Astron. Astrophys. 358, L9-L12 (2000)

3. Spatial distribution

The contribution due to Galactic foreground stars has not been subtracted from the maps. Its influence is most clearly seen in Fig. 2 in the direction of the Galactic Plane. In the other maps, the foreground contribution is rather constant and does not affect the morphology of the Clouds.

3.1. Structure of the LMC

The lower contours in Figs. 2-4 show an almost circular outline (axial ratios consistent with an inclination ) centered in all three cases near with major axis at about . Westerlund (1997) gives a similar diameter for the stars of the old disk. This stellar disk also coincides in shape and extent with the HI disk (Kim et al. 1998). The center of the disk is offset from the center of the Bar by about 30 to the north (see Fig. 2). We confirm the conclusion by Westerlund (1997) that the LMC consists of two systems: a circular disk and an off center bar. Half of the total number of stars are in the bar and this factor (Fig. 2) increases for younger objects. Unless this is a transient configuration, it thus seems that the LMC must be embedded in a gravitational potential produced by an unseen mass component (see also Sofue 1999). This is in agreement with the conclusion by Stil (1999) that the class of dwarf galaxies to which the LMC belongs (`fast rotators') is dominated by dark matter.

The youngest component (younger than 0.5 Gyr) is composed of very bright main-sequence dwarf stars, blue-loop stars and supergiants. Their distribution (Fig. 2) is clumpy and irregular. The Bar, extending over about , is prominent and contains a well defined nuclear concentration at its center. The region of 30 Dor is represented by the small feature just above the northeastern side of the Bar, and the Shapley Constellation III is the large structure at . Elongations at either end of the Bar indicate the presence of spiral arms most clearly seen in the northwest at the location of the giant HII region complex N 11. Similar structures are seen in the distribution of stellar complexes (Maragoudaki et al. 1998), associations and HII regions (Bica et al. 1999). Clusters (Bica et al. 1995, Kontizas et al. 1990) have a distribution more similar to the one of AGB/RGB stars.

The distribution of AGB stars (Fig. 3), also relatively young (around 1 Gyr) likewise reveals a prominent Bar and nucleus. Shapley Constellation III is inconspicuous in AGB stars. A broad and faint spiral arm begins at the northwestern end of the Bar and bifurcates around . The spiral arm feature originating at the southeastern end of the Bar is clearly delineated in the AGB star population and can easily be followed to . It was noted before by Bothun & Thompson (1988) in their surface photometry study of the Magellanic Clouds - see their diagram. 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 well matches the carbon stars by Kunkel et al. (1997).

The oldest population (from 1 to 5 Gyr), represented by RGB stars (Fig. 4), once again reveals a prominent Bar which is significantly broader than that defined by the younger populations. Galactic foreground stars may affect the outermost contours. The southern spiral arm is inconspicuous, but the two faint northern spiral arms seen in Fig. 3 (AGB) have weak counterparts in the form of extensions at .

Bothun & Thompson (1988) conclude that the LMC has a relatively large scale length more appropriate for galaxies with obvious spiral structure than for other dwarf galaxies. It is interesting that the asymmetric spiral structure delineated by the different components in Figs. 2-4 is in fairly good agreement with the HI map shown by Gardiner et al. (1998) and is nicely reproduced by their dynamical model.

3.2. Structure of the SMC

The structure of the SMC is still not understood (Westerlund 1997). Our maps show that populations of different age have different distributions. The youngest component has an asymmetric distribution (Fig. 5) elongated along a NE-SW axis (PA ). In the south, the outermost contour defines four protuberances which might be associated with tidal features: at least the eastern (coincident with the SMC Wing) and western protuberances are aligned with that of the Magellanic Cloud Bridge (cf. Staveley-Smith et al. (2000). Higher contours show an extension in the northeast, aligned with the main body of the SMC Bar. The Bar structure itself is similar to that seen in the distribution of young clusters (Bica & Dutra 2000) and in the upper main-sequence map by Zaritsky et al. (2000). Clusters, associations and HII regions (Bica & Schmitt 1995) are also found at the locations of the southern protuberances. The young stars are strongly concentrated in the southwestern part of the SMC Bar. Outside the main body of 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 by Stanimirovic et al. (1998) outline the distribution of the young stars quite well.

The AGB stars have a more regular distribution (Fig. 6) with two prominent central concentrations matching the carbon stars by Hardy et al. (1989). The easternmost also coincides with the peak of the young-star distribution. The AGB distribution axis is much less inclined (PA ) than that of the younger and very similar to that of the RGB star distribution. As in the case of the LMC, the stellar distributions become more regular and smoother with increasing age, also apparent in the B and V band images by Zaritsky et al. (2000) and for the outer contour in the carbon stars by Kunkel et al. (2000); carbon stars by Rebeirot et al.  (1983) fill the second level countour (Fig. 6).

The distribution of RGB stars (Fig. 7) is similar to that of the AGB stars and also exhibits two major concentrations. The western most is more pronounced in RGB than in AGB stars. The eastern concentration on average appears to be significantly younger than the western concentration dominated by the older stars. Remarkably, the strongest HI concentration in the SMC map by Stanimirovic et al. (1998) appears to be just between the concentration of younger stars and that of older stars. It is also remarkable that the older star distribution extends over the full length of the suspected southwestern tidal feature, about from the main body of the Bar. With respect to the overall distribution of the older stars, that of the HI appears to be displaced towards the east. The SMC Wing, prominent in HI and also traceable in the younger stellar population, has no counterpart in the older stars.

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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