Asymmetric Warfare: M31 and its Satellites

Photometric surveys of M31's halo vividly illustrate the wreckage caused by hierarchical galaxy formation. Several of M31's satellites are being disrupted by M31's tidal field, among them M33 and And I, while other tidal structures are the corpses of satellites already destroyed. The extent to which M31's satellites have left battle scars upon it is unknown; to answer this we need accurate orbits and masses of the perturbers. I focus here on M31's 150-kpc-long Giant Southern Stream (GSS) as an example of how these can be determined even in the absence of a visible progenitor. Comparing N-body models to photometric and spectroscopic data, I find this stream resulted from the disruption of a large satellite galaxy by a close passage about 750 Myr ago. The GSS is connected to several other debris structures in M31's halo. Bayesian sampling of the simulations estimates the progenitor's initial mass as log(Mstar/Msun) = 9.5 +- 0.2, showing it was one of the most massive Local Group galaxies until quite recently. The stream model constrains M31's halo mass to be (1.8+-0.5)x10^{12} Msun. While these small uncertainties neglect several important degrees of freedom, they are likely to remain good even with a more complete model. Future work on M31's satellites and streams will provide independent constraints on M31's mass, and reveal the shared history of M31 and its halo components.