The dynamical and thermodynamic effects of turbulence on the cosmic baryonic fluid

TL;DR

This paper tackles the missing baryons problem by analyzing turbulence in the cosmic baryonic fluid using IllustrisTNG simulations. It employs a multi-scale, wavelet-based decomposition to separate turbulent from bulk motions and quantifies dynamical and thermodynamic contributions via $Q$ terms and energy densities, across environments defined by dark-matter density. The main results show turbulent heating dominates over shocks in both low to high-density regions, driving gas detention in under- and intermediate-density zones and converting more gas into the WHIM as redshift decreases, with a quasi-steady balance between turbulent energy injection and dissipation from $z\sim 1$ to $z\sim 0$. The findings are robust across multiple simulations (TNG50-1, TNG50-2, TNG100-1, WIGEON, EAGLE), challenging the traditional shock-heating paradigm and highlighting turbulence as a key driver of cosmic structure formation and baryon distribution.

Abstract

Both simulations and observations indicate that the so-called missing baryons reside in the intergalactic medium known as the warm-hot intergalactic medium (WHIM). In this study we employed the IllustrisTNG50-1 simulation to demonstrate that knowledge of the turbulence in the cosmic baryonic fluid is crucial for correctly understanding both the spatial distribution and the physical origins of the missing baryons in the Universe. First, we find that dynamical effects cause the gas to be detained in low-density and intermediate-density regions, resulting in high baryon fractions, and prevent the convergence of the gas in high-density regions, leading to low baryon fractions. Second, turbulent energy is converted into thermal energy, and the injection and dissipation of turbulent energy have essentially reached a balance from $z=1$ to $0$. This indicates that the cosmic fluid is in a steady state within this redshift range. Due to turbulent heating, as the redshift decreases, an increasing amount of warm gas is heated and converted into the WHIM, and some even into hot gas. We find that, compared with turbulence in the cosmic fluid, shocks are unimportant in intermediate-density regions and even negligible in high-density regions, both dynamically and thermodynamically. This finding accounts for the origin of the WHIM in terms of both dynamics and thermodynamics, calls into question the traditional view of shock-heating, and highlights the importance of turbulence in shaping the large-scale structure of the Universe, particularly in the evolution of galaxies and galaxy clusters. In addition to TNG50-1, we validated our key findings with TNG50-2, TNG100-1, WIGEON, and EAGLE simulations, demonstrating that the spatial resolution, box size, and sub-grid-physics variations do not affect our main conclusions.

Figures (13)

• Figure 1:
• Figure 2: Mean dynamical effects as a function of dark matter density. The mean dynamical effects of turbulence and thermal motion in the cosmic fluid, as well as the gravitational effect, the cosmic expansion effect, and their combined effects, are represented by $Q_{\rm turb}$, $Q_{\rm th}$, $Q_{\rm grav}$, $Q_{\rm exp}$, and $Q_{\rm tot}$, respectively. Panels (a) to (f) are for $z = 4$ to $0$, respectively. We chose the units of velocity, $\nabla$, and time as ${\rm km}/s$, $h/{\rm Mpc}$, and $({\rm Mpc}/h/{\rm km})s$, respectively, which results in the vertical label shown in the figure.
• Figure 3: Mean energy densities and energy ratios as a function of dark matter density. The dimensionless baryonic density $\Delta_{\rm b}\equiv\rho_{\rm b}({\bf x})/\bar{\rho_{\rm b}}$ is used to compute the energy density. Panels (a) and (b) show the mean energy densities of thermal and turbulent energy at different redshifts, respectively. Panel (c) shows the energy ratios of turbulence to thermal energy. The lines represent the mean values.
• Figure 4: $z$-evolution of environment-dependent wavelet energy spectra of turbulence. Panels (a), (b), and (c) show the energy spectra of three intervals of dark matter density ($\Delta_{\rm dm}$), as marked. The energy spectra $E_{\rm turb}(k, \delta)$ are obtained from the env-WPS in Eq. (\ref{['eq:env-wps']}) as $E_{\rm turb}(k, \delta) = k^2 S_{\rm turb}(k, \delta)$. The relevant power-law fits for the scale ranges that are smaller and larger than the peak positions ($k_{\rm peak}$) are indicated.
• Figure 5: Panel (a): Averaged dynamical effects of $z=0$ contributed by turbulence and shocks. Panel (b): Averaged thermodynamic effects of $z=0$ contributed by turbulence and shocks. The shock-finding algorithm of Schaal2015 and Schaal2016 was used to identify shock zones, with a minimum Mach number ($\mathcal{M}_{\rm min}$) of $1.0$.