A panoramic VISTA of the stellar halo of NGC 253

3.5 MB·

by  Laura Greggio

Astronomy&Astrophysicsmanuscriptno.paper˙rev (cid:13)c ESO2014
January9,2014
A panoramic VISTA of the stellar halo of NGC 253 (cid:63)
L.Greggio1,M.Rejkuba2,3,O.A.Gonzalez4,M.Arnaboldi2,E.Iodice5,M.Irwin6,M.J.Neeser2,andJ.Emerson7
1 INAF,OsservatorioAstronomicodiPadova,Vicolodell’Osservatorio5,35122Padova,Italy
e-mail:[email protected]
2 ESO,Karl-Schwarzschild-Strasse2,D-85748Garching,Germany
e-mail:[mrejkuba],[marnabol],[mneeser]@eso.org
3 ExcellenceClusterUniverse,Boltzmannstr.2,D-85748,Garching,Germany
4 ESO,Ave.AlonsodeCordova3107,Casilla19,Santiago19001,Chile
e-mail:[email protected]
5 INAF,OsservatorioAstronomicodiCapodimonte,80126,Napoli,Italy
6 InstituteofAstronomy,MadingleyRoad,CambridgeCB030HA,UK
4 7 AstronomyUnit,SchoolofPhysicsandAstronomy,QueenMaryUniversityofLondon,MileEndRoad,London,E14NS,UK
1
0 Draftdate12.09.2013;Receiveddate;accepteddate
2
ABSTRACT
n
a
Context.Outskirtsoflargegalaxiescontainimportantinformationaboutgalaxyformationandassembly.Resolvedstarcountstudies
J
canprobetheextremelylowsurfacebrightnessoftheouterhalos.
8
Aims.NGC253isanearlyedge-ondiskgalaxyintheSculptorgroupwhereweresolvedthehalostarsfromground-basedimages,

withtheaimofstudyingitsstellarpopulationcontent,thestructureandtheoverallextentofthehalo.
]
Methods.WeuseZandJ-bandimagesfromtheVIRCAMcameramountedontheVISTAtelescopetoconstructthespatiallyresolved
A
Jvs.Z-Jcolour-magnitudediagrams(CMDs).Theverydeepphotometryandthewideareacoveredallowsustotracetheredgiant
G branch(RGB)andasymptoticgiantbranch(AGB)starsthatbelongtothehaloofNGC253outto50kpcalongthegalaxyminor
. axis.
h Results. We confirm the existence of an extra planar stellar component of the disk, with a very prominent southern shelf and a
p symmetricalfeatureonthenorthside.Theonlyadditionalvisiblesub-structureisanoverdensityinthenorth-westpartofthehalo
- ∼28kpcdistantfromtheplaneandextendingover20kpcparallelwiththedisk.Wemeasurethetransitionfromthedisktothehalo
o
ataradialdistanceofabout25kpcwithaclearbreakinthenumberdensityprofile.Theisodensitycontoursshowthattheinnerhalo
r
t isaflattenedstructurethatblendswithamoreextended,diffuse,rounderouterhalo.Suchexternalstructurecanbetracedtothevery
s
edgeofourimageoutto50kpcfromthediskplane.Thenumberdensityprofileofthestarsinthestellarhalofollowsapowerlaw
a
withindex−1.6,asfunctionofradius.TheCMDshowsaveryhomogeneousstellarpopulationacrossthefield;bycomparingwith
[
isochronesweconcludethattheRGBstarsare∼ 8Gyroldormore,whiletheAGBstarstraceapopulationofabout2×108 M
(cid:12)
1 formedfrom∼0.5toafewGyrago.Surprisingly,partofthislatterpopulationappearsscatteredoverawidearea.Weexploreseveral
v ideastoexplaintheoriginofthisrelativelyyoungcomponentintheinnerhaloofNGC253.
5
Keywords.Galaxies:spiral–Galaxies:Individual:NGC253–Galaxies:starclusters
6
6
1
. 1. Introduction eralsubstructures,scatteredoverthewholeprobedvolume,that
1
areremnantsofinteractionsofM31withitslessmassiveneigh-
0
The impressive evidence of stellar streams in the outer regions
4 bours.AsimilarsituationlikelyholdsfortheMilkyWay,asindi-
of galaxies (e.g. Ferguson et al. 2002; Mart´ınez-Delgado et al.
1 catedbythepresenceoftheSagittariusDwarf(Ibataetal.1994),
2008, 2009, 2010; Mouhcine et al. 2010; Chonis et al. 2011;
: and other structures (e.g. Yanny et al. 2003; Ibata et al. 2003;
v Miskolczi et al. 2011) dramatically shows the importance of
Belokurovetal.2006;Juric´etal.2008;Grillmair2009,andref-
i mergingintheirformation,andatthesametimediscloseshow
X erences therein). For both the Milky Way and the Andromeda
ar tbheeesfftuedcytivoef tfhoersutenldlaerrsptaonpduilnagtiotnhseingatlhaexyhafloorsmoaftigoanlaxpireoscecsasn. wgahlaicxhy,ctahnesheasrudblystrhuacvteuroesrigseineamtetdofrreosmidethaebolvaeteaddisiffruupsteiohnaloof,
Indeed, although the halo contains only a small fraction of the
dwarfs, due to the very long dynamical timescale in such low
massofthegalaxy,thelongdynamicaltimeskeepthememory
densityregionsofgalaxies.Thepresenceofthisunderlyinghalo
of the assembly history of the galaxy over a considerable frac-
is particularly puzzling in the frame of the current picture of
tion of its life. It is then of great interest to probe the extent of
galaxy formation in a Cold Dark Matter universe, and substan-
thehalo,itsstructureandamountofsubstructure,anditsstellar tialeffortisinvestedtohuntforthiselusivecomponentinnearby
populationcontent.
galaxies(Rejkubaetal.2009;Jablonkaetal.2010;Tanakaetal.
The panoramic view of the M31 - M33 complex 2011;Barkeretal.2012;Cockcroftetal.2013;Monachesietal.
(McConnachie et al. 2009) has revealed the presence of sev- 2013).
Sendoffprintrequeststo:L.Greggio The amount of substructure in the halo traces its buildup
(cid:63) BasedonobservationstakenwithintheVISTAScienceVerification throughhierarchicalaccretionandmergerepisodes.IntheMilky
ProgramID60.A-9285(A) Way, Bell et al. (2008) measured 30-40% RMS fluctuation in
1
L.Greggioetal.:StellarhaloofNGC253
thestellardistributionofthehalostarswithrespecttoasmooth
model, while Starkenburg et al. (2009) estimated a minimum
fraction of accreted stars in the halo of 10%. Evidence of in-
completely mixed sub-populations was also found in the inner
haloofNGC891,wheretheRMSfluctuationinstellardistribu- D2
tion is 14% (Ibata et al. 2009). An additional feature which is
FF0077
gainingreliabilityintheliteratureisthatthestellarhalomayac-
FF0066
tuallyconsistoftwocomponents,onemoreinternal,elongated,
FF0055
andrelativelymetalrich,andoneexternal,almostsphericaland FF0044
FF0033 FF0088 N
metalpoor(Carolloetal.2007). D3 B+FF001FF2200111
C+01
Onthetheoreticalside,sophisticatedgalaxyformationmod- M+FF000599 D1 E
els in a ΛCDM universe have been developed, with specific FF1100
predictions about the stellar population properties and surface
brightness profiles which can be compared to observations
(Abadietal.2006;Zolotovetal.2009;Cooperetal.2010;Font
etal.2011).
An analysis of the simulated stellar halos of Cooper et al.
(2010), that were formed via satellite galaxy accretions within
the cosmological N-body simulations of the Aquarius project,
showedasystematicallylargerRMSinthestellardensitydistri-
butionwithrespecttotheonemeasuredfortheMilkyWayhalo
byBelletal.(2008)ontheSloanDigitalSkySurveydata.Based
on that Helmi et al. (2011) estimated that ∼ 10% of the Milky Fig.1. VISTA image of NGC 253 with over plotted the field
Way halo stars formed in situ. According to Font et al. (2011) ofviewofpreviousresolvedstellarpopulationstudiesfromthe
simulations,whoinsteadusedtheGIMICsuiteofcosmological literature; specifically IMACS@Magellan (Bailin et al. 2011,
hydrodynamical simulations, the inner halo stars are predomi- cyan circle), 3 WIRCam@CFHT fields (Davidge 2010, large
nantlyformedinsitu,whiletheouterregionsofthehaloshould blue squares), NTT SUSI2 (Comero´n et al. 2001, yellow rect-
be mostly populated with stars originating from accreted satel- angles),andmanyACSfields(GHOSTsurvey,Radburn-Smith
lites. The in situ component in these simulations comes from a et al. 2011, small green squares). The HST WFC2 field from
proto-diskformedathighredshift,whichlaterhaditsstarsdis- Mouhcine et al. (2005a, small red square) partially overlaps
persed to form the inner halo via dynamical heating associated withACS,SUSI2,andIMACSfields.Thewidecoverageofthe
withmassaccretion(McCarthyetal.2012).Asaconsequence, VISTAdatacanbereadilyappreciated.
thesemodelsarecharacterizedbyachangeofslopeofthesur-
face brightness increase with radius, which occurs at about 30
kpc for a Milky Way type of galaxy. The different proportions
ofinsitustarformationwouldalsoaccountforageneralmetal-
NGC 253 is the brightest (M ∼ −20) member of the
licitygradientwithhighmetallicitystarsfoundpreferentiallyin B
Sculptor group, which is an extended filament of galaxies with
theinnerregions.
distances ranging from ∼ 2 to 4.5 Mpc (Jerjen et al. 1998;
Studying the stellar distribution in the outskirts of galaxies Karachentsev et al. 2003). NGC 253 is a luminous infrared
has great potential to constrain these models. To this end, the sourcewhichhostsarecentstarburstinitscentralpartandsus-
bestsuitedobjectsare(massive)spiralgalaxiesviewededgeon, tains a galactic wind (Rieke et al. 1980, 1988). Its inner re-
becauseofthefavorablegeometrytotracethestellarhalo.NGC gions and X-ray properties were studied by many authors (e.g.
253isonesuchgalaxy.Itissufficientlyneartoallowustomap Fabbiano&Trinchieri1984;Westmoquetteetal.2011,andref-
its oldest stellar population, traced by bright Red Giant Branch erences therein). The strong nuclear outflow in NGC 253 is
(RGB)stars.ThewidefieldofviewoftheVISTAtelescopeof- traced by huge lobes of diffuse X-ray, UV, Hα, HI, and far-
ferstheopportunitytostudythestellarcontentintheoutskirtsof IR dust emission that extend up to∼9 kpc away from the disk
itsdiskandinitshaloinoneshot.Noticethattheextendedhalo (Strickland et al. 2002; Bauer et al. 2008; Hoopes et al. 2005;
is characterized by very low surface brightness, down to µV (cid:39) Boomsmaetal.2005;Kanedaetal.2009).
33 mag/square arcsec or fainter. Reaching these levels (many
Thisoutflowisdrivenbywidespreadstarformationactivity,
magnitudesfainterthanthesky)withsurfacephotometryisex-
which is not only confined to the nuclear starburst but is also
tremelychallenging(Zackrissonetal.2012),whilephotometry
evidenced by bubble like structures visible on the disk (Sofue
of individual stars has proved very successful (e.g. Ibata et al.
etal.1994),aswellasbytheyoungsupergiantstarsinColour-
2007).
MagnitudeDiagrams(CMDs)ofresolvedstars(Dalcantonetal.
Inthesekindsofstudiesforegroundandbackgroundcontam- 2009;Davidge2010;Radburn-Smithetal.2011).Comero´netal.
inationisanimportantissue,duetotheintrinsicallylowsurface (2001)findyoungstarsalongtheminoraxis,upto15kpcaway
densityofthesoughtafterstellarhalo.Inthisrespect,NGC253 fromthegalaxyplane,suggestinglocalstarformationintheout-
isfavourablylocatedathighGalacticlatitude(b = −88◦),such flow(seealsoComero´netal.2003).Thisstrongactivitymaybe
that the foreground extinction is very low (E(B−V) = 0.02) surprisinggiventhatthereisnoclosecompanion,butNGC253
andweexpecttheforegroundsourcestobefairlyuniformlydis- showsclearsignsofinteraction,askinematicallydistinctstruc-
tributedintheVISTAfieldofview.Thisisanadvantagewithre- turesinthecentreofthegalaxy(Pradaetal.1998),extra-planar
specttostudyingthehalosoflargeLocalGroupgalaxies,where gasandstars,andpronouncedasymmetriesinthestellardistri-
the wide solid angle is more likely to intercept local structures bution (Davidge 2010). The evidence suggests that this galaxy
andforegroundinhomogeneities. hasexperiencedasecondaryeventduringitsformation.
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L.Greggioetal.:StellarhaloofNGC253
Pencilbeam,deepHSTobservationsofNGC253havebeen
made by Mouhcine et al. (2005a,b) with the WFPC2 camera,
while the ANGST (Dalcanton et al. 2009) and the GHOSTS
teams(Radburn-Smithetal.2011)observedwiththeACS.The
location of the tip of the RGB in the ACS data indicates a
distance modulus of 27.7, slightly larger but fully consistent
with the distance determined by Mouhcine et al. (2005a) from
WFPC2 data (27.59 ± 0.06 ± 0.16(syst)). Some of these HST
fields are located in the periphery of the disk, but are close
to it, while the outer regions of the galaxy were more exten-
sively probed with wide field imaging from the ground. Malin
&Hadley(1997)tracedanextendedlowsurfacebrightnessen-
velope of about 28 mag arcsec−2 out to 25(cid:48) (corresponding to
(cid:39) 25 kpc) distance around NGC 253, noticing the asymmetri-
calnatureofthisstellarhalo,andconcludingthatitmighthave
beendistortedbytheinfallofacompaniongalaxy.Deeperimag-
ingthatresolvedindividualstarsinthehalowerepublishedby
Davidge (2010), and Bailin et al. (2011). The former study is
not deep enough to sample the RGB stars, so that most of the
information concerns the disk and the extra-planar young and
intermediate-agecomponent,astracedbyyoungredsupergiants
andasymptoticgiantbranch(AGB)stars.Thisstudyrevealsthat
thediskofNGC253isdisturbed.Bailinetal.(2011)areinstead
able to trace the RGB population in a wide region which com-
prisestheinnerhaloforwhichtheyfindaratherflattenedgeom- Fig.2. Cumulative distribution along the vertical coordinate of
etry. Both studies find a stellar over density in the south, outer the VISTA tile for sources with different classifications on the
diskregion,confirmingearlyfindingbyBecketal.(1982). VDFS catalogue: stellar in both J and Z (red solid line), ex-
InthispaperwepresentdeepZandJbandphotometryofthe tendedinboth J andZ (blackdot-dashedline),stellarin J and
outerdiskandhaloofNGC253obtainedwiththeVISTAtele- extended in Z (magenta long-dashed line), stellar in Z and ex-
scopeintheframeworkoftheScienceVerificationProgramme. tended in J (blue short-dashed line). A flat spatial distribution
In Fig. 1 we provide an overview of the several pointings from correspondstoalineat45degrees(dottedline).Theshadedre-
the literature, which were used to investigate resolved stellar gion indicates the location of the disk of NGC 253, where the
populationsinNGC253,superimposedontheVISTAimage.In surfacebrightnessistoohightomeasureindividualsourcesand
comparisontothesepreviousworks,ourdataoffertheopportu- numbercountsareheavilyaffectedbycrowdingeffects.
nitytostudythewholeregionaroundNGC253,yieldingawide
and continuous view of the halo out to a distance of ∼ 50 kpc.
In Section 2 we describe the data, and in Section 3 present the tools for the public surveys which started on the VISTA tele-
Colour-MagnitudeDiagram(CMD)constructedwiththestellar scopeimmediatelyaftertheSV(Arnaboldietal.2012).
source catalogue. In Section 4 we discuss the stellar disk and
the halo components of NGC 253 plus the perturbation to the
2.1. Observations
disk, i.e. the southern shelf, and report about the detection for
thefirsttimeofasubstructureinthehalo,atabout30kpcfrom DuringtheSVperiod(fromOctober16toNovember2,2009),
themainplaneofthedisk.InSection5wediscussthestarfor- NGC 253 was observed in the first part of the night, until the
mation history in the outer disk and halo of NGC 253, and the secondSVtarget,Orion,becameobservable.
spatial distribution of both AGB and RGB population in these VISTA is a 4m alt-azimuth telescope with a single instru-
two components. Our results are summarized and discussed in ment:VIRCAM(Emersonetal.2006;Daltonetal.2006).The
Section6,andconclusionsaredrawninSection7.Throughout camerahasa1.65degreediameterfieldofviewwhichissparsely
thispaperweadoptadistancemodulustoNGC253of27.7mag, populated with 16 2k × 2k Raytheon VIRGO detectors with
whichisequivalenttoadistanceof3.47Mpc,yieldinganimage pixels of mean size ∼ 0(cid:48).(cid:48)34. Each individual exposure images
scaleof16.8pcperarcsecond. a (disconnected) total area of 0.59 deg2. To sample a contigu-
ous field of ∼ 1.5×1 deg2 at least 6 individual exposures (so
called pawprints) are necessary with large offsets. Since in this
2. Thedata
project we aimed to get deep photometry, the single tile cen-
The data presented in this paper were collected as part of the tred on RA=00:46:59.86, DEC=-25:16:31.8 was imaged many
VISTA Science Verification (SV) programme (Arnaboldi et al. timesoverthetwoweeklongobservingrun.Eachobservingse-
2010) which was designed to test the performance of the inte- quencetypicallyconsistedofseveral(2-5)jitteredexposuresat
grated telescope-instrument system with two projects: (i) one onepawprintposition,beforemakingalargeroffsettothenext
aimed at deriving very deep images in the smallest contiguous pawprint position to cover the gap. At this second position the
area that uses all VISTA’s large field of view (a tile in VISTA samenumberofjitteredexposuresweretakenandrepeateduntil
nomenclature)whoseresultsarethesubjectofthispaper;(ii)the sixpawprintswereaccumulatedtofillthetile.Thesesequences
othercoveringa30deg2areaaroundOrionBeltstarswithamo- werethenrepeatedwithdifferentfilters.
saic of 20 tiles having shallower exposures (Petr-Gotzens et al. Inspiteofthelargeextentofthetargetonthesky,(NGC253
2011). The two projects required different observing strategies, is one of the largest southern galaxies), the observing strategy
therefore providing tests of observing templates and operation did not require extra offset empty sky fields due to the very
3
L.Greggioetal.:StellarhaloofNGC253
large size of each VIRCAM detector, which covers an area of movethestripingpatternfromthegalaxyimages.Thepipeline
11.(cid:48)6 × 11.(cid:48)6 on sky. The galaxy major axis was oriented at then combines the individual jittered exposures for each OB
PA= 51.95◦, aligned with the shorter side of the tile, such that to make deeper individual pawprint stacks, and the six stacked
thegalaxycovereddetectors10and11inpawprints2,4,and6, pawprintsarecombinedtoproduceatileimage(andcatalogue)
while it was kept in the gap between the detectors in the other foreachOB.
three pawprints. Hence the jittered exposures of the odd paw- Thefinaldeepstackedtileimageandcatalogueusedforthe
printscouldbeusedtocreateskyimagesforthedataprocessing, scienceanalysiswascreatedbyfirstcombiningallsuitablesets
whileatthesametimebeingusedinthefinaldeepstack. of 6 pawprint stacks to make deeper pawprint stacks. Prior to
We acquired deep images of NGC 253 in three filters: Z, J makingthefinaltileimageanybackgroundvariationinthepaw-
and in NB118. The latter is a narrow band filter centred on the printcomponentimageswasremovedusingtheso-callednebu-
redshifted wavelength of Hα emitters at redshift z ∼ 0.84 and losity filter developed by CASU1. This filter differentially re-
Lyα emitters at z ∼ 8.8 (Milvang-Jensen et al. 2013). While movesallsmoothlyvaryingbackgroundonaspecifiedscale,in
thedatatakenwiththatfilterwerefocusedonthehighredshift this case set to ≈30 arcsec. In addition to generic background
universe behind NGC 253, with the deep J and Z-band images variationsthefilteralsodrasticallyreducestheimpactoflarge-
weexplorethestellarpopulationcontentandthestructureofthe scalereflectionhalosaroundbrightstarsandalsoremovesmost
haloofNGC253.TheJ-bandimagesconsistedof5exposures, oftheunresolveddisk/halolightfromNGC253.
each with a detector integration time (DIT) of 45 sec. The Z- These ”nebulised” pawprint images were then combined to
band images had 3 × 60 sec exposure, while several different make the final deep tile image. A few individual OB expo-
DITswereusedforNB118images:270,450and860sec.The sures were affected by electronic noise appearing occasionally
final deep J-band stack was made combining 20 tile sequences onchannel14ofdetector6.However,thetotalareaaffectedby
of 6 pawprints with a varying number of jitters that add up to thatextranoisewaslessthan0.4%ofthewholetile,andasmost
22.1htotalaccumulatedexposuretime.Thetotalexposuretime imagesdidnothavethisextranoisepresentthiswaseffectively
of all Z-band sequences was 9.6h, and 5.8h were accumulated removedduringthedatareduction.Incontrasttheupperhalfof
withtheNB118filter.Ontargetexposuretimeis1/3ofthetotal detector16(whichcoversthesoutherncornerofthetileinFig.
accumulated exposures due to large offsets necessary to fill the 1) is unstable, particularly in the bluer bands, and was masked
gaps between detectors. These integration times allowed us to offintheconfidencemapspriortomakingthefinaltileimages.
reachJ=23.5andZ=24.5(ontheVegamagnitudesystem)with Althoughthewholetileimageisavailable,objectsfallingonthis
aS/Nof3. partofdetector16haveshortereffectiveintegrationtimes,bya
In addition, we acquired shallower exposures in all the factor 2, compared to the rest of the tile resulting in shallower
VISTA broad-band filters (Z, Y, J, H and Ks) with the aim of photometryintheaffectedregions.Finally,priortomakingthe
studying the central part of the disk of NGC 253 and its struc- deepZ-andJ-bandcataloguesthecentralregionofNGC253was
ture. In this paper we analyze the deep Z and J-band images, masked out. No other masks, e.g. for bright foreground stars,
while the shallow broad-band imaging data are described in a wereused.
subsequentpaper(Iodiceetal.,inprep). The cataloguing and photometric measurements were done
During the observing run the seeing and sky transparency withtheVDFSpipelineusingthestandardCASUimcorepack-
varied, but the conditions were generally very good, with only age. This package measures fluxes in a range of apertures and
two nights having thin clouds (but excellent seeing) and two enables shape characterization for all objects. The deblending
nightsbeinglostduetostrongwind.Thefinaldeepstackedim- option was used to enable better measures of stellar sources in
agehasanaverageseeingof∼ 0(cid:48).(cid:48)9inthe J-bandand∼ 1(cid:48).(cid:48)2in thecrowdedouterdiskregionsofthegalaxy.Theshapeparame-
theZ-band. tersandseriesofaperturesfluxeswereusedtogeneratemorpho-
Figure1showsthefinalstackedJ-bandimage,mappedwith logicalclassificationinformationandtocomputestellaraperture
(cid:39)13000×16000pixels.AtthedistanceofNGC253theimage corrections.Photometric(andastrometric)calibrationwasbased
samplesanareaofabout74kpc×91kpcaroundNGC253. onunsaturated2MASSstarsconvertedtotheVISTA(Vega-like)
system.
The photometric errors as a function of magnitude are well
2.2. DataReductionandPhotometry
fittedwithanexponentialfunction:
Data reduction was carried out with the VISTA Data Flow σ =5.58×10−10×e0.803×J+1.17 (1)
System(VDFS,Lewisetal.2010)attheCambridgeAstronomy J
σ =1.87×10−9×e0.774×Z−0.026. (2)
Survey Unit (CASU). The reduction procedure consists of the Z
following standard steps: dark correction which removes the
The catalogue from the VDFS pipeline includes more than
dark current and also corrects other additive electronic effects; 400,000sourcesmatchedontheJandontheZtiles,but∼30%
linearitycorrectiontoaccountforthenon-linearoverallresponse of them are either spurious sources, or their magnitude and/or
of the VISTA detectors; flatfield correction using stacked twi-
shapeareveryuncertain.Mostoftheseobjectsareclassifiedas
lightflatswhichisalsousedtogain-normaliseallthedetectors
noiseineitherband,andafewofthemareinsteadsaturatedor
to a common internal system; and sky background correction
contain a bad pixel. We do not consider these detections in the
to remove the large-scale spatial background variation from at- followinganalysis.Inaddition,for∼ 10%oftheremainingob-
mosphere variations. In addition, the VISTA detector IRACE
jects,thepositionalcoincidenceofthesourceonthetwotilesis
controllers imprint a low-level horizontal stripe pattern on the
particularly poor. We also discard these cases, since we aim at
images. While the position and amplitude of the stripes varies
theconstructionofacatalogueofNGC253bonafidestarmem-
from one exposure to the next, it is the same along the row
bers, and the poor quality of the positional match may indicate
across the four detectors read out through each IRACE con-
the presence of an extended object or a blend of two or more
troller. Therefore, even though the extended and bright galaxy
stars.Afterthiscleaning,thecatalogueincludesabout253,500
wasoccupyingthewholeofdetectornumber10andalargefrac-
tionofdetector11,theothertwodetectorscouldbeusedtore- 1 seehttp://casu.ast.cam.ac.uk/publications/nebulosity-filter
4
L.Greggioetal.:StellarhaloofNGC253
Fig.3.ObservedCMDsforobjectswithdifferentclassificationsfromthereductionpackage:stellarsourcesonbothfilters(panel
(a),55025points);extendedsourcesinbothfilters(panel(b),151924points);stellaronthe JandextendedontheZ tile(panel(c),
27687 points); and extended on the J and stellar on the Z image (panel (d), 18932 points). In panel (a) the open (orange) circles
show the evolutionary path of a simple stellar population (SSP) model with metallicity Z=0.008 as the population ages. The size
of the circles increases with the age of the model plotted at (4, 10, 30, 100 and 300) Myr and at (1, 3 and 10) Gyr. The model
was obtained using the CMD on-line tool by L. Girardi at stev.oapd.inaf.it/cmd, and refers to an SSP born with 105M of stars
(cid:12)
distributedbetween0.1and100M withaSalpeterIMFflattenedbelow0.5M .
(cid:12) (cid:12)
sourcesclassifiedasstellarorextendedineitherthe J orZ tile, kinds of detections show quantitative differences, as illustrated
orboth.Inspectionofthespatialdistributionofthevariouscat- inFig.2.Sourcesclassifiedasextendedonbothtilesdeviatethe
egories of sources shows that all of them are concentrated to- leastfromaflatdistribution,whilesourcesclassifiedasstellaron
ward the disk of NGC 253. This is due to crowding, since the bothtilesdeviatethemost,withastrongexcessintheshadedre-
de-blending of stellar sources becomes progressively more dif- gion,wherethediskislocated.Sourcesclassifieddifferentlyon
ficult as the surface brightness increases. This effect enhances the two tiles exhibit a behavior which is intermediate between
thenumberofsourcesclassifiedasextended asthegalaxydisk the flat distribution and the population concentrated toward the
is approached. However, the spatial distribution of these four diskofNGC253.
5
L.Greggioetal.:StellarhaloofNGC253
Fig.5. Spatial distribution of catalogue A stars on the VISTA
Fig.4. Luminosity functions of the different kinds of sources
tile (J band) oriented along the detector coordinates. The up-
whoseCMDsareshowninFig.3.Theredthicklineshowsthe
per and right axis are labelled with physical coordinates in kpc
distribution of sources classified as stellar in both filters corre-
centredonthegalaxycentre.Holesareclearlyvisibleinthespa-
sponding to panel (a) and the black thick line is used for the
tial distribution. These are due to the masking of bright fore-
sources classified as extended in both filters, corresponding to
ground stars. The inner part of the disk has also been masked,
the panel (b). The thin magenta line and the shaded blue his-
because source crowding prevents accurate photometry on our
togramsrefertothesourcesshowninpanels(c)and(d)ofFig.
images. In the upper right corner, two white rectangles show
3. Labels in the figure follow the order of the histograms from
other regions with defects, where the photometry could not be
top to bottom. The arrow indicates the magnitude at which we
performed.Thecumulativeareaofthemaskedregionsisasmall
expecttheTipoftheRGBforastellarpopulationwithmetallic-
fractionofthetotalsurveyedarea.Solidlinesshowtherectangu-
ityZ =0.008,andadistancemodulusof27.7.
larsub-regionsintowhichwedividethetiletodrawthespatially
resolvedCMDsshownonFig.11andFig.19.Thesub-regions
aredrawnsoastosampleastatisticallysignificantpopulationof
InFig.3weplottheCMDofthesefourkindsofsources.In stars,andareidentifiedwithprogressivenumbersandletters,for
panel(a)weplottheCMDforsourcesthatarestellarinbothJ thehaloandthediskregionsrespectively.
andZ;herethestellarpopulationofNGC253isclearlyvisible
at magnitudes fainter than J ∼ 20, with the vertical plumes of
foregroundstarssuperimposed.Inpanel(b)weplottheCMDof
ground stars, the CMDs of extended sources in both bands are
sources extended in J and Z. This CMD is very different from
veryunlikelytocontainyoungandblue(globular)starclusters.
thatinpanel(a),suggestingthatthemajorityofthesourcesclas-
Indeed, we do expect globular clusters to appear as stellar-like
sified as extended in both filters are background galaxies. The sources in our image, because the median effective radius of
CMDsshowninpanels(c)fortheobjectsthatarestellarinJand
MilkyWayGlobularClustersis3.2pc(Harris1996),whichcor-
extendedinZ,andin(d)forobjectsextendedinJandstellarinZ responds to ∼ 0(cid:48).(cid:48)2 at the distance of NGC 253, well within the
containmostlyfaintsources,whoselightdistributionhaslower
seeingdisc.
S/N.InFig.4weshowtheluminosityfunctions(LF)ofthefour
In what follows, we consider as bona fide stars the sources
kindsofsources.Itappearsthatobjectswiththesamemorpho-
classifiedasstellarinbothfilters,andwhoseCMDisplottedon
logicalclassificationonthe J tilehaveasimilarmagnitudedis-
panel(a);thisisourcatalogueA.However,afairfractionofthe
tribution.Inparticular,theLFsofthesourcesclassifiedasstellar
sourcesclassifiedasstarsinJandextendedinZcouldbestellar
onthe J bandimage(plottedinredandmagenta)presentadis-
membersofNGC253also.Wetakethesesourcesintoaccount
continuityat J (cid:39) 22.5wheretheTipoftheRGBisexpectedif
whendiscussingthehaloofthegalaxy,itsextensionandshape.
adistancemodulusof27.7isappliedtothePadovaisochrones.
WerefertothecumulativecatalogueascatalogueB.
Wethenconcludethatmostofthesourcesinpanel(c)ofFig.3
arelikelystars,andtheyappearextendedontheZimagebecause
oftheworseseeing.Differently,mostofthesourcesinpanel(d) 3. TheColour-Magnitudediagramofstellarsources
arelikelytobecompactbackgroundgalaxies.
inNGC253
BecauseNGC253isundergoingastarburst,weconsiderthe
presenceof(young)globularclustersinourarea.Inpanel(a)of Fig.5showsthespatialdistributionofcatalogueAstars.Anex-
Fig.3weshowtheevolutionarytrackappropriateforaglobular tended stellar disk is easily identified at 1000 (cid:46) X (cid:46) 11000,
clusterof105 M withmetallicitysimilartothatoftheLMC;for 7000 (cid:46) Y (cid:46) 11000, behind a sheet of foreground stars and
(cid:12)
asolarmetallicity,suchpathwouldbeshiftedtoreddercolours unresolved background galaxies. The disk of the galaxy falls
by∼0.15mag.Whileitispossiblethatsomeclustersarepresent in the upper left section of the tile, with the centre located at
in the CMD of the stellar sources, confused among the fore- X (cid:39)5300,Y (cid:39)8750.Thereforeweprobethegalaxydiskuptoa
6
L.Greggioetal.:StellarhaloofNGC253
Fig.6. CMD of catalogue A stars with theoretical loci super-
Fig.7. CMD of the expected foreground stellar population as
imposed. The models are computed with the CMD tool at
stev.oapd.inaf.it/cmdandshiftedbyadistancemodulusof27.7. derived from the TRILEGAL simulation. The colour and point
type encode the Milky Way component, as labelled. The solid
Open circles, connected with a dotted line, show the loca-
line mimics the faint limit of the observed CMD of NGC 253.
tion of the RGB Tip of 10 Gyr old isochrones with metallic-
ity Z=(0.0001, 0.0004, 0.001, 0.004, 0.008, 0.019 and 0.03), Thesimulationhasbeencalculatedfora1.46deg2 FoVcentred
falling, respectively, at J=(23, 22.79, 22.65, 22.4, 22.18, 22.09 at the Galactic coordinates: l = 97.37, b = −87.96, adopting a
Chabrier IMF, and keeping all other parameters at their default
and 22.04). Solid thick red lines show the RGB portion of 10
GyroldisochroneswithZ=0.0001andZ=0.008;thedashedor- values.
angelineshowsa1GyroldisochronewithZ=0.008.Thegreen
solidlineshowstheboundarybelowwhichmoststellarsources
oftheRGBstarscoverawiderangeduetoacombinationofa
are likely members of NGC 253 (see text). 1σ error bars are
metallicityspreadandphotometricerrors.UsingWFPC2V and
shownfortwovaluesofthestar’scolor:thoseontheleftreferto
IdataofahalofieldofthisgalaxyMouhcineetal.(2005b)mea-
J−Z =0.5,whilethoseontherighttoJ−Z =1.2mag. sured a metallicity distribution ranging from [Fe/H] ∼ −2.3 to
[Fe/H]∼−0.2.Theoverplotted10Gyrisochrones(Fig.6)show
consistency between our data and this metallicity range, taking
distanceofmorethan40kpcalongthemajoraxis(South-West intoaccountthephotometricerrors.Theluminosityextensionof
direction), and the halo up to a distance of (cid:39)50 kpc, along the theAGBcomponentindicatesthepresenceofintermediateage
minoraxis(North-Westdirection).Theinnerpartofthediskhas (∼1Gyrold)stars.
been masked and shows up as white region on the figure. Few TheCMDinFig.6showswherethestellarmembersofNGC
verybrightforegroundMilkyWaystarsarealsovisibleaswhite 253arelocated.Thisistheregionbelowthesolidgreenline.The
”holes”.Tworectangularwhitestripesintheupperrightcorner restoftheCMDisdominatedbytheforegroundcomponent,and
areduetothelowersensitivityofdetector16(Sec.2.2),andthis some contamination will also be present in the selected region.
regionisnotconsideredinthefurtheranalysis.Thedistribution In the next section we assess the contribution from the Milky
of the stars on the disk appears asymmetrical, with an excess Way(MW)stars.
of sources on the upper/right side of the disk. This is the well
knownsouthernshelf alreadyrecognizedintheliterature(Beck
3.1. Foregroundcontamination:theTRILEGALmodel
et al. 1982; Davidge 2010; Bailin et al. 2011). Some other dis-
tortionsofthediskmayberecognized(e.g.ontheleftsideofthe InspiteofthehighGalacticlatitudeofNGC253,theshearsize
disk along the North direction), but it is difficult to assess their of our field and its photometric depth provide a large popula-
reliabilityinthisfigureduetotheforegroundcontamination. tionofMWforegroundstars.Thiscomponentmaybeestimated
The CMD of the bona fide stars (catalogue A) is shown fromthestellarcountsattheedgesofthetiles,butsincewedo
in Fig. 6, superimposed with a few theoretical loci from the not know how extended the halo of NGC 253 is, and measur-
Padovadatabase,shiftedtoadistancemodulusof27.7.Thedot- ing its radius is one goal of the current project, we first eval-
tedlineconnectsthetheoreticalRGBTipmodelsof10Gyrold uate the foreground contribution in an independent way. Fig. 7
isochrones with metallicity from ∼ 1/200 Z(cid:12) to ∼ 1.5Z(cid:12). The shows the CMD of a simulated population of foreground stars
stellar population of NGC 253 stands out from the foreground as obtained using the TRILEGAL tool version 1.52 (Girardi
contaminationasastrongenhancementoffaintandredsources. etal.2005)forafieldwithanareaof1.46deg2 centredonthe
The comparison with the evolutionary models reveals that our Galactic coordinates of NGC 253. The foreground population
CMD contains a bright RGB component plus an AGB compo-
nentextendingbeyondtheRGBTip(i.e. J (cid:46)22.5).Thecolours 2 stev.oapd.inaf.it/cgi-bin/trilegal
7
L.Greggioetal.:StellarhaloofNGC253
400 16<J<18
300
200
100
0
400 18<J<19
300
200
100
0
400 19<J<20
300
200
100
0
Fig.8. Luminosity function of the expected foreground stellar 400 20<J<20.5
population shown in Fig. 7 (black line) from the TRILEGAL 300
simulation compared with the data (red shaded histogram). No
scalingfactorhas beenappliedtobring themodelandthe data 200
intoagreement.
100
0
400 20.5<J<21
consists of a disk plus a halo component which, according to
300
the simulation, contribute respectively 36% and 64% to the to-
tal foreground counts within the CMD region considered here 200
(i.e. −1 ≤ (Z − J) ≤ 3;14 ≤ J ≤ 24). The great majority of
model stars in Fig. 7 are Main Sequence objects with mass up 100
to ∼ 1 M(cid:12), except for the bluest points (Z − J <∼ 0.5) which 0
arewhitedwarfs;theredtailofobjectsatZ − J ≥ 1.2aredisk
0 0.5 1 1.5 2 2.5
reddwarfswithmassesbetween0.07and0.15M .Fig.8shows
(cid:12)
theluminosityfunctionofthesimulatedforegroundpopulation.
The agreement with the observed counts at J (cid:46) 20 is remark-
able, given that this simulation is not a fit to our data, and the
TRILEGALdefaultparameterswerecalibratedondifferentdata Fig.9.Colourfunctionsintheindicatedmagnituderanges.The
sets. At fainter magnitudes the observed star counts exceed the redshadedhistogramsarecomputedfromthedataincatalogue
simulated ones. While there is no guarantee that the model is A; the solid black lines show the expected foreground stellar
correct at these faint magnitudes, which are poorly constrained populationascomputedusingTRILEGAL,seeFig.7.
fromobservations,thestrongexcessinourobservedluminosity
function sets in at a magnitude consistent with the expectation
forbrightstarsatadistanceofNGC253.
lowthislimittheforegroundcontaminationisstillpresent,butit
ToconstrainthebrightestmagnitudeatwhichtheNGC253
affectsmostlytheblueendofthecolourdistribution.Thesolid
members are found, we compare the colour functions of the
TRILEGAL simulated MW stars to the data in different mag- greenlineinFig.6showsouradoptedselectiontoisolatebona
fide stellar members of NGC 253 on the CMD. The validity of
nitude bins in Fig. 9. For magnitudes brighter than J ∼ 20 the
ourchoiceisfurtherstrengthenedbythecumulativedistributions
TRILEGAL MW model and the data are in very good agree-
showninFig.10,wherestarsbrighterthanJ =20.5closelyfol-
ment, considering also that the synthetic diagram does not in-
lowtheuniformdistributiononthetile,asexpectedfortheMilky
cludeanyphotometricerror,whichwouldwidenthemodeldis-
Wayforeground.Conversely,starsbelowthislocusontheCMD
tributions.Infact,thegoodagreementbetweentheTRILEGAL
(RGBandAGB)areconcentratedtowardsthediskofNGC253.
modelanddata,inFig.8and9,confirmsthatourphotometryis
quiteaccurateforstarsbrighterthanJ (cid:39)20,asshownalsobythe
small error-bars in Fig. 6. In the range 20.5 < J < 21 the data
3.2. TheCMDofNGC253starmembers
show a strong excess of red stars compared to the model, and
someexcessispossiblypresentalsointherange20< J <20.5. TheselectioncriteriadefinedinSection3.1forthestellarpop-
Therefore,weconsiderthelocation J = 20.5astheupperlimit ulation in NGC 253 can be used to verify whether there are
totheapparentluminosityoftheNGC253stellarmembers;be- sub-regionsinthetilewherethispopulationisscarceorabsent
8
L.Greggioetal.:StellarhaloofNGC253
Fig.11.SpatiallyresolvedCMDsovertheVISTAtile(haloregions).EachpanelshowstheCMDofthestarsfallinginarectangular
sub-regionlabelledinthebottomrightcornerwiththesamenumberasinFig.5.Thesolidlineshowsthedividinglineofthelikely
membersofNGC253,asinFig.6.Thenumberintheupperrightcorneristheratiobetweenthestarsbelowthisenvelopeandthe
totalnumberofstarsintheselectedregionofthetile.NoticethattheCMDsareplottedonlyuptoJ =18,whilestarsaremeasured
upto J =14.TheverticaldottedlineatZ−J =1.1marksthecolouratwhichtheNGC253stellarpopulationgreatlyoutnumbers
theforegroundcontamination(seeFig.9).
andderiveanempiricalevaluationofthecontaminationbyfore- 1. X < 1000 and X > 11500 because these pixels received
groundstarsandbackgroundunresolvedgalaxies. shorterexposuresduetothepawprintsampling;
2. the region with 7000 < Y < 11500 is dominated by the
We show in Fig. 11 the spatially resolved CMDs over the diskofNGC253,henceitisnotusefultoevaluatethefore-
portionoftheVISTAtilewhichmapsthehaloaroundthegalaxy, ground/backgroundcontamination;
i.e. for the rectangular sub-regions drawn on Fig. 5 and identi-
fied with progressive numbers. The following sub-regions are
excluded:
9
L.Greggioetal.:StellarhaloofNGC253
Actually,thedetailedcomparisonofthestellarpopulationin
sub-regions1+4withtheTRILEGALmodel,suitablyre-scaled
by the ratio of the sampled areas, shows a remarkable match
of the luminosity function and color distribution at J <∼ 20.5,
butstillthereisanexcessatfaintmagnitudes.Sincethemodel
doesnotincludethecontributionofthebackgroundunresolved
galaxies(andmayhaveuncertaincalibrationatthesefaintmag-
nitudes as mentioned above), we prefer to rely on the empiri-
cal estimate of the foreground+background contamination us-
ing sub-regions 1+4 (1000 ≤ X ≤ 3500;Y ≤ 2500 plus
8500 ≤ X ≤ 11500;Y ≤ 2500). There are 621 and 683 stel-
lar sources in the regions 1 and 4 of the tile, respectively. The
total number of sources is then 1304 over a combined (region
1+4) area of 0.123 deg2 and the surface density of foreground
starsincludedinourcatalogueofbonafidestars,i.e.ourphoto-
metriccatalogueA,isempiricallyestimatedtobe0.1×105stars
perdeg2,onaverage.Thisvaluewillbeconsideredlater,when
discussingtheextentoftheNGC253stellarhalo.
4. NGC253stellardiskandhalostructure
Detailed analysis of the inner regions and of the disk structure
inNGC253ispresentedinIodiceetal.(inprep.).Herewedis-
cuss primarily the structure of the halo, but to do that we need
Fig.10. Cumulative distributions along the vertical (Y) coordi- toknowhowextendedthediskis,andwherethedisk-halotran-
nateofthetileforstellarsourceswithdifferentlocationsonthe sitionoccurs.SinceNGC253isnotperfectlyedgeon,wemust
CMD. The short dashed (magenta) line refers to stars brighter evaluatehowfarthediskprojectsontheVISTAtileandextends
than J = 20.5; RGB (red, long dashed) and AGB (blue, dot- along the projected minor axis. We do that assuming that the
dashed) stars are located below the solid green line in Fig. 6, diskiscircular,bymeasuringitsextensionalongthemajoraxis
respectivelybelowandabovethedottedredlineconnectingthe andthenprojectingitalongtheminoraxisusingtheinclination
RGBtipsfromevolutionarymodels. angleofthedisk.
4.1. Thestellardisk
3. wealsodiscardtheupperrightcorneroftheVISTAtile(at InFig.12weshowthestellardensityprofilealongthetwosides
X > 8500,Y > 11500)becauseofthedefectsofdetector16 ofthemajoraxis(openandfilledcircles),whichturnouttobe
(seeSec.2.2). quitesmoothandsymmetric.Thesolidlineshowsthecountsof
theNGC253bonafidemembersalongtheSouth-Westdirection,
Thesub-regionstargetingthehaloofNGC253donothave whilethedashedlineshowstheprofilealongtheNorth-Eastdi-
the same area because we aim at maximizing the contrast be- rection.TheprofilesfromcatalogueBshowverysimilartrends.
tweenthegalaxypopulationandtheforegroundcontamination. WhentheSouth-WestandNorth-Eastsemi-majoraxisnum-
Therefore we consider wider areas in the outer portions of the berdensityprofilesareplottedontopofeachotherasinFig.12,
tile. The solid line plotted on each CMD of Fig. 11 shows the a change in the radial slopes appears at a distance of approxi-
locusbelowwhichweexpectthestarsmembersofNGC253to mately 25 kpc from the centre. The South-West profile is more
dominate,accordingtothediscussioninSection3.1. extendedthantheNorth-Eastone,becausethegalaxyisnotcen-
The number listed on the upper right corner of each panel tred in the tile (the disk centre is at X=5300, Y=8750 coordi-
of Fig. 11 is the fraction of stars in the spatially selected sub- nate).Weinterpretthechangeofslopeat∼25kpcassignalling
regionwhichfallsbelowthesolidline,withrespecttothetotal the location of the outer edge of the disk. This interpretation is
number of stars in catalogue A in the same spatial region. The promptedalsobytheanalysisofthesurfacebrightnessprofileof
verticaldottedlinehelpsustoevaluatethecontributionfromthe theentirediskinIodiceetal.(inprep.).Sincetheinclinationof
NGC 253 members, that appear as a red population and dom- NGC253isapproximately76deg,theprojectedcomponentof
inate the foreground/background contaminants (see Fig. 9). On thediskalongtheminoraxisis∼6kpc,correspondingto±1000
Fig. 11 the number of red stars associated with the NGC 253 pixfromthediskplane.Thisimpliesthattheregionsofthetile
population increases in sub-regions closer to the disk, as also atY <7500andY >10000inFig.5,donotincludestellarpop-
tracedbythefractionofstarsbelowthesolidline. ulationsfromthecircularthindisk.However,weknowthatthe
Forcomparison,theTRILEGALsimulationshowninFig.7, diskofNGC253appearsquitedisturbed,hencethoseregionsof
hasafractionof0.32,havingtakenintoaccountthemagnitude the tile closer to its edge may likely include some extra-planar
limit drawn as a solid black line in this figure. This fraction is stellarcomponentfromthedisk.
similartothevaluemeasuredonthebottomcornersofthetile, Fig. 13 shows the complete radial profile along the major
i.e. in sub-regions 1 and 4. Although the colour distribution in axis of NGC 253 out to 40 arcmin, i.e. 40.3 kpc, constructed
regions 1 to 4 and 16 to 18 may be consistent with that of the by combining two surface brightness profiles (small coloured
foregroundpopulation,theexcessofstarsbelowthesolidlinein dots)extractedalongNorth-EastandSouth-Westmajoraxesin
regions 2, 3, 16, 17 and 18 suggests that a component of NGC the J-band images with the number density from the J-band
253membersislikelypresentinthesesub-regions. starcountscorrectedforcrowdingeffects(blackdotswitherror
10