Clumps and streams in the local dark matter distribution (0805.1244v2)

Abstract: In cold dark matter cosmological models, structures form and grow by merging of smaller units. Numerical simulations have shown that such merging is incomplete; the inner cores of halos survive and orbit as "subhalos" within their hosts. Here we report a simulation that resolves such substructure even in the very inner regions of the Galactic halo. We find hundreds of very concentrated dark matter clumps surviving near the solar circle, as well as numerous cold streams. The simulation reveals the fractal nature of dark matter clustering: Isolated halos and subhalos contain the same relative amount of substructure and both have cuspy inner density profiles. The inner mass and phase-space densities of subhalos match those of recently discovered faint, dark matter-dominated dwarf satellite galaxies and the overall amount of substructure can explain the anomalous flux ratios seen in strong gravitational lenses. Subhalos boost gamma-ray production from dark matter annihilation, by factors of 4-15, relative to smooth galactic models. Local cosmic ray production is also enhanced, typically by a factor 1.4, but by more than a factor of ten in one percent of locations lying sufficiently close to a large subhalo. These estimates assume that gravitational effects of baryons on dark matter substructure are small.

• The paper demonstrates that the Via Lactea II simulation resolves over 40,000 dark matter subhalos with dense, cuspy profiles.
• The paper reveals that tidal disruptions generate coherent phase-space streams that boost gamma-ray annihilation signals by factors of 4–15.
• The paper suggests that the observed substructure may account for gravitational lensing flux anomalies within galactic halos.

Insights into Dark Matter Halo Substructure from the "Via Lactea II" Simulation

The paper presented by Diemand et al. provides a comprehensive analysis of dark matter within the framework of the cold dark matter (CDM) cosmological model. The paper centers on the intricate substructure of dark matter halos, as elucidated through the Via Lactea II simulation, the most detailed computational endeavor of its kind targeting the Galactic CDM halo. This work embodies a significant stride in understanding the hierarchical assembly and local distribution of dark matter.

Overview of Simulation and Methodology

The Via Lactea II simulation utilizes a high precision technique to explore the formation of a Milky Way-sized dark matter halo in a $\Lambda$CDM universe. The simulation's high fidelity is underscored by its deployment of 1.1 billion particles, each with a mass resolution of 4,100 $M_\odot$. By resolving substructures deep within the Galactic halo, it extends our comprehension of dark matter's fractal-like clustering.

A pivotal aspect of this simulation includes the use of the PKDGRAV2 code which, along with detailed cosmological parameters from WMAP data, provides unparalleled insight into the local abundance of subhalos. The simulation progresses from a redshift of 104.3 to present, thus offering a robust platform to investigate small-scale dark matter substructures that influence galactic halos.

Key Findings

The Via Lactea II simulation yields several critical insights:

1. Subhalo and Stream Identification: The simulation resolves over 40,000 subhalos within a radius of 402 kpc from the galactic center. These subhalos share a self-similar distribution with isolated halos, characterized by dense cusps and steep inner density profiles, aligning with observations of ultra-faint dwarf galaxies.
2. Phase-space Structures: In addition to subhalos, the simulation detects coherent streams, which are elongated features in phase space resulting from the tidal disruption of larger halos. These structures are significant in context of direct dark matter detection experiments.
3. Annihilation Signal Enhancement: Remarkably, the detailed substructure boosts the gamma-ray production from dark matter annihilation by factors of 4-15. This enhancement is primarily due to the high local densities within subhalos. The model also indicates a significant, albeit smaller, local cosmic ray production.
4. Density Profile Analysis: The paper reveals that CDM subhalos have inwardly dense cores, demonstrated through precise integration of subhalo density profiles. These profiles are consistent with a composite cuspy structure, countering suggestions for large core profiles.
5. Implications for Observations: The observed subhalo populations might explain anomalies in gravitational lensing phenomena. The projected substructure within 10 kpc is sufficient to account for lensing flux anomalies, suggesting a correspondence with observational data.

Implications and Future Directions

This research provides vital constraints on the nature of dark matter substructure and its small-scale interactions, reinforcing the validity of the CDM model. By advocating a self-similar pattern of clustering, the paper suggests continued exploration into deeper, unresolved scales of dark matter could yield further insights. The implications of such findings are vast, with potential impacts on indirect dark matter detection methods, astronomical observations of gamma-rays, and cosmic ray investigations.

In future research, continued simulation efforts at higher resolutions could explore even finer sub-hierarchies, and the inclusion of baryonic processes could clarify their influence on dark matter profiles. Moreover, further theoretical work might address the impact of different dark matter particle candidates on substructure evolution and annihilation signals. The complexities of gravitational interactions within subhalos and the interplay between these structures and luminous matter remain significant areas for investigation, promising further revelations in cosmic structure formation.