Velocity fields and turbulence from cosmic filaments to galaxy clusters

TL;DR

The paper analyzes velocity fields and turbulence along Virgo-connected cosmic filaments using a constrained, high-resolution Virgo replica simulation. By extracting longitudinal and transverse cuts and applying a Hodge-Helmholtz decomposition, the authors quantify how compressive and solenoidal motions evolve from filaments into the cluster core via 2D velocity power spectra. They find a transition from compressive-dominated flows in the filaments to predominantly solenoidal turbulence in the Virgo ICM, with amplitude and spectral slopes indicating stronger energy injection at the virial boundary and a Burgers-like regime near spine regions and a Kolmogorov-like state in the core. These results enhance understanding of turbulence generation in the cosmic web, with implications for ICM physics and potential observational diagnostics in X-ray and 21 cm surveys.

Abstract

Galaxy clusters are currently the endpoint of the hierarchical structure formation; they form via the accretion of dark matter and cosmic gas from their local environment. In particular, filaments contribute grandly by accreting gas from cosmic matter sheets and underdense regions and feeding it to the galaxy clusters. Along the way, the gas in filaments is shocked and heated, which, together with the velocity structure within the filament, induces swirling and, thus, turbulence. In this work, we study a constrained hydrodynamical simulation replica of the Virgo cluster to characterise the velocity field in the two cosmic filaments connected to the cluster with unprecedented high resolution. First, we conduct a qualitative examination of slices extracted from the simulation. We study the temperature, the velocity field, and derived quantities in longitudinal cuts to study the general structure of the filaments and in transverse cuts to study their inner organisation and connection to cosmic matter sheets and underdense regions. Then, we conduct a quantitative study of velocities in Virgo's filaments by computing the 2D energy spectrum from 1 and 5~Mpc square maps extracted from the slices and centred on the core of the filaments. We show that the velocity field goes from mostly compressive far in the filaments to mostly solenoidal in Virgo's core. Moreover, we observe that the total energy spectrum in the filaments gains in amplitude and steepens towards Virgo.

Figures (17)

• Figure 1: From left to right, longitudinal cuts of temperature, electron density, Mach number and velocity in Virgo replica and the two filaments to which it is connected. The maps are about 22 Mpc wide, contain $\mathrm{15728^2}$ pixels, and are centred on Virgo's core. On the third panel, Mach number, colour bands are used to display the large range of values: the black-to-white is on a linear scale in the [0.1,1] range, and the white-to-blue is on a log scale in the [1,100] range. On the right panel, the background map is the velocity along the x-axis of the simulation box, $v_x$, and the foreground arrows represent the norm of the velocity field in the x-y plane, $||\vec{v}||$. The background (foreground) filament is on the left (right) part of each panel. The buoyant bubble is located roughly at a position $(x=-5.0; y=0.5)$ Mpc. Multiple galaxies are visible on the density map, for example, at positions $(-2.5; -0.5)$ and $(-6.0; 1.0)$ Mpc.
• Figure 2: Zoom on the background filament in the longitudinal cut. The maps are $11.061\times12$ Mpc large with the same resolution as those presented in Fig. \ref{['long_cut']}. The left panel shows the velocity field, similar to the right panel of Fig. \ref{['long_cut']}, the central panel shows the divergence, and the right panel shows the z component of the vorticity. Virgo's $R_{500}$ and virial radius, $R_{\mathrm{vir}}$, are shown as black circle arcs on each map. The merger-accelerated accretion shock is located roughly in the range x=[-5,-2] Mpc and y=[-5,-1] Mpc.
• Figure 3: Zoom on the foreground filament in the longitudinal cut. The maps are $11.061\times12$ Mpc large with the same resolution as those presented in Fig. \ref{['long_cut']}. The left panel shows the velocity field, similar to the right panel of Fig. \ref{['long_cut']}, the central panel shows the divergence, and the right panel shows the z component of the vorticity. Virgo's $R_{500}$ and virial radius, $R_{\mathrm{vir}}$, are shown as black circle arcs on each map.
• Figure 4: Transverse cuts along the background filament. From left to right, we present cuts at distances of 8, 6, 4 and 2 Mpc from Virgo's centre. From top to bottom, we present the temperature, the velocity field with the background map being the velocity along the $z$ simulation box axis, $v_z$, and the foreground arrows representing the norm of the velocity field in the $zy$ plane, $||\vec{v}||$, the divergence of the velocity field and the $x$ component of its vorticity. Similarly to longitudinal cuts, the maps are 22.122 Mpc wide, contain $\mathrm{15728^2}$ pixels, and are centred on Virgo's centre.
• Figure 5: Transverse cuts along the foreground filament. From left to right, we present cuts at distances of 2, 4 and 6 Mpc from Virgo's centre. From top to bottom, we present the temperature, the velocity field with the background map being the velocity along the $z$ simulation box axis, $v_z$, and the foreground arrows representing the norm of the velocity field in the $zy$ plane, $||\vec{v}||$, the divergence of the velocity field and the $x$ component of its vorticity. Similarly to longitudinal cuts, the maps are 22.122 Mpc wide, contain $\mathrm{15728^2}$ pixels, and are centred on Virgo's centre.