MultiDark simulations: the story of dark matter halo concentrations and density profiles (1411.4001v2)

Abstract: Accurately predicting structural properties of dark matter halos is one of the fundamental goals of modern cosmology. We use the new suite of MultiDark cosmological simulations to study the evolution of dark matter halo density profiles, concentrations, and velocity anisotropies. The MultiDark simulations cover a large range of masses 1e10-1e15Msun and volumes upto 50Gpc**3. The total number of dark matter halos in all the simulations exceeds 60 billion. We find that in order to understand the structure of dark matter halos and to make ~1% accurate predictions for density profiles, one needs to realize that halo concentration is more complex than the traditional ratio of the virial radius to the core radius in the NFW profile. For massive halos the averge density profile is far from the NFW shape and the concentration is defined by both the core radius and the shape parameter alpha in the Einasto approximation. Combining results from different redshifts, masses and cosmologies, we show that halos progress through three stages of evolution. (1) They start as rare density peaks that experience very fast and nearly radial infall. This radial infall brings mass closer to the center producing a high concentrated halo. Here, the halo concentration increases with the increasing halo mass and the concentration is defined by the alpha parameter with nearly constant core radius. Later halos slide into (2) the plateau regime where the accretion becomes less radial, but frequent mergers still affect even the central region. Now the concentration does not depend on halo mass. (3) Once the rate of accretion slows down, halos move into the domain of declining concentration-mass relation because new accretion piles up mass close to the virial radius while the core radius is staying constant. We provide accurate analytical fits to the numerical results for halo density profiles and concentrations.

• The paper demonstrates that dark matter halo concentrations require the Einasto profile with a shape parameter, moving beyond the traditional NFW model.
• The paper employs extensive MultiDark simulations with ~3840³ particles to classify halos by mass and redshift, revealing distinct evolutionary stages.
• The paper finds that massive halos may show an upturn in concentration, challenging prior models and suggesting new research directions in cosmology.

An Analysis of Dark Matter Halo Characteristics in MultiDark Simulations

The paper examines the structural properties of dark matter halos, utilizing data from the expansive MultiDark suite of cosmological simulations. The paper specifically addresses the evolution of dark matter halo density profiles, concentrations, and velocity anisotropies. These elements are analyzed to produce 1–2% accurate predictions for halo density profiles, highlighting that halo concentration cannot be merely captured by simple ratios as described by the Navarro-Frenk-White (NFW) profile. Instead, massive halos exhibit configurations where the average density profile diverges from the NFW shape, necessitating inclusion of the shape parameter $\alpha$ within the Einasto framework to accurately define concentration.

Methodology and Halo Concentration Dynamics

The discussion unfolds around the evolving understanding of halo concentrations through three distinct stages: rare density peaks with fast infall leading to highly concentrated halos; a plateau stage where accretion becomes less radial yet is still influenced by mergers; and, finally, a slowing accretion rate stage where concentration shifts in relation to the concentration-mass graph. The paper provides analytical fits for these developmental regimes, enhancing predictive capacity regarding halo structures.

Across the paper, intricate methodologies are conveyed, including the classification of halos by mass and redshift, and their alignment with density peaks in the linear density perturbation field. The document leverages extensive datasets, characterized by simulations containing approximately $3840^3$ particles over varying cosmological model frameworks with identical Gaussian fluctuations used to mitigate cosmic variance – a critical consideration when simulating large volumes.

Results and Significance

The findings presented offer insight into the dependencies of halo concentrations on mass, redshift, and cosmological parameters. Earlier studies had identified that halo concentration declines with increasing mass; however, MultiDark simulations unveil a possible increase in concentration for the highest-mass halos. A significant issue the authors discuss is understanding whether this upturn is an artifact or an inherent trait of massive halos. Challenges in analytical approximations like distinguishing between Einasto's and NFW profile fits are underscored, as the implications on how concentration is reported have notable impacts on interpretations of simulated data.

Implications and Future Research Trajectories

The implications of this research are twofold: practically, it aids in more precise estimations of halo structural properties, which is essential given the precision demands of contemporary and future galaxy surveys; theoretically, it frames new questions on the underlying processes contributing to halo formation characteristics and their evolutions. Moreover, these results suggest future avenues in AI and astrophysics, aiming to refine simulation methodologies and improve cosmological models to better account for the observed phenomena of dark matter.

The paper implicitly invites further inquiry into the complexities of halo dynamics, concentration-mass relations, and the role of cosmic variance across simulations. With upcoming advancements in computational power and new observational data, subsequent research can explore these findings, clarifying the landscape of dark matter halo characteristics within evolving cosmological frameworks.