The SRG/eROSITA all-sky survey

View of the Fornax galaxy cluster

¹ Argelander-Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Max-Planck-Institut für extraterrestrische Physik, Gießenbachstr. 1, 85748 Garching bei München, Germany
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁵ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
⁶ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, via Piero Gobetti 93/3, 40129 Bologna, Italy
⁷ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China

^⋆ Corresponding author: reiprich@uni-bonn.de

Received: 27 February 2025
Accepted: 8 August 2025

Abstract

Context. The Fornax cluster is one of the closest X-ray-bright galaxy clusters; as such, we can study the system at high spatial resolution. However, previous observations of the intracluster medium were limited to less than R₅₀₀.

Aims. We aim to significantly extend the X-ray coverage of the Fornax cluster and to search for features in the X-ray surface brightness distribution beyond R₅₀₀ induced by the gravitational growth of this system.

Methods. We used data from five SRG/eROSITA all-sky surveys and performed a detailed one- and two-dimensional X-ray surface brightness analysis, tracing hot gas emission from kiloparsec to megaparsec scales with a single instrument. We compared the results to those from a recent numerical simulation of the local Universe (SLOW) and correlated the X-ray emission distribution with that of other tracers, including cluster member galaxies, ultra-compact dwarf galaxies, intracluster globular clusters, and HI-tail galaxies.

Results. We detect X-ray emission out to well beyond the virial radius, R₁₀₀ = 2.2 deg. In the inner regions within R₅₀₀, we see previously known features, such as a large-scale spiral-shaped edge; however, we do not find obvious evidence of the bow shock several hundred kiloparsecs south of the cluster center predicted by previous numerical simulations of the Fornax cluster. Instead, we discover emission fingers beyond R₅₀₀ to the west and southeast and excesses that stretch out far beyond the virial radius. They might be due to gas being pushed outward by the previous merger with NGC 1404 or due to warm-hot gas infall along large-scale filaments. Intriguingly, we find the distributions of the other tracers – galaxies and globular clusters – to be correlated with the X-ray-excess regions, favoring the infall scenario. Interestingly, we also discover an apparent bridge of low-surface-brightness emission beyond the virial radius connecting to the Fornax A galaxy group, which is also traced by the member galaxy and globular cluster distribution. This X-ray bridge furthermore approximately coincides with a region of enhanced Faraday depth detected previously. The gas distribution in the SLOW simulation shows similar features as those we have discovered with SRG/eROSITA.

Conclusions. SRG/eROSITA has enabled us to tremendously expand the view of the intracluster medium of the Fornax cluster. We witness the growth of a cluster along large-scale filaments.