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.
