The Uchuu Simulations: Data Release 1 and Dark Matter Halo Concentrations

Abstract: We introduce the Uchuu suite of large high-resolution cosmological $N$-body simulations. The largest simulation, named Uchuu, consists of 2.1 trillion ($12800^3$) dark matter particles in a box of side-length 2.0 Gpc/h, with particle mass $3.27 \times 10^{8}$ Msun/h. The highest resolution simulation, Shin-Uchuu, consists of 262 billion ($6400^3$) particles in a box of side-length 140 Mpc/h, with particle mass $8.97 \times 10^{5}$ Msun/h. Combining these simulations we can follow the evolution of dark matter halos and subhalos spanning those hosting dwarf galaxies to massive galaxy clusters across an unprecedented volume. In this first paper, we present basic statistics, dark matter power spectra, and the halo and subhalo mass functions, which demonstrate the wide dynamic range and superb statistics of the Uchuu suite. From an analysis of the evolution of the power spectra we conclude that our simulations remain accurate from the Baryon Acoustic Oscillation scale down to the very small. We also provide parameters of a mass-concentration model, which describes the evolution of halo concentration and reproduces our simulation data to within 5 per cent for halos with masses spanning nearly eight orders of magnitude at redshift 0<z\<14. There is an upturn in the mass-concentration relation for the population of all halos and of relaxed halos at z\>0.5, whereas no upturn is detected at z<0.5. We make publicly available various $N$-body products as part of Uchuu Data Release 1 on the Skies & Universes site. Future releases will include gravitational lensing maps and mock galaxy, X-ray cluster, and active galactic nuclei catalogues.

Overview of the Uchuu Simulations: Data Release 1 and Insights into Dark Matter Halo Concentrations

The paper introduces the Uchuu suite of cosmological N-body simulations, focusing on analyzing dark matter halo concentrations and offering data products for public access. Uchuu comprises various simulations, with the largest involving 2.1 trillion dark matter particles in a 2.0-1 box, and the highest resolution simulation, Shin-Uchuu, containing 262 billion particles in a smaller 140-1 box. The research aims to track dark matter halos across volumes hosting dwarf galaxies to massive galaxy clusters, leveraging high-resolution simulations to improve theoretical models.

Key Aspects and Results

1. Power Spectrum Analysis:

   • The simulations accurately measure the power spectrum over an extensive range of wave numbers, demonstrating convergence between both Uchuu and Shin-Uchuu results in the overlapping region. The models predict the power change with a deviation of less than 5 percent up to certain scales, revealing Uchuu's precision in managing the Baryon Acoustic Oscillation (BAO) scale to small structures.

2. Halo Mass and Subhalo Mass Function:

   • The halo mass function spans nearly eight orders of magnitude in mass. For simulations like Uchuu, the accuracy of halo mass functions differs by 10 percent at extreme masses, especially at higher redshifts. The subhalo mass function also exhibits nuances in high-mass scenarios, suggesting softer declines in these relations.

3. Halo Mass-Concentration Model Calibration:

   • Simulations enabled the refinement of mass-concentration models, showing accurate 5 percent fitting errors across a vast mass scale and redshift range. An upturn in halo concentration for massive halos is observed above redshift 0.5, consistent with theoretical expectations of hierarchical growth models.

Implications

The release of data products such as power spectra and halo/subhalo catalogs aids other researchers by providing high-fidelity templates critical for understanding galaxy formation processes and tuning galaxy formation models. Notably, the simulations map halos precisely down to small scale structures, enabling detailed mock catalogs to assess observations from deep sky surveys.

Conclusion

The Uchuu simulations mark a significant advancement in simulating cosmological phenomena over large scales with high resolution. This facilitates enhanced predictive modeling of galaxy formation, integration into ongoing sky survey data, and serves as a basis for further exploration in gravitational lensing and active galactic nuclei cataloging. Future developments are expected to delve deeper into subhalo properties with implications for dark matter research and astrophysical simulations, promising valuable insights into the hierarchical nature of cosmic structure formation.