Cold Dark Matter Substructure and Galactic Disks. I. Morphological Signatures of Hierarchical Satellite Accretion

We conduct dissipationless N-body simulations to investigate the cumulative effect of substructure impacts onto thin disk galaxies in the context of the ΛCDM paradigm. Our simulation campaign is based on a hybrid approach combining cosmological simulations and controlled numerical experiments. Substructure properties are culled from cosmological simulations of galaxy-sized CDM halos. We demonstrate that accretions of massive subhalos onto the central regions of host halos, where the galactic disk resides, since z ~ 1 should be common occurrences. In contrast, extremely few satellites in present-day CDM halos are likely to have a significant impact on the disk structure. One host halo merger history is subsequently used to seed controlled N-body experiments of repeated satellite encounters with an initially thin Milky-Way (MW) type disk galaxy. These simulations track the effects of six dark matter substructures, with initial masses in the range ~(0.7-2) × 10¹⁰ M_⊙ (~20%-60% of the disk mass), crossing the disk in the past ~8 Gyr. We demonstrate that these accretion events produce several distinctive morphological signatures in the disk, including long-lived, low surface brightness, ringlike features in the outskirts; significant flares; bars; and faint filamentary structures above the disk plane. The final distribution of disk stars exhibits a complex vertical structure that is well described by a standard "thin-thick" disk decomposition. We compare one of the resulting ringlike features in our 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. We conclude that satellite-disk interactions of the kind expected in ΛCDM models can induce morphological features in galactic disks that are similar to those being discovered in the Milky Way, M31, and other disk galaxies. 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.