Higher-order initial conditions for mixed baryon-CDM simulations

Abstract

We present a novel approach to generate higher-order initial conditions (ICs) for cosmological simulations that take into account the distinct evolution of baryons and dark matter. We focus on the numerical implementation and the validation of its performance, based on both collisionless N-body simulations and full hydrodynamic Eulerian and Lagrangian simulations. We improve in various ways over previous approaches that were limited to first-order Lagrangian perturbation theory (LPT). Specifically, we (1) generalize nth-order LPT to multi-fluid systems, allowing 2LPT or 3LPT ICs for two-fluid simulations, (2) employ a novel propagator perturbation theory to set up ICs for Eulerian codes that are fully consistent with 1LPT or 2LPT, (3) demonstrate that our ICs resolve previous problems of two-fluid simulations by using variations in particle masses that eliminate spurious deviations from expected perturbative results, (4) show that the improvements achieved by going to higher-order PT are comparable to those seen for single-fluid ICs, and (5) demonstrate the excellent (i.e., few per cent level) agreement between Eulerian and Lagrangian simulations, once high-quality initial conditions are used. The rigorous development of the underlying perturbation theory is presented in a companion paper (Rampf et al. 2020). All presented algorithms are implemented in the Monofonic Music-2 package that we make publicly available.

Quantifying the impact of baryon-CDM perturbations on halo clustering and baryon fraction

“…We now turn to a detailed description of our set of simulations. As was discussed in [16][17][18], obtaining the correct evolution of each species in 2-fluid simulations, even at linear order only, is already a nontrivial task which is why we thoroughly present a number of tests to validate our setup in this section. For the sake of shortness we present less important tests results in appendix B.…”

“…This assumption is however not exactly true, and recently substantial effort was put in correctly describing and simulating the evolution of the two fluids across cosmic history. In this process, so-called 2-fluid simulations where the baryons and CDM fluid are initialized with different transfer functions have become an essential tool [14][15][16][17][18][19][20]. In this…”

Simulating intergalactic gas for DESI-like small scale Lymanα forest observations

“…codes consistently is a non-trivial problem with accurate solutions only having been developed very recently (e.g. [54,55]). We therefore leave more careful analyses of the impact of initial conditions to future work.…”

“…for feedback, inhomogeneous reionization) will have to be taken into account. There are also recent developments regarding Baryonic ICs that we have not studied here [54,55].…”

“…https://www.camb.info 2 https://cosmo.nyu.edu/roman/2LPT/ 3 note that the approach used for this task within 2lpt-ic is not ideal and will lead to unphysical changes in the growth of Baryons and dark matter at later times. Possible solutions to this issue have been developed by[54] and[55], but have not yet been used for this work.…”