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[*] Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and the Intrinsic Elongation of the LMC
van der Marel R.P.
AJ, 122, 1827-1843, 2001

[*] Citations to this paper in the ADS

I construct a near-IR star count map of the LMC and demonstrate, using the viewing angles derived in Paper I, that the LMC is intrinsically elongated. I argue that this is due to the tidal force from the Milky Way.

The near-IR data from the 2MASS and DENIS surveys are ideally suited for studies of LMC structure, because of the large statistics and insensitivity to dust absorption. The survey data are used to create a star count map of RGB and AGB stars. The resulting LMC image shows the well-known bar, but is otherwise quite smooth. Ellipse fitting is used for quantitative analysis. The radial number density profile is approximately exponential with a scale-length r_d = 1.3-1.5 kpc. However, there is an excess density at large radii that may be due to the tidal effect of the Milky Way. The position angle and ellipticity profile both show large radial variations, but converge to PA_maj = 189.3 +/- 1.4 degrees and epsilon = 0.199 +/- 0.008 for r > 5 degrees. At large radii the image is influenced by viewing perspective (i.e., one side of the inclined LMC plane being closer to us than the other). This causes a drift of the center of the star count contours towards the near side of the plane. The observed drift is consistent with the position angle Theta = 122.5 +/- 8.3 degrees of the line of nodes inferred in Paper I.

The fact that Theta differs from PA_maj indicates that the LMC disk is not circular. Deprojection shows that the LMC has an intrinsic ellipticity epsilon" = 0.31 in its outer parts, considerably larger than typical for disk galaxies. The outer contours have a more-or-less common center, which lies approximately 0.4 kpc from the center of the bar. Neither agrees with the kinematic center of the HI gas disk. The LMC is elongated in the general direction of the Galactic center, and is elongated perpendicular to the Magellanic Stream and the velocity vector of the LMC center of mass. This suggests that the elongation of the LMC has been induced by the tidal force of the Milky Way.

The position angle of the line of nodes differs from the position angle Theta_max of the line of maximum line of sight velocity gradient. Results from HI gas and discrete tracers indicate that Theta_max - Theta = 20-60 degrees. This could be due to one or more of the following: (a) streaming along non-circular orbits in the elongated disk; (b) uncertainties in the transverse motion of the LMC center of mass, which can translate into a spurious solid-body rotation component in the observed velocity field; (c) an additional solid body rotation component in the observed velocity field due to precession and nutation of the LMC disk as it orbits the Milky Way, which is expected on theoretical grounds.


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Last modified May 29, 2002.
Roeland van der Marel, marel@stsci.edu.
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