The Missing Satellites of the Magellanic Clouds: Testing LCDM Predictions on Small Scales

According to LCDM theory, hierarchical evolution occurs on all mass scales, implying that satellites of the Milky Way should also have companions. The recent discovery of ultra-faint dwarf galaxy candidates in close proximity to the Magellanic Clouds provides an opportunity to test this theory. Missing in this regard have been the proper motions, which would allow us to measure the 3D velocities of these galaxies, and thereby test whether any may be bound to the Magellanic Clouds system as opposed to the Milky Way system. Recently, we have been able to use Gaia data release 2 to measure the proper motions for 13 of the 32 new dwarf galaxy candidates. All 13 also have radial velocity measurements. We compare the measured 3D velocities of these dwarfs to those expected at the corresponding distance and location for the debris of a Large Magellanic Cloud  analog in a cosmological numerical simulation. We conclude that four of these galaxies (Hor1, Car2, Car3, and Hyi1) have come in with the Magellanic Clouds, constituting the first confirmation of the type of satellite infall predicted by LCDM.  I will discuss the implications and future prospects for this work, and the trade-off between Gaia, space missions (JWST, WFIRST, EUCLID), and ELTs.