Are the Magellanic Clouds on Their First Passage about the Milky Way?

摘要

Recent proper-motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil and coworkers suggest that the 3D velocities of the Clouds are substantially higher (~100 km s-1) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we reexamine the orbital history of the Clouds using the new velocities and a ΛCDM-motivated MW model with virial mass Mvir = 1012 M☉ (e.g., Klypin and coworkers). We conclude that the LMC and SMC are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances 200 kpc (i.e., Mvir 2 × 1012 M☉), that their orbital period and apogalacticon distance must be a factor of 2 larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of H I gas trailing the LMC and SMC that extends ~100° across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.