieldofstandardstarsfromthelistofSmith&etal.(2002)was
observedforourGMOSprogramme,duringthesamenightsthan
thefieldN3610F. Weobtainedthegrowthcurve,andhenceaper-
2.4 Completenessanalysis
turecorrections, fromthestandard stars’ aperturephotometry for
severalapertureradii.Then,wefittransformationequationsofthe InordertoestimatethephotometriccompletenessforourGMOS
form: fields,weadded 250 artificialstarstotheimages of thethreefil-
ters,distributedovertheentirefieldsinanhomogeneousway.Their
coloursspantheexpectedrangesforGCsandmagnitudesequally
mstd =ZP +minst−KMK×(X−1) generated for 21 < i′0 < 26. As this process was repeated 40
where mstd and minst are the standard and instrumental magni- times,weachievedawholesampleof10000artificialstars.Their
tudes, respectively, and ZP are the photometric zero-points for photometrywasperformedfollowingexactlythesamestepsasfor
each band. KMK isthe corresponding mean atmospheric extinc- the science images. After selecting the definitepoint-sources, we
tion at Mauna Kea, obtained from the Gemini Observatory Web obtained the completeness curves shown in Fig.2. For the field
Page1,andX theairmass.Fortheg′filter,partoftheprogramme N3610F, we discriminated between artificial stars located at less
was scheduled in February, while the rest of the observations than1arcminfromtheNGC3610centre(opensquares),andfur-
were obtained a month later, together with a photometric stan- therthanthislimit(filledsquares).Thecompletenessfunctionsfor
dardsfield.Thezero-pointsforFebruaryandMarchobservations the outer region of N3610F and the field WestF are very similar,
wereZPg′ = 28.29 0.03,andZPg′ = 28.30 0.03,respec- achievingthe70percentati′0 ∼ 25.Thisvaluehasbeenconsid-
tively. The final assum±ed zero-points for the g′, r±′, and i′ filters eredasthefaintmagnitudelimitinthefollowinganalysis.
wereZPg′ = 28.30 0.03,ZPr′ = 28.38 0.02andZPi′ = InthecaseoftheACSFfield,weadded50artificialstarsper
28.49 0.03,respecti±vely.Inthenextstep,we±appliedtheGalactic image,spanningthecolourrangeofGCsand21.5<I0<27.We
± repeated the process toachieve afinal sample of 50000 artificial
extinctioncorrectionsobtainedfromSchlafly&Finkbeiner(2011)
stars, which allowed us to calculate the completeness curves for
tothecalibratedmagnitudes.Finally,consideringthepoint-sources
differentgalactocentricranges (Fig.3).Aswellasfor theGMOS
in common in both fields N3610F and WestF, we calculated the
fields, the photometry was developed in the same manner as for
f∆olil′ow=in0g.1z3e.roW-peoainptpldiiefdfetrheensceeso,ff∆segt′s=tot−he0.W06e,s∆tFrc′a=talo0g.1u0eaanndd thesciencefield.WeselectedasmagnitudelimitI0∼25.5,which
mightrepresentaconservativeselectionfortheoutergalactocentric
referredthephotometrytothefieldN3610F.
ranges, but corresponds to 60 per cent in the range 15arcsec
∼
< Rg <30arcsec.FortheGMOSandACSfields,itisclearthe
1 http://www.gemini.edu/sciops/instruments/gmos/calibration decreaseofthecompletenessasweapproachthegalaxycentre.
MNRAS000,1–14(2016)
4 L. P. Bassinoet al.
n 1 1
o − −
cti8 0 WestF 0 N3610F
a.
r0
F 1 1
ss −i’)0 2 −i’)0 2
e4 g’ g’
en0. ( 3 ( 3
et N3610F
pl 4 4
m WestF
o0 5 5
C0.21 22 23 24 25 26 −1 0 1 2 3 −1 0 1 2 3
(r’−i’) (r’−i’)
0 0
i’
0
0 WestF 0 N3610F
0. 0.
Figure2. Completeness curves forthe twoGMOS fields, asfunction of
ig′0uimshaegdnbiteutdwee.eTnhaertbifiinciwalidsttharissa0t.l2e5ssmthagan.F1oarrcthmeinfieflrdomN3N6G10CF3,6w1e0dciesntitnre- −r’)0 0.5 −r’)0 0.5
(opensquares),andfurtherthanthislimit(filledsquares). (g’ 1.0 (g’ 1.0
5 5
n 1. 1.
o
acti8 5 4 3 2 1 0 −1 5 4 3 2 1 0 −1
Fr0. (g’−i’)0 (g’−i’)0
s
s R > 60
e g
n4 Figure4.Colour-colourdiagramsforbothGMOSsciencefields.Solidlines
ete0. 4350 << RRg << 6405 indicatethecolourrangesofselectedGCcandidates(seeSection3.1).
pl g
m 15 < R < 30
Co0.020 21 2g2 23 24 25 26 27 fhualvfiell(ogu′r−coi′l)o0ur∼cr1it,e(rgia′.−r′)0 ∼0.7and(r′−i′)0 ∼0.3,which
I In the ACS photometry, we selected as GC candidates the
0
point-sourcesormarginallyresolvedsourceswithcoloursranging
0.75 < (V I)0 < 1.4(Bassinoetal.2008).InFig.5,theleft-
nFiitguudree,f3o.rCdoimffeprleentetngeaslsacctuorcveenstfroicrtrhaengAeCsS(RFgfieilnd,aarcssaecf)u.nTcthioenboinfIw0′idmthagis- hmaangdnistiuddeeand−diagmriadmdsle(CpaMneDls)sfhoorwthethtewio′0GveMrsOusS(fig′el−dsi.′T)0hecoblloaucrk-
0.25mag. dots represent the objects that fulfill the point-source criteria, af-
tertheχandsharpselection.Thefilledcirclesidentifythosethat
fulfilltheGCcandidates’coloursandmagnitudecriteriaindicated
3 GLOBULARCLUSTERSYSTEMOFNGC3610
before. The right-hand side panel shows the I0 versus (V I)0
−
3.1 SelectionofGCcandidates CMDfortheACSfield,withGCcandidateshighlightedwithfilled
circles.
rT′h)e0cvoelrosuusr-(cgo′lo−uri′d)i0afgorramalslt(hge′p−oiin′t)-0sovuerrcseussi(nr′bo−thi′G)0MaOndS(fige′ld−s selecItendotrhdeer62toGcCalccualnadteidtahtees(wV,itIh)atvoai(lga′b,lie′)cotrlaonusrfsoirnmbaotitohnp,hwoe-
anraerrporwesreanntgedesi,nwFeigc.a4n.uAsestthheesceodloiaugrrsaomfsbtoondai-sfitdinegGuiCshstfhaellmwfirtohmin tometricsystems,withi′0 <24mag.Fig.6shows(g′−i′)0versus
(V I)0 colours for these objects. The solid line represents the
thecontamination (e.g.Casoetal.2015, andreferencestherein). −
transformationobtainedwithalinearleast-squaresfit,
We selected as GC candidates the point-sources in the following
ranges:0.4 < (g′ i′)0 < 1.4,0.3 < (g′ r′)0 < 1and0 <
(r′ i′)0 <0.5.Th−eseadoptedlimitingcolou−rsaresimilartothose (g′ i′)0 =1.26 0.06 (V I)0 0.43 0.07 (1)
− − ± × − − ±
chosenbyCasoetal.(2015)andEscuderoetal.(2015).
Asacomparison,fromanequivalent transformationpresentedby
Straderetal.(2003,2004)inferredagesandmetallicitiesfora
Faiferetal.(2011),theirequation3,wecanderivethecoefficients
sampleof13GCcandidatesinNGC3610,bymeansofLick/IDS
1.25and0.4,respectively.Accordingtothistransformation,weim-
indices,resultinginamajorityofoldGCs.However,twoGCcan-
d(ida2teGsyinr athnedir[Zsa/mHp]letur0n.e5d).oIuntotordbeerytooucnognfiarnmdvthearyt emveetnalt-hreicshe pagroreveewouitrhGthCecoonloesuradliompittesdasfo0r.6(g6′<−i(′V)0−.BIe)t0w<een1.I405asnodthi′0atththeerye
∼ ∼ isonlyazero-pointdifference,
GCswouldbeidentifiedwiththepresentselectioncriteria,weob-
tainedtheexpectedcoloursinourbandpassesfromthetheoretical
umsoindgeltshoefirswinegble-bsatseelldartopoolp2u.laIftiwonesc(oSnSsPid)ebryaBCrheassbarnieret(2a0l.0(12)0l1o2g)-, i′0 =I0+0.43±0.01 (2)
normal IMF and the reddening corrections applied to our point-
sources catalogue, aSSPwiththese ages and metallicitieswould 3.2 Colourdistribution
In order to analyse the GC colour distribution, we separated the
2 http://stev.oapd.inaf.it/cgi-bin/cmd sample in four radial regimes, corresponding to 15 < Rg <
MNRAS000,1–14(2016)
NGC3610anditsglobularclustersystem 5
20 N3610F 20 WestF 20 ACSF 15’’ < Rg < 30’’ 30’’ < Rg < 1’
0
2
21 21 21 15
N
0
2 1
2 2 2
2 2
5
i’0 i’0 I023
3 3
2 2
5 1’ < R < 2’ 2’ < R < 4’
1 g g
4
2
4 4
2 2
0
5 1
2 N
5 5
2 2
6 5
2
0.5 1.0 1.5 0.5 1.0 1.5 0.6 1.0 1.4
(g’−i’)0 (g’−i’)0 (V−I)0
0.4 0.6 0.8 1.0 1.2 1.4 0.4 0.6 0.8 1.0 1.2 1.4
Figure 5. Colour-magnitude diagrams for the two GMOS fields and the (g’−i’) (g’−i’)
0 0
ACSfield.Filledcircleshighlightthoseobjectsthatfulfillthecoloursand
magnitudecriteriaappliedtoselectGCcandidates.
Figure7. Background-correctedcolourdistribution(dashedlines)forthe
GCcandidatesfromACSphotometry(upperleftpanel),overlappingACS
andGMOSdata(upperrightpanel,GMOSsampleisthesmallerone),and
2
. GMOSphotometry(lowerpanels),forfourradialintervalsasindicatedin
1
therespective panels.SolidlinesshowtheresultsfromGMMfittingand
0 verticaldot-dashedlinesindicatethetypicalcoloursofblueandredpeaks
−i’) .0 ((g′−i′)0=0.82and1.10,respectively).Please,noticethedifferentscales
1
g’ inupperandlowerpanels.
(
8
.
0
bimodaldistributionprovidesarealisticfit(ameaningfulbimodal
1.0 1.1 1.2 1.3 caseisacceptedforDD > 2),andthekurtosis(κ),whichisex-
(V−I) pected to be negative for bimodal distributions. For the sake for
0
comparison, we have included in Table 1 the results of the uni-
Figure6.(g′−i′)0 versus(V −I)0 coloursforasampleofbrightGC modalfitseveninthecaseswhereDDandκpointedtoabimodal
candidateswithavailablemagnitudesinbothphotometricsystems. fit. According to the calculated DD and κ parameters, the three
innersubsamplesseemtobebetterdescribedbybimodaldistribu-
tions,but justasingleGaussianshould befittedtotheoutermost
30arcsec, 30arcsec < Rg < 1arcmin, 1 < Rg < 2arcmin and one. Moreover, it isalsonoticeable from Fig.7how different the
2< Rg <4arcmin.Tothisaim,wecombined GMOSandACS GCcolourdistributionsarewhenwediscriminatethemaccording
data, transforming ACS photometry into (g′ −i′) colour and i′0 toRg.
magnitude bymeans of equations(1) and (2). Bothdata setswill For the inner radial subsample, 15 < Rg < 30arcsec, the
overlap in only one radial range (30arcsec < Rg < 1arcmin). colour distribution is better fitted by two Gaussians, with ex-
Fig.7 shows the background-corrected (g′ i′) colour distribu- pected values for the blue and red peaks (mean colours), i.e.
tion(dashedlines)forGCcandidatesinthes−efourdifferentradial (g′ i′)0 0.8 and 1.1 (e.g. Escuderoetal. 2015). This distri-
ranges,withi′ <25andapplyingabinwidthof0.08mag. buti−onisdo∼minatedbytheredGCsub-population thatrepresents
0
The same procedure was applied in the four radial ranges. more than 70per cent of the subsample. The GC distribution for
First, we obtained a clean sample of GC candidates, randomly 30arcsec< Rg <1arcminisalsobetterfittedbytwoGaussians,
selecting sources from the comparison region and deleting GC asindicatedbytheDDandκparametersforbothACSandGMOS
candidates with similar (g′ i′) colours, until we reached the data.BothdistributionsareshowninFig.7(upperrightpanel)and
−
expected number of objects due to contamination. Then, we ap- the histograms look similar, with a dominant red sub-population
pliedtoeachsampletheGaussianMixtureModelingtest(GMM, andafewmoreGCsintheACSsample.However,thefitsarenotso
Muratov&Gnedin 2010) to determine whether their respective similar,asonlytheACSdatahaveblueandredpeaksinagreement
colourdistributionsarelikelytoberepresentedbythesumoftwo withtheonesestimatedfortheprevious(innermost)radialrange.
Gaussians.Foreachcase,thisprocedurewasrepeated25times,and ThecolourofthebluerpeakestimatedfortheGMOSsubsampleis
themeanresultsarelistedinTable1,includingthemeancolours, inthemiddlebetweentheusualblueandredpeaks,thoughitmust
dispersions,andthefractions(f)foreachsub-population.Thetwo betakenintoaccountthatthedifferencesmaybeduetofewGCs.
lastcolumnscorrespondtotheDDparameter,whichisrelatedto Anyway,thecolour distributionforbothsubsamplesofACSdata
theseparationofthemeanvaluesandindicateswhetheranspecific agreeswiththatfortheinnersubsampleofGMOSdata,inthesense
MNRAS000,1–14(2016)
6 L. P. Bassinoet al.
1.4 2]) 2 All GCs
-n a =−2.8±0.2
mi
c
ar 1
0.4 N
n [
(0 0
1
g
o
l 1
−
−0.5 0.0 0.5
log (r [arcmin])
10
Figure8.ProjectedspatialdistributionforGCcandidates fromACSand
GMOSdata.Thecentreofthegalaxyishighlighted withanopencircle.
Northisup,Easttotheleft.Thefieldofviewis10.2×5.5arcmin2.The 2]) 2 Blue GCs
colourbardepictedonthetopright-handsidecorrespondsto(g′−i′)0. -min a =−2.3±0.2
c 1
ar
N
thattheyarealldominatedbyredGCs.Thisisalsoevident from (n [0 0
theradial projecteddistributiondepictedinFig.9(middlepanel), g1
o
wherethereisacleardeficiencyofblueGCsintheinnerregionas l 1
−
comparedtotheprofileforlargerRg. −0.5 0.0 0.5
Theintermediateradial GMOS subsample (lower leftpanel) log (r [arcmin])
10
showsthemoreclearbimodaldistribution,withblueandredpeaks
in the expected colours (indicated in all panels with dot-dashed
vreedrticclaulstleinrsesatroef6a0cilaintadte4c0ompeprarciesnotn,).reTshpeecftrivacetliyo,nwsiothf bthlueebalnude -2n]) 2 Ra =e−d3 G.1C±s0.3
sub-populationpresentinganextensiontowardsverybluecolours. mi 1
c
Finally, the outermost GMOS subsample (lower right panel) is ar
quiteanomalousasaunimodaldistributionispresent,withamean N 0
(g′ −i′)0 = 0.87. Weare aware that this isa small subsample, (n [10 −1
where a bunch of blue clusters also seems to be present, but the g
o
groupisdominatedbyGCsof‘intermediate’colours.Theirmean l 2
−
colour agrees with the bluer peak obtained for the inner GMOS −0.5 0.0 0.5
log (r [arcmin])
subsample(30arcsec<Rg <1arcmin),sotheymaybeprobable 10
presentatsmallerradiitoo.WehavealreadydetectedGCsof‘inter-
mediate’coloursinothergalaxiesthatexperiencedrecentmergers, Figure9.RawandbackgroundcorrectedradialdistributionsforGCcan-
andwillcomebacktothisintheDiscussion. didates(openandfilledcircles,respectively).Trianglesandcirclesidentify
Summarising, thanks to the larger FOV of GMOS we have thecombineddata,fromACSandGMOS,respectively.Thesolidstraight
beenabetostudytheGCcolourdistributionoverthewholeradial lines represent therespective power-laws, fittedbyleast-squares, andthe
extent, reaching the outer sub-populations never analysed before. respectiveslopesαaredepictedineachpanel.Thedashedlinescorrespond
WerecoveredthedominantinnerredGCsub-populationinagree- totherespectivebackgroundlevels,whilethedottedonesindicate30per
centofthebackgroundlevel,usedtodefinetheGCSextension.Thesolid
mentwiththeresultsfromtheACSdata,andfoundabimodalGC
curvedlineintheupperpanelshowsthefitofaHubbleprofile(seetextfor
distributionatintermediateradiiaswellasasmallnumberofclus-
theresults).Please,noticethedifferentscaleinthelowerpanel.
terswith‘intermediate’ coloursintheoutskirts,though theymay
bepresentatinnerregionstoo.
areingood agreement withintherangeof overlappingRg.Here,
wedonotconsiderseparatelytheintermediatecolourclusters(lo-
3.3 Projectedspatialandradialdistributions
catedintheouterregion)astheyaretoofewtoderivearadialpro-
TheprojectedspatialdistributionforGCcandidatesinbothGMOS file. The adopted colour limit between both GC sub-populations
fields,combinedwiththoseselectedfromtheACSdataintheinner is (g′ i′)0 = 0.95 (Faiferetal. 2011), which corresponds to
−
region,isshowninFig.8.Thecolourrange,asdepictedintheFig- (V I)0 = 1.1. Therespectivebackground levelsareindicated
ure,spans0.4 < (g′ i′)0 < 1.4andthecentreofNGC3610is with−dashed lines, and were determined from the comparison re-
−
indicatedwithanopencircle.ItcanbeseenthatredGCsaremore giondescribedinSection2.3.
concentratedtowardsthegalaxy,whileclustersofbluercoloursap- The background-corrected radial profiles were fitted by
peartodominateatlargerradii. power-lawswithslopes 2.8 0.2, 2.3 0.2and 3.1 0.3for
− ± − ± − ±
Fig.9 presents the raw and background-corrected radial dis- theentire sample, blue and red GCs, respectively (indicated with
tributionsfortheGCcandidatesfromGMOSdata(openandfilled thesolidlines).Duetoincompletenessinthecentralregion,thefits
circles,respectively),andcorrectedbycompleteness.Theopenand wereperformedforradii0.5<r<4arcmin.Theseresultsclearly
filledtrianglesrepresenttheanalogousdistributions,butfromACS confirmthat theredsub-population ismoreconcentrated towards
data.UpperpanelcorrespondstothewholeGCsample,whilemid- thegalaxywhilethebluerclustersoneextendsfurtheraway.
dleandlowerpanelspresentsimilarplotsbutforblueandredGCs, WeestablishedtheGCSextentasthegalactocentricdistance
respectively.ItcanbeseenthatresultsfromGMOSandACSdata atwhichthebackground-correctedsurfacedensityisequaltothe30
MNRAS000,1–14(2016)
NGC3610anditsglobularclustersystem 7
Table1.ParametersoftheGMMfittingtothecolourdistribution,consideringdifferentradialranges.µj,σj andfj correspondtothemean,dispersionand
fractionforeachGaussiancomponent(j=1:blueGCsandj=2:redGCs).WerefertothetextforthemeaningsofDD,andκ.
µ1 σ1 f1 µ2 σ2 f2 DD κ
ACSdata
15′′ <Rg <30′′ 3.19±0.46 -0.67
Unimodal 1.03±0.02 0.15±0.01 − − − −
Bimodal 0.82±0.03 0.06±0.02 0.27±0.09 1.09±0.03 0.11±0.02 0.73±0.09
30′′ <Rg <1′ 2.89±0.45 -0.81
Unimodal 1.02±0.02 0.15±0.01 − − − −
Bimodal 0.84±0.03 0.07±0.02 0.32±0.11 1.10±0.02 0.10±0.02 0.68±0.11
GMOSdata
30′′ <Rg <1′ 2.15±0.17 -0.55
Unimodal 1.03±0.01 0.13±0.01 − − − −
Bimodal 0.97±0.03 0.10±0.01 0.72±0.05 1.16±0.02 0.04±0.01 0.28±0.05
1′ <Rg <2′ 3.15±0.13 -0.97
Unimodal 0.93±0.01 0.16±0.01 − − − −
Bimodal 0.82±0.02 0.09±0.01 0.60±0.03 1.10±0.01 0.07±0.01 0.40±0.03
2′ <Rg <4′ 1.82±0.20
Unimodal 0.87±0.02 0.11±0.01 − − − −
per centof thebackground level (dottedlines).Thiscriterionhas 3.4 LuminosityfunctionandGCpopulation
been previously used insimilar studies (e.g. Bassinoetal.2006),
andimpliesanewdeterminationoftheGCSextentof 4arcmin, Fig.10 shows the background and completeness corrected GCLF
i.e., 40kpcattheadopteddistanceforNGC3610. ∼ fortheGCcandidatesselectedfromtheGMOSdata.Theerrorbars
∼ForbothGCsub-populations, theradialdensityprofilesflat- assumePoissonuncertaintiesforscienceandbackgroundmeasure-
ten towards the galaxy centre. As the completeness analysis was ments, and the bin width is 0.25mag. The bins filled with verti-
performed for different galactocentric radii, in order to take into cal grey lines have not been considered in the GCLF fitting, due
accounttheeffectofthegalaxysurface-brightnessprofile,theflat- to the declining completeness (i.e. according the limit i′0 = 25
tenedradialprofilesmightimplyarealpaucityofGCsintheinner adoptedinSection2.4).Theturn-overmagnitude(TOM)anddis-
regionsofthegalaxy.Inordertoconsidertheslopechange,wefit- persionobtainedfromtheleast-squaresfitofaGaussianmodelare
tedtotheradialprofileoftheentiresample(r<4arcmin)aHubble i′0,TOM =24.6±0.25andσ=0.9±0.24.
profile(Binney&Tremaine1987;Dirschetal.2003)oftheform: Old GC populations in early-type galaxies usually present a
GaussianGCLF,withaTOMintheV-bandofMVTOM ∼ −7.4
(e.g.Richtler2003;Jordánetal.2007),denotingtheuniversalityof
theGCLFthatisareliabledistanceindicator.OnthebasisofACS
r 2 b data,Goudfrooijetal.(2007)foundthattheGCLFforredGCsin
n(r)=a 1+ (3)
(cid:18)r0(cid:19) ! NGC3610 deviates from the usual Gaussian distribution and can
befittedbyapower-law.Ithasbeenclearlyshownthatpower-law
luminosityfunctionscorrespondtoyoungstellarclusters,notbona-
fideoldGCs(e.g.Whitmoreetal.2014).OurGMOSphotometry
wherea = 152 13,r0 = 0.62 0.05,andb = 1.42 0.08
± ± − ± isnotasdeepastheACSone,sowecannotcomparedirectlythe
(solid curved line in Fig.9, upper panel). This profile provides a
behaviourofbothGCLFs.Inaddition,ifwewanttousetheACS
much better fit to the inner radial distribution. As well as GCs
photometrythatwehavere-done,thebackgroundcorrectiontobe
formation requires specific environmental conditions and merger
appliedtothesedatawasobtainedfromtheGMOSimages,sothe
remnantsaretheplaceswhereYMC(youngmassiveclusters)are
samelimitingmagnitudeisvalid.
found(e.g.Kruijssen2014),theseconditionsalsofavourtheirtidal
disruption(Kruijssenetal.2012;Kruijssen2015).Brockampetal. We have adopted for NGC3610 the SBF distance modulus
(2014) also points to the relevance of the erosion of GCs in the (m M 32.7), whichimpliesa‘standard’VTOM 25.3ac-
− ∼ ∼
present-day characteristics of its GCS. The erosion might be re- cordingtotheMVTOMquotedabove.Ifweusethecolourandmag-
sponsible for the evolution of GCs mass function from the ini- nitudetransformationsderivedinSection3.1andconsiderforthe
tial power-law to abell-shaped distribution. TheGCsdestruction GCcandidatesameancolour(g′ i′) 0.96(estimatedfroma
mainlyoccursuptothegalaxyhalf-lightradius,withsmallerup- cleansample),theresultingTOM−isi′ ∼24.6,inagreementwith
0 ∼
perlimitradiusandefficiencywhenwemovetowardsmoremas- thevalueobtaineddirectlyfromthedata.Asweareanalysingthe
siveandextendedellipticalgalaxies.Thedegreeoftheinitialradial global GCLF, and not those of blue and red GCs separately, the
anisotropyinthevelocitydistributionoftheGCSalsoplayamain TOMfromthefittedGaussianagreeswithwhatwouldbea‘stan-
roleinthefractionoferodedGCsandtheradiusatwhichitiseffi- dard’TOM,accordingtotheSBFdistance.Weunderstandthisre-
cient(Brockampetal.2014). sult as just a consequence that we are considering the whole GC
MNRAS000,1–14(2016)
8 L. P. Bassinoet al.
o
50 0.10
40 o c]0.05
e
N 2030 o o o DR [arcseff−0.050.00
0 o
1 o o o o o o −0.10
o o o 21 22 23 24 25 26
0 I0
21 22 23 24 25
i’ Figure11.Left:differencebetweenReff measuredinV andI filtersfor
GCcandidatesfromACSdata,inarcsec.Theverticaldashedlinerepresents
themagnitudelimituptowhichReffisreliable.Right:Reffdistributionfor
Figure 10. Background and completeness corrected luminosity function GCcandidatesbrighterthanI0=24.
(GCLF) for GCs based on the GMOS data. The errorbars assume Pois-
sonuncertainties forscience and background measurements, andthe bin
widthis0.25.Theverticallinesindicatetheluminosityrangefainterthan
i′ =25,thathasnotbeenincludedintheGCLFfitting.
0 ingthesamemonthasthoseofNGC3610.WeselectedaKing30-
profile,i.e.aKingprofilewithconcentrationparameterc=30,be-
ingctheratiooftidalovercoreradius(King1962,1966).Wedid
sample, which is dominated by truly ‘old’ GCs, and we are not
notconsideranypossibleeccentricity,butlargevaluesarenotex-
able to study separately the LF of the intermediate-age GC sub-
pected for GCs nor for UCDs (e.g. Harris 2009; Chiboucasetal.
population.
2011).Theleft-handpanelofFig.11showsthedifferencebetween
InordertoestimatethetotalGCpopulation,withtheadvan-
tageofhavingalargeFOVwiththeGMOSdata,wefirstconsider the Reff measured in V and I for the GC candidates as a func-
thenumberofGCsobtainedfromtheintegrationoftheHubblera- tion of I0, which presents an even distribution around zero. The
dialprofile.Then,weapplyacorrectiontotakeintoaccountthatwe mean Reff between those obtained inboth filterswill be adopted
asthefinalvalues. Larsen(1999)indicatedthatstructural param-
arecoveringafractionofthefiducialGaussianadoptedasGCLF,
uptoourmagnitudelimiti′ 25.Withthisprocedure, theesti- eters calculated by ISHAPE are reliable when the object Reff is
0 ∼ atleastonetenthofthePSF,forS/N 50orhigher.Thislatter
mated‘old’GCpopulationofNGC3610results500 110mem- ∼
bers. ± conditionisfulfilledinbothfiltersforGCsbrighterthanI0 = 24,
indicatedwithaverticaldashed lineintheFigure.Inbothfilters,
Assuming the total V0 magnitude of NGC3610, obtained
the derived PSF had FWHM 0.08arcsec, which implies for
mfromMNE=D,3a2s.7Vtot00.1=,th1e0a.7bs±olu0t.e1V3,manadgntihtueddeisotfanthceegmaoladxuyluiss theKing30-profileReff ∼ 0.12∼arcsec.Hence,reliableReff mea-
MV−= 22.0 0±.16.Then,thespecificfrequency,i.e.thenumber surementswereobtainedforGCsbrighterthanI0 = 24andwith
ofGCs−peruni±tgalaxyluminosity(Harris&vandenBergh1981), Reff > 0.01arcsec. This agrees with the results, as small differ-
fortheoldGCpopulationresultsSN = 0.8 0.4.Itisapproxi- encesintheReff calculatedwithbothfilterswereobtainedforGCs
± brighterthanthequotedmagnitudelimit.
matelyinagreement, withintheerrors,withthespecificfrequen-
ciesobtained by Whitmoreetal. (1997) (SN = 0.6 0.14) and The right-hand panel of Fig.11 shows the Reff distribu-
Goudfrooijetal.(2007)(SN =1.4 0.6).Suchvalues±correspond tion for GC candidates brighter than I0 = 24. The binwidth
± is 0.0025arcsec. The distribution maximum is about 0.015
tothelowerlimitoftherangeofSN obtainedforellipticalgalaxies −
0.0175arcsec( 0.35pixels),whichcorrespondsto2.5 3pcat
ofsimilarluminosity(Harrisetal.2013).Infact,themostfrequent ∼ −
galaxytypewithSN < 1,inthebrightness rangecorresponding the distance of NGC3610, i.e. similar to the mean Reff obtained
withACSdataforGCsinnearerearly-typegalaxies,liketheones
to massive galaxies (MV < 20), are not ellipticals but spirals
− inVirgoandFornaxclusters(e.g.Jordánetal.2005;Mastersetal.
(Harrisetal.,theirfig.10).
2010).
In order to explore relations between size, colour, and lu-
3.5 Effectiveradiiofglobularclusters minosity of GCs, the left-hand panel of Fig.12 shows Reff ver-
sus MV for the 13 clusters in NGC3610 spectroscopically con-
ThesizeofGCs,measuredastheireffective(Reff)orhalf-lightra- firmed by Straderetal. (2003, 2004) (highlighted with open cir-
dius,isoneoftheparametersthatcharacterisethesestellarsystems cles), together with GCs/UCDs from Virgo (Brodieetal. 2011),
andhelpsourunderstandingoftheirformationandevolution(e.g. Fornax(Mieskeetal.2004),Antlia(Casoetal.2013a,2014),Hy-
Puziaetal.2014,andreferencestherein).Asaconsequenceofthe dra(Misgeldetal.2011),andComa(Chiboucasetal.2011).The
outstanding resolution of the ACSdata, it ispossible tocompute colour bar represents (V I)0 colours. For Virgo objects, the
Reff ofGCsuptofewparsecs,atsimilardistancesthanNGC3610 colourwasobtainedfromg−′i′-bandphotometryapplyingthetrans-
(e.g.Casoetal.2013a,2014).Forthispurpose, weusedthesoft- formationsderivedinthispaper.Asexpected,thereseemstobeno
wareISHAPE(Larsen1999),designedtocalculatestructuralpa- correlation between mean size and luminosity for the MV range
rameters for marginally resolved objects by fitting their surface- oftypicalGCs(e.g.Puziaetal.2014),excludingthesocalled‘ex-
brightnessprofileswithanalytical models, convolved withaPSF. tendedclusters’ (EC).TheECshavesimilarbrightness thanGCs
As mentioned in Section2.2, for each filter we derived the PSF butpresentlargersizes,i.e.Reff > 10pc(Brüns&Kroupa2012;
from observations of 47Tucoutskirts, withimages obtained dur- Forbesetal. 2013, and references therein). For brighter objects,
MNRAS000,1–14(2016)
NGC3610anditsglobularclustersystem 9
0 0
2. 2.
5 5
)eff1. )eff1.
R R
( (
g g
o o
l 0 l 0
1. 1.
1.50
5 5
0. 0.
0.70
−9 −10 −11 −12 −13 −8 −9 −10 −11 −12
M M
V V
Figure12.Left:Reff[pc]versusMV forNGC3610spectroscopically confirmedGCs(Straderetal.2003,2004,highlighted withopencircles), together
with GCs/UCDs from Virgo (Brodieetal. 2011), Fornax (Mieskeetal. 2004), Antlia (Casoetal. 2013a, 2014), Hydra (Misgeldetal. 2011), and Coma
(Chiboucasetal.2011).ThetwoclustersidentifiedwithdoublecirclescorrespondtoW6andW11,theyoungandmetal-richonesfromtheStradersample.
Right:SimilarplotforNGC3610GCcandidates,includingthesamespectroscopicallyconfirmedGCs(opencircles),andthreeGC-likeobjectswithavailable
Reffmeasurements(triangles),whosepositionsmatchwithX-raypoint-sourcesLiu(2011).Thecolourbarrepresentsthe(V−I)0coloursofGCsandUCDs
inbothpanels.
confirmedclusters(identifiedwithopencircles).SeveralECcandi-
Table 2.Magnitudes, colours (from GMOSdata), andeffective radii for
the spectroscopically confirmed clusters (Straderetal. 2004). Identifica- datesarealsopresent,withintheluminosityrangetypicalofGCs.
tions(ID)aretakenfromStraderetal. Asquotedabove,theclusternamedW30isincludedamongthem.
ThisEChas intermediatecolours and, according totheLick/IDS
ID i′0 (g′−i′)0 Reff analysis performed by Straderetal. (2004), it has a typical GC-
(mag) (mag) (pc) like age. There is a small group of both confirmed objects and
W3 20.84±0.01 1.07±0.01 2.86±0.77 candidatesonthebright side,closetoMV ∼ −11, whicharein
thebrightnesslimitbetweenmassiveGCsandUCDs.Theyoung
W6 21.01±0.01 1.06±0.01 3.22±0.09
metal-richclustersW6andW11arelocatedinthesameplaceas
W9 21.09±0.02 0.98±0.02 2.97±0.09
W10 21.39±0.01 0.92±0.02 4.38±0.17 this group and, comparing with the analogous plot presented by
W11 21.24±0.01 1.20±0.01 7.36±0.26 Norrisetal.(2014)intheirfig.11,theymayfallonanextension
W12 21.48±0.01 0.94±0.01 2.37±0.26 ofthesequenceofYMCstowardsfainterandsmallerclusters.As
W22 22.01±0.01 0.89±0.02 2.98±0.26 partofthisgroup, thereare3GC-likeobjectswithavailableReff
W28 22.23±0.01 1.02±0.02 3.83±0.09 measurementsandwhosepositionsmatchwithX-raypoint-sources
W30 22.39±0.01 1.00±0.02 11.55±0.34 fromLiu(2011)(identifiedwithopentriangles).Outoftheselat-
W31 22.44±0.01 1.15±0.02 4.98±0.68 terobjects,oneisthespectroscopicallyconfirmedGCidentifiedas
W32 22.43±0.01 1.14±0.02 3.22±0.26
W9byStraderetal.(2003).FromtheLick/IDSindices,theseau-
W33 22.60±0.01 0.97±0.02 3.47±0.60
thorsshowedthatW9isoldandred(metal-rich),withmetallicity
W40 22.84±0.01 1.16±0.02 6.02±0.60
[Fe/H] = 1.2 0.2. The two remaining GC candidates lack
− ±
spectroscopicconfirmation,buttheirphotometricagesandmetal-
licitieswerederivedbyGeorgievetal.(2012).Theycorrespondto
intheUCD’sdomain, thesizegloballyincreaseswithluminosity youngobjects:G8, 2Gyrand[Z/H] 0.2,andG14, 5Gyr
(Brüns&Kroupa2012;Norrisetal.2014). and[Z/H] 0.4∼. ∼− ∼
Among the NGC3610 confirmed clusters (Straderetal. Thus,m∼o−stoftheGCcandidatesbrighterthanI0 = 24have
2004),andaccordingtothe(V I)colourlimitsadoptedinthis sizesintheexpectedrangeforGCSsofnearbyearly-typegalaxies,
−
paper,2clustersareblue,8clustersarered,and3haveintermediate butwealsodetectedagroupofECs.Amongthe13spectroscopi-
colours.Table2givestheirmagnitudesandcoloursfromtheg′i′- callyconfirmedGCs,onlyoneseemstobeanECandtheresthave
bandphotometrycalculatedinthispaper,aswellastheirestimated normalGC-sizes.Inparticular,oneofthetwoyoungandmetal-rich
effectiveradiiandcorresponding errors.Allofthemhaveradiiin ones(W11,Reff =7.36pc)ismarginallylargerthanthemean.
therangeoftypicalGCswiththeexceptionofasingleobject,spec-
troscopicallyconfirmedandidentifiedasW30(Straderetal.2003),
thatismarginallylarger(Reff =11.55pc)andmightbeclassified 4 SURFACEPHOTOMETRYOFNGC3610
as EC. The two young and metal-rich clusters are in the reddest
subsample,withsizesReff = 3.22and7.36pcforW6andW11, Fig.13showsthesurface-brightnessprofileofNGC3610(surface
respectively. brightnessversusequivalentradius,beingreq =√ab=a√1 ǫ,
−
Theright-handpanelofFig.12presentsananalogueplotbut whereaistheisophotesemi-majoraxisandǫitsellipticity)inthe
for NGC3610 GC candidates, including the 13 spectroscopically g′filter,obtainedwiththeIRAFtaskELLIPSE.Thegalaxyprofile
MNRAS000,1–14(2016)
10 L. P.Bassinoet al.
8 4
1 0.
2] 0
-c 2 2
e e0.
s
c 2
r 2
[mag ag’02624 0.0 20 40 req6 [0arcsec]80 100 120
m
8 40
2 1
0 50 100 150 200
req[arcsec] PA [º]100
Figure 13. Surface-brightness profile of the NGC3610, g′-band. The 0
6
dashedlinesrepresenttwoSérsicmodelsfittedtothegalaxyprofile,while
20 40 60 80 100 120
thesolidlineindicatesthesumofthem.
req [arcsec]
4
Tthaebgle′fi3.ltPera.rBamotehterr0soanfdthreetfwfoaSreéresxicprmesosdeedlsinfiattrecdsetco.thegalaxyprofilein 0.0
2
0
A40.
Component µ0 r0 n reff
Inner 19.2±0.2 10.6±1.3 0.8±0.08 13.6 01
Outer 21.1±0.2 23.2±2.8 1.1±0.05 47.3 −0. 20 40 60 80 100 120
req [arcsec]
Figure14.Ellipticityǫ,positionanglePA,andA4Fouriercoefficientof
hasbeenfittedwithtwoSérsicmodels(Sérsic1968)expressedin theellipticalisophotesversusequivalentradiusreq
surface-brightnessunits(magarcsec−2)
.
1
µ(r)=µ0+1.0857 req n , (4)
(cid:18)r0 (cid:19)
whereµ0 isthecentralsurface-brightness,r0 isascaleparameter
andandnistheSérsicshapeindex(i.e.n = 1correspondstoan
exponential profile and n = 4 to a de Vaucouleurs profile). The
resultingparametersfortheinnerandoutercomponentsarelisted
in Table 3, where we have also included the respective effective
radii,accordingtotherelation
reff =bnnr0 (5)
wherebn isafunctionofthenindex,thatmaybeestimatedwith
theexpressiongivenbyCiotti(1991).
If we compare our two-component fit with the photometric
analysis performed by Whitmore et al., we are not able to de-
tecttheirsmalltwisted‘innerdisc’within3arcsec(Whitmoreetal.
2002).Wejustattempttofittheinnerpartexcludingtheverycen-
tral20arcsec,wheretheprofilegetssteeper,aswearemostlyin-
terestedinthelarge-scalebrightnessdistribution. Figure15.GMOSimageofNGC3610(N3610F)intheg′-bandwithlight
Fig.14 shows the parameters obtained with ELLIPSE that
contoursoverplottedinred(solidlines).Threeconcentric circles (dashed
characterisethefittedellipticalisophotes:ellipticityǫ,positionan- lines)areshownat60,90,and120arcsecfromthegalaxycentre,inorder
glePA(measuredpositivefromNtoE),andtheA4Fouriercoef- tofacilitatethecomparisonwithotherfiguresandthetext.
ficientthatisrelatedtodiskiness(A4>0discyisophotes,A4<0
boxyisophotes),asafunctionofreq.Thesurface-brightnesscon-
toursarepresentedinFig.15superimposedtoag′ GMOSimage. PApresentsaclearvariationfrom132to92deg,thatisachange
Thebehaviour ofalltheseparameters,whichcanbegloballyfol- of 40deg between 40 and 60arcsec. The global behaviour of
∼
lowedonFig.15,isadirectconsequenceofthecomplexstructure ǫand PA agree withthose presented by Goudfrooijetal. (1994)
ofthegalaxy.Theǫdecreasesfromacentralvalueof 0.4down withintheirmorelimitedradiusof 40arcsec.Finally,theA4co-
∼ ∼
to 0.05whilereaching50arcsec.Thisradialrangecorresponds efficientconfirms earlierstatementsthat thereareboxy isophotes
∼
totheellipticalisophotesofourinnercomponent,i.e.adiscwewill (A4<0)inNGC3610,aspointedoutbyScorza&Bender(1990)
describebelow.Furtherout,ǫrisesslightlybutremainingsmaller forinstance,whichisconsideredasaclearevidenceofpastmerg-
than0.1,andfinallyapproaches0between110and120arcsec.The ers.Theseboxyisophotesarepresentbetween 20(thelowerlimit
∼
MNRAS000,1–14(2016)