Speaker
Description
We present a comprehensive numerical investigation into a minor merger event in the Andromeda Galaxy (M31), which we propose as a unified origin for four prominent stellar substructures: the Andromeda Giant Southern Stream (AGSS), Eastern Extent (EE), North-Eastern Shelf (NES), and Western Shelf (WS). This scenario provides a unified and self-consistent framework for understanding their formation, helping to resolve a long-standing issue regarding the relationship between halo substructure and galaxy assembly. Conducting $N$-body simulations, we model the dynamical interaction between M31 and an accreting progenitor, 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 model. Our results demonstrate that the progenitor with a dark matter halo mass of several $10^9$ solar masses, which experienced a first collision with M31 around 1 Gyr ago, can simultaneously reproduce the observed spatial distribution of the AGSS, EE, NES, and WS. We find that NES and WS are independent of the gravitational potential of the progenitor’s dark matter halo across the limited range of parameters considered in this study. In contrast, the position of the EE is noticeably affected by variations in the dark matter halo potential of the progenitor, with shallower potential shifts EE farther north. Another important result of our research is the clarification of the spatial relationship between EE and the Stream Cp which is a metal-poor component of the Stream C. Our findings indicate that EE is located several tens of kiloparsecs closer to us than Stream Cp, whose farther distance suggests overlapping debris from distinct collision events, while both remain closely aligned in celestial coordinates. This suggests that caution is required when interpreting their distribution in these coordinates. Moreover, we predict the existence of a previously undetected positive stream along the AGSS, characterized by positive line-of-sight velocities relative to M31. This feature would represent a kinematic counterpart to the already observed negative stream exhibiting negative line-of-sight velocities. Confirming the positive stream would provide strong support for our merger scenario. Moreover, we propose that three stellar streams, namely Stream B, a metal-rich component of Stream C referred to as Stream Cr, and EE, construct the Andromeda Giant Southern Arc (AGSA) connected to the AGSS. While the full extent of the AGSA and the existence of the positive stream remain to be observationally verified, we anticipate that future spectroscopic surveys and advances in theoretical studies will play a crucial role in confirming their nature.
In this study, the progenitor could have been placed on a radial infall trajectory, plunging toward M31 center from north to south at high velocity. This accretion event, traced through its stellar debris, offers insight into the hierarchical growth of galactic halos, contributing to our broader understanding of cosmic structure formation. This progenitor represents a key system for understanding the relationship between satellite accretion and stellar halo formation. Furthermore, M31 is an ideal laboratory for testing these scenarios through high-precision observations due to its proximity and providing a comprehensive view of the entire system.
