Merger of massive galaxy cluster CL0238.3+2005 at z~0.4: just after pericenter passage?

TL;DR

This paper reports on a rare, massive galaxy cluster observed shortly after a near head-on merger, combining multi-wavelength data to analyze its dynamic state and potential for studying dark matter and plasma physics.

Contribution

It presents a detailed multi-wavelength analysis of a rare post-merger galaxy cluster, providing insights into merger dynamics and its suitability for various astrophysical studies.

Findings

Cluster CL0238.3+2005 is in an intermediate merger phase.

The merger occurred less than 0.1 Gyr ago.

The merger axis is neither along the line of sight nor in the plane of the sky.

Abstract

Massive clusters of galaxies are very rare in the observable Universe. Even rarer are mergers of such clusters observed close to pericenter passage. Here, we report on one such case: a massive (~ $10^{15}\,M_\odot$) and hot (kT ~ 10 keV) cluster CL0238.3+2005 at $z\approx 0.42$. For this cluster, we combine X-ray data from SRG/eROSITA and Chandra, optical images from DESI, and spectroscopy from BTA and RTT-150 telescopes. The X-ray and optical morphologies suggest an ongoing merger with the projected separation of subhalos of $\sim 200$ kpc. The line-of-sight velocity of galaxies tentatively associated with the two merging halos differs by 2000-3000 km/s. We conclude that, most plausibly, the merger axis is neither close to the line of sight nor to the sky plane. We compare CL0238 with two well-known clusters MACS0416 and Bullet, and conclude that CL0238 corresponds to an intermediate phase between the pre-merging MACS0416 cluster and the post-merger Bullet cluster. Namely, this cluster has recently (only $\lesssim 0.1$ Gyr ago) experienced an almost head-on merger. We argue that this "just after" system is a very rare case and an excellent target for lensing, Sunyaev-Zeldovich effect, and X-ray studies that can constrain properties ranging from dynamics of mergers to self-interacting dark matter, and plasma effects in intracluster medium that are associated with shock waves, e.g., electron-ion equilibration efficiency and relativistic particle acceleration.