Merging clusters of galaxies, which are objects emerging during the Large-scale structure formation process, converts large amounts of gravitational energy into gas heating and even non-thermal energy such as turbulence, particle acceleration and magnetic field amplification which emits synchrotron radiation. However, how much of this energy is distributed to each component is not well known. In order to
estimate the energy input and compare with energy outputs, we need to understand the structure of galaxy clusters at various merger stages. An early phase merging cluster, CIZA J1358.9 -4750 (hereafter CIZA J1359) is located very close (z=0.07) and is a major merger with a mass ratio of almost one. The southeast and northwest clusters are separated by ~1.4 Mpc in the sky, and Suzaku observations showed that
there is a candidate shock wave of Mach 1.4 in the middle of the bridging structure between the two clusters (Kato+ 2016). The post-shock region is also one of the brightest post-shock regions known to date. In this study, we attempted to clarify
the 3D structure of CIZA 1359 to determine how much amount of kinetic energy is distributed.
We made a 2D temperature map of CIZA J1359 by dividing it into small regions. While we confirmed that the two clusters are relatively undistorted except for the bridge region, we found that the high-temperature region in the center of the bridge extends over a width of 700 kpc. In other words, the shock identified in the Suzaku data is the southeast end of the hot region, and there is also another shock wave candidate to the northwest. The pseudo-density map shows a low-density region in between the two shocks. This structure is sometimes called "channel", and has been reported in several clusters of galaxies, including A85 (Ichinohe+ 2015). We calculated the Mach numbers from the temperature and density in these two shock fronts, but the two values were inconsistent.
To tackle this problem, we made a toy-model geometry assuming that the hot ("shocked") and cold ("un-shocked") components overlap in the line of sight, and constructed a simple 3D model to determine the depth in line of sight of the hot region. Here, cold region was modeled by extrapolating the undisturbed cluster models of the two clusters. We also tied the kT-density relation using Rankine-Hugoniot equation. As a result, we found a 3D model which can consistently reproduce the 2D temperature map and the shock condition. The post shock region thus estimated near the shock waves has a depth of ~1 Mpc. The post shock temperature is estimated to be 7.0 keV and a density as 8.0×10-4 cm-3.
Using this post-shock condition and assuming it is roughly uniform, the shock waves' age was calculated to be 360 Myr, and the total kinetic energy to be 3.9×1060 erg. The y-parameter calculated from this 3D model is consistent with the y-parameter observed in Planck within the temperature fluctuations of CMB radiation, although its limited angular resolution and fluctuation does not allow further investigation. We expect that detectors with high energy resolution will solve various problems such as turbulence energy estimation.
