Baryonification: An alternative to hydrodynamical simulations for cosmological studies

TL;DR

This work advances the baryonification approach by introducing a component-wise BFC model that splits each N-body particle into dark-matter and baryonic parts, displaces them independently, and assigns gas pressure via hydrostatic equilibrium and the ideal gas law. Calibrated against FLAMINGO and TNG hydrodynamical simulations, the model reproduces gas, stellar, and dark matter density profiles and achieves power-spectrum suppressions accurate to about $2\%$ up to $k\approx 5\,h\mathrm{Mpc}^{-1}$ across redshifts, with a compact 2+1 parameter version capturing redshift evolution. The authors also present pressure profiles in good agreement with simulations and perform a map-level comparison that visually matches dark matter, gas, stellar, and Compton-$y$ fields, supporting multi-probe cosmology at the field level. This framework offers a fast, flexible alternative to full hydrodynamics for ongoing and future Stage-IV surveys, enabling efficient multi-probe analyses and cosmological inference using three-dimensional density and pressure fields. Future work will quantify observables and extend field-level validations for robust cosmological applications.

Abstract

We present an improved baryonification (BFC) model that modifies dark-matter-only $N$-body simulations to generate particle-level outputs for gas, dark matter, and stars. Unlike previous implementations, our approach first splits each simulation particle into separate dark matter and baryonic components, which are then displaced individually using the BFC technique. By applying the hydrostatic and ideal gas equations, we assign pressure and temperature values to individual gas particles. The model is validated against hydrodynamical simulations from the FLAMINGO and TNG suites (which feature varied feedback prescriptions) showing good agreement at the level of density and pressure profiles across a wide range of halo masses. As a further step, we calibrate the BFC model parameters to gas and stellar mass ratio profiles from the hydrodynamical simulations. Based on these calibrations, we baryonify $N$-body simulations and compare the resulting total matter power spectrum suppressions to the ones from the same hydrodynamical simulation. Carrying out this test of the BFC method at each redshift individually, we obtain a 2 percent agreement up to $k=5\,h$/Mpc across all tested feedback scenarios. We also define a reduced, 2+1 parameter BFC model that simultaneously accounts for feedback variations (2 parameters) and redshift evolution (1 parameter). The 2+1 parameter model agrees with the hydrodynamical simulations to better than 2.5 percent over the scales and redshifts relevant for cosmological surveys. Finally, we present a map-level comparison between a baryonified $N$-body simulation and a full hydrodynamical run from the TNG simulation suite. Visual inspection of dark matter, gas, and stellar density fields, along with the integrated pressure map, shows promising agreement. Further work is needed to quantify the accuracy at the level of observables.

Figures (18)

• Figure 1: Baryonified density fields of the gas (left), dark matter (centre), and stellar components (right). Here we show a randomly selected sub-box of $L=24$ Mpc/$h$ from a periodic simulation box of $L=128$ Mpc/$h$ with $N=512^3$ particles.
• Figure 2: Truncated NFW profile with two-halo term from the BFC model (solid line) compared to the mean profiles (dotted line) and 68 % scatter (grey band) from the DMO only counterparts of the FLAMINGO m8 (top panels) and TNG-300 (bottom panels) simulations.
• Figure 3: Gas profiles from the BFC model (solid line) compared to measurements from hydrodynamical simulations (dashed line with 68 % scatter band). The top panels show results from FLAMINGO m8, while the bottom panels are from the TNG-300 simulations. The BFC model parameters are obtained via a simultaneous fit to the gas and the stellar profiles for each simulation.
• Figure 4: Stellar profiles from the BFC model (solid line) compared to measurements from hydrodynamical simulations (dashed line with 68% scatter band). The top panels show results from the FLAMINGO m8 run, while the bottom panels are from the Illustris TNG-300 simulation. The BFC model parameters are selected via a simultaneous fit to the gas and the stellar profiles for each simulation.
• Figure 5: Dark matter (DM) profiles from the BFC model (solid line) compared to measurements from hydrodynamical simulations (dashed line with 68 percent scatter band). The top panels show results from the FLAMINGO m8 run, while the bottom panels are from the Illustris TNG-300 simulation. The BFC model parameters are selected via a simultaneous fit to the gas and the stellar profiles for each simulation.