Orbits of Massive Satellite Galaxies: I. A Close Look at the Large Magellanic Cloud and a New Orbital History for M33

Abstract

The Milky Way (MW) and M31 both harbor massive satellite galaxies, the Large Magellanic Cloud (LMC) and M33, which may comprise up to 10 per cent of their host's total mass. Massive satellites can change the orbital barycentre of the host-satellite system by tens of kiloparsecs and are cosmologically expected to harbor dwarf satellite galaxies of their own. Assessing the impact of these effects depends crucially on the orbital histories of the LMC and M33. Here, we revisit the dynamics of the MW-LMC system and present the first detailed analysis of the M31-M33 system utilizing high precision proper motions and statistics from the dark matter-only Illustris cosmological simulation. With the latest Hubble Space Telescope proper motion measurements of M31, we reliably constrain M33's interaction history with its host. In particular, like the LMC, M33 is either on its first passage (t_{inf} < 2 Gyr ago) or if M31 is massive (>=2x10^12 Msun), it is on a long period orbit of about 6 Gyr. Cosmological analogs of the LMC and M33 identified in Illustris support this picture and provide further insight about their host masses. We conclude that, cosmologically, massive satellites like the LMC and M33 are likely completing their first orbits about their hosts. We also find that the orbital energies of such analogs prefer a MW halo mass ~1.5x10^12 Msun and an M31 halo mass >=1.5x10^12 Msun. Despite conventional wisdom, we conclude it is highly improbable that M33 made a close (< 100 kpc) approach to M31 recently ( t_{peri} < 3 Gyr ago). Such orbits are rare (< 1 per cent) within the 4$\sigma$ error space allowed by observations. This conclusion cannot be explained by perturbative effects through four body encounters between the MW, M31, M33, and the LMC. This surprising result implies that we must search for a new explanation for M33's strongly warped gas and stellar discs.