The LMC consists of an outer body that appears elliptical in projection on the sky, with a pronounced, off-center bar. The appearance in the optical wavelength regime is dominated by the bar, regions of strong star formation, and patchy dust absorption. The LMC is generally considered an irregular galaxy as a result of these characteristics. It is in fact the prototype of the class of galaxies called "Magellanic Irregulars" (de Vaucouleurs & Freeman 1973). Detailed studies of the morphological characteristics of the LMC have been performed using many different tracers, including optically detected starlight (Bothun & Thompson 1988; Schmidt-Kaler & Gochermann 1992), stellar clusters (Lynga & Westerlund 1963; Kontizas et al. 1990), planetary nebulae (Meatheringham et al. 1988) and non-thermal radio emission (Alvarez, Aparici & May 1987). Recent progress has come primarily from studies of stars on the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) and from studies of HI gas.
2.1. Near-Infrared Morphology
Recently, two important near-IR surveys have become available for studies of the Magellanic Clouds, the Two Micron All Sky Survey (2MASS; e.g., Skrutskie 1998) and the Deep Near-Infrared Southern Sky Survey (DENIS; e.g., Epchtein et al. 1997). Cross-correlations of the data from these surveys and from other catalogs are now available as well (Delmotte et al. 2002). The surveys are perfect for a study of LMC morphology and structure. Near-IR data is quite insensitive to dust absorption, which is a major complicating factor in optical studies (Zaritsky, Harris & Thompson 1997; Zaritsky 1999; Udalski et al. 2000; Alcock et al. 2000b). The surveys have superb statistics with of the order of a million stars. Also, the observational strategy with three near-IR bands (J, H and Ks in the 2MASS survey; I, J and Ks in the DENIS survey) allows clear separation of different stellar populations. In particular, the data are ideal for studies of evolved RGB and AGB stars, which emit much of their light in the near-IR. This is important for studies of LMC structure, because these intermediate-age and old stars are more likely to trace the underlying mass distribution of the LMC disk than younger populations that dominate the light in optical images.
Figure 1 shows the (J - Ks, Ks) color-magnitude diagram (CMD) for the LMC region of the sky. Several finger-like features are visible, each due to different stellar populations in the LMC or in the foreground (Nikolaev & Weinberg 2000; Cioni et al. 2000a; Marigo, Girardi & Chiosi 2003). The LMC features of primary interest in the present context are indicated in the figure, namely the Red Giant Branch (RGB), the Tip of the RGB (TRGB), the oxygen-rich AGB stars, and the carbon-rich AGB stars ("carbon stars"). Van der Marel (2001) used the color cut shown in the figure to extract a sample from the 2MASS and DENIS datasets that is dominated by RGB and AGB stars. These stars were used to make the number-density map of the LMC shown in Figure 2. Kontizas et al. (2001) showed the distribution on the sky of ~ 7000 carbon stars identified by eye from optical objective prism plates. Their map does not show all the rich detail visible in Figure 2, but it is otherwise in good agreement with it.
Figure 1. Near-IR (J - Ks, Ks) CMD from 2MASS data for the LMC region of the sky. Only a quarter of the data is shown, to avoid saturation of the grey scale. The features due to the Red Giant Branch (RGB), the Tip of the RGB (TRGB), the oxygen-rich AGB stars [AGB(O)], and the carbon-rich AGB stars [AGB(C)] are indicated. The region enclosed by the solid lines was used to extract stars for creation of the LMC number density map in Figure 2.
Figure 2. Surface number density distribution on the sky of RGB and AGB stars in the LMC from van der Marel (2001). North is to the top an east is to the left. The image covers an area of 23.55° × 21.55°. The Galactic foreground contribution was subtracted. A color version of the image is available at http://www.stsci.edu/~marel/lmc.html .
The near-IR map of the LMC is surprisingly smooth. The morphology is much less irregular than it is for the the younger stellar populations that dominate the optical light. Apart from the central bar there is a hint of some spiral structure, as discussed previously by, e.g., de Vaucouleurs & Freeman (1973). However, the spiral features all have very low contrast with respect to their surroundings, and there is certainly no well organized spiral pattern in the LMC. Quantitative analysis can be performed on the basis of ellipse fits to the number density contours. This yields a surface number density profile that can be fit reasonably well fit by an exponential with a scale length of 1.4° (1.3 kpc). The radial profiles of the ellipticity (defined as 1 - q, where q is the axial ratio) and the major axis position angle PAmaj both show pronounced variations as function of distance from the LMC center, due to the presence of the central bar. However, at radii r 4° the contour shapes converge to an approximately constant position angle PAmaj = 189.3° ± 1.4° and ellipticity = 0.199 ± 0.008.