Discovery of a nearby early-phase major cluster merger CIZA J1358.9-4750

Abstract

CIZA J1358.9-4750 is a nearby (z = 0.074) pair of clusters of galaxies located close to the Galactic plane. It consists of two X-ray extended humps at the northwest and the southeast separated by 14′ (∼ 1.2 Mpc), and an X-ray bright bridgelike structure in between. With Suzaku, the south east hump was shown to have a temperature of 5.6 ± 0.2 keV and the northwest one 4.6 ± 0.2 keV. Neither hump exhibits significant central cool components. The bridge region has a temperature higher than 9 keV at the maximum, and this hot region is distributed almost perpendicular to the bridge axis, in agreement with the shock heating seen in numerical simulations at an early phase of a head-on major merger. This resemblance is supported by good positional coincidence between the X-ray peaks and cD galaxies associated with each cluster. In a short exposure XMM-Newton image, a significant intensity jump was found at a position where the Suzaku-measured temperature exhibits a steep gradient. These properties indicate the presence of a shock discontinuity. The Mach number is estimated to be 1.32 ± 0.22 from the temperature difference across the identified shock front, which gives a colliding velocity of approximately 1800 km s-1. From optical redshifts of the member galaxies, the two clusters are indicated to be merging nearly on the sky plane. Thus, CIZA J1358.9-4750 is considered as a valuable nearby example of early-phase merger with a clear shock feature.