Bloodhound unleashed: particle-based substructure tracking for cosmological simulations

ABSTRACT Modern studies of galaxy formation rely heavily on numerical simulations, which in turn require tools to identify and track self-bound structures in stars and dark matter. In this paper, we present Bloodhound, a new halo tracking algorithm optimized to track and characterize substructure in cosmological simulations, a regime that is crucial for studies of the nature of dark matter but where standard methods often have difficulties. Using simulations of Milky Way-mass haloes, we demonstrate that Bloodhound extends subhalo tracking by $3{\text{-}}4\, \mathrm{Gyr}$ on average, and significantly longer for subhaloes with small pericentres, relative to the widely used Rockstar $+$ consistent-trees halo tracking pipeline. We also show that Bloodhound provides continuous tracking, mitigating an issue for the standard technique where subhaloes can be lost and then found again – but assigned to a new merger tree – after several snapshots. This improved tracking leads to a substantially larger number of surviving subhaloes in the inner regions of dark matter haloes, which has several implications for studies of the Milky Way’s satellite galaxy system and its use for constraining properties of dark matter. For example, within the radius where current surveys are complete to ultra-faint galaxies ($D_{\rm MW} \lesssim 50$ kpc), Bloodhound finds more than twice as many subhaloes above the atomic cooling scale relative to the standard tracking method. Our results underscore the importance of robust subhalo tracking techniques in advancing our understanding of galaxy formation and cosmological models.