Barry Chiang (Yale University)

Dynamics of Dark Matter Subhalos: Velocity Anisotropy, Numerical Convergence, and Cluster Lenses

In the hierarchical structure formation model of ΛCDM cosmology, dark matter halos grow by accreting lower-mass subhalos and exhibit non-trivial velocity anisotropy due to aspherical collapse and radial mass infall. Once accreted, the tidal evolution of these subhalos is a highly non-linear process that necessitates numerical simulations. Previous idealized simulations have only investigated the tidal evolution of isotropic subhalos, while state-of-the-art cosmological simulations often lack the resolution to reliably track low-mass substructures. In [arXiv:2411.03192], we present the first high-resolution idealized simulations of anisotropic subhalos, finding a significant anisotropy-dependence in subhalo bound mass fraction and density profile evolution, thereby invalidating previous empirical results derived from isotropic subhalos. Notably, for subhalos with high radial anisotropy at accretion, tidal stripping and subsequent re-virialization can drive a cusp-core transformation without baryonic feedback. Building on this, in [arXiv:2510.26901], we establish universal force and mass numerical convergence criteria, validated across subhalos with varying anisotropies, concentrations, sizes, masses, orbits, and density profiles. Applying these criteria, we find that subhalos with pericenter distances less than ~0.2 R_{vir} in current cosmological simulations are mostly force-unresolved and susceptible to runaway numerical disruption. As a concrete example, in [arXiv:2511.14726], we curate a sample of well-observed massive galaxy cluster lenses and their TNG-Cluster analogs to compare subhalo demographics. We demonstrate in [arXiv:2601.07909] that these simulated analogs lack inner substructures compared to observed clusters, by about an order of magnitude within the central projected 0.2 R_{vir}, underscoring the need to guard against numerical artifacts.

Biography: Barry T. Chiang is currently a third-year PhD student in Astronomy at Yale, with main research interests in dark matter, galactic dynamics, numerical simulations, and gravitational lensing. Prior to Yale, he was a visiting researcher with Jeremiah P. Ostriker at Columbia University (2022), a research assistant with Hsi-Yu Schive at National Taiwan University (2020-2023), and a research assistant with Jessie Shelton at the University of Illinois (2018-2020). He completed a master’s degree in Applied Mathematics (Part III) at the University of Cambridge in 2021 and three bachelor’s degrees in Astronomy, Engineering Physics, and Music Composition from the University of Illinois in 2020.