Discovery of a pre-merger shock in an intercluster filament in Abell 98

TL;DR

This paper reports the first clear detection of a merger shock in Abell 98, revealing early cluster formation stages, electron heating mechanisms, and intercluster filament properties through deep Chandra X-ray observations.

Contribution

It presents the first unambiguous detection of an axial merger shock in Abell 98 and characterizes the intercluster filament, advancing understanding of early cluster mergers and the warm-hot intergalactic medium.

Findings

Detection of a shock with Mach number ~2.3 in Abell 98.

Observation of a 1.07 keV intercluster gas filament.

Electron temperature data supports collisionless shock heating models.

Abstract

We report the first unambiguous detection of an axial merger shock in the early-stage merging cluster Abell 98 using deep (227 ks) Chandra observations. The shock is about 420 kpc south from the northern subcluster of Abell 98, in between the northern and central subclusters, with a Mach number of M $\approx$ 2.3 $\pm$ 0.3. Our discovery of the axial merger shock front unveils a critical epoch in the formation of a massive galaxy cluster, when two subclusters are caught in the early phase of the merging process. We find that the electron temperature in the post-shock region favors the instant collisionless model, where electrons are strongly heated at the shock front, by interactions with the magnetic field. We also report on the detection of an intercluster gas filament, with a temperature of kT = 1.07 $\pm$ 0.29 keV, along the merger axis of Abell 98. The measured properties of the gas in the filament are consistent with previous observations and numerical simulations of the hottest, densest parts of the warm-hot intergalactic medium (WHIM), where WHIM filaments interface with the virialization regions of galaxy clusters.