Large Magellanic Cloud: Revealing the bar structure
The LMC, a small irregular galaxy, is dominated in its appearance by a stellar bar. The old stellar population shows a bar-dominated featureless distribution, whereas the young stars indicate a barless, single-armed spiral like structure. This dual appearance of the LMC has been a long standing puzzle. The brightness of core helium- burning red clump stars in the bar region of the LMC was used as a probe to understand the bar structure. These stars are standard candles to estimate absolute distances. The LMC bar was found to have a structure and a warp, indicating that the bar is dynamically disturbed (Subramaniam 2003, ApJL, 598, L19). This was later found to be due to the presence of a mis-aligned structure in the central region, termed as a secondary bar (Subramaniam 2004). The central bar is inclined in the line of sight with trailing pattern on both ends, suggesting a possible counterrotation for the inner bar. The inner structure in the LMC is likely to be formed from early mergers.
Kinematic analysis of stellar rotation in the inner regions was done to verify the counterrotation. Stellar radial velocity in the central region was used to estimate the radial velocity curve along various position angles including the line of nodes. The central part of the radial velocity profile, along the major axis shows a V-shaped deviation, instead of a straight line profile. This is a clear indication of counterrotation. The kinematics of stars and gas in the disk was also modeled, so as to explain the differences shown by these entities (Subramaniam & Prabhu 2005, ApJ 625, L47). A counterrotating contribution is added to the main stellar disk of the LMC along with a gas rich disk. The inner gas-rich disk of the LMC could be formed from this infalling gas tidally removed from the SMC. This shows that the LMC is in the process of accreting gas. This model explains the dual appearance of the LMC in gas and stars, it points out the source of gas for the formation of stars and explains the origin of recent star formation in the LMC.
All the above results clearly indicate that the nearest dwarf neighbour shows how a galaxy evolves through its lifetime. Even though it is much smaller than our Galaxy, the LMC seems to have gone through at least one merger and one gas accretion event in its lifetime.( A. Subramaniam )
