X-ray and optical analysis of the distant, merging double cluster SPT-CLJ2228-5828, its gas bridge, and shock front
Galaxy cluster mergers are excellent laboratories for studying a wide variety of different physical phenomena. Such a unique system is the distant SPT-CLJ2228-5828 cluster merger located at $z\approx 0.77$. Previous analyses via Sunyaev-Zeldovich and weak lensing data suggested that the system potentially was a dissociative cluster post-merger. In this work, we use new, deep XMM-Newton data to study the hot gas in X-rays, spectroscopic Gemini data to precisely determine the redshift of the two mass concentrations, and new HST data to improve the total mass estimates of the two components. We find that SPT-CLJ2228-5828 constitutes a pre-merging, double cluster system, instead of a post-merger. The merging process of the two clusters has started with their outskirt gas colliding with a $\sim 22^{\circ}-27^{\circ}$ on the plane of the sky. We fully characterize the surface brightness, gas density, temperature, pressure, and entropy profiles of the two merging clusters. The two systems have very similar X-ray properties with a moderate cluster mass of $M_{\text{tot}}\sim (2.1-2.4)\times 10^{14}\ M_{\odot}$. A $\approx 333$ kpc long gas bridge connecting the two clusters is detected at a $5.8\sigma$ level. The baryon overdensity of the excess bridge gas is $\delta_{\text{b}}\sim (75-320)$ across the length of the bridge and its gas mass is $M_{\text{gas}}\sim 1.4\times 10^{12}\ M_{\odot}$. Gas density and temperature jumps are also found across the gas bridge, revealing the existence of a weak shock front with a Mach number $\mathcal{M}\sim 1.1$. The gas pressure and entropy are also increased at the position of the shock front. We estimate the age of the shock front to be $\lesssim 100$ Myr and its kinetic energy $\sim 2.4\times 10^{44}$ erg s$^{-1}$. SPT-CLJ2228-5828 is the first such high-$z$ pre-merger with a gas bridge and a shock front to be studied in X-rays.