The Rotation Curve of the Large Magellanic Cloud and the Implications for Microlensing

Abstract

The rotation of the disk of the Large Magellanic Cloud (LMC) is derived from the radial velocities of 422 carbon stars. New aspects of this analysis include the propagation of uncertainties in the LMC proper motion with a Monte Carlo, and a self-consistent modeling of the rotation curve and disk kinematics. The rotation curve is well fit by a truncated, finite-thickness exponential disk model with no dark halo. The velocity dispersion in radial bins decreases from 22 to 15 km/s, then increases to 20 km/s at larger radii. Constant-thickness models in virial equilibrium cannot be reconciled with the data even if the effects of LMC or Galactic dark halos are included. If the disk is virialized, the LMC disk is flared. We model the velocity dispersion at large radii (R > 6 kpc) as a maximal flared disk under the influence of the Galactic dark halo, which favors a mean density for the latter of 0.00025 M_sol/pc^3 at the LMC distance. LMC stellar kinematics play an important role in elucidating the nature of MACHOs, a dark population inferred from LMC microlensing. We have constructed a truncated and flared maximal disk model for the LMC which is kinematically based. Our model does not include a nonvirialized component such as tidal debris. Our upper limit on the self-lensing optical depth is in good agreement with that obtained from less sophisticated models, and is an order of magnitude too small to account for the MACHO microlensing signal [Abridged]