Milky Way '08 Seminar

Date: Thursday,
Speaker: Stelios Kazantzidis (Ohio State University)
Title: "Cold Dark Matter Substructure and Galactic Disks"
Abstract: 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, numerical simulations I investigate whether the abundance of substructure predicted by CDM models is in conflict with the existence of thin, dynamically fragile galactic 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 onto initially-thin, fully-formed disk galaxies. In contrast to what can be inferred from statistics of the z=0 surviving substructure, I show that accretions of massive subhalos onto the central regions of host halos, where the galactic disks reside, since z ~ 1 should be common. I demonstrate that these accretion events severely perturb the galactic disks and produce a wealth of distinctive morphological and dynamical signatures including: (1) long-lived, low-surface brightness, ringlike features in the outskirts; (2) considerable thickening and heating at all radii; (3) prominent flaring; (4) long-lived, lopsidedness of typical amplitude omparable to that measured in observational samples of galactic disks; and (5) substantial tilting and warping. The final distribution of disk stars exhibits a complex vertical structure that is well described by a standard ''thin-thick'' disk decomposition. I compare one of the resulting ringlike features in the simulations to the Monoceros Ring stellar structure in the MW. The comparison shows quantitative agreement in spatial distribution and kinematics, suggesting that such observed complex stellar components may arise naturally as disk stars are excited by encounters with CDM substructure. These results suggest that satellite-disk interactions of the kind expected in LCDM models play a significant role in setting the structure of disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys and strometric 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.