Indications of an offset merger in Abell 3667

¹ Departure of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
² Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 229-8510, Japan
⁴ International Center for Quantum-field Measurement Systems for Studies of the Universe and Particles (QUP), The High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
⁵ Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
⁶ Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷ Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁸ Core Research for Energetic Universe, Department of Physics, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁹ Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan
¹⁰ Department of Physics, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan
¹¹ SRON Netherlands Institute for Space Research, Utrecht, The Netherlands
¹² Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹³ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹⁴ RIKEN Nishina Center for Accelerator-Based Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
¹⁵ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
¹⁶ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹⁷ Mizusawa VLBI Observatory, National Astronomical Observatory Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Received: 15 March 2024
Accepted: 17 June 2024

Abstract

Context. Cluster mergers are the most energetic events, releasing kinetic energies of up to 10⁶⁴ erg and involving megaparsec(Mpc)-scale shocks in their intra-cluster medium (ICM). In merging clusters, cold fronts are frequently observed, which are characterized by temperature and density jumps while maintaining constant pressure. They, together with the overall morphology of the ICM, provide important information for our understanding of the merging structure, such as velocity, impact parameter, and mass.

Aims. Abell 3667 is a nearby (z = 0.056) merging cluster with a prominent cold front and a pair of two bright radio relics. Assuming a head-on merger, the origin of the cold front is often considered to be a remnant of the cluster core stripped by its surrounding ICM. Some authors have proposed an offset merger scenario in which the subcluster core rotates after the first core crossing. This scenario can reproduce features such as the cold front and a pair of radio relics. To distinguish between these scenarios, we reanalyzed the ICM distribution and measured the line-of-sight bulk ICM velocity using the XMM-Newton PN data.

Methods. We created an unsharp masked image to identify ICM features, and analyzed X-ray spectra to explore the ICM thermodynamical state. Applying the new XMM-Newton European Imaging Camera (EPIC)–PN calibration technique using background emission lines, the line-of-sight bulk ICM velocities were also measured.

Results. In the unsharp masked image, we identify several ICM features, some of which we detect for the first time. We confirm the cold front and note an enhanced region extending from the cold front to the west (named “CF-W tail”). There is an enhancement of the X-ray surface brightness extending from the first brightest cluster galaxy (BCG) to the cold front, which is named the “BCG-E tail”. The notable feature is a “RG1 vortex”, which is a clockwise vortex-like enhancement with a radius of about 250 kpc connecting the first BCG to the radio galaxy (RG1). It is particularly enhanced near the north of the 1st BCG, which is named the “BCG-N tail”. The thermodynamic maps show that the ICM of the RG1 vortex has a relatively high abundance of 0.5−0.6 solar compared to the surrounding regions. The ICM of the BCG-E tail also has a high abundance and low pseudo-entropy and can be interpreted as a remnant of the cluster core’s ICM. Including its arc-like shape, the RG1 vortex supports the idea that the ICM around the cluster center is rotating, which is natural for an offset merger scenario. The results of the line-of-sight bulk ICM velocity measurements show that the ICM around the BCG-N tail is redshifted with a velocity difference of 940 ± 440 km s⁻¹ compared to the optical redshift of the first BCG. We obtain other indications of variations in the line-of-sight velocity of the ICM and discuss these in the context of an offset merger.