5.3. HI Kinematics
HI gas provides another powerful method to study the kinematics of the LMC. High quality data is available from, e.g., Kim et al. (1998). Unfortunately, the kinematical analysis presented by Kim et al. was not as general as that discussed above for carbon stars. They did not leave wts as a free parameter in the fit. Instead, they corrected their data at the outset for the transverse motion of the LMC using the proper motion measured by Jones et al. (1994), (µW, µN) = (- 1.37 ± 0.28, -0.18 ± 0.27) mas yr-1, and assumed that di / dt = 0. This fixes wts = - 175 ± 72km s-1, which is inconsistent with the value inferred from the carbon star velocity field. Kim et al. obtained a kinematic line of nodes that is both twisting with radius and inconsistent with the value determined from the carbon stars. In addition, they inferred a rotation curve for which the amplitude exceeds that in Figure 8 by ~ 40%. It is likely that these results are affected by the imposed value of wts. A more general analysis of the HI kinematics is therefore desirable, but unfortunately, is not currently available. The same limitations apply to many of the other published studies of LMC tracer kinematics cited at the start of Section 5.
Independent of how the data are analyzed, it is likely that collisionless tracers provide a more appropriate means to study the structure of the LMC using equilibrium models than does HI gas. The dynamical center of the carbon star velocity field is found to be consistent (to within the ~ 0.4° per-coordinate errors) with both the center of the bar and the center of the outer stellar contours. However, it has long been known that the center of the HI rotation velocity field does not coincide with the center of the bar, and it also doesn't coincide with the center of the outer contours of the stellar distribution. It is offset from both by ~ 1 kpc. This indicates that the HI gas may not be in equilibrium in the the gravitational potential. Added evidence for this comes from the fact that the LMC and the SMC are enshrouded in a common HI envelope, and that they are connected by a bridge of HI gas (see Figure 5). Even at small radii the LMC gas disk appears to be subject to tidal disturbances (see Figure 4) that may well affect the velocity field.