The spatial distribution of sources populating different regions of the
colour-magnitude diagram (I-J, I) extracted from the DENIS catalogue towards
the Magellanic Clouds (DCMC -- Cioni et al. 2000) reveal significantly
different morphologies. Each region is associated to a different age group. The
Large Magellanic Cloud (LMC) shows an extended circular shape with a prominent,
off center bar, a nucleus and irregular spiral arms. The Small Magellanic Cloud
shows a perturbated structure with a prominent central concentration of stars.
Old and young populations are offset from one another.Comment: 4 pages and 7 figures, accepted for publication in A&A Journal
  Letter

Cioni, Maria-Rosa L.

Habing, Harm J.

Israel, Frank P.

Astronomy and Astrophysics

English

arXiv

The spatial distribution of sources populating different regions of the colour–magnitude diagram (I −J,I) extracted from the DENIS catalogue towards the Mag- ellanic Clouds (DCMC – Cioni et al. 2000) reveal signifi- cantly different morphologies. Each region is associated to a different age group. The Large Magellanic Cloud (LMC) shows an extended circular shape with a prominent, off center bar, a nucleus and irregular spiral arms. The Small Magellanic Cloud shows a perturbated structure with a prominent central concentration of stars. Old and young populations are offset from one another.Peer reviewe

Cioni, M-R.L.

Habing, H.J.

Israel, F.P.

University of Hertfordshire Research Archive

arXiv:astro-ph/0005057v1  3 May 2000A&A manuscript no.(will be inserted by hand later)Your thesaurus codes are:11(11.13.1)ASTRONOMYANDASTROPHYSICSThe morphology of the Magellanic Clouds revealed by starsof different age: results from the DENIS surveyMaria-Rosa L. Cioni, Harm J. Habing, and Frank P. IsraelSterrenwacht Leiden, Postbus 9513, 2300 RA Leiden, The NetherlandsReceived 20 April 2000 / AcceptedAbstract. The spatial distribution of sources populatingdifferent regions of the colour–magnitude diagram (I − J ,I) extracted from the DENIS catalogue towards the Mag-ellanic Clouds (DCMC – Cioni et al. 2000) reveal signifi-cantly different morphologies. Each region is associated toa different age group. The Large Magellanic Cloud (LMC)shows an extended circular shape with a prominent, offcenter bar, a nucleus and irregular spiral arms. The SmallMagellanic Cloud shows a perturbated structure with aprominent central concentration of stars. Old and youngpopulations are offset from one another.Key words: Galaxies: Magellanic Clouds1. IntroductionThe Magellanic Clouds are our closest neighbors allow-ing direct observation of individual constituent objects.They are bound to the Galaxy and show signs of stronginteraction with the Milky Way about 0.2 Gyr ago (West-erlund 1997). The LMC is classified as an irregular dwarfgalaxy, its most prominent feature is a central bar, muchlike those found in barred spiral galaxies. Its eastern sideis closer than its western side (Caldwell & Coulson 1986).Underlying the bar is a circular disk of older stars (West-erlund 1997). The appearance of the SMC is characterizedby a much less pronounced bar, and an eastern extensioncalled the Wing. Lines-of-sight through the SMC appearto cover extensive depths; the Wing and the northeasternpart of the Bar are closer than the southern parts (West-erlund 1997).Newly obtained large photometric data sets at differ-ent wavelengths and with improved sensitivity and spatialcoverage allow us to investigate the large scale propertiesof the Magellanic Clouds. In particular, data in the nearinfrared allow us to access stages of stellar evolution thatare marginally covered by optical data, such as the RGBand AGB phases.Send offprint requests to: mrcioni@strw.leidenuniv.nlVery recently Zaritsky et al. (2000) found that theasymmetric appearance of the SMC is primarily causedby the distribution of young stars, and that the olderstars have a very regular distribution. It is not possiblefrom their Figures to evaluate the behavior of the densitytowards the center of the Cloud. Weinberg & Nikolaev(2000) point out the presence of intervening tidal debrisup to ≈ 15 kpc from the LMC.2. The DataOur morphological study of the Magellanic Cloud is basedon a sample of stars extracted from the DCMC catalogue(Cioni et al. 2000). The sample includes all sources de-tected in both I and J , irrespective of detection in KS .The DCMC catalogue contains sources detected in at leasttwo of the three DENIS bands (I: 0.8µm, J : 1.25µm andKS : 2.15µm) within a surface area of 20 × 16 squaredegrees centered on (α, δ) = (5h27m20s,−69◦00′00′′) to-ward the LMC and 15 × 10 square degrees centered on(α, δ) = (1h02m40s,−73◦00′00′′) toward the SMC; J2000coordinates are used throughout this paper. The obser-vations have been performed with the DENIS instrument(Epchtein et al. 1997) on the 1m–ESO telescope.We have used the (I − J , I) colour–magnitude di-agrams (Figure 1 for the LMC only) to select threeclasses of objects in each Cloud. Sources labelled (A)with I < −4.64× (I − J) + 19.78 represent the youngestpopulation in the Magellanic Clouds: the brightest dwarfstars, blue–loop stars and supergiants (third vertical se-quence from the left), together with an unrelated fore-ground component of dwarfs and giants (first two ver-tical sequences from the left). Sources labelled (B) withI > −4.64× (I − J) + 19.78, located above the tip of thered giant branch (TRGB – Cioni et al. 2000a) are mainlyasymptotic giant branch stars (AGB). Sources labelled (C)with I > −4.64× (I−J)+19.78 located below the TRGBare mostly red giant branch stars (RGB) and representthe oldest population in the Clouds. For the sake of clar-ity, we have plotted in Fig. 1 only those sources that weredetected in all three wave bands and that occur in the verycentral part of the Cloud. Sources detected only in I and2 Maria-Rosa L. Cioni et al.: The morphology of the Magellanic Clouds from DENISFig. 1. Colour–Magnitude Diagram (I − J ,I) of sourcesdetected simultaneously in I, J and KS towards the LMC(−69 < δ < −67). The horizontal line marking the po-sition of the TRGB (Cioni et al. 2000a), and the slantedline at I < −4.64× (I − J) + 19.78 define the classes A,B, and C discussed in the text.J predominantly populate the lower part of the diagram.The position of the TRGB is indicated by a horizontalline. The I, J and KS sensitivity limits are 18, 16 and 14mag respectively. Photometric errors widen the sequencestowards fainter magnitudes.For each class of objects in each of the two Clouds, weshow their distribution in the plane of the sky by count-ing the sources in bins of 0.2◦ × 0.2◦, applying a lightsmoothing to the resulting structure (Figs. 2–7); the con-tour values increase logarithmically. Regions correspond-ing to missing data (strips at constant RA indicated bydiamonds) were filled in by interpolation. Their effect ismostly negligible except in Fig. 4 where strips of possiblylower photometric quality may be causing discontinuitiesin the outermost contours.3. Spatial distributionThe contribution due to Galactic foreground stars hasnot been subtracted from the maps. Its influence is mostclearly seen in Fig. 2 in the direction of the GalacticPlane. In the other maps, the foreground contribution israther constant and does not affect the morphology of theClouds.3.1. Structure of the LMCThe lower contours in Figs. 2–4 show an almost circu-lar outline (axial ratios consistent with an inclinationi = 30◦ − 40◦) centered in all three cases near α =5h20m, δ = −69◦ with major axis at about 13◦. West-erlund (1997) gives a similar diameter for the stars of theold disk. This stellar disk also coincides in shape and ex-tent with the HI disk (Kim et al. 1998). The center ofthe disk is offset from the center of the Bar by about 30′to the north (see Fig. 2). We confirm the conclusion byWesterlund (1997) that the LMC consists of two systems:a circular disk and an off center bar. Half of the total num-ber of stars are in the bar and this factor (Fig. 2) increasesfor younger objects. Unless this is a transient configura-tion, it thus seems that the LMC must be embedded in agravitational potential produced by an unseen mass com-ponent (see also Sofue 1999). This is in agreement with theconclusion by Stil (1999) that the class of dwarf galaxiesto which the LMC belongs (‘fast rotators’) is dominatedby dark matter.The youngest component (younger than 0.5 Gyr) iscomposed of very bright main-sequence dwarf stars, blue–loop stars and supergiants. Their distribution (Fig. 2) isclumpy and irregular. The Bar, extending over about 4◦, isprominent and contains a well defined nuclear concentra-tion at its center. The region of 30 Dor is represented bythe small feature just above the northeastern side of theBar, and the Shapley Constellation III is the large struc-ture at δ ≈ −67◦. Elongations at either end of the Barindicate the presence of spiral arms most clearly seen inthe northwest at the location of the giant HII region com-plex N 11. Similar structures are seen in the distributionof stellar complexes (Maragoudaki et al. 1998), associa-tions and HII regions (Bica et al. 1999). Clusters (Bica etal. 1995, Kontizas et al. 1990) have a distribution moresimilar to the one of AGB/RGB stars.The distribution of AGB stars (Fig. 3), also relativelyyoung (around 1 Gyr) likewise reveals a prominent Barand nucleus. Shapley Constellation III is inconspicuous inAGB stars. A broad and faint spiral arm begins at thenorthwestern end of the Bar and bifurcates around α =05h10m, δ = −66.5◦. The spiral arm feature originatingat the southeastern end of the Bar is clearly delineatedin the AGB star population and can easily be followed toα = 4h45m, δ = −73◦. It was noted before by Bothun &Thompson (1988) in their surface photometry study of theMagellanic Clouds – see their 1.1 < B−R < 1.35 diagram.At least this spiral arm might be due to tidal action, as itappears to be connected to the Magellanic Cloud Bridge(cf. Staveley-Smith et al. 2000). The outernmost contourwell matches the carbon stars by Kunkel et al. (1997).The oldest population (from 1 to 5 Gyr), representedby RGB stars (Fig. 4), once again reveals a prominentBar which is significantly broader than that defined bythe younger populations. Galactic foreground stars mayaffect the outermost contours. The southern spiral armis inconspicuous, but the two faint northern spiral armsseen in Fig. 3 (AGB) have weak counterparts in the formof extensions at α = 5h → 6h, δ = −64◦ → −68◦.Bothun & Thompson ( 1988) conclude that the LMChas a relatively large scale length more appropriate forgalaxies with obvious spiral structure than for other dwarfgalaxies. It is interesting that the asymmetric spiral struc-ture delineated by the different components in Figs. 2–4Maria-Rosa L. Cioni et al.: The morphology of the Magellanic Clouds from DENIS 3Fig. 2. Star count of the LMC – class A (Fig. 1); contoursare at: 100, 125, 150, 200, 300, 400, 500, 800 per 0.04 deg2.is in fairly good agreement with the HI map shown byGardiner et al. (1998) and is nicely reproduced by theirdynamical model.3.2. Structure of the SMCThe structure of the SMC is still not understood (Wester-lund 1997). Our maps show that populations of differentage have different distributions. The youngest componenthas an asymmetric distribution(Fig. 5) elongated along aNE–SW axis (PA ≈ 45◦). In the south, the outermost con-tour defines four protuberances which might be associatedwith tidal features: at least the eastern (coincident withthe SMC Wing) and western protuberances are alignedwith that of the Magellanic Cloud Bridge (cf. Staveley-Smith et al. (2000). Higher contours show an extensionin the northeast, aligned with the main body of the SMCBar. The Bar structure itself is similar to that seen in thedistribution of young clusters (Bica & Dutra 2000) and inthe upper main-sequence map by Zaritsky et al. (2000).Clusters, associations and HII regions (Bica & Schmitt1995) are also found at the locations of the southern protu-berances. The young stars are strongly concentrated in thesouthwestern part of the SMC Bar. Outside the main bodyof the SMC, the two Galactic globular clusters NGC 104= 47 Tuc (west) and NGC 362 (north) can be discerned.The HI column density contours in the map presented byStanimirovic` et al. (1998) outline the distribution of theyoung stars quite well.The AGB stars have a more regular distribution(Fig. 6) with two prominent central concentrations match-ing the carbon stars by Hardy et al. (1989). The eastern-most also coincides with the peak of the young–star dis-tribution. The AGB distribution axis is much less inclined(PA ≈ 75◦) than that of the younger and very similar tothat of the RGB star distribution. As in the case of theLMC, the stellar distributions become more regular andFig. 3. Star count of the LMC – class B (Fig. 1); contoursare at: 3, 5, 7, 10, 20, 30, 50, 100, 150 per 0.04 deg2.Fig. 4. Star count of the LMC – class C (Fig. 1); contoursare at: 100, 150, 200, 250, 350, 500, 600, 800, 1000, 1200per 0.04 deg2.smoother with increasing age, also apparent in the B andV band images by Zaritsky et al. (2000) and for the outercontour in the carbon stars by Kunkel et al. (2000); car-bon stars by Rebeirot et al. (1983) fill the second levelcountour (Fig. 6).The distribution of RGB stars (Fig. 7) is similar tothat of the AGB stars and also exhibits two major concen-trations. The western most is more pronounced in RGBthan in AGB stars. The eastern concentration on aver-age appears to be significantly younger than the westernconcentration dominated by the older stars. Remarkably,the strongest HI concentration in the SMC map by Sta-nimirovic` et al. (1998) appears to be just between theconcentration of younger stars and that of older stars. Itis also remarkable that the older star distribution extendsover the full length of the suspected southwestern tidalfeature, about 2◦ from the main body of the Bar. With4 Maria-Rosa L. Cioni et al.: The morphology of the Magellanic Clouds from DENISFig. 5. Star count of the SMC – class A; contours are at:20, 30, 45, 60, 75, 100, 125, 150, 200, 250 per 0.04 deg2.Fig. 6. Star count of the SMC – class B; contours are at:2, 5, 10, 15, 20, 25, 50 per 0.04 deg2.respect to the overall distribution of the older stars, that ofthe HI appears to be displaced towards the east. The SMCWing, prominent in HI and also traceable in the youngerstellar population, has no counterpart in the older stars.4. ConclusionsCounts of sources towards the Magellanic Clouds ex-tracted from the DCMC (Cioni et al. 2000) allow dif-ferentiation in the (I − J , I) colour–magnitude diagraminto three groups of objects with different mean ages. Thespatial distribution of the three age groups is quite dif-ferent: in either Cloud, the youngest stars exhibit an ir-regular structure characterized by spiral arms and tidalfeatures while the older stars are smoothly and regularlydistributed. The distribution of younger stars is well–correlated with those of clusters, associations, HII regionsand HI. The significant offset of the LMC Bar with re-spect to the overall circular disk suggests that the LMCFig. 7. Star count of the SMC – class C; contours are at:50, 60, 75, 100, 125, 150, 200, 225 per 0.04 deg2.potential is dominated by dark matter. The well–definedsouthern spiral arm may be due to tidal interaction withthe SMC. The nature of the two northern spiral arms isuncertain. In the SMC, the regular, but double–peaked,structure of the AGB and RGB stars is remarkable, as isits offset from the HI distribution, and the mean age dif-ference of the two maxima. Relatively faint east–west fea-tures in the younger star population (including the Wing)are probably also due to tidal interaction.ReferencesBica E.L.D., Dutra C.M., 2000, AJ 119, 1214Bica E.L.D., Schmitt H.R., 1995, AJS 101, 41Bica E.L.D., Schmitt H.R., et al., 1999, AJ 117,238Bothun G.D., Thompson I.B., 1988, AJ 96, 877Caldwell J.A.R, Coulson I.M., 1986, MNRAS 218, 223Cioni M.R., Loup C., Habing H.J., et al., 2000, A&AS acceptedCioni M.R., van der Marel R.P., Loup C., Habing H.J., A&AsubmittedEpchtein N., De Batz B., et al., 1997, The Messenger 87, 27Gardiner L.T., Turfus C., Putman M.E., 1998, ApJ 507, L35Hardy E., Suntzeff N.B., Azzopardi M., 1989, ApJ 344, 210Kim S., Dopita M.A., et al., 1999, AJ 118, 2797Kontizas M., Morgan D.H., et al., 1990, A&ASS 84, 527Kunkel W.M., Demers S., Irwin m.J., AJ submittedKunkel W.M., Irwin M.J., Demers S., 1997, A&ASS 122, 463Maragoudaki F., Kontizas M., et al., 1998, A&A 339, L29Rebeirot E., Martin N., Mianes P., et al. 1983, A&AS 51, 277Sofue Y., 1999, PASJ 51, 445Stanimirovic` S., Staveley-Smith L., et al., 1998, MNRAS 302,417Staveley-Smith L., Gibson B., et al., 2000, IAU 190, p. 37Stil J.M., 1999 Ph. D Thesis Leiden University (NL), Ch. 4Weinberg M.D., Nikolaev S., ApJ submittedWesterlund B.E., 1997, The Magellanic Clouds, Camb. Astr.Se. 29Zaritsky D., Harris J., et al., ApJ Lett. accepted

The morphology of the Magellanic Clouds revealed by stars of different age: results from the DENIS survey

Cioni, M.-R.L.

NARCIS 

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Leiden University Scholary Publications

Astron. Astrophys. 358, L9–L12 (2000) ASTRONOMYANDASTROPHYSICSLetter to the EditorThe morphology of the Magellanic Clouds revealed by stars ofdifferent age: results from the DENIS surveyM.-R.L. Cioni, H.J. Habing, and F.P. IsraelSterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The NetherlandsReceived 11 April 2000 / Accepted 3 May 2000Abstract. The spatial distribution of sources populating dif-ferent regions of the colour–magnitude diagram (I − J , I)extracted from the DENIS catalogue towards the MagellanicClouds (DCMC – Cioni et al. 2000) reveal significantly dif-ferent morphologies. Each region is associated to a differentage group. The Large Magellanic Cloud (LMC) shows an ex-tended circular shape with a prominent, off center bar, a nucleusand irregular spiral arms. The Small Magellanic Cloud shows aperturbated structure with a prominent central concentration ofstars. Old and young populations are offset from one another.Key words: galaxies: Magellanic Clouds1. IntroductionThe Magellanic Clouds are our closest neighbors allowing directobservation of individual constituent objects. They are bound tothe Galaxy and show signs of strong interaction with the MilkyWay about0.2 Gyr ago (Westerlund 1997). The LMC is classi-fied as an irregular dwarf galaxy, its most prominent feature is acentral bar, much like those found in barred spiral galaxies. Itseastern side is closer than its western side (Caldwell & Coulson1986). Underlying the bar is a circular disk of older stars (West-erlund 1997). The appearance of the SMC is characterized by amuch less pronounced bar, and an eastern extension called theWing. Lines-of-sight through the SMC appear to cover exten-sive depths; the Wing and the northeastern part of the Bar arecloser than the southern parts (Westerlund 1997).Newly obtained large photometric data sets at differentwavelengths and with improved sensitivity and spatial coverageallow us to investigate the large scale properties of the Magel-lanic Clouds. In particular, data in the near infrared allow us toaccess stages of stellar evolution that are marginally covered byoptical data, such as the RGB and AGB phases.Very recently Zaritsky et al. (2000) found that the asymmet-ric appearance of the SMC is primarily caused by the distribu-tion of young stars, and that the older stars have a very regulardistribution. It is not possible from their Figures to evaluate theSend offprint requests to: mrcioni@strw.leidenuniv.nlbehavior of the density towards the center of the Cloud. Wein-berg & Nikolaev (2000) point out the presence of interveningt dal debris up to≈ 15 kpc from the LMC.2. The dataOur morphological study of the Magellanic Cloud is based ona sample of stars extracted from the DCMC catalogue (Cioniet al. 2000). The sample includes all sources detected in bothI and J , irrespective of detection inKS . The DCMC cata-logue contains sources detected in at least two of the three DE-NIS bands (I: 0.8µm, J : 1.25µm andKS : 2.15µm) within asurface area of20 × 16 square degrees centered on(α, δ) =(5h27m20s,−69◦00′00′′) toward the LMC and15 × 10 squaredegrees centered on(α, δ) = (1h02m40s,−73◦00′00′′) towardthe SMC; J2000 coordinates are used throughout this paper. Theobservations have been performed with the DENIS instrument(Epchtein et al. 1997) on the 1m–ESO telescope.We have used the (I − J , I) colour–magnitude diagrams(Fig. 1 for the LMC only) to select three classes of objects in eachCloud. Sources labelled (A) withI < −4.64× (I −J)+19.78represent the youngest population in the Magellanic Clouds:the brightest dwarf stars, blue–loop stars and supergiants (thirdvertical sequence from the left), together with an unrelatedforeground component of dwarfs and giants (first two verti-cal sequences from the left). Sources labelled (B) withI >−4.64 × (I − J) + 19.78, located above the tip of the redgiant branch (TRGB – Cioni et al. 2000a) are mainly asymp-totic giant branch stars (AGB). Sources labelled (C) withI >−4.64 × (I − J) + 19.78 located below the TRGB are mostlyred giant branch stars (RGB) and represent the oldest populationin the Clouds. For the sake of clarity, we have plotted in Fig. 1only those sources that were detected in all three wave bandsand that occur in the very central part of the Cloud. Sourcesdetected only inI andJ predominantly populate the lower partof the diagram. The position of the TRGB is indicated by a hor-izontal line. TheI, J andKS sensitivity limits are18, 16 and14 mag respectively. Photometric errors widen the sequencestowards fainter magnitudes.For each class of objects in each of the two Clouds, weshow their distribution in the plane of the sky by counting theLETTERL10 M.-R.L. Cioni et al.: The morphology of the Magellanic Clouds from DENISFig. 1.Colour–Magnitude Diagram (I−J ,I) of sources detected simul-taneously inI, J andKS towards the LMC (−69 < δ < −67). Thehorizontal line marking the position of the TRGB (Cioni et al. 2000a),and the slanted line atI < −4.64× (I −J)+19.78 define the classesA, B, and C discussed in the text.Fig. 2. Star count of the LMC – class A (Fig. 1); contours are at: 100,125, 150, 200, 300, 400, 500, 800 per0.04 deg2.sources in bins of0.2◦ × 0.2◦, applying a light smoothing tothe resulting structure (Figs. 2–7); the contour values increaselogarithmically. Regions corresponding to missing data (stripsat constant RA indicated by diamonds) were filled in by interpo-lation. Their effect is mostly negligible except in Fig. 4 wherestrips of possibly lower photometric quality may be causingdiscontinuities in the outermost contours.3. Spatial distributionThe contribution due to Galactic foreground stars has not beensubtracted from the maps. Its influence is most clearly seen inFig. 2 in the direction of the Galactic Plane. In the other maps,the foreground contribution is rather constant and does not affectthe morphology of the Clouds.Fig. 3. Star count of the LMC – class B (Fig. 1); contours are at: 3, 5,7, 10, 20, 30, 50, 100, 150 per0.04 deg2.3.1. Structure of the LMCThe lower contours in Figs. 2–4 show an almost circular outline(axial ratios consistent with an inclinationi = 30◦ − 40◦) cen-tered in all three cases nearα = 5h20m, δ = −69◦ with majoraxis at about13◦. Westerlund (1997) gives a similar diameterfor the stars of the old disk. This stellar disk also coincides inshape and extent with the HI disk (Kim et al. 1998). The centerof the disk is offset from the center of the Bar by about 30′ tothe north (see Fig. 2). We confirm the conclusion by Westerlund(1997) that the LMC consists of two systems: a circular diskand an off center bar. Half of the total number of stars are inthe bar and this factor (Fig. 2) increases for younger objects.Unless this is a transient configuration, it thus seems that theLMC must be embedded in a gravitational potential producedby an unseen mass component (see also Sofue 1999). This isin agreement with the conclusion by Stil (1999) that the classof dwarf galaxies to which the LMC belongs (‘fast rotators’) isdominated by dark matter.The youngest component (younger than0.5 Gyr) is com-posed of very bright main-sequence dwarf stars, blue–loop starsand supergiants. Their distribution (Fig. 2) is clumpy and irregu-lar. The Bar, extending over about4◦, is prominent and containsa well defined nuclear concentration at its center. The region of30 Dor is represented by the small feature just above the north-eastern side of the Bar, and the Shapley Constellation III is thelarge structure atδ ≈ −67◦. Elongations at either end of theBar indicate the presence of spiral arms most clearly seen inthe northwest at the location of the giant HII region complexN 11. Similar structures are seen in the distribution of stel-lar complexes (Maragoudaki et al. 1998), associations and HIIregions (Bica et al. 1999). Clusters (Bica et al. 1995, Konti-zas et al. 1990) have a distribution more similar to the one ofAGB/RGB stars.The distribution of AGB stars (Fig. 3), also relatively young(around1 Gyr) likewise reveals a prominent Bar and nucleus.Shapley Constellation III is inconspicuous in AGB stars. ALETTERM.-R.L. Cioni et al.: The morphology of the Magellanic Clouds from DENIS L11Fig. 4. Star count of the LMC – class C (Fig. 1); contours are at: 100,150, 200, 250, 350, 500, 600, 800, 1000, 1200 per0.04 deg2.broad and faint spiral arm begins at the northwestern end ofthe Bar and bifurcates aroundα = 05h10m, δ = −66.5◦. Thespiral arm feature originating at the southeastern end of the Baris clearly delineated in the AGB star population and can easilybe followed toα = 4h45m, δ = −73◦. It was noted before byBothun & Thompson (1988) in their surface photometry studyof the Magellanic Clouds – see their1.1 < B − R < 1.35diagram. At least this spiral arm might be due to tidal action,as it appears to be connected to the Magellanic Cloud Bridge(cf. Staveley-Smith et al. 2000). The outernmost contour wellmatches the carbon stars by Kunkel et al. (1997).The oldest population (from1 to 5 Gyr), represented byRGB stars (Fig. 4), once again reveals a prominent Bar which issignificantly broader than that defined by the younger popula-tions. Galactic foreground stars may affect the outermost con-tours. The southern spiral arm is inconspicuous, but the two faintnorthern spiral arms seen in Fig. 3 (AGB) have weak counter-parts in the form of extensions atα = 5h → 6h, δ = −64◦ →−68◦.Bothun & Thompson (1988) conclude that the LMC has arelatively large scale length more appropriate for galaxies withobvious spiral structure than for other dwarf galaxies. It is in-teresting that the asymmetric spiral structure delineated by thedifferent components in Figs. 2–4 is in fairly good agreementwith the HI map shown by Gardiner et al. (1998) and is nicelyreproduced by their dynamical model.3.2. Structure of the SMCThe structure of the SMC is still not understood (Westerlund1997). Our maps show that populations of different age havedifferent distributions. The youngest component has an asym-metric distribution(Fig. 5) elongated along a NE–SW axis (PA≈ 45◦). In the south, the outermost contour defines four protu-berances which might be associated with tidal features: at leastthe eastern (coincident with the SMC Wing) and western protu-berances are aligned with that of the Magellanic Cloud BridgeFig. 5.Star count of the SMC – class A; contours are at: 20, 30, 45, 60,75, 100, 125, 150, 200, 250 per0.04 deg2.(c . Staveley-Smith et al. (2000). Higher contours show an ex-tension in the northeast, aligned with the main body of the SMCBar. The Bar structure itself is similar to that seen in the distri-bution of young clusters (Bica & Dutra 2000) and in the uppermain-sequence map by Zaritsky et al. (2000). Clusters, associ-ations and HII regions (Bica & Schmitt 1995) are also foundat the locations of the southern protuberances. The young starsare strongly concentrated in the southwestern part of the SMCBar. Outside the main body of the SMC, the two Galactic glob-ular clusters NGC 104 = 47 Tuc (west) and NGC 362 (north)can be discerned. The HI column density contours in the mappresented by Stanimirovic̀ et al. (1998) outline the distributionof the young stars quite well.The AGB stars have a more regular distribution (Fig. 6) withtwo prominent central concentrations matching the carbon starsby Hardy et al. (1989). The easternmost also coincides with thepeak of the young–star distribution. The AGB distribution axisis much less inclined (PA≈ 75◦) than that of the younger andvery similar to that of the RGB star distribution. As in the caseof the LMC, the stellar distributions become more regular andsmoother with increasing age, also apparent in theB andV bandimages by Zaritsky et al. (2000) and for the outer contour in thecarbon stars by Kunkel et al. (2000); carbon stars by Rebeirote al. (1983) fill the second level countour (Fig. 6).The distribution of RGB stars (Fig. 7) is similar to that ofthe AGB stars and also exhibits two major concentrations. Thewestern most is more pronounced in RGB than in AGB stars.The eastern concentration on average appears to be significantlyyounger than the western concentration dominated by the olderstars. Remarkably, the strongest HI concentration in the SMCmap by Stanimirovìc et al. (1998) appears to be just between theconcentration of younger stars and that of older stars. It is alsoremarkable that the older star distribution extends over the fulllength of the suspected southwestern tidal feature, about2◦ fromthe main body of the Bar. With respect to the overall distributionof the older stars, that of the HI appears to be displaced towardsthe east. The SMC Wing, prominent in HI and also traceable inLETTERL12 M.-R.L. Cioni et al.: The morphology of the Magellanic Clouds from DENISFig. 6. Star count of the SMC – class B; contours are at: 2, 5, 10, 15,20, 25, 50 per0.04 deg2.the younger stellar population, has no counterpart in the olderstars.4. ConclusionsCounts of sources towards the Magellanic Clouds extractedfrom the DCMC (Cioni et al. 2000) allow differentiation in the(I−J ,I) colour–magnitude diagram into three groups of objectswith different mean ages. The spatial distribution of the threeage groups is quite different: in either Cloud, the youngest starsexhibit an irregular structure characterized by spiral arms andtidal features while the older stars are smoothly and regularlydistributed. The distribution of younger stars is well–correlatedwith those of clusters, associations, HII regions and HI. Thesignificant offset of the LMC Bar with respect to the overallcircular disk suggests that the LMC potential is dominated bydark matter. The well–defined southern spiral arm may be dueto tidal interaction with the SMC. The nature of the two northernspiral arms is uncertain. In the SMC, the regular, but double–peaked, structure of the AGB and RGB stars is remarkable, as isits offset from the HI distribution, and the mean age differenceof the two maxima. Relatively faint east–west features in theyounger star population (including the Wing) are probably alsodue to tidal interaction.Fig. 7. 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The morphology of the Magellanic Clouds revealed by stars of different age: results from the DENIS survey

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