Abstract
Current experiments are providing measurements of the flux power spectrum from the Lyman-α forests observed in quasar spectra with unprecedented accuracy. Their interpretation in terms of cosmological constraints requires specific simulations of at least equivalent precision.In this paper, we present a suite of cosmological N-body simulations with cold dark matter and baryons, specifically aiming at modeling the low-density regions of the inter-galactic medium as probed by the Lyman-α forests at high redshift. The simulations were run using the GADGET-3 code and were designed to match the requirements imposed by the quality of the current SDSS-III/BOSS or forthcoming SDSS-IV/eBOSS data. They are made using either 2 × 7683 ≃ 1 billion or 2 × 1923 ≃ 14 million particles, spanning volumes ranging from (25 Mpc h−1)3 for high-resolution simulations to (100 Mpc h−1)3 for large-volume ones. Using a splicing technique, the resolution is further enhanced to reach the equivalent of simulations with 2 × 30723 ≃ 58 billion particles in a (100 Mpc h−1)3 box size, i.e. a mean mass per gas particle of 1.2 × 105M⊙ h−1. We show that the resulting power spectrum is accurate at the 2% level over the full range from a few Mpc to several tens of Mpc.We explore the effect on the one-dimensional transmitted-flux power spectrum of four cosmological parameters (ns, σ8, Ωm and H0) and two astrophysical parameters (T0 and γ) that are related to the heating rate of the intergalactic medium. By varying the input parameters around a central model chosen to be in agreement with the latest Planck results, we built a grid of simulations that allows the study of the impact on the flux power spectrum of these six relevant parameters.We improve upon previous studies by not only measuring the effect of each parameter individually, but also probing the impact of the simultaneous variation of each pair of parameters. We thus provide a full second-order expansion, including cross-terms, around our central model. We check the validity of the second-order expansionwith independent simulations obtained either with different cosmological parameters or different seeds. Finally, a comparison to the one-dimensional Lyman-α forest power spectrum obtained with BOSS by [1] shows an excellent agreement.
Highlights
Current experiments are providing measurements of the flux power spectrum from the Lyman-α forests observed in quasar spectra with unprecedented accuracy
To model of the physics of the Universe, we introduced two categories of parameters that are varied in the simulations: cosmological parameters that describe the cosmological model in the simplest case of ΛCDM assuming a flat Universe with mass-less neutrinos, and astrophysical parameters that model the astrophysics within the intergalactic medium (IGM) and the relation between temperature and density of the gas
We first verified that the power spectrum of independent simulations obtained either with different cosmological parameters or different seeds is consistent with the power spectrum derived from the Taylor expansion of Sec. 2
Summary
In order to derive confidence intervals on each parameter of a cosmological model with eight to ten free parameters, one would like to compute theoretical predictions for thousands of models, exploring most of the parameter space. Statistical frameworks have been studied to optimize the precision of the model for a reduced number of simulations, such as Latin hypercube sampling [46] While this method is superior to a random sampling of the parameters for instance as regards the attained precision [47], it still requires a large number of simulations. For cosmological predictions of the power spectrum in the Lyman-alpha regime where hydrodynamical simulations are required, the grid approach as presented in [50] is generally adopted (cf [51] for instance for a recent application). This is the method we have selected for this work
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