Local Group analogues in a cosmological context -- I. Relating velocity structure to the cosmic web

Abstract

Our Local Group, dominated in mass by the Milky Way (MW) and M31, provides a unique laboratory for testing $\hat{I}›$CDM cosmology on small scales owing to its proximity. However, its connection to the surrounding large-scale environment, which is essential for interpreting its properties, is inadequately understood. In this work, we explore the connection between Local Group analogues (LGAs) and their surrounding large-scale environments using the ABACUSSUMMIT simulation suite, highlighting the key role of the coupling energy of the MW-M31 orbit, $E_{\mathrm{coupling}}$. We find that LGAs with high $E_{\mathrm{coupling}}$ preferentially reside in denser regions, whereas those with low $E_{\mathrm{coupling}}$ tend to occupy low-density environments. Furthermore, LGAs with low $E_{\mathrm{coupling}}$ exhibit strong alignment with cosmic filaments, manifested as a pronounced polar anisotropy in the distribution of tracer haloes. By contrast, LGAs with high $E_{\mathrm{coupling}}$ show a weaker polar anisotropy but an enhanced azimuthal anisotropy, with large-scale tracer haloes preferentially lying in the plane spanned by the halo pair and the orbital spin vector. Within this framework, our Local Group is characterised by typical $E_{\mathrm{coupling}}$ residing in a relatively under-dense environment, yet it remains consistent with the 95\% range of analogue systems identified in the simulation.

17 pages, 13 + 2 figures, 1 table, comments are welcome!