8.4. Non-planar Structure in the LMC Disk
The studies discussed in Sections 5 and 6 indicate that the overall structure of the LMC is that of a (thick) disk. From Section 8.3 it follows that there is no strong evidence for unexpected material far from the disk plane. However, this does not mean that there may not be non-planar structures in the disk itself. For example, it was already discussed in Section 6 that there might be warps and twists in the disk plane.
Early evidence for non-planar material came from HI observations. One of the most prominent optical features of the LMC is the star forming 30 Doradus complex, located just north of the eastern tip of the bar. This region is a very strong source of UV radiation, yet the HI gas disk of the LMC does not show a void in this part of the sky. This indicates that the 30 Doradus complex cannot be in the plane of the LMC disk, but must be at least 250-400 kpc away from it (Luks & Rohlfs 1992). HI channel maps show that there is a separate HI component, called the "L-component", that is distinct from the main LMC disk. It has lower line-of-sight velocities than the main disk by 20-30 km s-1, contains some 19% of the HI gas in the LMC, and does not extend beyond 2°-3° from the LMC center (Luks & Rohlfs 1992). Absorption studies indicate that this component is behind the LMC disk (Dickey et al. 1994). The 30 Doradus complex is spatially located at the center of the L-component. It is probably directly associated with it, given that the L-component shows a hole of HI emission at the 30 Doradus position as expected from ionization. These results suggest that in the central few degrees of the LMC an important fraction of the gas and stars may not reside in the main disk.
It has now proven possible to test some of these ideas more directly by using large stellar databases. Section 6 already mentioned the study of more than 2000 Cepheids by Nikolaev et al. (2004). They obtained accurate reddening-corrected (relative) distances to each of the Cepheids. The distance residuals with respect to the best-fitting plane do not follow a random Gaussian distribution. Instead they show considerable structure, as shown in Figure 10. The two-dimensional distribution of the residuals on the sky was interpreted as a result of two effects, namely a symmetric warp in the disk, and the fact that the bar is located ~ 0.5 kpc in front of the main disk. Interestingly, an offset between the LMC bar and disk had previously been suggested by Zhao & Evans (2000) as an explanation for the observed LMC microlensing optical depth. If the disk and the bar of the LMC are not dynamically connected, then this may also explain why in projection the LMC bar appears offset from the center of the outer isophotes.
Figure 10. Map of the average vertical distances of Cepheids (in kpc) from a best-fitting plane solution as function of in-plane coordinates (x0, y0) from Nikolaev et al. (2004). The orientation of the bar in this representation is similar as in Figure 2. Negative (positive) distances denote material behind (in front of) the fitted plane. The cross indicates the HI rotation center according to Kim et al. (1998), the star shows the geometric center of the Cepheid sample, and the triangle gives the center of the outer carbon star isophotes from van der Marel & Cioni (2001). A color version of the image is available in the Nikolaev et al. paper.
The variation of the reddening-corrected Red Clump magnitude over the face of the LMC has also been used to study vertical structures in the LMC disk. Olsen & Salyk observed 50 randomly selected fields in the central 4° of the LMC at CTIO. They found that fields between 2°-4° south-west of the LMC center are 0.1 mag brighter than expected from the best plane fit. They interpreted this to indicate that stars in these fields lie some 2 kpc above the LMC disk, and argued for warps and twists in the LMC disk plane. Subramaniam (2003) used Red Clump magnitudes determined from stars in the OGLE database to address the same issue. This yielded a map of the Red Clump magnitude along the length and width of the LMC bar. This map shows considerable structure and clearly cannot be fit as a single plane. Subramaniam (2004) suggested that the residuals can be interpreted as the result of a misaligned secondary bar inside the primary bar.
Eclipsing binaries have also provided interesting information on this subject. These binaries can be modeled in detail to yield a fairly accurate distance. The distances of sources studied so far seem to indicate a slightly lower LMC distance modulus of m - M 18.4 (Ribas et al. 2002) than has been inferred from other tracers (see Section 4). However, it is possible to obtain somewhat higher values with a slightly different analysis (Groenewegen & Salaris 2001; Clausen et al. 2003) so this is no great cause for concern (Alves 2004b). What is interesting though is that four eclipsing binaries analyzed by the same team with the same method show a considerable spread in distance. This has been interpreted to mean than one of the binaries lies ~ 3 kpc behind the LMC disk plane (Ribas et al. 2002) and that another one lies ~ 4 kpc in front of it (Fitzpatrick et al. 2003).
The above studies indicate that there is considerable and complicated vertical structure in the central few degrees of the LMC disk. However, this is a rapidly developing field, and many important questions remain open. In particular, it is unclear whether the features reported in the various studies are actually the same or not. Qualitative comparison is not straightforward. Different authors study different areas of the LMC, they plot residuals with respect to different planes, and they present their results in figures that plot different types of quantities. However, cursory inspection of the various papers shows very few features that are obviously in common between the studies. Quantitative comparison is therefore needed. Direct comparison to the results for the outer parts of the LMC, where there is little evidence for extra-planar structures (van der Marel & Cioni 2001), is also important. If discrepancies emerge from such comparisons, then this can mean two things. Either some studies are in error (e.g., due to use of inaccurate dust corrections, or by incorrect interpretation of stellar evolutionary variations as distance variations) or different tracers do not trace the same structure. The latter might well the case. Cepheids are young stars with ages less than a few times 108 years whereas stars on the RGB and AGB are typically older than 1 Gyr. Since the structure of the LMC and its gaseous component vary with time as a result of tidal interactions with the Milky Way and the SMC, one wouldn't necessarily expect stars formed at different epochs to trace identical structures.
Another open question is what the physical and dynamical interpretation is of the extra-planar structures that are being detected. Many authors have used the term "warp". However, the residuals that have been reported do not much resemble the smoothly varying residuals that are expected from a single plane with a global warp. The structures appear to be both different and more complicated than a single warp. If a decomposition into different components is attempted, then such a decomposition must use entities that can be understood dynamically (disk, bar, bulge, warp, etc.) and that make sense in the context of the overall evolutionary history of the Magellanic System. Components with holes or sharp edges (e.g., Subramaniam 2004) are not particularly realistic from a dynamical viewpoint. Phase mixing of stellar orbits quickly removes such sharp discontinuities. Also, if the vertical structures detected in the inner region of the LMC disk are due to components that are not connected to the main disk plane, then why does the projected image of the LMC (e.g., Figure 2) look so smooth? And why is there so little evidence from stellar kinematics for components with decoupled kinematics (e.g., Figure 7; Zhao et al. 2003)? Clearly, many questions remain to be answered before we can come to a full understanding of the vertical structure of the LMC.