case western reserve university



Cold Dark Matter Substructure and Galactic Disks: Morphological and Dynamical Signatures of Hierarchical Satellite Accretion

Stelios Kazantzidis (OSU)

The Cold Dark Matter (CDM) model of hierarchical structure formation has emerged as the dominant paradigm in galaxy formation theory owing to its remarkable ability to explain a plethora of observations on large scales. Yet, on galactic and sub-galactic scales the CDM model has been neither convincingly verified nor disproved, and several outstanding issues remain unresolved. Using high-resolution N-body simulations I investigate whether the abundance of substructure predicted by CDM models is in conflict with the existence of thin, dynamically fragile galactic stellar disks. The simulation campaign is based on a hybrid approach combining cosmological simulations of galaxy-sized CDM halos to derive the properties of infalling subhalo populations and controlled numerical experiments of repeated satellite impacts on an initially-thin Milky Way-type disk galaxy. In contrast to what can be inferred from statistics of the z=0 surviving substructure, accretions of massive satellites onto the central regions of host halos, where the galactic disk resides, since z ~ 1 should be common. I show that these accretion events severely perturb the galactic disk and produce a wealth of distinctive morphological and dynamical signatures including: long-lived, low-surface brightness, ring-like features in the outskirts; significant flares; central bars; faint filamentary structures that (spuriously) resemble tidal streams in configuration space; tilting; warping; thickening; and heating. The final distribution of disk stars exhibits a complex vertical structure that is well-described by a standard ``thin-thick'' disk decomposition, where the ``thick'' disk component has emerged primarily as a result of the interaction with the most massive subhalo. Subhalo passages are also responsible for causing a velocity offset and displacement of the galactic disk with respect to the center of the host dark matter halo. These results highlight the significant role of CDM substructure in setting the structure of disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys and astrometric satellites may be able to distinguish between competing cosmological models by testing whether the detailed structure of galactic disks is as excited as predicted by the CDM paradigm.