Simultaneous formation of the Andromeda Giant Southern Stream and the substructures in the Andromeda Halo

Abstract We investigate a minor merger event in M 31 that simultaneously forms the Andromeda Giant Southern Stream (AGSS), the Eastern Extent (EE), the North-Eastern Shelf (NES), and the Western Shelf (WS), offering a unified model for these substructures. By varying the scale radius and mass of the dark matter halo associated with the progenitor, around the range predicted by the $\Lambda$ cold dark matter (CDM) model, we successfully reproduce the spatial features of these substructures. Across the limited range of parameters considered in this study, our analysis shows that the spatial evolution of the NES and the WS is independent of the gravitational potential of the dark matter halo associated with the progenitor, while a shallower potential shifts the EE further north. The simulations clearly demonstrate that the progenitor with a dark matter halo mass of $9\times 10^9$ M$_\odot$ colliding with M 31 850 Myr ago could simultaneously form the observed substructures, including the AGSS, EE, NES, and WS. The simulation results indicate that the EE lies several 10 kpc closer to us than the aligned Stream Cp, which is actually a metal-poor component of Stream C, whose farther distance suggests overlapping debris from distinct collision events, while both remain closely aligned in celestial coordinates. Furthermore, we predict the existence of a positive stream along the AGSS, characterized by positive line-of-sight velocities relative to M 31, which complements an already observed negative stream exhibiting negative line-of-sight velocities. Finally, we propose that three objects—Stream B, a metal-rich component of Stream C known as Stream Cr, and the EE—are components of the Andromeda Giant Southern Arc (AGSA) connected to the AGSS. Although the existence of the positive stream and a complete picture of the AGSA have yet to be confirmed observationally, we anticipate that future spectroscopic observations and further advances in theoretical studies will verify their existence.