X-ray and optical analysis of the distant merging double cluster SPT-CLJ2228-5828, its gas bridge, and its shock front

Galaxy cluster mergers are excellent laboratories for studying a wide variety of different physical phenomena. An example of such a cluster system is the distant SPT-CLJ2228-5828 merger located at z ≈ 0.77. Previous analyses via the thermal Sunyaev-Zeldovich effect and weak lensing (WL) data suggested that the system was potentially a dissociative cluster post-merger, similar to the Bullet cluster. In this work, we perform an X-ray and optical follow-up analysis of this rare system. We used new deep XMM-Newton data to study the hot gas in X-rays in great detail, spectroscopic Gemini data to precisely determine the redshift of the two mass concentrations, and new Hubble Space Telescope 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 as previously thought. The merging process of the two clusters has started, with their gas on the outskirts colliding with a ∼22° −27° on the plane of the sky. Both clusters have a similar radius of R500 ∼ 700 kpc, with the two X-ray emission peaks separated by ≈1 Mpc (2.1′). We fully characterized the surface brightness, gas density, temperature, pressure, and entropy profiles of the two merging clusters for their undisturbed non-interacting side. The two systems have very similar X-ray properties, with a moderate cluster mass of Mtot ∼ (2.1 − 2.4)×1014 M⊙ according to X-ray mass proxies. Both clusters show good agreement with known X-ray scaling relations when their merging side is ignored. The WL mass estimate of the western cluster agrees well with the X-ray-based mass, whereas the eastern cluster is surprisingly only marginally detected from its WL signal. A gas bridge with ≈333 kpc length connecting the two merging halos is detected at a 5.8σ level. The baryon overdensity of the excess gas (not associated with the cluster gas) is δb ∼ (75 − 320) across the length of the bridge, and its gas mass is Mgas ∼ 1.4 × 1012 M⊙. The gas density and temperature jumps at ∼10−3 cm−3 and ∼5.5 keV, respectively, are also found across the gas bridge, revealing the existence of a weak shock front with a Mach number ℳ ∼ 1.1. The gas pressure and entropy also increase at the position of the shock front. We estimate the age of the shock front to be ≲100 Myr and its kinetic energy ∼2.4 × 1044 erg s−1. SPT-CLJ2228-5828 is the first such high-z pre-merger with a gas bridge and a shock front, consisting of similarly sized clusters, to be studied in X-rays.