Assembling Andromeda: What Orbital Dynamics Reveal About the M31 System

Ekta Patel (Villanova University)

March 16, 2026

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Abstract: The Milky Way–Large Magellanic Cloud interaction has reshaped our understanding of how massive satellites drive disequilibrium in their host systems — and Andromeda (M31) tells a strikingly parallel, yet distinct, story. With 6D phase space data now available for nearly a dozen M31 satellites, we can for the first time begin to reconstruct the complex interaction history of the M31 system in detail. In this talk, I will present orbital modeling results for M31 and its most massive satellite, M33. Contrary to the long-held assumption that M33 experienced a recent close encounter with M31, I will show that M33 is most likely on its first infall into M31’s halo — a result further corroborated by M33’s untruncated HI disk and the orbital histories of cosmological analogs. I will also present preliminary orbital results on M32, a prominent dwarf elliptical galaxy situated near M31’s disk. Drawing on insights from the MW-LMC system, I will discuss how M33’s passage may be displacing M31’s barycenter and influencing the dynamics of its broader satellite population, and what this means for interpreting M31’s assembly history.