UNIT project: Universe N-body simulations for the Investigation of Theoretical models from galaxy surveys

Abstract

We present the UNIT N-body cosmological simulations project, designed to provide precise predictions for nonlinear statistics of the galaxy distribution. We focus on characterizing statistics relevant to emission line and luminous red galaxies in the current and upcoming generation of galaxy surveys. We use a suite of precise particle mesh simulations (FastPM) as well as with full N-body calculations with a mass resolution of ∼ 1.2×10 9 h −1 M to investigate the recently suggested technique of Angulo & Pontzen 2016 to suppress the variance of cosmological simulations We study redshift space distortions, cosmic voids, higher order statistics from z = 2 down to z = 0. We find that both two-and three-point statistics are unbiased. Over the scales of interest for baryon acoustic oscillations and redshift-space distortions, we find that the variance is greatly reduced in the two-point statistics and in the cross correlation between halos and cosmic voids, but is not reduced significantly for the three-point statistics. We demonstrate that the accuracy of the two-point correlation function for a galaxy survey with effective volume of 20 (h −1 Gpc) 3 is improved by about a factor of 40, indicating that two pairs of simulations with a volume of 1 (h −1 Gpc) 3 lead to the equivalent variance of ∼150 such simulations. The N-body simulations presented here thus provide an effective survey volume of about seven times the effective survey volume of DESI or Euclid. The data from this project, including dark matter fields, halo catalogues, and their clustering statistics, are publicly available: http://www.unitsims.org.

The cosmology dependence of galaxy clustering and lensing from a hybrid N-body–perturbation theory model

“…These ratios are thus consistent with unity at small wavenumbers, and while they deviate significantly from unity at high k they have significantly less dynamic range than the basis spectra. In order to de-noise these ratios, we apply a Savitsky-Golay (Savitzky & Golay 1964) filter of order three using an 11-point window in k. Doing so dramatically reduces the amount of noise in the spectra, and is a simple alternative to reduce noise at high k, where techniques such as fixed amplitude, paired phase simulations do little to reduce variance (Angulo & Pontzen 2016;Villaescusa-Navarro et al 2018;Chuang et al 2019). As a final preprocessing step, we also take the base-10 logarithm of these smoothed ratios in order to further decrease the dynamic range.…”

Linear bias and halo occupation distribution of emission-line galaxies from Nancy Grace Roman Space Telescope

“…In particular, we use the Galacticus SAM to perform a large-scale galaxy simulation. The model processes all the dark matter merger trees distributed within the 1 h −1 Gpc box of the N-body simulation UNIT (Chuang et al 2019). We then construct a light-cone catalogue using the method in Kitzbichler & White (2007).…”

“…The key ingredient for SAMs-like Galacticus is the set of merger trees of dark matter haloes, which can be approximately constructed using the Press-Schechter formalism (Press & Schechter 1974), or come from a cosmological N-body simulation. Here we have chosen the later for high fidelity, and use the merger trees extracted from the UNIT simulation 1 (Chuang et al 2019) that assumes a spatially flat CDM model with parameters consistent with Planck 2016 measurement (Planck Collaboration XIII 2016). The simulation contains 4096 3 particles with a box-size of 1 h −1 Gpc.…”

“…The large volume and high resolution make this simulation sufficient for the next generation galaxy surveys, including the Dark Energy Spectroscopic Instrument (DESI), and those planned for Roman and Euclid. We refer the readers to Chuang et al (2019) for more details of the UNIT simulation and its data products. The merger trees of the dark matter haloes are constructed using the CONSISTENT TREES software (Behroozi et al 2013) and the final product contains more than 160 million merger trees.…”

Full-sky photon simulation of clusters and active galactic nuclei in the soft X-rays for eROSITA

“…All simulations are consistently post-processed with the rockstar software (Behroozi et al 2013) to obtain haloes and sub-haloes. The halo mass function is correct to a few percent down to halo masses of M vir ∼ 10 10 M for SMDPL, M vir ∼ 10 11 M for MDPL2, UNIT1, UNIT1i and M vir ∼ 10 13 M for HMDPL, see details in Comparat et al (2017); Chuang et al (2019).…”