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

Recent proper motion measurements of the Large and Small Magellanic Clouds(LMC and SMC, respectively) by Kallivayalil et al (2006a,b) suggest that the 3Dvelocities of the Clouds are substantially higher (~100 km/s) than previouslyestimated and now approach the escape velocity of the Milky Way (MW). Previousstudies have also assumed that the Milky Way can be adequately modeled as anisothermal sphere to large distances. Here we re-examine the orbital history ofthe Clouds using the new velocities and a LCDM-motivated MW model with virialmass Mvir = 1e12 Msun (e.g. Klypin et al 2002). We conclude that the Clouds areeither currently on their first passage about the MW or, if the MW can beaccurately modeled by an isothermal sphere to distances >200 kpc (i.e., Mvir >2e12 Msun), that their orbital period and apogalacticon distance must be afactor of two larger than previously estimated, increasing to 3 Gyr and 200kpc, respectively. A first passage scenario is consistent with the fact thatthe LMC and SMC appear to be outliers when compared to other satellite galaxiesof the MW: they are irregular in appearance and are moving faster. We discussthe implications of this orbital analysis for our understanding of the starformation history, the nature of the warp in the MW disk and the origin of theMagellanic Stream (MS), a band of HI gas trailing the LMC and SMC that extendsroughly 100 degrees across the sky. Specifically, as a consequence of the neworbital history of the Clouds, the origin of the MS may not be explainable bycurrent tidal and ram pressure stripping models