Carbon Stars and C/M Ratio in the WLM Dwarf Irregular Galaxy

0.42 MB·English

by  A. T. Valcheva

Astronomy&Astrophysicsmanuscriptno.wlm February5,2008
(DOI:willbeinsertedbyhandlater)
Carbon Stars and C/M Ratio in the WLM Dwarf Irregular Galaxy
A.T.Valcheva1,V.D.Ivanov2,E.P.Ovcharov1,andP.L.Nedialkov1
7
0
0 1 DepartmentofAstronomy,UniversityofSofia,5JamesBourchier,Sofia1164,Bulgaria
2 e-mail:[email protected], [email protected], [email protected]

n 2 EuropeanSouthernObservatory,Ave.AlonsodeCordova3107,Casilla19,Santiago19001,Chile
a e-mail:[email protected]
J

5 Received18November2005/accepted18January2007
2

Abstract. WeidentifytherichCarbonstarpopulationoftheMagellanic-typedwarfirregulargalaxyWLM(Wolf-Lundmark-
1
Melotte) and study its photometric properties from deep near-IR observations. The galaxy exhibits also a clear presence of
v
Oxygen rich population. We derive a Carbon to M-star ratio of C/M=0.56±0.12, relatively high in comparison with many
7
2 galaxies.ThespatialdistributionoftheAGBstarsinWLMhintsatthepresenceoftwostellarcomplexeswithasizeofafew
7 hundredparsecs.
1 Using the H map of WLM and the derived gas-to-dust ratio for this galaxy N(H)/E(B − V)=60±10 [1021 at. cm−2
0 mag−1] we re-determined the distance modulus of WLM from the IR photometry of four known Cepheids, obtaining
7 (m−M) =24.84±0.14mag.Inaddition,wedeterminethescalelengthof0.75±0.14kpcofWLMdiskinJ-band.
0
0
h/ Keywords.Galaxies:individual:WLM,distance,gas-to-dustratio,scalelength,C/Mratio–Galaxies:LocalGroup,irregular
p –Stars:AGB,Carbon
-
o
r
t 1. Introduction becausetheenhancedstellarabundancewouldputWLMwell
s
a abovethemetallicity-luminosityrelation.
WLM (Wolf-Lundmark-Melotte; also DDO221; Wolf 1909,
: These results make it important to study the Carbon star
v Melotte 1926) is a dwarf irregular Local Group member.
i in WLM because of their sensitivity to the metal abundance.
X Earlier photographic surveys of the galaxy were presented in
Besides, AGB stars are representative for the stellar popula-
Ables & Ables (1977) and Sandage & Carlson (1985). The
r tion with ages between 1 – 10Gyr and can be used as a ob-
a first CCD observations of WLM were performed by Ferraro
servational constraint to the properties of the post-main se-
et al. (1989) who reported significant variations in the re-
quencestellar evolution.Initially,the behaviorofthe C/M ra-
cent star-formation rate across the galaxy. They also detected
tio is understood at least qualitatively (Scalo & Miller 1981,
uniform underlyingrelatively old stellar population(∼1Gyr).
Iben& Renzini1983). Largenumberofstudies ofAGB stars
Lateron,Minniti&Zijlstra(1997)derivedfromV andI-band
in Local group galaxies was performed using optical narrow
photometry a distance modulus (m-M) =24.75±0.1 mag and
0 bandimaging.Itis turn outto be an easy way to separateM-
[Fe/H]=−1.45±0.2.Hodgeetal.(1999)reportedthefirstHST
typefromCarbonstars(Albertetal.2000,Battinelli&Demers
observationsofWLM.Theyresolvedthesoleglobularcluster
2004).Theprogressintherecentyearsmadeitpossibletopro-
(Ables&Ables1977)andobtained(m-M) =24.73±0.07mag,
0 ducethefirstobservationalcalibrationsoftheC/Mratioversus
[Fe/H]=−1.52±0.08 and an age of 14.8±0.6Gyr, typical for
metallicityrelation(Groenewegen2006,Cioni&Habing2005,
theoldglobularsintheMilkyWay.Dolphin(2000)concluded
Battinelli&Demers2005).
thattheWLMstartedtoformstarsabout12Gyrago,withap-
AsAGB starsare almostthebrightestcoolstarsit iseasy
proximatelyhalfofthestar-formationoccurringduringthelast
tobeinvestigatedinthenear-IRrange.Theinterestinthisre-
9Gyr,alsobasedonHSTimaging.
gion of the spectrum grow up strongly in the last three years
Recently, Venn et al. (2003) determined
(Cioni&Habing2003,2005,Kangetal.2005,2006,Sohnet
[Fe/H]=−0.38±0.29 from high-resolution spectroscopy
al.2006).HerewereportanalysisoftheCarbonstarsinWLM
of two WLM blue supergiants. They found depleted stellar
fromnear-IRimaging.
Oxygenabundancebyafactoroffive,incomparisonwiththe
Oxygen abundance of the H regions. Even though later Lee
et al. (2005) reducedthe discrepancy,some unusualchemical 2. Observationsanddatareduction
evolution history is required to explain the observations
TheobservationsofWLMweremadeinDec2004undernon-
Sendoffprintrequeststo:A.Valcheva photometric conditions. In order to estimate the foreground
2 Valchevaetal.:Near-IRviewofWLM:CarbonstarsandC/Mratio
contamination an additional field located 14.8 arcmin South
from the center of the galaxy, far enough to eliminate the
galaxycontribution,wasobservedinJul2006(hereafter”clear
sky” field). The near-IR imaged and spectrograph SofI (Son
of ISAAC) at the ESO NTT (New TechnologyTelescope) on
LaSillawasused.Theinstrumentisequippedwith1024×1024
HawaiiHgCdTearraywith18.5micronsquarepixelsandscale
of 0.288 arcsec pixel−1, giving a field of view of 4.92×4.92
arcmin2.Wealternatedbetweenthetargetandaclearpatchof
sky nearby.To minimize the effectsof arraycosmeticsandto
improvetheskysubtractionweintroducedasmallrandomjit-
ter of up to a few tens of arcseconds between the sequential
images. In total, we collected 10 images for both fields in J
S
and 20 on the galaxy and 15 on the ”clear sky” field in K .
S
Eachoftheimagesin J istheaverageof2×30sexposures
S
andof8×8sexposuresinK .Thetotalintegrationtimewas
S
10min in J (hereafter J for simplicity) and in K 21.33min
S S
onthegalaxyand16minonthe”clearsky”.
Thedatareductionfollowedthetypicalsteps.First,wecre-
ated a sky for each filter by median combination of the off-
target images using suitable upper limits and rejection algo-
rithms to exclude the stars present in the sky field. Next, we
subtractedtheskyfromeachimage,anddivideditbythedome
flat.Thedomeflats arepreferableincomparisonwiththe sky
flats because they allow us to remove the variable bias level,
Fig.1. Near-IR image of WLM. Isolines show the H map of
foundinsomeHawaiidetectors.Toaccountfortheunevenil-
Jacksonetal.(2004)atlevelsfrom25to95%ofthemaximum
luminationofthedomeflatscreen,weappliedanillumination
columndensity(2.16×1021cm−2),inincrementsof10%.The
correction. It was derived from the photometry of a standard
squaremarkstheboundaryofthe”inner”field(seeSection2).
star, placed on 16 positions across the field of average view,
ina4×4grid.Finally,theindividualimageswerealignedand
combinedtoformthefinalimage(Fig.1).
The photometriccalibrationwas performedby comparing
our instrumentalphotometrywith the measurementsfromthe
2MASSPointSourceCatalog:
J = j−5.55(±0.05) (1)
K =k−8.65(±0.18) (2)
S
based on 18 common stars. Here J and K are the 2MASS
S
systematicmagnitudes,and jandk aretheinstrumentalmag-
nitudes.Nostatisticallysignificantcolortermswerefound.
ThephotometrywasperformedwiththeIRAFDAOPHOT
package.Ingalaxyfield1550starsweredetectedin J and739 Fig.2. Photometric errors as a function of the magnitude for
inK withthresholdofrespectively4σand3σabovetheback- J (top)and K (bottom)band.Thecompletenesslimitsofour
S S
ground.The instrumentalmagnitudesfor each filter were ob- photometry are ∼20.1 and ∼17.9mag for J and K -band, re-
S
tainedbyusingconstantGaussianPSF.Thepositionsandmag- spectively.
nitudesofallmeasuredstarsmatchedinbothJ andK frames
S
withmaximumtoleranceof3pixels(0.86arcsec)aregivenin
Tabl.1.ThemeasurementerrorsareplottedinFig.2.
Theluminosityfunctionsin J and K are shownin Fig 3.
The reduction and calibration procedures for ”clear sky” S
The numberofstars in each bin is normalizedto 1 squarear-
fieldfollowtheanalogicalsteps.Thephotometricequationsob-
cmin. We tested the completeness limits variations across the
tainedfrom72MASSstars
galaxy by splitting the field into an “inner” part, containing
J = j−3.12(±0.03) (3) the area with maximum H column densities and an “outer”
part, respectively 5.76 and 14.77 square arcmin (see Fig 1).
K =k−5.34(±0.04) (4)
S The crowding effects are limited to ≤ 0.2mag. We adopted
wereusedtotransformthemagnitudesof89starsin J and67 Jlim∼20.1mag and Klim∼17.9mag. For comparison the lumi-
S
in K into 2MASS magnitudes. Matching procedure gives in nosityfunctionof”clearsky”isalsoshowninFig3.Onaver-
S
total50commonstarsin JandK . agetheLFisnearlyanorderlowerthenthegalaxyone.
S
Valchevaetal.:Near-IRviewofWLM:CarbonstarsandC/Mratio 3
Table 1.Photometryof555WLMstarswith J and K -bandphotometry.Thecolumnscontain:identificationnumber,RA and
S
DEC(J2000),JandK magnitudes(the1σerrorsaregiveninbrackets.Keystothenotes:V-variablestar,withidentifierfrom
S
thesourcepaper(SC85forSandage&Carlson1985andM05forMoraetal.inpreparation),C-Carbonstarcandidate(BD04
forBattinelli&Demers2004),G-globularcluster.Thepapereditionofthejournalcontainsonlythefirst20entries.Therestof
thetableisavailableintheelectronicversionofthejournalonly.
No R.A. Dec. J K Note
S
1 00:01:45.62−15:30:06.619.713(0.135)18.268(0.131)M05LC120
2 00:01:46.86−15:29:44.018.150(0.047)16.737(0.058) C
3 00:01:47.40−15:29:49.119.762(0.119)18.376(0.155) C
4 00:01:47.50−15:27:54.819.222(0.073)18.402(0.140)
5 00:01:47.99−15:27:47.519.408(0.092)17.487(0.065) BD04
6 00:01:48.20−15:26:49.019.327(0.088)17.855(0.097) C
7 00:01:48.34−15:28:08.519.402(0.107)17.462(0.096) C
8 00:01:48.42−15:27:08.619.078(0.069)18.273(0.118)
9 00:01:48.79−15:27:35.219.800(0.130)18.849(0.232)
10 00:01:48.80−15:27:16.819.132(0.068)18.037(0.108) BD04
11 00:01:49.09−15:27:06.319.712(0.109)18.985(0.255)
12 00:01:49.11−15:29:03.319.708(0.114)19.501(0.418)
13 00:01:49.26−15:26:53.413.177(0.029)12.683(0.004)
14 00:01:49.33−15:28:24.119.495(0.092)18.920(0.253)
15 00:01:49.50−15:26:06.418.821(0.056)18.021(0.103)
16 00:01:49.56−15:27:31.015.615(0.222)14.631(0.042) G
17 00:01:49.57−15:28:00.618.921(0.075)17.781(0.090)
18 00:01:49.79−15:29:42.119.211(0.076)18.764(0.217)
19 00:01:49.88−15:26:32.318.833(0.091)17.404(0.090) C
20 00:01:49.94−15:26:20.719.196(0.076)18.640(0.192)
3. Color-MagnitudeDiagram
Thecolor-magnitudediagramofWLMisshowninFig.4(top
panel).Weuseddistancemodulus(m−M) =24.85mag–anav-
0
erageoftheestimatesof:(i)Minniti&Zijlstra(1997)whode-
rived(m−M) =24.75±0.1magfromtheI-magnitudeofthered
0
giantbranch(hereafterRGB)tipand(ii)Rejkubaetal.(2000)
whoobtained(m−M) =24.95±0.13magfromV-bandphotom-
0
etry of the horizontal branch. These two values agree within
theerrorsandtheyarederivedfromdifferentmethodsanddata
sets. Corrections for the Galactic extinction were made using
E(B−V)=0.037mag (Schlegeletal. 1998). Here andthrough-
out the paper we use the extinctionlaw of Rieke & Lebofsky
(1985).
For the high Galactic latitude of WLM (b= –73.62◦) we
expect the foregroundcontamination to be negligible. This is
confirmed from the number of stars found under similar ob-
servationalconditionsin our ”clear sky”, which is just 9% of
allstarsinthegalaxyfield.Color-magnitudediagramwascon-
Fig.3.Luminosityfunctionsfor J (upperpanel)and K (bot-
S
structusingthesamedistancemodulusandGalacticextinction
tompanel)withamagnitudebinof0.2mag.LogNisthenum-
andisshowninFig.4(rightpanel).Insuchwayatrueestima-
berofstarsnormalizedfor1squarearcmin.Eachpanelshows
tionoftheGalaxycontaminationcanbemadebycountingthe
the luminosity functions for the whole field (solid lines), the
starsin1magnitudebinsinM .Thenumbersaregiveninthe
innerpart(dashedlines)andouterpart(dottedlines). TheLF KS
rightontheFig.4(leftpanel).
for the ”clear sky” field is also shown with dot-dashed lines.
Vertical lines represent our galaxy 100 % photometry com- The comparison with isochrones of Bertelli et al. (1994)
pletenessforeveryband. indicate that the majority of the stars are 1-10Gyr old,
with a significant population of AGB and Carbon stars
with (J−K ) >1mag. Only a handful of blue stars with
S 0
(J−K ) <1magand M >−8magaredetected.Wealsoiden-
S 0 KS
4 Valchevaetal.:Near-IRviewofWLM:CarbonstarsandC/Mratio
tified on ourimagesa numberof variablestars fromSandage
&Carlson(1985)andMoraetal.(inpreparation).
The RGB properties versus metallicity calibrations of
Ivanov & Borissova (2002) predict that the RGB tip for the
WLMmetallicityislocatedatM >−6.14mag.Unfortunately,
KS
theincompletenessofthephotometryinthismagnituderange
preventsusfromanalysisoftheRGBproperties.
4. CarbonstarsidentificationandC/Mratiointhe
WLM
WLM is unusually rich of Carbon star, along with two other
dwarfgalaxies–IC1613andNGC6822– accordingtoCook
et al. (1986). Recently, Battinelli & Demers (2004) surveyed
WLM using narrow band CN-TiO filters and identified 149
Carbon stars and practically no M-type AGB stars (77 iden-
tified Carbon stars in our field are marked in Tabl. 1 with
BD04).TheyalsofoundanextremelyhighC/MratioofWLM:
12.4±3.7.
The IR photometryallows us to carry out an independent
censusoftheCarbonstars,followingthemethodappliedonthe
MagellanicClouds(Cioni&Habing2003),NGC6822(Cioni
&Habing2005,Kangetal.2006),NGC185(Kangetal.2005)
and NGC147 (Sohn et al. 2006). It is found that the color
distribution of all AGB stars have a well pronounceddiscon-
Fig.6. Color-magnitude M versus (J − K ) diagram for
tinuity at some point, M-type stars have a peak follow by a KS S 0
AGBstarsinWLM.Thetwodottedlinesrepresentthe100%
red tail of Carbon stars. We separated the Carbon rich from
and 25% photometrycompletenesslimits. Six variablesfrom
the Oxygen rich stars using the color distribution of the stars
Sandage & Carlson (1985) are plotted with solid squares and
abovethe RGB tip (Fig. 6, the inset) and foundcolor limit at
77 Carbon stars from Battinelli & Demers (2004) are plotted
(J−K ) =1.20mag. A similar dereddenedlimit was foundby
S 0 with solid triangles. The inset is a color distribution of the
Cioni& Habing(2005) forNGC6822and Sohn et al. (2006)
starsabovetheRGBtip,usedtoseparatetheOxygenrichand
forNGC147:(J−K ) =1.24and(J−K ) =1.28,respectively.
S 0 S 0 Carbon rich stars. See Sec. 4 for details. The separation line
(dashedline)(J−K ) =1.20isshownonbothpanels.
S 0
20
all stars with spectral types M0 and later (M0, M1, M2 etc.)
15 whichsometimesisdesignatedasM0+.Usingthesamecriteria
onthe“clearsky”fieldweobtain31Cstarsand19Mstarsas
allofthemareabovetheRGBtip.Takingintoaccountthelatter
N 10 numbersthis will reduceourC stars to 115and the M star to
240.
The K luminosity functions for our Carbon star sam-
S
5 ple and for the 77 cross-identified Carbon stars (shown in
Fig. 6 with solid triangles) from Battinelli & Demers (2004)
are shown in Fig. 5. To eliminate the binning effects we cre-
0 ated for each data set two luminosity functions shifting the
15 16 17 18 19 20
K bin centers by half of the bin width and averaged them. We
S
fitted the luminosity function with a Gaussian, in two itera-
Fig.5.LuminosityfunctionsoftheCarbonstarsinWLM.The
tions, excluding the values that deviated more than 3σ. The
solidlinerepresentsthedistributionofCarbonstarsabovethe
finalfitshavemeanof<K >=17.35mag,σ=0.71magforour
RGB tip. The dashed line shows the 77 Carbon stars from S
sampleand<K >=17.33mag,σ=0.51magforthe77Carbon
S
Battinelli&Demers(2004)thatweidentifiedinourfield.The
stars from Battinelli & Demers (2004) which corresponds to
bin width is 0.2mag. The final Gaussian fits to the data are
< M >=−7.51magand<M >=−7.53mag,respectively.
shown(seeSec.4fordetails). KS KS
Previous studies in some LG galaxies in the near
IR lead to identical mean absolute K magnitude of
Integrating over the two parts of the histogram we obtain the C stars: <M >=−7.93±0.36mag (Kang et al.
KS
146Carbonstarsand259Mstars.Thelatternumberincludes 2005), <M >=−7.56±0.47mag (Sohn et al. 2006) and
KS
Valchevaetal.:Near-IRviewofWLM:CarbonstarsandC/Mratio 5
Fig.4.Near-IR M versus(J −K ) magcolor-magnitudediagramofWLM(leftpanel)andthe”clearsky”field(rightpanel;
KS S 0
see Sec. 3 fordetails). The typicalmeasurementerrorsforstars with (J − K ) =1magare shownon the right. Isochronesfor
S 0
the10,20,30,100Myr,1and10GyrandZ=0.001fromBertellietal.(1994)areplotted.Leftpanel:theopentriangleindicates
thelocationoftheWLMglobularcluster.SixvariablestarsfromSandage&Carlson(1985)areplottedwithsolidsquaresand
77 Carbonstars of Battinelli & Demers(2004) are shownwith solid triangles. The levelof the RGB tip is also markedin the
right.Thenumberofforegroundstarsinevery1-magnitudebinalong M obtainedfromCMDof”clearsky”aregivenonthe
KS
right.
<M >=−7.60±0.50mag (Kang et al. 2006) for NGC185, sorption feature– prominentfor spectraltypes later than M3.
KS
NGC147 and NGC6822, respectively. <M > in WLM is TheirmethodprobablyomitsearlyMtypestars.Furthermore,
KS
comparable with these values in 1σ level and support this the low metallicity of WLM diminishes additionally the TiO
assumption. The sample of galaxies include dwarf irregulars featurestrenghtandmakesthedetectionofMtypestarsmore
anddwarfellipticalsbutisnotrepresentativeforwiderangeof difficult.
metallicityand a possibleinfluenceoverthe meanabsoluteK
magnitudecannotbeentirelyexcluded.
Next,wederivedaC/Mratioof0.56±0.12(foregroundcor-
rectedratio0.48),usingtheCarbonandM-typestarsweiden-
tified above the RGB tip. However, Cioni & Habing (2005) 5. Spatialdistributionofthestellarpopulations
pointed that this classification method omits the early type
Carbonstars that mightbe bluerthan the adoptedcolor limit. OurdatacovermostofthemainbodyofWLM,allowingusto
Theincompletenessduetothiseffectcanbeestimatedbycom- studythespatialdistributionofstellarpopulations.Wecreated
parisonofourCarbonstarlistandthatofBattinelli&Demers 2-dimensionalhistograms(Fig.7)with22.5×22.5arcsec2bins,
(2004) – it yielded additional 15 Carbon stars among the M correspondingtolinearscaleof100×100pcatthedistanceof
stars,bringingtheC/Mratioupto0.66±0.11(0.58).Thediffer- thegalaxy.Thedistributionsweresmoothedbyaboxcarfunc-
encebetweenthetwovaluesissmall,consideringthePoisson tionofwidth2.
uncertaintiesgivenhere.
The histograms indicate no major difference between the
Bothvaluesobtainedherearesmallerbyafactorof20than stellar distributions. There are two peaks, corresponding to
the result of Battinelli & Demers (2004): C/M0+∼12.4. The complexes with typical sizes of a few hundred parsecs. The
large difference is due to the presence of M-type AGB popu- C/M ratioexhibitsa morecomplicatedbehavior– the peak is
lation,whichis almostentirelyabsentinBattinelli &Demers slightlyoffsetfromthepeakofthestellardistributions.Further
(2004) survey of WLM. We can only speculate that this dif- studiesarenecessarytoverifyifthisreflectsarealmetallicity
ferenceisrelatedtothenarrowbandfiltercenteredatTiOab- oranagegradientacrossthegalaxy.
6 Valchevaetal.:Near-IRviewofWLM:CarbonstarsandC/Mratio
Fig.7.Spatialdistributionofallstars(left),AGBstars(middle)andC/Mratio(rightpanel).Theisolinesareatlevels:1to6with
astepof1,1to5withastepof1,and0.2to1.4withastepof0.3,respectivelyforthethreeplotsfromlefttoright.
6. Miscellaneous
6.1.CepheiddistancetoWLMinthenear-IR
We identified 5 Cepheids from the list of Sandage & Carlson
(1985)onourJ-bandimage(theK imageistooshallowtode-
S
tectthem).WederivedadistancetoWLMadoptingtheperiod-
luminosityrelationofGieren,Fouque&Gomez(1998):
M =−5.240(±0.028)−3.129(±0.052)×(logP−1.0) (5)
J
where M is the absolute J-band magnitude of the Cepheids
J
andPistheperiodindays.
The distances to individualCepheidsare listed in Tabl. 2.
WeremovedtheMilkyWayextinctionusingA =0.033mag.To
J
correctfortheinternalextinctiontowardsindividualCepheids
we used the H map of Jackson et al. (2004). The hydrogen
surfacedensitieswereconvertedintoextinctionassuminggas-
to-dust ratio N(H)/E(B−V)=60±10.0 [1021 at. cm−2 mag−1].
Thisvalueisanextrapolationbasedonalinearleastsquaresfit
(Fig.8)ofthegas-to-dustratioversuslog(O/H)+12:
N(Hi)/E(B−V)=−(58±5)×[log(O/H)+12]+(517±44)(6) Fig.8. Gas-to-dust ratio versus log(O/H)+12 relation. Data
pointsarelabeled(seeSec.6.1fordetails).Thesolidlineisthe
withr.m.s.=2. linearleastsquaresfit,andtheverticaldashedlineindicatesthe
This relation is based on gas-to-dust ratio estimates for: WLMabundancefromLeeetal.(2005).
the solar neighborhood (Bohlin et al. 1978; marked MW in
Fig.8),theM31outskirts(Cuillandreetal.2001;M31 ),LMC
o
(Junkkarinen et al. 2004; LMC) and SMC (Jakobsson et al.
RejectingtheoutlierNo.66(seeTable2),weobtainame-
2004;SMC).TheabundancesaretakenfromRussell&Dopita
diandistancemodulusof(m−M) =24.84±0.14mag.Wetenta-
(1990)forLMCandSMC.IncaseoftheMilkyWayweused 0
tivelyassumethattheerrorisgivenbythemaximumdeviation.
theGalacticmetallicitygradientfromEstebanetal.(2005),and
Inaddition,therearesystematicerrorsof0.03magforthezero-
forM31–themetallicitygradientofSmarttetal.(2001).Note
pointoftheperiod-luminosityrelation(Equ.5),and0.02mag
thatthis is a preliminarycalibration,based on a limited num-
fortheLMCdistanceadoptedinGierenetal.(1998).
berofmeasurements.Moregas-to-dustratiosarenecessaryto
deriveareliablerelation.
Finally, we assumed that the Cepheids are located on the
6.2.VariablestarsinWLM
WLM mid-plane and therefore, the extinctions are due only
to half of the totalhydrogencolumndensities. The extinction SomevariablestarsfromthelistsofSandage&Carlson(1985)
intrinsicto WLM doesnotexceed0.015mag,dueto the high andMoraetal.(inpreparation)wereincludedinourfield.They
gas-to-dustratioofthegalaxy. aremarkedinTabl.1.
Valchevaetal.:Near-IRviewofWLM:CarbonstarsandC/Mratio 7
Table2.CepheiddistancetoWLM.Thecolumnscontain:identificationnumber,J-bandmagnitude(the1σerrorsaregivenin
brackets),periodindaysfromSandage&Carlson(1985),absolutemagnitudecalculatesfromEqu.5,apparentdistancemodulus,
totalextinctioninA derivedfromgas-to-dustratio(includingMilkyWayextinctionofA =0.033mag,Schlegeletal.1998)and
J J
truedistancemodulus(m−M) .
0
No J Period,days M (m−M) A (m−M)
J J J 0
7 20.108(0.155) 7.6712 -4.880 24.988 0.048 24.940
8 20.016(0.150) 7.3754 -4.827 24.843 0.047 24.796
21 19.829(0.124) 9.33306 -5.146 24.975 0.044 24.931
48 19.724(0.160) 8.53000 -5.024 24.748 0.048 24.700
66 20.197(0.189) 4.54714 -4.170 24.367 0.040 24.327
6.3.StructuralparametersofWLM
Structuralparametersofgalaxiesaredeterminedmorereliably
fromIRimagesratherthanopticalonesbecausetheeffectsof
theintrinsicextinctionarediminished.Ourimagesdonoten-
compasstheentiregalaxybutneverthelessweareabletodeter-
mine approximatelythe scale length along the major axis. To
dothatweaveragedthepixelvaluesina63.4-arcsecwidestrip
(∼300pc at the distance of WLM), crossing the center of the
galaxyinNorth-Southdirection.WeusedtheJ-bandimagebe-
causeofthehighersignal-to-noiseratio.Theaveragefluxval-
uesalongthesemi-majoraxisareshowninFig.9.Anexponen-
tiallawfitoverthecentral1kpcregionofWLMyieldedascale
lengthof0.75±0.14kpc,similar to the scale lengthof0.9kpc
determinedbyFisher&Tully(1975).TheinsetinFig.9shows
histogramsofthescalelengthsfordwarfgalaxiesintheLocal
GroupfromthecompilationofMateo(1998).
Thebright–andyounger–starsinthegalaxyareresolved
on our images, but the fainter – and older ones– not. We ad-
dressedthequestionifthereisadifferentscalelengthfordif-
ferentstellarpopulationsbyexcludingthebrightstarsfromthis
analysis:weaveragedtheJ-bandimagewitha61×61pixelme-
dianfilter,effectivelyremovingtheresolvedstars.Thederived Fig.9.ScalelengthoftheWLMdiskin J-band.Thedotsindi-
scale length for “older” population was 0.71±0.04kpc, again catestheaveragevaluesalongthesemi-majoraxis,thedashed
for the central 1kpc of the galaxy. Our estimates have to be lineisthesameforthemedian-averagedimage.Thesolidlines
treated with caution because the strip is not perfectly aligned arethefits.Theinsetshowsthedistributionofthescalelengths
alongthemajoraxis. forthedwarfirregulars(solidline)anddwarfspheroidalgalax-
ies(dashedlines)in theLocalgroup.Theleftarrowindicates
our estimate and the right arrow shows estimate of Fisher &
7. Discussionandsummary Tully(1975).
This work adds anotherobjectto the list of galaxieswith ho-
mogeneouslydeterminedC/M0+ratiosfromnear-IRphotom- to-dustratiofortheWLMis60±10[1021at.cm−2mag−1].This
etry: LMC, SMC, NGC6822, NGC147, NGC185 and now allows usto measurethe extinctionalongthe line ofsight to-
the WLM. This is an opportunity for independent check of wards individual Cepheids and by period-luminosity relation
the results from narrow band Carbon star surveys. The num- for the Cepheids (Gieren 1998) in the IR we determined the
ber of data points is still limited and did not allow us to test distance to the WLM – (m−M) =24.84±0.14mag, similar to
0
if using only C/M ratios derived from IR observations can the previous estimates. To probe the significance of used PL
reduce the spread of the relation but this is the objective of relation we re-determined the distance modulus using the PL
our furtherstudies. We obtainedfor the WLM a C/M ratio of relationcalibratedbyTestaetal.(2006)fortheCepheidsintwo
∼0.6±0.1andapeakoftheCarbonstarsluminosityfunctionof LMCyoungclusters.Thisresultsindistancemodulusbetween
<M >=−7.51mag. 24.76and25.00,dependingontheassumedpulsationmodels.
KS
To measure the intrinsic reddening in WLM we used the Finally, we fitted the surface brightnessdistributionalong the
H map of WLM (Jackson et al. (2004), and we adopted a main axis of WLM with an exponential low yielding a scale
linear dependence of the gas-to-dust ratio on the metallicity: length of 0.75±0.14kpc, typical value for the dwarf irregular
N(H)/E(B−V)=−(58±5)×[log(O/H)+12]+(517±44).Thegas- galaxies.
8 Valchevaetal.:Near-IRviewofWLM:CarbonstarsandC/Mratio
Acknowledgements. This work obtained a limited support by the Sohn, Y.-J., Kang, A., Rhee, J., Shin, M., Chun, M.-S., Kim, Ho-Il
grants F-1302/03 and VU-F-201/06 of the Bulgarian Scientific 2006,ApJ,445,69
Foundation. Also, this publication makes use of data products from Testa, V., Ferraro, F. R., Ripepi, V., Musella, I., DallO´ra, M.,
the Two Micron All Sky Survey, which is a joint project of the Mucciarelli,A.,Marconi,M.,Mateo,M.2006,MmSAI,77,263
University of Massachusetts and the IR Processing and Analysis Venn,K.A.,Tolstoy,E.,Kaufer,A.,Skillman,E.D.,Clarkson,S.M.,
Center/California Institute of Technology, funded by the National Smartt,S.J.,Lennon,D.J.,&Kudritzki,R.P.2003,AJ,126,1326
Aeronautics and Space Administration and the National Science Wolf,M.1909,Astron.Nachr,183,187
Foundation.ThisresearchhasmadeuseoftheSIMBADdatabase,op-
eratedatCDS,Strasbourg,France.TheauthorsthankDr.M.-R.Cioni
for the useful comments and the anonymous referee for the help in
improvingthearticle.
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