The SMC structure and kinematics are less well studied and understood than those of the LMC. The morphological appearance in blue optical light is patchy and irregular. Kinematical observations of HI and young stars reveal ordered rotation that indicates that these tracers may reside in a disk. However, detailed velocity field fits using equation (1) have not been attempted. Stanimirovic et al. (2004) found that the HI rotation curve in the SMC rises almost linearly to Vrot 50 km s-1 at the outermost datapoint (~ 3.5 kpc), with no signs of flattening. The implied dynamical mass inside this radius is 2.4 × 109 M. By contrast, the total stellar mass of the SMC is ~ 3.1 × 108 M and the mass of the neutral gas is 5.6 × 108 M. The combined mass of the visible material in the SMC is therefore insufficient to explain the dynamically inferred mass, and the SMC must be embedded in a dark halo. 1
Evans & Howarth (2008) obtained velocities for 2045 young (O, B, A) stars in the SMC, and found a velocity gradient of similar slope as seen in the HI gas. Surprisingly though, they find a position angle for the line of maximum velocity gradient that is quite different, and almost orthogonal to that seen in the HI. This may be an artifact of the different spatial coverage of the two studies (Evans & Howarth did not observe in the North-East region where the HI velocities are largest), since it would be difficult to find a physical explanation for a significant difference in kinematics between HI gas and young stars.
When the old red stars that trace most of the stellar mass are isolated using CMDs, the morphological appearance of the SMC is more spheroidal (Zaritsky et al. 2000, 2002, Cioni, Habing & Israel 2000b; Maragoudaki et al. 2001). Harris & Zaritsky (2006) studied the kinematics of 2046 RGB stars and inferred a velocity dispersion = 27.5 ± 0.5 km s-1. This is similar to the dispersion of the young stars observed by Evans & Howarth (2008), but unlike the young stars, the older RGB stars do not show much rotation. Their low Vrot / is consistent with what is typical for dE and dSph galaxies. Hence, the SMC may be more akin to those galaxy types than to other more irregular systems.
Studies of the distances of individual tracers in the SMC have shown it to be much more vertically extended than would be expected for a disk galaxy. Crowl et al. (2001) mapped the distances of star clusters using red clump magnitudes. They argued that the SMC has axial ratios of 1:2:4, and is viewed almost pole on. While different authors have found a range of other axial ratios using different types of tracers, most authors agree that the SMC has a considerable line-of-sight depth.
1 These values are based on the analysis in Stanimirovic et al. (2004), although those authors do not draw the same conclusion. Back.