When was the Large Magellanic Cloud accreted onto the Galaxy ?

Abstract

Using fully self-consistent N-body models for the dynamical evolution of the Large Magellanic Cloud (LMC) in the Galaxy, we show that if the LMC initially has an extended old stellar halo before its commencement of tidal interaction with the Galaxy, physical properties of the stars stripped from the LMC stellar halo can have fossil information as to when and where the LMC was accreted onto the Galaxy for the first time. If the epoch of the first LMC accretion onto the Galaxy from outside its viriral radius is more than ~4 Gyr ago (i.e., at least two pericenter passages), the stars stripped from the stellar halo of the LMC can form an irregular polar ring or a thick disk with a size of ~100 kpc and rotational kinematics. On the other hand, if the LMC was first accreted onto the Galaxy quite recently (~ 2 Gyr ago), the stripped stars form shorter leading and trailing stellar stream at R=50-120 kpc. Also distributions of the stripped stars in phase space between the two cases can be significantly different. The derived differences in structure and kinematics of the stripped stars therefore suggest that if we compare the observed three-dimensional (3D) distribution and kinematics of the outer Galactic stellar halo along the polar-axis, then we can give strong constraints on the past orbit of the LMC. We find that the orbital properties of the LMC in the successful formation models of the Magellanic Stream (MS) are consistent with those predicted from recent LambdaCDM cosmological simulations. We conclude that the LMC was accreted onto the Galaxy more than ~4 Gyr ago so that interaction between the LMC, the Small Magellanic Cloud (SMC), and the Galaxy could form the MS and its Leading Arms (LAs).