Cosmological simulation of radio synchrotron bridge between pre-merging galaxy clusters

Abstract

Radio bridges are diffuse synchrotron emission observed between merging galaxy clusters. Recent radio observations have reported both detections and non-detections of radio bridges between clusters. The detections imply the presence of cosmic rays (CRs) and magnetic fields permeating the cosmic web that produce synchrotron emission observable with current facilities, whereas the non-detections suggest that specific physical conditions are required for their formation. We study the CR reacceleration by solenoidal turbulence in the filament connecting two massive clusters at an early stage of the merger. Our aim is to test whether this mechanism can generate diffuse emission in the inter-cluster region. We perform a cosmological magneto-hydrodynamical (MHD) simulation using the Enzo code. We improved a run-time Lagrangian tracer method implemented in Enzo, and follow the trajectories of baryonic matter using $N=\mathcal{O}(10^7)$ tracer particles. In post-processing, we conduct a parallel computation of the Fokker-Planck (FP) equation for all tracers, with cooling and reacceleration efficiencies evaluated from the local quantities recorded along each tracer trajectory. Our simulation generate a Mpc-sized radio bridge in the early stage of the cluster merger. Within a reasonable parameter range, the reacceleration model produces a broad variety of spectra. In our fiducial model, the simulated bridge matches several properties of the one found between Abell 399 and Abell 401, such as its spectral shape, intensity profile, and pixel-by-pixel correlation between radio and X-ray intensities. The inter-cluster region is filled with turbulence induced by infalling mass clumps and subsequently amplified by the approaching motion of the clusters. The CR reacceleration by the turbulence is a viable mechanism to power a Mpc-sized synchrotron emission observed as radio bridges.

Figures (15)

• Figure 1: Evolution of the projected gas density map along Z axis in our Enzo simulation. The maps of zoomed-in region centered on the inter-cluster filament are shown. The white square shows the (1.2 Mpc)$^3$ box region where a radio bridge forms (Sect. \ref{['sec:bridge']}).
• Figure 2: Time evolution of magnetic field (left panel) and timescale of reacceleration (right panel) from $z=0.3$ to 0.1 along the trajectories of $N=5.1\times10^5$ tracer particles that end up in the bridge region (white box in Fig. \ref{['fig:Enzo_map_z']}). In both panels, the solid line and the shaded region show the median value and 1$\sigma$ range of the distribution. In the left panel, the red and blue lines show the dynamo magnetic field calculated with $\eta_B = 0.05$ and the original magnetic field in the Enzo simulation, respectively. The dashed and dotted lines show the median values for different parameters.
• Figure 3: An example of CRE spectrum in a tracer simulated with our FP code. Shown is the evolution from $z =1$ to $z = 0.02$ (from blue to red).
• Figure 4: Evolution of the radio intensity map at 140 MHz. The white contour shows the projected gas density at logarithmically spaced eight levels between $3\times10^{12}$$M_\odot$/Mpc$^2$ and $2\times10^{14}$$M_\odot$/Mpc$^2$. The mesh size of the radio map is 16 (comoving) kpc.
• Figure 5: From top to bottom, we show time evolution of the X-ray peak distance between the clusters, baryon mass, solenoidal turbulent energy flux, and the 140 MHz radio power, and the typical equilibrium energy of CREs. The latter four quantities are calculated for the (1.2 Mpc)$^3$ box region extracted from the inter-cluster filament (Fig. \ref{['fig:Enzo_map_z']}).