Population gradient in Sextans dSph: Comprehensive mapping of a dwarf galaxy by Suprime-Cam

0.88 MB·

by  S. Okamoto

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed18January2017 (MNLATEXstylefilev2.2)
Population gradient in Sextans dSph:
Comprehensive mapping of a dwarf galaxy by
⋆
Suprime-Cam
7
1
0
2 S. Okamoto1†, N. Arimoto2,3,4, E. Tolstoy5, P. Jablonka6,7, M. J. Irwin8,

n
Y. Komiyama4, Y. Yamada4, and M. Onodera2
a
J 1Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Rd. Shanghai, 200030 China
6 2Subaru Telescope, 650 North Aohoku Place, Hilo, HI 96720, USA
1 3The Graduate Universityfor Advanced Studies, Osawa 2-21-1, Mitaka, Tokyo181-8588, Japan
4National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo181-8588, Japan
] 5KapteynInstitute, University of Groningen, Postbus 800, 9700AV Groningen, the Netherlands
A 6Laboratoire d’astrophysique, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix,
G Switzerland
7GEPI, Observatoire de Paris, CNRS, Universit´e de Paris Diderot, F-92195 Meudon, Cedex, France
h. 8Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, U.K.
p
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o
18January2017
r
t
s
a
[ ABSTRACT
We present the deep and wide V and Ic photometry of the Sextans dwarf spheroidal
1 galaxy(dSph)takenbySuprime-CamimagerontheSubaruTelescope,whichextends
v
outtothetidalradius.Thecolour-magnitudediagram(CMD)reachestwomagnitudes
2
belowthemainsequence(MS)turn-off,showingasteepredgiantbranch,blueandred
2
horizontal branch (HB), sub-giant branch (SGB), MS, and blue stragglers (BS). We
4
constructthe radialprofile of eachevolutionaryphase and demonstrate that blue HB
4
0 starsaremore spatiallyextended, while redHB stars aremorecentrallyconcentrated
. than the other components. The colour distribution of SGB stars also varies with the
1
galactocentricdistance;theinnerSGBstarsshiftbluerthanthoseintheoutskirt.The
0
radial differences in the CMD morphology indicate the existence of the age gradient.
7
The relatively younger stars (∼ 10 Gyr) are more centrally concentrated than the
1
: older ones (∼ 13 Gyr). The spatial contour maps of stars in different age bins also
v showthattheyoungerpopulationhashigherconcentrationandhigherellipticitythan
i
X the older one. We also detect the centrally concentrated bright BS stars, the number
of which is consistent with the idea that a part of these stars belongs to the remnant
r
a of a disrupted star cluster discovered in the previous spectroscopic studies.
Key words: galaxies: dwarf – galaxies: individual: name: Sextans – galaxies:stellar
content – galaxies: structure – galaxies: Local Group
1 INTRODUCTION Since 2005, analyses of resolvable stellar distribution
based on large photometric survey programs have led
Dwarf galaxies are the most simple and numerous galaxies
to discoveries of numerous ultra faint galaxies (UFDs)
in the Universe and their stellar populations provide clues
(e.g. Willman et al. 2005; Belokurov et al. 2006). The deep
about the formation and evolution of galaxies in various
colour-magnitude diagrams (CMDs) show that they have
environments. Within the currently favoured cosmological
quiteanoldandmetal-poorpopulation(e.g.Okamoto et al.
Cold Dark Matter scenario, stellar halos of large galaxies
2008, 2012; Brown et al. 2012). On the other hand, radial
werebuiltupthroughmergersoflow-masssystems,suchas
population gradients and distinct dynamical components
thedwarfspheroidal(dSph)galaxiesaroundtheMilkyWay.
are found in many brighter dwarf satellites, such as Fornax
andSculptordSph(e.g.Tolstoy et al. 2004;Battaglia et al.
2006, 2008). The differences in stellar populations, chemi-
⋆ BasedondatacollectedatSubaruTelescope,whichisoperated cal compositions and the structural properties between the
bytheNational AstronomicalObservatoryofJapan classical dSphsand UFDsgivehintstounderstandthestar
† E-mail:[email protected]
2 S. Okamoto et al.
formation inlow masssystemsintheearly Universeaswell
as the formation of theMilky Way and its satellites.
The Sextans dSph was discovered from UK Schmidt
Telescope (UKST) sky survey (Irwin et al. 1990). It has
the lowest central surface brightness (Σ0 = 18.2 ±
0.5magarcmin−2)oftheGalacticdSph,excludingthemost
recently discovered ones, and has a high mass-to-light ratio
(M/LV ∼40M⊙/L⊙,V)(Mateo et al.1991;Mateo1998).It
liesatadistanceof94±8kpcanditstidalradiusestimates
vary between ∼ 80 and ∼ 160′ (Irwin & Hatzidimitriou
1995; Roderick et al. 2016). Due to the large extent in the
sky, it is difficult to study the global stellar population by
most current cameras and spectrographs on large aperture
telescopes.Theobservationalstudiescoveringthecentralre-
gion showed some evidence of thepresence of multiple stel-
lar populations (e.g. Bellazzini, Ferraro, & Pancino 2001;
Harbeck et al. 2001; Kleynaet al. 2004). Leeet al. (2003)
(hereafterL03)presentedastudyofthewide-fieldBVIpho-
tometryreachingtothemain-sequence(MS).Theycovered
the central 42′ ×28′ region using CFH12K camera on the
Canada-France-Hawaii Telescope and reported the differ-
ent spatial distributions of red and blue horizontal branch
stars(HB);theredHBstarsarecentrallyconcentratedand
blueHBstarsaremoreextended.Subsequently,asynthetic Figure1.ThetargetfieldsofSextansdSph.Theboxrepresents
modelwasadaptedtotheirphotometriccataloguetoreveal thefieldofviewoftheSuprime-Cam(34′′×27′′).Theblueand
redboxesindicateourobserved26fieldswhicharegood(<1.2′′)
the star formation history (SFH) and chemical enrichment
and bad (> 1.2′′) seeing, respectively. The gray boxes show the
historyasafunctionofadistancefromthecentre(Lee et al.
unobserved field. The tidal radius from (Irwin&Hatzidimitriou
2009). It showed that, within the half-light radius of Sex-
1995)havebeenoverlaidasasolidline.
tans, star formation was more effective and lived longer in
the central regions than the outer regions. More recently,
Roderick et al.(2016)investigatedtheareawithin4×Rh of 2×Rc (or1.5×Rh)andsamplingfieldstobeyondthetidal
Sextans using DECam on CTIO 4m Blanco telescope, and
radius. Using Subaru/Suprime-Cam, we reveal the spatial
found substructures at around Rt. differences of stellar populations from the innermost to the
The medium-resolution spectroscopic studies (MSR), outskirt region. The observation and data analysis proce-
based on both CaII triplet and the synthetic methods, re- duresaredescribedinSection2.InSection3,wepresentthe
vealed that the radial gradient of the metallicity is very resulting CMDs, and derivethedistanceand thestructural
small in the Sextans dSph, but the metallicity dispersion properties. The spatial difference of the stellar population
decreaseswiththedistancefromthecentre(Battaglia et al. and the bluestraggler (BS) stars are discussed in Section 4
2011; Kirby et al. 2011). The inner region (r<0.8◦) shows and 5. Finally we summarize our conclusion in Section 6.
thewhole rangeof [Fe/H] from −3.2 to−1.4, with an aver-
age [Fe/H] = −1.9, while at larger radii only stars of more
poorthan[Fe/H]∼−2.2 arepresent(Battaglia et al. 2011).
High resolution spectroscopic studies of a number of red 2 OBSERVATION AND DATA REDUCTION
giant branch (RGB) stars confirmed the range of metallici-
2.1 Observation
ties estimated by those MSR studies (Shetroneet al. 2001;
Aokiet al.2009;Tafelmeyer et al.2010;Hondaet al.2011). We observed 26 fields in and around the Sextans dSph by
These photometric and spectroscopic studies indicate that usingtheprimefocusimagerSuprime-Cam(Miyazaki et al.
thestarformationperiodismoreextendedinthecentralre- 2002) on the Subaru Telescope during nights of December
gions than in the outer region, at least within the observed 30, 2005 to January 2, 2006 (PI. N. Arimoto), with the
area. seeing ranging from 0.6′′to 1.5′′. The Suprime-Cam cam-
Kleyna et al. (2004) found a cold substructure in the eraprovidesafield-of-view of34′×27′ with apixelscale of
central 5′ of Sextans, from seven stars with radial velocity 0.202′′.Toavoidthesaturationofbrightstars,wetooklong
measurements. The subsequent study with a larger spec- andshortexposureimageswithJohnsonV (10×50secand
troscopic sample confirmed this central substructure, the 3×10sec)andCousinsI (10×110secand3×30sec)filters.
total luminosity and metallicity of which were estimated The details of theobservations are summarized in Table 1.
as 2.2 × 104L⊙ and [Fe/H] = −2.6 ± 0.15, respectively In Figure 1, the target fields (R01-R27) and the con-
(Battaglia et al. 2011). Walkeret al. (2006) also found an- trol fields (A01-A04) of the original observation plan are
other kinematically cold substructure of ∼ 104L⊙ luminos- shown. We chose nine fields at the galaxy centre and 18
ityataroundthecoreradius,whiletheydidnotconfirmthe fieldsalongthedirectionofmajorandminoraxis.Theblue
central one. and red boxes represent the observed 26 fields with good
In this paper, we present the deep and wide field pho- (FWHM<1.25′′;blue)andbad(FWHM>1.25′′;red)see-
tometry of the Sextans dSph covering entire region within ing conditions, respectively, and the gray boxes show the
Population gradient in Sextans dSph 3
Table 1.Informationabouttheobservationandtargetfields
R05 I-band
R05 V-band
R25 I-band
Field Filter Short Long R25 V-band
time(sec) FWHM time(sec) FWHM 1
RR0021 IIVVcc 11330000××××113333 1100′′′′....′′′′00990685 252530305×5×××151050 0110′′′′....′′′′90088268 ection ratio 00..68
R03 Ic 30×3 0′.′70 235×5 0′.′68 Det
V 10×3 0′.′99 50×5 1′.′02 0.4
R04 Ic 30×3 0′.′70 235×5 0′.′66
V 10×3 0′.′94 50×5 0′.′90 0.2
R05 Ic 30×3 0′.′66 235×5 0′.′70
V 10×3 0′.′92 50×5 0′.′77 0
R06 Ic 30×3 0′.′64 235×5 0′.′94 20 21 22 23 24 25 26
V 10×3 0′.′72 50×5 0′.′74 Magnitude
R07 Ic 30×3 0′.′80 235×5 0′.′82 Figure2.Thecompletenessofdetectedstellarobjects.Thesolid
V 10×3 0′.′78 50×5 0′.′77 anddottedlinesarethedetectionratioofI-andV-bandimagesin
R08 Ic 30×3 0′.′84 235×5 0′.′94 theR05field,respectively,whichrepresentthecaseofgoodseeing
V 10×3 1′.′20 50×5 1′.′34 condition (∼ 0.7′′). The dashed and dash-dotted lines indicate
R09 Ic 30×3 1′.′14 235×5 0′.′80 thoseintheR25fieldwhichistheworstcaseusedinthedetailed
V 10×3 1′.′26 50×5 1′.′36 analysis.The90%complete-levelisshownasahorizontaldotted
R11 Ic 30×3 0′.′76 235×5 0′.′78 line.
V 10×3 1′.′34 50×5 1′.′38
R12 Ic 30×3 0′.′92 235×5 0′.′80
V 10×3 1′.′00 50×5 0′.′94
R13 Ic 30×3 0′.′62 235×5 0′.′78 2004). Each image was processed and calibrated in the
V 10×3 1′.′14 50×5 1′.′34
samemannerasforSuprime-CamimagesinOkamoto et al.
R14 Ic 30×3 0′.′70 235×5 0′.′78 (2008).Fortheseprocessedimages,theDAOPHOTinIRAF
V 10×3 1′.′28 50×5 1′.′30
R15 Ic 30×3 0′.′80 235×5 0′.′82 packagewasusedtoobtainthepoint-spread-function(PSF)
V 10×3 0′.′80 50×5 0′.′86 photometryoftheresolvedstars(Stetson1987).Toseparate
R16 Ic 30×3 0′.′82 235×5 0′.′82 thepoint sources from theextendedonesand noise-like ob-
V 10×3 1′.′12 50×5 1′.′26 jects,weselectedthesourceshavingtheDAOPHOTparam-
R17 Ic 30×3 0′.′63 235×5 0′.′70 eter χ2 and SHARP within 3σ of the clipped-mean values
V 10×3 0′.′76 50×5 0′.′82 ofartificialstarsatthesamemagnitudeintheartificial-star
R18 Ic 30×3 0′.′86 235×5 0′.′84 test. The positions of detected stellar objects in each pro-
V 10×3 1′.′04 50×5 0′.′86 cessed image were cross-correlated (within 1′′) to make a
R19 Ic 30×6 0′.′60 235×5 0′.′92 composite catalogue of long and short exposure images.
V 10×3 0′.′70 50×5 0′.′72
We checked the consistency of the magnitude of stars
R21 Ic 30×3 0′.′96 235×5 0′.′90
V 10×3 0′.′84 250×3 0′.′82 commonly detected in both long and short exposure im-
R22 Ic 30×3 0′.′66 235×5 0′.′70 ages (typically 20-22 mag stars), and in both side by side
V 10×3 0′.′76 50×5 0′.′90 images. The magnitude difference between the long and
R24 Ic 30×3 0′.′90 235×5 0′.′86 short exposures is ∆Mshort−long < 0.02, and between the
V 10×3 0′.′94 50×5 0′.′84 side by side images is ∆M < 0.05. The Galac-
neighbour
R25 Ic 30×3 1′.′22 235×5 1′.′04 tic extinction is taken from Schlegel, Finkbeiner, & Davis
V 10×3 1′.′08 50×5 1′.′02 (1998) for the direction of each observed field in
R26 IVc 1300××33 00′′..′′8788 25305××55 00′′..′′9846 Sextans. The assumed extinction law is the Rv=3.1
(Cardelli, Clayton, & Mathis 1989) and the relation of
R27 IVc 1300××33 10′′..′′0964 25305××55 10′′..′′0984 AI/AV=0.594 (Schlegel, Finkbeiner,& Davis1998).
A01 Ic 30×3 0′.′68 235×5 1′.′04 The completeness and photometric errors were derived
V 10×3 0′.′74 50×5 0′.′70 using the artificial-star test with the ADDSTARroutine in
A03 Ic 30×3 0′.′64 235×5 0′.′70 DAOPHOT.Weadded7000artificialstarstoeachimagein
V 10×3 0′.′88 50×5 0′.′92 every 0.5 magnitude interval from 18 mag to 24 mag and
in every 0.25 magnitude interval from 24 mag to 26 mag.
Weprocessedtheresultingimagescontainingartificialstars
in the same way as for the original images to estimate the
unobserved field. In this paper, we use the regions which
haveFWHM<1.25′′ in both V- and Ic-band images. detection ratios. The ratio, N(recovered)/N(added) of the
best seeing field (R05) and the worst seeing field (R25) are
plotted in Figure 2, which indicates that ourphotometry is
atleast90%completeat24maginbothbandscoveringthe
2.2 Data reduction
entireregion.ThemeanphotometricerrorsplottedinFigure
DatawereprocessedusingapipelinesoftwareSDFREDded- 3inSection3arebasedonthedifferencebetweentheinput
icated to the Suprime-Cam (Yagi et al. 2002; Ouchiet al. magnitudeand theoutputmagnitudeof theartificial stars.
4 S. Okamoto et al.
Figure3.Thecolour-magnitudediagramofthecentral80′×100′andwithinr=17′areaofSextansdSph.Errorbarsshowthephotometric
error at each magnitude level based on the artificial-star test. The boundaries marked in the left panel are used to select as sub-giant
branch(SGB),redgiantbranch(RGB),blueandredhorizontalbranch(BHB,RHB)andbluestraggler(BS)starsbelongingtoSextans.
3 COLOUR-MAGNITUDE DIAGRAM AND fields which located at the outside of the tidal radius of
STRUCTURAL PROPERTIES Irwin & Hatzidimitriou (1995).
3.1 Colour-magnitude diagram
3.2 Structural properties
Figure 3presentsthede-reddenedCMD of thestar-likeob-
jects found in the central nine fields (R01, R02, R07, R12, To investigate the distance to Sextans, we use the V-band
R18, R24, R25, R26, R27; hereafter the core region) that magnitudeoftheHBstars. WeestimatetheHBmagnitude
cover the central 80′ ×100′ area of the Sextans dSph. Er- as V0,HB = 20.33±0.02 derived from the 391 RHB stars
ror bars show the photometric errors at each magnitude with20.1<V0 <20.5 and0.55<(V −Ic)0 <0.85, and the
with (V − Ic)0 = 0.75 based on the artificial star test. 139BHBstarswith20.1<V0 <20.5and0.0<(V −Ic)0 <
TheCMDcontainsapproximately74,000sources.Itshowsa 0.45 in the core region. The metallicity distribution of the
well-populatedRGBextendingdownwardfromV0 ∼17,the SextansdSphwasdeterminedbyBattaglia et al.(2011).We
redandblueHB(RHB,BHB)atV0=20.3,thewell-defined adopt[Fe/H]=−2.0fortheaveragetoassumeatheoretical
sub-giant branch (SGB) to MS traced below V0 ∼ 23, to- value of the absolute HB magnitude to be MV,HB = 0.496.
gether with the numerous BS candidates. All the features Wethenobtainadistancemodulus(m−M)0=19.83±0.05
show that theSextansis basically dominated by old metal- (corresponding to a distance of 92.5±2.2 kpc). This value
poor population, but the existence of both RHB and BHB agrees with the previous estimates of L03; they gave (m−
and the thickness of SGB and MS suggest that there are M)0 =19.90±0.06 from theIc-band magnitude of TRGB,
multiplepopulationsinthisgalaxy(seeSection4).Unfortu- and (m−M)0 =19.89±0.04 based on theHB magnitude.
nately,starsbrighterthanIc∼16weresaturatedinourIc- Figure 4 shows the spatial distribution of the star-like
bandimages,sothatthetipofRGB(Ic,TRGB =15.95±0.04 objectswithintheboundariesinFigure3,whichweredrawn
in L03) can hardly be identified. Figure 3 includes fore- by hand. The stars within the areas marked by white lines
ground Galactic stars which mainly distribute uniformly at andgraycolourareusedtoinvestigatetheCMDmorpholo-
V0 > 0.6, and background unresolved objects. To estimate gies at various distances in Section 4. In order to estimate
the significance of the contamination, we use A01 and A03 structural properties, we chose these member stars in the
Population gradient in Sextans dSph 5
elliptical radius [kpc]
0.27 2.7
King
Exponenial
Plummer
10
-2n]
mi
arc
nsity [
e 1
d
er
b
m
u
n
field level=0.203
0.1 Rc Rh Rt
1 10 100
elliptical radius [arcmin]
Figure5.Radialprofilederivedbycalculatingtheaveragenum-
berdensitywithinellipticalannuli.Best-fittingKingprofilewith
coreradiusRc=17.91′±0.65′andtidalradiusRt=120.5′±7.7′,
exponential profile with the effective radius re = 15.15′±0.30′
andPlummerprofilewiththePlummerradiusb=26.60′±0.43′
areoverlaidassolid,dashedanddotted line,respectively.
Carlomethod.TheerrorbarstakeintoaccountthePoisson
errors,theuncertaintiesinthefieldlevelestimation,andthe
areaestimation.WefittheradialprofilewithstandardKing,
exponentialandPlummermodelsusingaleast-squaresmin-
imizationtechnique.Thebest-fittingmodelsareoverlaidon
Figure 4. The spatial distribution of the member stars of Sex- the radial profile in Figure 5 as solid, dashed and dotted
tans. The plotted stars arelocated within the boundaries inthe lines, respectively. The derived parameters, the core radius
CMD shown in Figure 3. The areas within the central dashed
Rc, the tidal radius Rt and the half-right radius Rh of the
white line (r = Rc), between two solid white lines (r ∼ Rh),
exponentialandPlummerprofilesarelistedinTable2with
and within the gray coloured regions (r ∼ 2×Rh, r ∼ Rt) are
thestandard error of the model fitting.
usedinthedetailedanalysisofthestellarcomponentsatvarious
Our estimation of the structural properties agree with
distances inFigure6.
Irwin & Hatzidimitriou (1995) and Roderick et al. (2016),
exceptfortheellipticityandthetidalradius.Ourellipticity
Table 2.ThestructuralpropertiesofSextans dSph e=0.20isslightlysmallerthantheirestimations.Although
it is within the margin of error. The estimated tidal radius
Parameter Value Rt = 120.5′ ±7.7′ is smaller than Irwin & Hatzidimitriou
(1995) estimation of Rt = 160′ ± ′50, and larger than
Coordinates(J2000) 10h13m04.9s,−01◦37′31.6′′ Roderick et al. (2016)’s Rt =83.2′±7.1′. These differences
Positionangleofmajoraxis 54◦.2 are mainly due to the area coverage and the image depth.
Ellipticity 0.20 They covered the entire region of Sextans, while we use
(m−M)0 19.83±0.05 deeper images of the central area. The best-fitting King
Distance 92.5±2.5[kpc]
modelintheleft panelofFigure8ofRoderick et al. (2016)
Rc 481±17[pc]
Rt 3.24±0.2[kpc] does not fit the data at large radii well, implying a larger
Rh (exponential) 685±14[pc] tidal radius. The contour map of Irwin & Hatzidimitriou
Rh (plummer) 715±12[pc] (1995) shows that the inner parts of Sextans are less el-
liptical than the outer parts. Our tidal radius estimation is
basedontheimagesextendedtowardtheminoraxis.There-
core region (80′×100′, equivalent to 2.1×2.4 kpc), to re- fore, if we assume that the outer ellipticity is e=0.35, the
duce contamination and to avoid the incompleteness of the tidalradiusincreasestoRt =148.3′±9.4′,whichagreeswell
photometryandthedifferenceofcompletenessbetweeneach with Irwin & Hatzidimitriou (1995).
region. The centroid, ellipticity and position angle are de-
rived from the density-weighted first and second moments
and are listed in Table 2.
4 THE STELLAR POPULATIONS
In Figure 5, thestellar radial profile in the logarithmic
form is constructedbycalculating theaverage numberden- Stellar populations of the “classical” dSphs are more com-
sity of member stars in elliptical annuli with the ellipticity plicated than fainter dwarf galaxies. Bright dSphs are
and the position angle derived above. The field contami- known to possess multiple and spatially distinct stellar
nation level is estimated from the A01 field. The observed components in the RGB and HB (e.g. Tolstoy et al. 2004;
areawithineachellipticalannulusiscalculatedbytheMonte Battaglia et al. 2006, 2008). Fornax and Sculptor dSphs
6 S. Okamoto et al.
Figure6.TheCMDsofstarsatvariousdistancesfromthecentreofSextans.Thetheoreticalisochronesof10.0Gyrand13.8Gyrwith
Z=0.0002areoverlaidasdashedandsolidlines,respectively.ThedistancefromthecentreofeachCMDisdescribedineachpanel.The
starswithinthesolidrectanglesareusedtoderivethecolourdistributionofHBandSGBstarsinFigure7and8.
(MV =−13.4,−11.1) are confirmed to have the population the faintest “classical” dSph, Sextans also shows the differ-
gradients; theold,metal-poor populationsexist at allradii, ent spatial distribution of RHBand BHB stars (L03).
whilemoremetal-rich,youngerstarsaremorecentrallycon-
To demonstrate the spatially difference of the stellar
centrated (deBoer et al. 2012a,b). Carina dSph shows at
population in Sextans, we compare the CMDs of star-like
least three distinct MS turn-offs (Monelli et al. 2003), and
objectsintheellipticalannuliatvariousdistances.Figure6
Population gradient in Sextans dSph 7
40 40
(a) 0’ < r < 17’ (b) 26’ < r < 30’ (a) 0’ < r < 17’ (b) 26’ < r < 30’
: control field : distribution of (a) 150 : field III : distribution of (a) 150
: control field I II IV V : field
HB 30 30 GB
mber of 20 20 mber of S 100 I II III IV V 100
Nu 10 10 Nu 50 50
0 0 0 0
40 40
(c) 52’ < r < 58’ (d) 95’ < r < 125’ (c) 52’ < r < 58’ (d) 95’ < r < 125’
: distribution of (a) : distribution of (a) 30 : distribution of 1/5*(a) : distribution of 1/5*(a) 30
: control field : control field : field : field
HB 30 30 GB
mber of 20 20 mber of S 20 I II III IV V 20
Nu 10 10 Nu 10 10
0 0 0 0
0.1 0.3 0.5 0.7 0.1 0.3 0.5 0.7 0.4 0.5 0.6 0.7 0.8 0.4 0.5 0.6 0.7 0.8
(V- I) (V- I) (V- I) (V- I)
c 0 c 0 c 0 c 0
Figure7.ThecolourdistributionoftheHBstarswithin20.1< Figure8.ThecolourdistributionsofthefaintSGBstarswithin
V0 <20.6in each CMD of Figure6 is shown as the shaded bar. 23.0 < V0 < 23.2 in the CMDs of Figure 6 are shown as the
The width of colour bin is 0.04 mag. The histograms with light samemannerasFigure7.Thehistogramswithlightgraycolour
gray colour in 7b-7dindicate the distribution of 7a as the basis. in Figure 8c and 8c show the distribution of Figure 8a with a
Thedarkgrayhistogramshows thedistributionofthefieldcon- scalefactorof1/5.Theerrorbarshowstheaveragephotometric
taminationestimatedbyusingtheregionof135′<r<175′from error, (V −Ic)0,err = 0.05, at V0 = 23.0 and (V −Ic)0 = 0.5.
thecentreofSextansdSph.Theerrorbarshowsthephotometric The arrows I to V indicate the theoretical colours of 10.0, 11.2,
error,(V −Ic)0,err =0.02,atV0=20.5and(V −Ic)0=0.5. 12.6,13.8,and15.5Gyrsoldstars,respectively.
showstheCMDsofthefourregionslocatedattheelliptical
confirmedmetallicityisbiasedsignificantly(Battaglia et al.
distance of (a) 0<r< Rc, (b) r∼Rh, (c) r∼2×Rh, (d)
2011).Figure6dincludesthestar-likeobjectsattheedgeof
sra∼meRsto,lridespaencgtlieve(l5y3.0Tthoe5s5e0lecatrecdmrieng2i)o,nhsecnocvee,rthalemnousmt btheer SextansdSph(Rt =124.7′±7.8′).Most of thecomponents
can hardly beidentified.
of foreground/background objects in four CMDs are con-
In these four CMDs, the HB and SGB colour distri-
sidered to be similar. Padova isochrones of 10.0 Gyr and
butions change with the distance from the galaxy centre.
13.8GyrwithZ=0.0002areoverlaidasguides(Marigo et al.
Figure 7 and Figure 8 show the colour distributions of HB
2008).
and SGB stars within the rectangles in Figure 6, which are
selected to enhance color differences with respect to age of
the populations and to minimize field contamination. As a
4.1 The spatial difference of CMD morphology
guide, the histograms with light gray colour in Figure 7b-d
In the innermost region, Figure 6a shows the dominant and in Figure 8b are the same as that of the innermost re-
RHB, a few BHB, the relatively wide sequence of SGB to gion, and those in Figure 8c and 8d are the distribution of
MSTO, and numerous BS stars. The overlaid 10 Gyr and theinnermost region with a scale factor of 1/5.
13.8Gyrisochroneswellreproducethedistributionofstars. While the BHB stars are seen in all panels in Fig-
Atthehalf-lightradius(Figure6b),theclumpofRHBfades ure 7, the RHB stars dominate within Rc, and decrease
intotheforegroundcontamination,andthewidthofMSTO rapidlywith thedistance. Thisspatial differenceof HBdis-
andSGBbecomesnarrow.Theyoungerisochronecouldnot tribution implies that the old/metal-poor population exist
fit to MSTO to SGB which seems to be shifted to redder in the whole region of Sextans dSph, while the relatively
and fainter than that of the inner region. The differences younger/metal-rich population is situated only in the cen-
between Figure 6a and Figure 6b still remain when we re- tral region as shown in L03.
sample the data as the same size. Figure 6c is similar to On the other hand, the peak of the (V −Ic)0 colour
Figure 6b, but there are only a few stars. The RHB, RGB, of SGB distribution becomes redder from the centre (0.62
BS stars almost disappear, but the BHB and the MS stars in r < Rc) to the outer region (0.70 at r = Rh, 0.72 at
canbeidentified.InFigure6bandFigure6c,eventheoldest r = 2 × Rh), and the bluer part of the SGB disappears
(13.8Gyr)isochroneshowsaslightlybluercolouratMSTO in Figure 8c and Figure 8d. This difference is larger than
thanthatofSextansdSph.Ifweadoptthemoremetal-rich the photometric error at this magnitude. The bluer SGB
isochrone(Z=0.0004) fortheseouterregions,thisdifference starsintheinnerregion indicateeitherarelatively younger
canbereduced,butitisnotlikelythatthespectroscopically or more metal poor population compared to the outer re-
8 S. Okamoto et al.
gion. However, since spectroscopic studies show a metallic- [kpc]
0 0.5 1
ity gradient from the metal-rich central part to the metal-
poor outer region in Sextans dSph, the latter case is un-
1.0
likely (Battaglia et al. 2011; Kirby et al. 2011). The theo-
retical colours of 10.0, 11.2, 12.6, 13.8, and 15.5 Gyrs old
stars with [Fe/H] = −2.0 (Z=0.0002 with [α/Fe] = 0.0) at y 0.8
sit
themagnitudebasedonPadovaisochronesareshownasar- n
e
d
rows I to V in Figure 8. The peaks between the innermost er
(r < Rc) and the outer (r ∼ 2×Rh) region corresponds to umb 0.6
about∼3Gyragedifference,assumingtheconstantmetal- e n
v
licity of [Fe/H]=−2.0. ulati 0.4
Note that if we adopt a metallicity gradient as found um
C
by previous spectroscopic observations, the spatial age dif- SGB
ference estimated here would become larger. This is due to 0.2 RHB
thefactthatforagivenage,thecolorofSGBstarsincreases BHB
Rc Rh
with metallicity, therefore a metal rich population needs to
0.0
have a much younger stellar age in order to produce a blue 0 10 20 30 40 50
color distribution. elliptical radius [arcmin]
Figure9.Cumulativeradialprofileofstellarcomponentsderived
4.2 The population gradient bycalculatingtheaveragenumberdensitywithinellipticalannuli.
Thetriangles,squares,andcirclesrepresenttheprofilesofRHB,
The spatial differenceof theCMD morphology reveals that
BHBandSGBstars,respectively.Theoverallcoreandhalf-light
the stellar population of Sextans is different from inside to
radiiarealsoindicated.
outside out to 2 × Rh. The innermost part possess both
young/metal-richandold/metal-poorpopulationswhichap-
pear in the HB morphology and color distribution of the foreground/background contaminants are almost negligible
SGB. While younger component fade away, the older pop- (<3%).
ulation still exists beyond the half light radius. Although In Figure 10, the younger SGB population has higher
theageandmetallicityofstarsaredegenerateincolourand concentrationandhigherellipticitythantheoldpopulation.
magnitude of HB and SGB to MSTO, the spatial differ- The density peak in thepanel (a) has slight offset from the
ences of both HBand faint SGBcolour distributionsin the galaxy centre. There is a secondary peak at the northeast
previous subsection indicate the existence of the radial age side of Rc in the panel (c) that has similar position to the
gradientontheassumptionoftheconstantmetallicityorthe coldsubstructurefoundbyWalkeret al.(2006).Wecannot
metallicity gradient in the Sextans. This spatial difference confirmthissecondarypeak inthedensitymapofallmem-
could also be seen in the radial profile of the RHB, BHB, ber stars in Figure 4, and there is no clear difference of the
and SGB stars. CMD morphology between this area and the surrounding
Figure 9 shows the cumulative radial number density area.Butifitistheremnantofadisruptedcluster,thestel-
profiles. Each star is selected by the colour and magnitude larageisasoldasUFDsandoldGalacticglobularclusters.
criteria in Figure 3. In Figure 9, the distribution of BHB The radial population gradients and two distinct dy-
starsismoreextendedthanSGBcomponent,andtheRHB namical components are found in many bright dwarf satel-
is concentrated toward the center. The SGB profile is inbe- lites. The brightest UFD, Canes Venatici I (CVn I) also
tweenBHBandRHB,becauseSGBincludesbothcentrally contains radially different stellar populations; the distribu-
concentrated young stars and spatially extended old popu- tion of RHB stars is more concentrated toward the centre
lations.TheRGBandBSstarshavesimilarprofilestoSGB thanthatofBHBstars(Okamoto et al.2012).Ontheother
stars. hands,faint satellites, suchas Boo¨tes I,havenospatial dif-
Spectroscopicobservationsshowawiderangeofmetal- ferenceintheCMDmorphology,andareallconsistentwith
licitydispersionsandametallicitygradientinSextansdSph anageof(∼13Gyr).From faint UFDstobrightdSphs,the
(Battaglia et al.2011;Kirby et al.2011).However,ifweas- complexity of stellar populations increase in proportion to
sume theconstant metallicity as [Fe/H]=−2.0, thespatial theincreasingtotalluminosity.Thestellarpopulationstruc-
distribution of stars within a specific age range could be tureofSextansdSphfollowsthistendency.Sextansdosenot
investigated using theoretical isochrones. Figure 10 shows show obvious evidence of young or intermediate age popu-
the spatial distributions of SGB stars within the magni- lation as the Sagittarius and Fornax dSphs, but shows the
tude range 22.7 < V0 < 23.5, which is bright enough age range larger than 3 Gyrat theinner most region.
to ignore the spatial disparities of the photometric errors Principally, stars in satellite galaxies were born in very
and the completeness, and is adjusted to reduce the fore- low mass halos. All the Galactic dSphs contain the oldest
ground/background contaminations. We divide SGB stars populationcomparabletothoseoftheoldGalacticglobular
into the age bins of (a) 8 to 11 Gyr, (b) 11 to 12.5 Gyr, clusters, which suggests that the local over-densities in the
(c) 12.5 to 14 Gyr, and (d) 14 to 15.5 Gyr using Padova primordial matter distribution started forming stars at a
isochrones. Then these stars are binned and smoothed by similartime.Then,thegaseousmatterinalowermasshalo
the Gaussian kernel with the bandwidth of 2′5 to draw was easily blown out by the supernova feedback when the
the spatial contour maps in Figure 10. The density scales star formation occurred, or was possibly photo-evaporated
in panel (b)-(d) are fixed to that of the panel (a). The byreionization (e.g. Mun˜oz et al. 2009; Guo et al. 2011).
Population gradient in Sextans dSph 9
Da [kpc] Da [kpc] 1/Rc
0.5 0 −0.5 0.5 0 −0.5
0 1 2 3
(a) 8Gyr − 11Gyr 689 stars (b) 11Gyr − 12.5Gyr 982 stars
Dd [arcmin] −20020 EN −0.500.5Dd [kpc] Frequency ( N / N )RGB 0000....1234 BS RHB 0000....1234
(c) 12.5Gyr − 14Gyr 1073 stars (d) 14Gyr < 1046 stars
20 0.5 0.0 0.0
[arcmin] 0 0Dd [kpc] N / N )RGB 0.2 0.2
Dd −20 −0.5 ency ( 0.1 0.1
u
q
e
20 0 −20 20 0 −20 Fr
aD [arcmin] aD [arcmin] bright BS faint BS
0.0 0.0
Figure 10. Thecontour maps of SGB stars inthe different age 0 10 20 30 40 50
bins. The density levels and colour scales of panels (b)-(d) are elliptical radius [arcmin]
fixedtothatofpanel(a).Thesolidanddashedwhitelinesshow
thecoreandhalf-lightradius,respectively. Figure11.Theradialdistributionsoftherelativefrequenciesof
BSstars(filledcirclesconnectedbysolidline),ofRHBstars(open
circlesconnectedbydottedline),ofbrightBSstars(filledsquares
connected by dashed line) and of faint BS stars (filled triangles
After UFDs stopped their star formations, some halos connected by dotted-dashed line). The numbers of stars within
ellipticalannuliarecountedandcorrectedforthecontamination
keptthegasandcontinuedthestarformationforafewGyrs
andnormalizedtothoseofRGBstars.
then became brighter systems. Rocha, Peter, & Bullock
(2012) have calculated the probability distribution of the
infall time for the Milky Way satellites based on the cos- 5.1 The radial distribution
mological simulation. Accordingtotheirestimates, Sextans
was accreted between 7and 9Gyrago, which was asimilar Inour catalogue, thereare 288 BS stars within thecore ra-
time as the end of forming stars at the inner most region. dius,and747BSstarsintheobservedregionwithinthetidal
It implies that the star formation in Sextans was gradu- radius. The contamination level is estimated using thecon-
ally quenched from outside to inside, then finally stopped trol field, about 2% within the core radius and 10% within
when it fell into the Milky Way. Or, multiple pericentric thetidalradius,respectively.WenotethattherearetwoBS
passages around the Milky Way could removed gas gradu- stars within r = 100′′ and within the selection criterion of
allyfrom Sextans,whichalsomadethepopulationgradient L03. These stars have not been found in L03, because they
(Nichols, Revaz, & Jablonka 2014). are located at the CCD gap of their images.
To investigate the radial difference between BS and
otherpopulations,andbetweenbright(V0 <22.5)andfaint
(V0 > 22.5) BS stars, we plot the relative frequencies of
BS stars normalized to RGB stars in Figure 11, together
withthatofRHBstarstoRGBstars.Thenumbersofstars
5 BLUE STRAGGLER STARS
within elliptical annuli are counted and corrected for the
Blue straggler (BS) candidates are found universally in the contamination, using the reference field outside of the tidal
GalacticdSphsatellites. ThedSphshavepredominatelyold radius. The errors include Poissonian statistics, uncertain-
populations,buttheymighthavehadalow-levelstarforma- ties in foreground subtraction and photometric errors.
tion in last few Gyrs at the inner region. Therefore, unlike In Figure 11, the radial frequency of NBS with respect
BS stars in the old globular clusters (GC), these stars can toNRGB isnearlyflatandsimilartothatofNRHB toNRGB,
be either genuine BS stars or substantially young MS stars showing the spatial distribution of the BS stars is similar
(∼ 2Gyr). L03 investigated the BS stars within central 20′ to those of the RGB and RHB stars. If the considerably
of Sextans and showed that the bright BS stars are more younger MS stars (∼2Gyr) exist in Sextans and are found
centrally concentrated than thefaint BS stars. asBScandidates,theywouldbeexpectedtobecentrallylo-
Inthissection,weprobethepropertiesofBSstarspop- cated, rather than distributed throughout the galaxy. Con-
ulated throughout the galaxy. The criterion for BS stars is sideringwiththelongdynamicaltimescaleandthelowstel-
shown in Figure 3. The magnitude and colour ranges are lar density,the uniform BS distribution indicates that they
similar to those of L03, but we modify theboundary of the are the mass-transfer BS stars evolved from primordial bi-
minimummagnitude.Thankstothedeeperphotometry,we naries, as those found in other dSphs(Momany et al. 2007;
can easily discriminate BS stars against MS stars and the Mapelli et al. 2007; Okamotoet al. 2012).
contaminations at thismagnitude level. As pointed out by L03, the relative frequency of the
10 S. Okamoto et al.
MV [mag] similar distribution that keeps increasing with decreasing
1.5 2 2.5 3 3.5 luminosity.
Withintheinnermostregion(r<0.5×Rc),weconfirm
120 120
: All ( R < 2rc) that the brighter BS stars are more centrally concentrated
than the fainter ones, which can be the result of either the
100 : R < 0.5rc 100
masssegregation,theexistenceofyoungerpopulationatthe
: 0.5rc < R < rc
: rc < R < 2rc centralregion, ortheremnantofadisruptedstellarcluster.
80 80 However, the first one is doubtful, because of the long dy-
namical relaxation timescale of SextansdSph.
N BS 60 60 A possible explanation is that the inner brighter BS
stars are younger than the fainter ones. Typical BS stars
formedviamasstransferaremassivethantheMSTOmass,
40 40
butnotmorethantwicetheMSTOmass(e.g.McCrea1964;
Shara, Saffer, & Livio1997).Althoughitisdifficulttodeter-
20 20 minethemassofbrightBSstars,thecentrallyconcentrated
younger population having higher MSTO mass would con-
0 0 tain thebrighterBS stars than thoseof theold population.
21 21.5 22 22.5 23
Kleyna et al. (2004) found a kinematically cold sub-
V0 [mag] structure in the central 5′, which may be a remnant of a
disruptedstarclusterwhichsankintotheSextanscentre.It
Figure 12.The luminositydistributionof BSstars. Theempty
hanisdtotghreamerrroerpbreasresnsthsotwhethalelPsaomispsolenioafnBeSrrsotrasr.sTwhiethgirnayr<col2o×urRedc,, wtaalslucmoninfiormsiteydwwaitshesatilmaragteerdspasec2t.r2o×sc1o0p4iLc⊙sa(mBpaltetaagnldiaitesttaol-.
heavilyandlightlyhatchedhistogramsrepresentBSstarslocated 2011).Inahighdensityregion,suchasastarcluster,bright
at 0 < r< 0.5×Rc, 0.5×Rc < r< Rc, and Rc <r < 2×Rc, BSstarsarethoughttobecreatedpreferentiallythroughthe
respectively. direct collision of two stars, while it would not be happen
in a low density system (e.g. Bailyn & Pinsonneault 1995).
Therefore, a part of centrally concentrated bright BS stars
bright BS stars (filled squares) seems to be decreasing could be originally formed in a star cluster that was dis-
slightly with increasing radius up to at r ∼ Rc, while that rupted and sunk into the Sextans centre. By subtracting
of the fainter BS stars (filled triangles) shows the flat dis- the primordial BS luminosity distribution estimated from
tribution. Note that we compare the radial frequency with theouterregion,thenumberofBSstarsofcluster-origin,in
that of BS stars based on theL03 definition and do not see r<0.5×Rc,canbeestimatedasN =17.9±1.2,whichisthe
any significant difference. typicalBSnumberinastarclusterof104L⊙ (Santana et al.
2013).
Although, only spectroscopic analyses can provide the
indisputable evidence to reveal the origin of bright BS dis-
5.2 The luminosity distribution
tribution in Sextans, our results are consistent with both
To probe theradial gradients of the bright BS frequencyin scenarios.
the innerregion, we compare theluminosity distribution of
BS stars in the innermost region (r<0.5×Rc) with those
of BS stars located in the outer regions (0.5×Rc <r<Rc 6 CONCLUSIONS
andRc<r<2×Rc).Theforegroundandbackgroundcon-
taminationsarecorrectedusingthefieldoutsideofthetidal We present the wide-field, deep photometry of the Sextans
radius.InFigure12,thenumberofthecentralBSstars(gray dSph,samplingfieldsextendedtothetidalradiusandreach-
coloured histogram) increases rapidlywithdecreasing lumi- ingtwomagnitudebelow themain-sequenceturn-off.Iten-
nosityandbecomesconstantatthemagnitudesfainterthan ablesustoderivetheglobalpropertiesofstellarpopulations.
V0 =22,whilethoseofBSstarsintheouterregions(heavily We estimate the distance, structural properties and probe
andlightlyhatchedhistograms)increasegraduallyandkeep the differences of the stellar populations with the distance
increasingatfaintermagnitudes.AtthemagnitudeV0=22, from thegalaxy centre.
one third of BS stars are located within r < 0.5×Rc. As The analysis of the radial distribution of each evolu-
theluminosity decreases, theproportion ofcentralBS stars tionaryphaseshowsthatblueHBstarshavemorespatially
to all BS stars decreases and it becomes 12% at V0 = 23. extendeddistribution,whileredHBstarsaremorecentrally
The different luminosity distributions are confirmed by the concentrated. The colour distributions of SGB stars also
two-sampleKolmogorov-Smirnovtest.Asimilar correlation showthespatialdifference;theSGBstarsincentralregions
between theradial distance andtheluminosity distribution present bluercolour than those in theoutskirt.
wasalsofoundinFornaxdSph,butnotfoundinDraco,Ursa Theseresults indicatetheagegradient of Sextans.The
Minor and Sculptor(Mapelli et al. 2009). relatively younger stars (∼10 Gyr) are more centrally con-
The luminosity distribution of BS stars located at the centratedthantheoldstars(∼13Gyr),andthestarforma-
outsideofthecoreradius(lightlyhatchedhistogram)isnot tion in the innermost region continued more than ∼3 Gyr.
significantlydifferentfromthatofBSstarsat0.5×Rc<r< The spatial maps of SGB stars in different age bins also
Rc (heavily hatched histogram) in Figure 12. We note that confirm that the younger population has the higher con-
theluminositydistributionofBSstarsbeyond2×Rchasalso centration and higher ellipticity than the old population.