The Illusion of Morphology in Tidal Structures: Changes to Stellar Shells and Streams in Non-Spherical Haloes

Abstract We identify shell-like tidal structures in flattened haloes that appear stream-like when viewed under different projections. This dependence on projection demonstrates how changes in the host halo can directly impact the formation and classification of tidal debris, highlighting the challenges of relying solely on visual inspections. To address this, we employ our clustering-based classification framework to systematically categorise the tidally disrupted satellites into stream-like and shell-like structures. Our host consists of a static, three component MW model with flattening introduced along the z-axis NFW dark halo. We consider three halo shape scenarios: a spherical halo q = 1 , an extremely oblate halo with q = 0.5 , and a prolate halo where q = 1.5. We evolve three types subhalos: a highly radial, massive subhalo favouring shell formation, an eccentric orbit leading to stream formation and an intermediate orbit case. We first classify the tidal structures visually using face-on and edge-on density projections of the 3D position distribution. This visual inspection reveals shell-like and stream-like formations across all the face-on projections of different halo shapes, while the edge-on projection leads to contrary classifications in some cases. To resolve these ambiguities, we apply the classification method developed in our earlier work analysing the structures in ordered density, radial and energy angle space. We further investigate the spatial dispersion of stream-like structures and the rate at which core density reduces as the flattening parameter varies. Our results demonstrate that the variations in halo shape can affect the formation and classification of tidal debris, as well as the spatial dispersion and core density evolution of streams. This offers insights on how not only the initial condition of the subhalo, but also the structural properties of the host halo, plays a crucial role in determining the morphology of tidal features. These findings offer new insights into the role of dark matter halo geometry in shaping the tidal structure formation and its contribution to hierarchical galaxy formation and evolution.