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

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, The Netherlands
⁴ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁵ International Gemini Observatory/NSF NOIRLab, Casilla 603, La Serena, Chile
⁶ Cerro Tololo Inter-American Observatory/NSF NOIRLab, Casilla 603, La Serena, Chile
⁷ High Energy Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
⁸ Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
⁹ Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
¹⁰ Max Planck Institute for Extraterrestrial Physics, Gießenbachstraße 1, 85748 Garching bei München, Germany
¹¹ Institute of Cosmology and Gravitation, University of Portsmouth, Burnaby Road, Portsmouth PO1 3FX, UK
¹² Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Cambridge, MA 02139, USA
¹³ School of Physics, University of Melbourne, Parkville, VIC 3010, Australia
¹⁴ Department of Astronomy, University of Michigan, 1085 S. University Ave, Ann Arbor, MI 48109, USA
¹⁵ Department of Physics, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand

^⋆ Corresponding author; kmigkas@strw.leidenuniv.nl

Received: 6 November 2024
Accepted: 16 December 2024

Abstract

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 M_tot ∼ (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 δ_b ∼ (75 − 320) across the length of the bridge, and its gas mass is M_gas ∼ 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.