Galaxy and Halo Root Systems: Fingerprints of Mass Assembly

Abstract

We discuss what we call halo or galaxy root systems, collections of particle pathlines that show the infall of matter from the initial uniform distribution into a collapsed structure. The matter clumps as it falls in; projected through time, it produces filamentary density enhancements analogous to tree roots and branches, or branching river networks. This relates to the larger-scale cosmic web, but is defined locally about one of its nodes: a physical, geometric version of a merger tree. We find dark-matter-halo root systems on average to exhibit more roots and root branches for the largest cluster haloes than in small haloes, indicating their more complicated assembly, even in dark matter without gas physics. Also, we find that high spin manifests as curvier roots, and that many have mass contributions that start far away from central, compact protohalo cores. Root systems also show sensitivity to anisotropic infall, which we see some evidence for in a simulation with rather low box size.