When was the Large Magellanic Cloud accreted on to the Galaxy?

Abstract

Using fully self-consistentN-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 on to the Galaxy for the first time. If the epoch of the first LMC accretion on to the Galaxy from outside its virial radius is more than ~4Gyr ago (i.e. at least two pericentre passages), the stars stripped from the stellar halo of the LMC can form an irregular polar ring or a thick disc with a size of ~100kpc and rotational kinematics. On the other hand, if the LMC was first accreted on to the Galaxy quite recently (~2Gyr ago), the stripped stars form shorter leading and trailing stellar stream atR= 50-120kpc. 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 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 also discuss whether the orbital properties of the LMC in successful formation models for the Magellanic Stream (MS) can be consistent with orbital properties of the LMC-type systems in Galaxy-type haloes predicted from recent high-resolution cosmological simulations in a Λ cold dark matter (ΛCDM) universe. We find that the orbital properties of the LMC in the successful formation models are consistent with those predicted from the cosmological simulations. We also find that the LMC cannot merge with the Galaxy within the last ~6Gyr in models consistent with predictions from the ΛCDM simulations. Given that the successful MS formation models predict at least two pericentre passages of the LMC in the Galaxy, we conclude that the LMC was accreted on to the Galaxy more than ~4Gyr ago so that interaction between the LMC, the Small Magellanic Cloud and the Galaxy could form the MS and its Leading Arms. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.